Shelf Life of an Extruded Corn Snack

SHELF LIFE DETERMINATION OF AN
EXTRUDED CORN SNACK FOOD
A Thesis (Research Paper)
Submitted in Partial Fulfilment of the Requirements for the
Degree of Master Science in Food Science and Technology
of
The University of the West Indies
Tamayo Hutton
2007
Department of Chemical Engineering
Food Science and Technology Unit
St. Augustine Campus

Acknowledgements i
ACKNOWLEDGEMENTS
Thank you very much Dr. G.H. Baccus-Taylor and Prof. J. Akingbala for
guidance and tremendous support in finally completing this research paper.
Thanks to Dr. G. Legall for last minute help and advice on Statistical Analysis
methods. Many thanks to Ms. Giselle Ramtahal and Ms. Karen Camejo,
laboratory technicians for support through the many difficulties experienced in the
lab.
Many regards to Ms. Patricia Bhairo-Beekhoo, Mr. Allan Dass and Mr.
Ian Currie for facilitating key aspects of this project. Finally, a very special thank
you to Ms. Colleen Norville, Ms. Lucy Brown and Mr. Clinton Hutton for
emotional support through extremely challenging times.

Table of Contents ii
TABLE OF CONTENTS
Page
Acknowledgements i
List of Figures vii
List of Tables ix
Abstract x
1.0 Introduction 1
1.1 Definition of Shelf Life 1
1.2 Definition of an Extruded Corn Snack Food 2
1.3 Project Outline 2
1.4 Problem Definition 3
1.5 Scope of Work 4
2.0 Literature Survey 5
2.1 Extruded Corn Snack 5
2.1.1 Brief Process Description 5
2.1.2 Cornmeal 6
2.1.3 Flavour 6
2.1.4 Palm Olein Oil 7
2.1.5 Nutritional Composition 7
2.1.6 Packaging Material 8
2.1.7 The Extrusion Process 9
2.2 Food Quality 10
2.3 Intrinsic Factors 12

Table of Contents iii
Page
2.3.1 Taste 13
2.3.2 Smell 14
2.3.3 Vision 15
2.3.4 Hearing 15
2.3.5 Kinesthesis & Somethesis 16
2.4 Extrinisic Factors 17
2.5 Shelf Life 18
2.6 Legislation 20
2.6.1 The Republic of Trinidad & Tobago 22
2.6.2 Other Caribbean Territories 23
2.6.3 North America 25
2.6.4 The European Union 25
2.7 Shelf Life Importance 27
2.8 Factors Affecting Shelf Life 28
2.8.1 Biological Decay – Pre-harvesting 29
2.8.2 Senescence 29
2.8.3 Water Activity & Moisture Migration 30
2.8.4 Transfer of Substances other than Moisture and/or
Water Vapour 31
2.8.5 Microbial Factors 32
2.8.6 Rancidity Development 34
2.9 Review of Research on Similar Corn Based Snack Foods 38
2.9.1 Shelf Life Determination of Lightly Salted Potato Chips 38
2.9.2 Mathematical Models for Estimating the Shelf Life of
Corn Flakes 39
2.9.3 Flavour Properties and Stability of a Corn-Based Snack 43
2.10 Shelf Life Determination 46

Table of Contents iv
Page
2.11 Determination of the End of the Product’s Shelf Life 47
2.12 Determination of Suitable Shelf Life Tests & Procedures 49
2.12.1 Lipid Oxidation & Off-Flavour Development 50
2.12.2 Extraction of Lipids 52
2.12.3 Moisture Migration and its effect on Texture 56
2.12.4 Sensory Evaluation 57
2.13 Accelerated Shelf Life 59
3.0 Methodology 61
3.1 Storage of Samples 61
3.2 Sensory Evaluation of a Locally Manufactured
Cheese-Flavoured Extruded Corn Snack by Trained Panellists 62
3.3 Determination of the Moisture Uptake in Stored Samples 65
3.4 Texture Analysis of Samples using the Penetrometer 65
3.5 Lipid Extraction from the Extruded Corn Snack Food 66
3.6 Peroxide Value Evaluation of Extracted Oil 69
3.7 Treatment of Results 69
4.0 Results & Discussion 71
4.1 Results of Sensory Evaluation of the Extruded Corn
Snack Food 71
4.1.1 Sensory Evaluation of the Texture Attribute 71
4.1.1.1 Evaluation of the Texture Attribute of Sample ‘A’ 72
4.1.1.2 Evaluation of the Texture Attribute of Sample ‘R’ 72
4.1.2 Sensory Evaluation of the Flavour Attribute 73
4.1.2.1 Evaluation of the Flavour Attribute of Sample ‘A’ 74
4.1.2.2 Evaluation of the Flavour Attribute of Sample ‘R’ 74
4.1.3 Sensory Evaluation of the “Overall Attribute” 74

Table of Contents v
Page
4.1.3.1 Evaluation of the “Overall Attribute” of
Sample ‘A’ 75
4.1.3.2 Evaluation of the “Overall Attribute” of
Sample ‘R’ 76
4.2 Statistical Analysis of the Sensory Evaluation Results of the
Extruded Corn Snack Food 76
4.2.1 Statistical Analysis of the Texture Attribute Sensory
Results 76
4.2.2 Statistical Analysis of the Flavour Attribute Sensory
Results 79
4.2.3 Statistical Analysis of the “Overall Attribute” Sensory
Results 82
4.3 Results of Moisture Uptake in an Extruded Corn Snack Food 85
4.4 Results of Texture Analysis of an Extruded Corn Snack
Food using the Penetrometer 86
4.5 Comparison of Results of Sensory Evaluation of the
Texture with Penetrometer & Moisture Uptake of the
Extruded Corn Snack Food 87
4.5.1 Graphical Comparison of the Texture Attribute Sensory
Results with Moisture Uptake of an Extruded Corn
Snack Food 87
4.5.2 Statistical Comparison of the Texture Attribute Sensory
Results with Moisture Uptake of an Extruded Corn
Snack Food 88
4.5.3 Graphical Comparison of the Texture Attribute Sensory
Results with Penetrometer Readings of an Extruded Corn
Snack Food 90

Table of Contents vi
Page
4.5.4 Statistical Comparison of the Texture Attribute Sensory
Results with Penetrometer Readings of an Extruded Corn
Snack Food 90
4.6 Results of the Peroxide Value of the Oil Extracted
from an Extruded Corn Snack Food 92
4.7 Comparison of the Flavour Attribute Sensory Results with
the Peroxide Value of Oil Extracted from an Extruded Corn
Snack Food 93
4.7.1 Graphical Comparison of the Flavour Attribute Sensory
Results with the Peroxide Value of Oil Extracted from
an Extruded Corn Snack Food 93
4.7.2 Statistical Comparison of the Flavour Attribute Sensory
Results with the Peroxide Value of Oil Extracted from
an Extruded Corn Snack Food 94
4.8 Evaluation of the Control of the Snack Food 96
5.0 Conclusion 97
6.0 Recommendations 98
References 99
Appendix: Raw Data 112
Appendix: Minitab Logic Regression Data 122
Appendix: Sensory Evaluation Score Sheet 125

List of Figures vii
LIST OF FIGURES
Page
2.1 Development of rancidity in foods, storage times given in
arbitrary values, PV – peroxide value; IP – induction period 36
2.2 Food stability as a function of water activity 37
2.3 Aqualab 3TE moisture analyser 41
2.4 Texture technologies TA.XT2i texture analyser 42
2.5 Osme equipment and Osmegram 44
2.6 Osmegram showing panel member’s scorecard 45
2.7 Glassware used in Soxhlet extraction 54
2.8 Arrhenius’ equation 59
3.1 Sensory evaluation test booth set up 63
3.2 Rotary evaporator 68
4.1 Percentage of panellist that failed the Texture Attributes of the
snack food during 9 weeks of storage under retail and ambient
conditions 78
4.2 Percentage of panellist that failed the Flavour Attributes of the
conditions 81
4.3 Percentage of panellist that failed the Overall Attributes of the
conditions 83

List of Figures viii
Page
4.4 Comparison of the texture attribute results of sensory evaluation
with the results of mean moisture increase in an extruded corn
snack food stored for 9 weeks in ambient and retail conditions 89
4.5 Comparison of the texture attribute results of sensory evaluation
with the results of penetrometer in an extruded corn snack food
stored for 9 weeks in ambient and retail conditions 91
4.6 Comparison of the flavour attribute results of sensory evaluation
with the results of peroxide value of oil extracted from an extruded
corn snack food stored for 9 weeks in ambient and retail
conditions 95

List of Tables ix
LIST OF TABLES
Page
2.1 The nutritional facts of the snack 7
2.2 Water activity (aw) values of some foods 33
2.3 Water activity (aw) limits for microbial growth 34
2.4 Guidelines that can be followed to set shelf life end-point 47
2.5 Ranking of lipid oxidation methods 51
4.1 Sensory evaluation of the texture of an extruded corn snack food 71
4.2 Sensory evaluation of the flavour of an extruded corn snack food 73
4.3 Sensory evaluation results of a snack food showing
“overall attribute” 75
4.4 Moisture increase in an extruded corn snack food for a 9
week storage period 85
4.5 Penetrometer distance (in mm) of a snack food for 5 seconds 87
4.6 Peroxide values of oil extracted from an extruded corn snack
food during storage for control, ambient and retail conditions 92

Abstract x
ABSTRACT
This study was an attempt to determine the shelf life of a locally
manufactured cheese-flavoured extruded corn snack, packaged in metallised
oriented polypropylene film, using sensory evaluation techniques. Twenty (20)
trained panel members were instructed to evaluate samples, stored under two
conditions against a ‘control’. The samples were stored under ambient conditions
(33o
C, relative humidity 100%), and retail conditions (18o
C, relative humidity
75o
C). The control was frozen at a temperature of -18o
C. Panellists evaluated
samples for texture, flavour and overall attributes (appearance, texture, sound,
flavour) using an 11-point scale. Objective testing of physicochemical properties
of the snack food were performed to correlate with the subjective sensory
evaluation. Regression was used to fit sensory data and to determine an
established manufacturer’s cut-off point (the point at which the quality of the
snack became unacceptable). Logistic regression was used to correlate moisture
uptake in the snack food, and penetrometer readings, with sensory evaluation of
the texture attribute and, peroxide value of oil extracted from the snack food, with
sensory evaluation of the flavour attribute. The shelf of the packaged snack food
was found to be 50 days under ambient conditions (33o
C, 100% RH), and 58 days
under retail conditions (18o
C, 75% RH). Penetrometer results correlated best with
sensory analysis, producing a Pearson p-value of 0.000 (95% confidence).

Chapter 1: Introduction 1
CHAPTER 1
INTRODUCTION
1.1 DEFINITION OF SHELF LIFE
Shelf life can be defined generally as the period of time following
manufacture, over which a food maintains the specified quality (Eskin and
Robinson, 2001). Guidelines of the Institute of Food Science and Technology
(1993) define shelf life as the time during which the product will remain safe; will
be certain to retain the desired sensory, chemical, physical and microbiological
characteristics; and finally will comply with any label declaration of nutritional
data when stored under the recommended conditions. At the end of the shelf life,
the food is deemed unacceptable for sale and hence consumption. Characteristics
of the food that determine the shelf life are the sensory, nutritional,
microbiological properties (Eskin and Robinson, 2001).

1.2 DEFINITION OF AN EXTRUDED CORN SNACK FOOD
Snack food may be defined as a type of food not meant to be eaten as a
main meal of the day such as breakfast, lunch or dinner (WFI 2006c). It is a food
intended to be consumed between meals, providing a brief supply of energy
(Farlex Incorporated, 2007). Processed snack foods are designed to be less
perishable and more durable than prepared foods. These foods generally have
little or no nutritional value and are consumed purely for the enjoyment of its taste
(WFI 2006c). An extruded corn snack food is a processed snack food, consisting
of a corn base made from cornmeal or corn grits, that is cooked and expanded via
the extrusion process and then flavoured (Lusas and Rooney, 2001).
1.3 PROJECT OUTLINE
With the rapid increase of convenience stores, packaged snack foods are
significant business. The snack food industry in market-driven societies generates
billions of dollars in revenue annually. The market for processed snack foods is
enormous, and many large corporations compete rigorously to capture larger
shares of the snack food market (WFI 2006c).

Two (2) of the leading snack food companies in the Republic of Trinidad
and Tobago produce different types of snacks such as, tortilla chips, potato chips,
assorted nuts, pop corn, biscuits and cookies. However, the type of snack food
that is sold in the largest volume for both companies is extruded corn snack foods
(Anonymous 2007b; Anonymous 2007c, personal correspondence). Extruded
corn snack foods are easily shaped (which is critical in attracting children), its
texture attribute is easily adjusted, and many variations of products may be
created from a single process line (Anonymous 2007a, personal correspondence).
1.4 PROBLEM DEFINITION
Both snack companies determine the shelf life of their snack foods under
uncontrolled environmental conditions using sensory evaluation techniques only.
This is not reproducible and is not reliable in ensuring the quality of the product
(Man 2000). The snack companies rely on the measurement of physical
parameters such as salt percentage, moisture content, and lipid content, to control
quality of the snack foods during production, but do not incorporate this in the
determination of the foods’ shelf lives (Anonymous 2007a; Anonymous 2007c,
personal correspondence). Correlating sensory evaluation results with
physicochemical properties is an established practice in shelf life determination
(Man 2000). As the competition among local and international companies

increase, for larger shares of the market, quality assurance, and hence more
reliable shelf life determination methods, becomes more critical.
1.5 SCOPE OF WORK
This study intends to analyze the shelf life of a popular locally
manufactured extruded corn snack food flavoured with powdered cheese.
Confidentiality agreements demand that names of employees and details of the
formulation and process are not disclosed, thus Anonymous is liberally cited
throughout this study. The objectives of this project were:
1. To use sensory evaluation tests and statistical analysis to determine the
length of time within which a locally manufactured cheese flavoured
extruded corn snack became unacceptable for consumption under the
following conditions.
a. At the most extreme storage conditions in ambient, tropical
conditions at 33o
C and 100% relative humidity.
b. Under typical retail outlet storage conditions at 18o
C and 75%
relative humidity.
2. To evaluate physical and chemical properties of the snack food to
determine any correlation between physicochemical properties and
sensory tests, in determining the product’s shelf life.

Chapter 2: Literature Survey: Extruded Corn Snack Food 5
CHAPTER 2
LITERATURE SURVEY
2.1 EXTRUDED CORN SNACK FOOD
2.1.1 Brief Process Description
Anonymous (2007a, personal correspondence) gave a brief description of
the process used for the locally manufactured extruded corn snack. Water is added
to dry corn meal with a dry moisture content of approximately 11% in order to
increase the moisture content to approximately 15%. The mixture is then extruded
through dies at high pressure and temperature which expand the base, forms the
snack’s shape and gives it the required density. The expanded product is baked to
the required specification, ensuring the right texture and mouth feel. It is also the
only ‘kill’ step in the process. A cheese powder-based flavour is then mixed with
palm olein oil. The resulting slurry is pumped and sprayed on the extruded corn
base. The product is finally packaged in film consisting of metallized coextruded
biaxially oriented polypropylene (metallized OPP). The finished product is a dry
cheese-flavoured extruded corn snack, with a moisture content of approximately
1.5%.

2.1.2 Cornmeal
Corn is the key component for the extruded base of the snack food
(Anonymous 2007a, personal correspondence). Corn is dry milled to corn meal
that has a granulation of 0.59 mm – 0.193 mm. Meal has less than 1% oil, low ash
and fibre content, a long shelf life and a bright colour without black specks (Lusas
and Rooney, 2001). Suppliers consistently provide high quality corn meal. As the
cornmeal does not vary significantly in quality within a shipment, or from
shipment to shipment, the cornmeal does not change the product’s shelf life
(Anonymous 2007a, personal correspondence).
2.1.3 Flavour
Historically, the most popular flavours for salty snack seasonings have
been cheese, barbeque, sour cream and onion, and ranch (Lusas and Rooney,
2001). The seasoning typically has a salt content of 10 – 15% (Lusas and Rooney,
2001). The extruded corn snack is flavoured with a powdered cheese seasoning
which is typical of cheese seasonings currently used on the market. The cheese
supplier, similarly to the corn meal supplier, consistently supplies a raw material
that conforms to the major food standards worldwide. Therefore, its percentage
moisture content and microbial load are consistent and do not fluctuate
significantly (Anonymous 2007a, personal correspondence).

2.1.4 Palm Olein Oil
Palm olein oil is used in the manufacture of the extruded corn snack. This
oil is used to aid in extrusion, but its primary purpose is in flavouring the extruded
corn base (Anonymous 2007a, personal correspondence). Palm olein is the liquid
fraction obtained by fractionation of palm oil after crystallization at controlled
temperatures. It is fully liquid in warm climate and has a narrow range of
glycerides (APOC 2007). Palm oil resists oxidation and rancidity, therefore
products made using palm oil have an extended shelf life. Palm oil contains a
balance of polyunsaturated, monounsaturated and saturated fatty acids (Wright).
The oil is consistently supplied at a typical acceptable industry standard range of
peroxide values and free fatty acid percentages (Anonymous 2007a, personal
correspondence).
2.1.5 Nutritional Composition
Table 2.1 shows the nutritional composition of the extruded corn snack
food. The percentage composition of the snack by mass is as follows:
Total fat = 7/19 x 100 = 36.8%*
Total carbohydrates = 10/19 x 100 = 52.6%*
Total Sodium = 0.11/19 x 100 = 0.6%*
Moisture = 1.5% (maintained by process)

TABLE 2.1 The Nutritional Facts of the Snack (Anonymous 2007c, personal
correspondence).
Serving Size 19 g
Amount per serving
Calories 110
Calories from fat 60
% Daily Value based on 2,000 calorie diet
Total Fat 7g 11%
Saturated Fat 4g 20%
Sodium 110mg 5%
Total Carbohydrates 10g 3%
Dietary Fibre 0g 0%
Sugar 0g
Protein 1g
Vitamin A 0% · Vitamin C 0%
Calcium 0% · Iron 0%
2.1.6 Packaging Material of the Extruded Corn Snack Food
The material that is used to package the extruded corn snack food is
typical metallised oriented polypropylene (OPP) that is used worldwide
(Anonymous 2007a, personal correspondence). Most snack food packaging use
OPP film in one of several forms. The resulting functional performance of OPP
meets the protection requirements of many snack foods (Lusas and Rooney,
2001).
The purpose of the packaging is to provide barriers to environmental
influences. The snack food requires barriers from moisture, oxygen and light from
entering the package, as well as flavour from leaving the package (Coles, et al.
2003). The shelf life of the snack food is to be determined within its packaging.

The packaging will serve the purpose of extending the life of the food, but will
not affect the types of degradation typical of snack foods (Lusas and Rooney,
2001).
2.1.7 The Extrusion Process
Many forms of extruders exist, but they all function similarly (Lusas and
Rooney, 2001). A barrel, in which a close-fitting screw is continually rotating,
moves product towards the discharge end. Product moves to a restricted opening,
and as more product is conveyed, less becomes available and compression of the
product occurs (Charalambous 1993). The compression produces heat energy,
which converts raw granular, starch material, into a smooth viscous dough. This
heat converts moisture in the dough to steam, which upon discharge, causes
product expansion due to the reduction in pressure (Charalambous 1993).
This extrusion process is continuous. The high temperatures that are
formed are able to inactivate most micro-organisms and enzymatic systems, but
are applied for a short enough time (typically less than 15 seconds), that minimal
nutrient losses and functional changes occur (Lusas and Rooney, 2001). The
process is highly versatile. Modifications in ingredients, moisture content, screw
speed and discharge size opening, may produce a wide variety of products that
may be partially cooked, fully cooked or totally expanded from the same unit
(Lusas and Rooney, 2001).

Chapter 2: Literature Survey: Food Quality 10
2.2 FOOD QUALITY
Food quality has different meanings for different food industry
professionals (Taub and Singh, 1998). Food quality is dependent on three main
characteristics. Nutritionists consider the nutritional value, microbiologists
consider the food’s safety, while chemists consider the food’s stability as
measures of its quality (Singhal, et al. 1997). Despite these valid interpretations of
food quality, ultimately it is the consumer, through the purchase of the product,
who determines foods’ quality (Taub and Singh, 1998).
Food quality is defined in the United States Department of Agriculture
(USDA) Marketing Workshop Report (1951) as “the combination of attributes or
characteristics of a product that have significance in determining the degree of
acceptability of a product to a user”. Quality is often measured in industry by
different physical, chemical and microbial properties of the food, and any other
distinctive attribute or characteristic of the product (Gould 1977). Alli (2004)
defines food quality as “the extent to which all the established requirements
relating to the characteristics of a food are met”. It is important to define and
assign values to attributes of the food which coincide with the consumer’s
requirements and expectations, or to set the food’s standards. The term quality,
without being defined in terms of some standard, means very little (Gould 1977).

Chapter 2: Literature Survey: Food Quality 11
Cardello (1995) defines food quality as “the acceptance of the perceived
characteristics of a product by consumers who are the regular users of the product
category or those who comprise the market segment”. This definition of quality
takes into consideration all characteristics of the food, not only the sensory
attributes, but also factors that include convenience, cost and value (Taub and
Singh, 1998).
The quality of processed food deteriorates from the point of manufacture
over time. When the quality of the food is no longer fit for consumption the
product’s shelf life has ended (Man 2000).

Chapter 2: Literature Survey: Intrinsic Factors 12
2.3 INTRINSIC FACTORS
The factors that affect a consumer’s perception of food and food quality
are numerous. Many of these factors are intrinsic to the food, and are related to its
physicochemical characteristics (Singhal, et al. 1997). These include such factors
as ingredient, processing and storage variables. These factors are some of the
most salient and important variables, determining both the acceptability and
perceived quality of the item to the consumer (Man and Jones, 2000). Consumer
enjoyment of snack foods for example results from several factors, primarily
taste, texture and size (Lusas and Rooney, 2001). It is usually through these
sensory characteristics and the internal changes that occur over time, that
consumers develop opinions about other aspects of food quality, such as, safety,
stability, and even the nutritional value (Man and Jones, 2000).
Food’s quality is perceived by the interaction between the
physicochemical properties of the food and human sensory receptor organs (Taub
and Singh, 1998). The food becomes a stimulus which triggers sensory
experiences of the consumer that create complex perceptions of quality (Man and
Jones, 2000). Taste, smell, texture and appearance of food contribute greatly to
the consumer’s perception of the quality of that food (Singhal, et al. 1997).
Understanding the relationships between, physicochemical characteristics of food,

the sensory and physiological mechanisms that convert these characteristics into
human perceptions of food attributes, and the effects of these perceived attributes
on acceptance and/or consumption of the item, is critical to an understanding of
what constitutes food quality (Taub and Singh, 1998).
2.3.1 Taste
Taste is a sensory experience that results from stimulation of
chemoreceptors located on the tongue, palate, pharynx, larynx and other areas of
the oral cavity. There are five distinct taste categories, salty, sour, sweet, bitter,
and umami (Jackson and Linskens, 2002).
Saltiness is stimulated by the sodium ion (Na+
) and cations of other low
molecular weight salts. Sourness is stimulated by the presences of hydrogen ions,
although the anion and undisassociated acid can modify its taste (Bessière and
Thomas, 1990). The sweet sensation may occur as a reaction to a variety of
organic and inorganic compounds (Taub and Singh, 1998). Bitter taste is more
complicated. It is triggered by alkaloids, heavy halide salts, as well as some amino
acids. It is believed that there may be three or more different bitter receptor
mechanisms (Bessière and Thomas, 1990).
Umami, the fifth recognised taste category is described as delicious or
savoury, and is associated with monosodium glutamate and the taste of meat

(UIC). Pungency and astringency are also associated with sensations of taste
(Taub and Singh, 1998). Pungency is the sensation associated with pepper or
spiciness. Taste receptors, especially the mucus membranes, react to the chemical
capsaicin (WFI 2006a). Astringency is a dry sensation that occurs when tannins
denature the salivary proteins, causing a rough ‘sandpapery’ feel in the mouth
(WFI 2006b). The sensations pungency and astringency are part of the ‘common
chemical sense’ (Green, et al. 1990).
2.3.2 Smell
Smell is the sensory experience which occurs when receptors in the
olfactory epithelium of the nose are stimulated by airborne compounds. The odour
of a food and its ‘taste’ are often confused, as during chewing of the food air from
the mouth is passed into the nasal cavity through the nasopharynx (Jacob).
There has been great difficulty over the years to classify the multitude of
smells into categories. Equally difficult has been the identification of the
attributes of the stimulus that elicit odour qualities (Taub and Singh, 1998).
It is now believed that there may be upwards of 300 to 1000 different
olfactory genes, a number so large that it could easily account for the over 10,000
perceptible odours. This large number of receptor types is unique in human
sensory systems, and accounts for the complexities involved in determining the

quality of smell in foods. It is also responsible for the vast differences among
persons in their perception of smells (Taub and Singh, 1998).
2.3.3 Vision
Visual perception occurs as a result of stimulation of receptors in the
retina of the eye by electromagnetic radiation (Taub and Singh, 1998). Colour is
critical in the appearance of food and the perception of its quality. However, other
visual attributes also play a critical role. All foods and beverages absorb some
light, and the rest is reflected or transmitted. The clarity and lustre of food and
beverages are thus also used as a means to determine food quality. Size, shape,
surface texture and wholeness (such as in nuts, potato chips and extruded snacks),
are also other visual attributes of food that determine the perception of quality
(Steele 2004).
2.3.4 Hearing
Hearing is the sensation that results from the stimulation of receptors in
the cochlea of the ear by sound waves (Vickers 1979). The sounds produced
during biting and mastication of food have significant effects on quality
perception in certain foods, such as cereals, potato chips and fresh fruits (OTA
1979). Recently, food has been analysed for their texture by the physical or
perceptual sounds emitted during mastication (Vickers 1979).

2.3.5 Kinesthesis & Somethesis
Food texture and quality may be perceived by kinesthesis, which is the
perception of limb position and limb movement (EBI 2007), as well as
somesthesis, which is the perception of pressure, pain and temperature. Since
foods provide resistance to active jaw movements (chewing), both kinesthetic
joint and muscle receptors are involved in the perception of food texture and
quality (Taub and Singh, 1998).
There are receptors that give rise to painful sensations as a result of
intense tactile, thermal, or chemical stimuli (Taub and Singh, 1998). The intensity
of the sensations may result in perceptions of stinging, chemical cool and
chemical warmth, even the perception of carbonation in soft drinks. These
sensations mediated by the trigeminal nerve, belong to what is called the
“common chemical sense” (Taub and Singh, 1998).

Chapter 2: Literature Survey: Extrinsic Factors 17
2.4 EXTRINSIC FACTORS
Although intrinsic factors are significant determinants of food quality,
there are several extrinisic factors that also play a role in consumers’ perception.
Attitudes, expectations, environmental conditions, biological factors (hunger,
thirst, health) and social or cultural influences may affect a consumer’s perception
of quality (Taub and Singh, 1998; Steele 2004).
These extrinisic factors must be considered and systematically eliminated
or regularised, so that the perception of the intrinsic factors of the food being
studied may be accurately determined. A well-trained panel will dramatically
reduce the likelihood of extrinisic factors affecting the quality of the sensory
results (Meilgaard, et al. 1999; Freitas, et al. 2003).

Chapter 2: Literature Survey: Shelf Life 18
2.5 SHELF LIFE
Except in the situations where microbiological deterioration has occurred
to the extent to which the product becomes unsafe, the definition of the shelf life
will be determined by the customer and the manufacturer (Kilcast and
Subramaniam, 2000; Man 2002). A lower quality, economy product may have a
longer shelf life than a high quality, premium product, even though the quality
index of the expired premium product may be higher than that of the economy
product at the start of its shelf life. The difference in the shelf life given to
different quality products, typically occur due to the expectations of the
consumer. The consumer of a premium product will have a higher expectation of
the product than the consumer of the economy product (Kilcast and
Subramaniam, 2000). However, a large degree of change is evidently tolerable to
many customers (Singhal, et al. 1997), and so acceptable sensory characteristics
are often defined by company policy (Kilcast and Subramaniam, 2000).
Shelf life is greatly affected by the storage conditions under which the
product is kept (Man 2002). IFST (1993) explicitly states that shelf life must be
defined under specific recommended conditions. Storage characteristics are
measured under carefully controlled environmental conditions, that are generally
not experienced for the period between product manufacture and consumption by

Chapter 2: Literature Survey: Shelf Life 19
the consumer. It is therefore important that the storage characteristics of the
product under different types of storage conditions are understood (Kilcast and
Subramaniam, 2000).
Shelf life is a complex concept that is dependent on the nature of the food
product under consideration, the preservation technologies applied, and the
environmental conditions to which the food product is exposed (Eskin and
Robinson, 2001). The packaging of the product also has a great influence in the
maintenance of the quality and the shelf life of the food, and is a major part of the
preservation system of the food (Eskin and Robinson, 2001).

Chapter 2: Literature Survey: Legislation 20
2.6 LEGISLATION
Food legislation in most countries requires most pre-packaged foods to
carry an “open date” or date of “minimum durability” (Man and Jones, 2000).
These dates help the consumer to decide how long the product may be stored prior
to consumption, and also help with stock rotation in grocery stores (Steele 2004).
According to Cadwallader and Weenen (2003), and LaBuza and Szybist
(2001), there are several methods in which the shelf life or open dating may be
employed.
.
 The “best before” or “best if used by” date is the date up to, and including
which, the food “can reasonably be expected to retain its specific
properties”, providing it has been stored under specified conditions. Food
may still be safe for consumption after this date, but its appearance and
quality may deteriorate beyond product specifications (Man and Jones,
2000).
 “Production date” or “pack date” is the actual date the product was
processed or harvested and packaged. This form of dating makes the
consumer aware of the age of the product in order to make a selection

judgement. This form of dating is used primarily for pre-packaged fresh
fruits and vegetables (Cadwallader and Weenen, 2003).
 The “sell by” or “pull” date (Freitas, et al. 2003) is used for perishable
processed foods. This helps the retailer in stock rotation to sell the product
at a point where the consumer may purchase product, and will still be able
to have adequate storage time at home, before the end of the shelf life
(Cadwallader and Weenen, 2003).
 The “used by” date may be a “sell by” date, with a warning to consume
within days of that date. It may also simply be a date determined by the
manufacturers as the end of the useful quality life of the product
(Cadwallader and Weenen, 2003).
 “Closed” or “Coded Date” is a date that is used by the industry that
indicates production lots. It may also represent a packing date, and is
useful to the manufacturer in the case of a recall, but is not intended to be
used by the consumer (Cadwallader and Weenen, 2003).
Therefore, it is critical that procedures are established for shelf life to be
evaluated, accurately in order to satisfy legislation, ensure customer satisfaction,
while maximising the sale life of the product (Cadwallader and Weenen, 2003).

Modern society demands foods that are safe, nutritious, aesthetically
appealing, readily available, convenient to use and reasonably priced
(Charalambous 1993). Evaluation of local, regional and international food
legislative requirements, revealed that most countries require some form of dating
of product, so that the customer may be able to determine the age, and hence
quality and safety of the food.
2.6.1 The Republic of Trinidad & Tobago
The Food and Drugs Act of Trinidad & Tobago states in Item 16(1)(b)(vi)
that “any expiry date or date mark required by these Regulations” should be
placed “on any panel except the bottom of the package” (The Government of
Trinidad and Tobago, 1980). This regulation was then amended in 2001, by
changing the subparagraph to “the expiry date or date mark” (The Government of
Trinidad and Tobago, 2001).
The regulations as they exist leave much room for interpretation of the
law. However the Draft Revised Food Labelling Regulations has far more specific
and detailed regulations regarding how dating of food should be done (Food
Advisory Committee of Trinidad and Tobago). Only food with a durable life of
less than 90 days is required to possess a date. The date the food was packaged, as
well as the durable life (shelf life), and instructions for proper storage, if different

from normal conditions, are required. The draft goes as far as to state how the
durable life dates should be written (Food Advisory Committee of Trinidad and
Tobago).
2.6.2 Other Caribbean Territories
Item 6.1 of Jamaica’s Standards Act, states that all processed food
packages must bear a code showing the date the food was packaged as a batch
number (Government of Jamaica, 1974). However, this was purely done with the
intent of traceability, and is clearly stated in Item 6.3 which reads, “manufacturer,
processor, importer or distributor of any processed food in relation to which the
code referred to in paragraph (1) is used shall, at the request of the Bureau, supply
to the Bureau the key to such code” (Government of Jamaica, 1974).
In 1988, The Jamaica Bureau of Standards amended the requirements for
the labelling of pre-packaged foods. The Bureau declared new requirements as a
standard specification pursuant to Section 7 of the Standards Act, 1968. Section 7
of the Standards Act refers to information that is required to be given on packages
of containers (Government of Jamaica, 1974). Item 3.2e of the Bureau’s standard
requires that a “datemark or date of minimum durability, where an indication of
the age of the goods is likely to be useful to the consumer of purchaser” (Jamaica
Bureau of Standards, 1988).

Guyana’s regulation, under The Food and Drugs Act, with respect to
dating, is identical to that of Jamaica. According to Item 18(2)(b)(vi), “the label
applied to a food shall carry on any panel the expiry date or date mark required by
these Regulations” (The Ministry of Health, Housing and Labour of Guyana,
1977). The same regulation is also required of importers of food products to the
country (Collins and Alves, 1993).
According to Item 4.7 of the Barbados National Standards Institution (2004)
Labelling Requirements of Prepackaged Foods, it is a requirement unless
otherwise specified in an individual Barbados National Standard, for the
following date markings to be applied as appropriate, along with any special
storage instructions.
a. the “date of minimum durability”;
b. the “date of manufacture” for all manufactured foods;
c. the “date of packaging” for all prepackaged foods not from a
manufacturing process.
These requirements are very similar to that have been proposed in Trinidad
and Tobago (Food Advisory Committee of Trinidad and Tobago).

2.6.3 North America
Canadian legislation requires the display of the “durable life” date (CFIA).
This country defines this period as “the period starting on the day a food is
packaged for retail sale, that the food will retain its normal wholesomeness,
palatability and nutritional value, when it is stored under conditions appropriate
for that product” (CFIA). The “durable life” date is required for prepackaged
foods with a durable life of 90 days or less, with a few exceptions. Storage
instructions are required to be displayed, if they differ from normal room storage
conditions on the package, if the food is packaged at a non-retail establishment.
Retail establishments may choose to place date and storage instructions on the
label, or on a poster beside the food (CFIA).
2.6.4 The European Union
The labelling requirements of the European Union are particularly detailed
(European Council). The European Union’s food labelling law was developed in
1979, and has been amended several times to inform and protect consumers of
changing threats through the years (European Council).
With respect to shelf life, Article 9 of the Directive 2000/13/EC defines
the date of minimum durability of a foodstuff, as the date until which the
foodstuff retains its specific properties when properly stored (European Council).

This date is required to be indicated as ‘Best before …’ when the date includes an
indication of the day, or ‘best before end …’ in other cases. If required, the date
must incorporate storage conditions required to keep the product for the specified
period (European Council).
These requirements are typical of most labelling requirements throughout
the world. However, the European Union has included additional requirements for
highly perishable foods in Article 10. The Article states, “in the case of foodstuffs
which, from the microbiological point of view, are highly perishable and are
therefore likely after a short period to constitute an immediate danger to human
health, the date of minimum durability shall be replaced by the use by date”
(European Council).

Chapter 2: Literature Survey: Shelf Life Importance 27
2.7 SHELF LIFE IMPORTANCE
Legislations clearly indicate that it is a requirement to communicate with
potential consumers, the date by which food is acceptable and safe for
consumption. It is essential firstly, to have an expiry date that is accurate, so that
customers will be able to accurately determine the quality of the food (Man 2002).
If the customers gain confidence in the quality of the product, this influences
satisfaction (LaBuza 1982). Customer satisfaction generally equates to repeat
customers and ultimately profits (Man and Jones, 2000).
It is necessary to also have the maximum possible shelf life in order to
increase the product’s availability for sale, as well as to increase the product’s
distribution (LaBuza and Szybist, 2001). Therefore, it is critical to develop an
optimal date which ensures satisfactory quality, yet allows the greatest window of
time for consumption (Man and Jones, 2000).

Chapter 2: Literature Survey: Factors Affecting Shelf Life 28
2.8 FACTORS AFFECTING SHELF LIFE
With few exceptions, food quality decreases with time of storage,
irrespective of the preservation methods used and the control of storage
conditions, even for foods held in a frozen state (Steele 2004). Storage may affect
texture, flavour, colour, appearance and the nutritive value and safety of the food
(Eskin and Robinson, 2001).
There are intrinsic and extrinsic factors that may affect shelf life of the
snack. The intrinsic factors are as follows: raw materials, product formulation and
composition, product make-up, water activity value, pH value, availability of
oxygen and redox potential (Eskin and Robinson, 2001). The rate of deterioration
is affected by extrinsic deteriorative factors, such as, moisture, oxygen, light,
temperature and aroma transfer. Other factors that will affect shelf life are the
processing method, hygiene, packaging materials and system, storage, distribution
and retail display (Man and Jones, 2000).
Food systems are very complex. Their deterioration can be
multidirectional and have multistage characteristics (Man and Jones, 2000).
Although theoretically possible, it is not realistic to describe all geometrical
attributes of appearance; to determine all chemical components; as well as all

physicochemical and biological processes of real food systems. Analysis of the
maximum number of identified compounds may not provide the most valuable
information with respect to perception of spoilage and shelf life determination
(Man 2002). The selection of properties which may encompass the sensory
experience of the food is most important. The mechanisms of these substances’
retention and release and their proportions, may significantly influence the quality
attributes of the food, and provide more accurate meaningful shelf life
determination (Man and Jones, 2000).
2.8.1 Biological Decay – Pre-harvesting
Prior to harvesting and slaughter, foods from animals or plants are subject
to many diseases, including viruses, parasites, yeasts, molds and bacteria (OTA
1979). Pre-harvest deterioration will certainly determine the initial quality which
the food will possess. The shelf-life of snack foods obtained from untainted corn
kernels would be longer than kernels that suffered disease, or any other form of
pre-harvest deterioration (OTA 1979).
2.8.2 Senescence
Upon harvest or slaughter, the plant or animal is separated from its source
of nutrients and water. However, enzymes continue to operate and utilise nutrients
stored (Karel and Lund, 2003). This enzymatic activity may be beneficial or

detrimental to the manufacturer and the shelf-life determination process (OTA
1979).
All foods are affected by enzymatic processes during postharvest and
result in the degradation of sensory quality, including loss of colour, flavour,
nutrients and texture. The breakdown products themselves also damage the tissues
such that the decaying process becomes more rapid (OTA 1979).
2.8.3 Water Activity & Moisture Migration
Water activity influences the storage stability of foods (Steele 2004). Most
physical changes or instabilities involve moisture or mass transfer of components
in the food. A frequent cause of degradation of food products is a change in their
water content (Man 2002). Moisture transfer occurs in foods due to gradients in
chemical potential, which is directly a function of the food’s water activity (aw).
Water activity is defined as the equilibrium relative humidity for a product
divided by 100 (Steele 2004).
The change in moisture can lead to the food becoming unacceptable,
particularly affecting the texture of the food (Steele 2004). Low moisture foods
such as extruded corn salty snacks are particularly susceptible to absorption of
moisture from the environment (Bourne 2002). It is also possible for moisture to
migrate from the powdered cheese flavour to the corn base, as the water activity

of the flavour is higher than that of the base (Ray 2001). Dry food products are
expected to be crisp, however, if they absorb water, they may undergo glass
transition to become tough and soggy. Moisture content and texture analysis are
critical in the determination of food quality (Bourne 2002), and hence the
determination of shelf life. An increase in moisture absorption in snack foods can
lead to other problems such as microbial or chemical degradation (Steele 2004).
2.8.4 Transfer of Substances Other Than Moisture and/or Water Vapour
The transfer either into or out of food, of substances other than moisture
which affects its safety and/or quality, is likely also to have an impact on its shelf
life (Man 2002).
Volatile flavour components in snack foods may diffuse through the
packaging material, and can affect the shelf life of the food. Diffusion occurs at a
slower rate than moisture migration or oxidation. The slower rate is as a result of
the barrier properties of the packaging material, as well as the larger molecule size
of the flavour components, in comparison with water and oxygen molecules
(Lusas and Rooney, 2001).
Taint and off-flavours may develop as the food absorbs foreign and
objectionable flavours, depending on the packaging used and the prevailing
environment. Foods that have a large surface area to volume ratio such as leaf tea,

or with high a fat content such as snack foods, are particularly susceptible (Man
2002).
2.8.5 Microbial Factors
Micro-organisms are responsible for quality loss of many foods,
particularly fresh foods. Microbes are ubiquitous and they grow rapidly, under the
correct conditions (OTA 1979). Potential food spoilage micro-organisms include
bacteria, fungi (molds and yeasts), viruses and parasites (Steele 2004).
Ramstad and Watson (1987) state, that the final moisture level of the
processed extruded corn snack food for optimum keeping quality characteristics
should be less than 2%. Savoury snack foods have a water activity of less than
0.60 as seen in Table 2.2 (Man 2000). According to Man (2000) this water
activity is below that required for the growth of osmophilic yeasts, which are the
hardiest micro-organisms as seen in Table 2.3. These yeasts can survive in a water
activity no lower than 0.6 (Steele 2004). The Office of Technology Assessment
(OTA) (1979) states, that no microbiological hazards are presented by low
moisture snack foods, as they would lose crispiness before microbes would grow,
and thus become unacceptable before they would become a microbiological
threat.

TABLE 2.2 Water activity (aw) values of some foods.
Source: (Man 2000).

TABLE 2.3 Water activity (aw) limits for microbial growth.
Source: (Man 2000).
2.8.6 Rancidity Development
During the processing of foods, tissue damage occurs that causes the
release of various food chemical constituents into the cellular fluid environment
(OTA 1979). These chemicals can then react with each other or with external
factors, leading to deterioration of the food and resulting in quality deterioration
(OTA 1979).
Many foods contain unsaturated fats that are important in the nutrition of
humans. These fats are subject to three types of rancidity – hydrolytic rancidity,

ketonic rancidity and oxidative rancidity or lipid oxidation (Allen and Hamilton,
1999).
Hydrolytic rancidity is caused by hydrolysis of the triglycerides in the
presence of moisture, which gives rise to the liberation of free fatty acids (FFA).
These free fatty acids are particularly troublesome in the lauric oils, as the fatty
acids have strong soapy off-flavours (Allen and Hamilton, 1999; Man 2002).
Ketonic rancidity occurs when there is a fungal attack on foods, in the
presence of limited amounts of oxygen and water. Methyl ketones and aliphatic
alcohols are ultimately formed and possess a strong off-flavour (Allen and
Hamilton, 1999).
In lipid oxidation, fats are subject to direct attack by oxygen, through an
autocatalytic-free radical mechanism that results in rancid off-flavours, making
the food undesirable to consumers (Man 2002). Very little fat has to oxidize for
the consumer to detect rancidity and reject the food, even though it may still be
edible and nutritious (OTA 1979).
Lipid oxidation in food products develops slowly initially, and then
accelerates at later stages during storage seen in Figure 2.1 (Frankel 1998). The
rate of reaction depends on temperature to some degree; the rate increases two to

three times for every 10o
C increase in storage temperature for dry foods (Frankel
1998). Lipid oxidation is also dependent on water activity (Man 2002). Foods if
too dry or not dried enough are more subject to rancidity as seen in Figure 2.2.
The extruded corn snack is particularly susceptible to lipid oxidation due to its
particularly low water activity. Knowledge of the rate of reactions of lipid
oxidation can be used to predict shelf life, along with the knowledge of how fast
oxygen permeates the food package (OTA 1979).
FIGURE 2.1 Development of rancidity in foods, storage times given in arbitrary values, PV –
peroxide value; IP – induction period (Frankel 1998).

FIGURE 2.2 Food stability as a function of water activity (Frankel 1998).

Chapter 2: Literature Survey: Review of Research on Similar Corn Based Snack Foods 38
2.9 REVIEW OF RESEARCH ON SIMILAR CORN-BASED SNACK FOODS
2.9.1 Shelf Life Determination of Lightly Salted Potato Chips (British
Cellophane Limited, 1985)
The British Cellophane Limited (BCL) (1985) performed a shelf-life study
on potato chips. BCL examined lipid oxidation, moisture uptake percentage, as
well as sensory evaluation over time of the food. Man (2000) suggested that the
method used was acceptable for determining deep fat fried, quick fried, extruded,
roasted and baked savoury snack foods, made from cereals or potato.
Packs of salted potato chips were collected and stored at controlled
environmental conditions of a temperature of 25o
C and a relative humidity of
75%, for a period of 12 weeks. The packs were stored flat and subjected to a 12-
hour cycling exposure to fluorescent lights. Similar packs were stored in a deep
freeze to be used as controls in subsequent sensory analysis (Man 2000). A
trained sensory panel conducted quantitative sensory evaluation every week,
comparing the deteriorating samples with the control. At the same time, the
peroxide value and free fatty acid value of oil extracted from the chips were
conducted to determine the state of lipid oxidation. The percentage moisture
uptake was determined by weighing packs before submitting them to the panel

each week and recording the difference from that of the weight of the packs when
they were obtained on the manufacture date (BCL 1985).
It was found that moisture uptake percentage showed a closer correlation
to sensory evaluation results, more so than the peroxide values of the extracted
oil. This occurred as peroxide values are a measure of the primary lipid oxidation
products, but these compounds normally decompose quickly to secondary and
tertiary oxidation products (Man 2000).
2.9.2 Mathematical Models for Estimating the Shelf Life of Corn Flakes
(Azanha and Faria, 2005)
These researchers performed shelf life studies on cornflakes. Four
deterioration factors were identified in storing dried cereals: (a) moisture gain,
resulting in loss of crispiness; (b) lipid oxidation, resulting in rancidity and off-
flavours; (c) loss of vitamins, resulting in the nutritional labelling being incorrect;
(d) breakage, resulting in an aesthetically undesirable product. Therefore, even
though the extruded corn snack food and the cornflakes were not identical, they
were both manufactured from the same raw materials and had similar
deteriorative factors.
After identifying the major deteriorative forces of cornflakes, Azanha and
Faria (2005) focussed on texture analysis of the cornflakes. The objective of the

study was to determine the critical moisture content of cornflakes and to evaluate
the adequacy of three mathematical models (linear, middle point and logarithm
interval), to estimate the shelf-life of cornflakes packaged in three different
flexible packages under simulated storage conditions.
Sensory evaluation with 32 trained panellists following the procedure of
Meilgaard, et al. (1999), differentiated between the control product at the initial
moisture content, and the ageing samples which were stored at 100% relative
humidity and 23o
C. Panellists evaluated the texture of the control and the
experimental samples using a 9-point scale, with a difference of 1 indicating
difference not detectable and 9 indicating extremely intense level of difference. At
the same time, the moisture content was measured by means of the Aqualab 3TE
as seen in Figure 2.3.

FIGURE 2.3 Aqualab 3TE moisture analyser (DDI 2007).
The texture was also analysed mechanically, with hardness determined as
a peak force, in gram force (gf), required to compress the product by 50%, and
overall crispness evaluated as the total number of positive peaks, using the
Texture Analyzer (TA.XT2i) as seen in Figure 2.4. The experimental moisture
points determined were then compared with points predicted by the three different
mathematical models, and the best fit determined (Azanha and Faria, 2005).

FIGURE 2.4 Texture technologies TA.XT2i texture analyser (TT 2007).
A similar predictive approach via mathematical modelling was also
explored, but for the growth of micro-organisms (McMeekin and Ross, 1996).
Steele (2004) also explored the use of several other deteriorative properties as a
basis for predictive analysis by mathematical modelling. This procedure may be
widely used for various applications in shelf life determination.

2.9.3 Flavour Properties and Stability of a Corn-Based Snack (Charalambous
1993)
A corn-based extruded snack was subjected to five separate types of tests,
all of which dealt with flavour analysis. Five separate batches of the snack were
stored at a controlled temperature for 0, 3, 6, 9 and 12 month periods, after which
they were frozen to prevent further ageing effects.
The first test was a descriptive sensory analysis by a trained panel of 27.
The panellists evaluated aroma and flavour of the snack food, for the different
batches. In a separate test, the volatile compounds in the sample were extracted
using methanol, which were then isolated in dichloromethane. The extract was
concentrated and its aroma also analysed by a sensory panel for the different
batches.
The extracts from 3 of the batches were separated by gas chromatography
and assessed by the Osme technique, as seen in Figures 2.5 and 2.6. Four panel
members documented the time and intensity of the aroma as it left the gas
chromatograph. The difference over time was analysed.

FIGURE 2.5 Osme equipment and Osmegram (Charalambous 1993).

FIGURE 2.6 Osmegram showing panel member’s scorecard (Charalambous 1993).
The peaks found in the gas chromatograph were tentatively identified by
matching published mass spectra. Pure chemicals were then purchased, and
chemical identities were confirmed by a match of retention indices and mass
spectra (Charalambous 1993).
Lastly, the hexanal content in all 5 samples was measured by static
headspace gas chromatograph analysis. The oxygen in the canisters headspace
was measured in the samples also by a Systeck instrument. The changes in
hexanal and oxygen content over time were analysed, and it was found that
hexanal content increased over time and was a reliable off-flavour predictor
(Charalambous 1993).

Chapter 2: Literature Survey: Shelf Life Determination 46
2.10 SHELF LIFE DETERMINATION
A shelf life study is done to determine as accurately as possible, under
specified storage conditions, the point in time at which the product has become
either unsafe, or in the case of the extruded corn snack food, unacceptable to the
target consumers (Man 2002). Different types of foods have different quality
expectations. Consumer enjoyment of snack foods results from several factors,
but the overwhelming key factors are taste, texture and size (Lusas and Rooney,
2001).

Chapter 2: Literature Survey: Determination of the end of the Product’s Shelf Life 47
2.11 DETERMINATION OF THE END OF THE PRODUCT’S SHELF LIFE
The period of time from manufacture or processing to the end point of the
food’s life, or the point which food is unacceptable, is found by different methods
depending on the type of food (Man 2002). Examples of established guidelines
for determining the end point of the shelf life of some foods may be seen in Table
2.4 (Man 2002).
Established guidelines for dry, salty snack foods, such as the one under
study, were not obtained from the reviewed literature, as was found for perishable
foods. However, Man (2002) states that where guidelines are not available,
manufacturers and processors have to establish their own end-points, using
microbiological examination, chemical analysis, physical testing and properly
designed and conducted sensory evaluation, to define product-specific sensory
criteria.

Chapter 2: Literature Survey: Determination of the end of the Product’s Shelf Life 48
TABLE 2.4 Guidelines that can be followed to set shelf life end-point.
Source: (Man 2002).

Chapter 2: Literature Survey: Determination of Suitable Shelf Life Tests & Procedures 49
2.12 DETERMINATION OF SUITABLE SHELF LIFE TESTS & PROCEDURES
As Lusas and Rooney (2002) stated, consumers choose snack foods,
because of taste, texture and size. Market research also showed that for the
particular target market of the product being tested, the taste of the snack, along
with the perception of value for money and quantity for price, were the key
characteristics that drove sales numbers (Anonymous 2007b, personal
correspondence). However, it was also found that the key complaints with respect
to the quality of the product, as a result of fluctuations during production and/or
changes that may have occurred during storage, were almost all texture related
(Anonymous 2007a, personal correspondence).
Changes in texture due to moisture migration, and off-flavour
development due to lipid oxidation, were found to be the two critical properties
responsible for consumer acceptability and hence shelf life determination. The use
of a trained sensory panel to complement the chemical and/or physical analysis is
always required (Man 2002).

2.12.1 Lipid Oxidation & Off-Flavour Development
The major contributor to rancidity and off-flavour development in
extruded corn snacks is lipid oxidation (Allen and Hamilton, 1999). Table 2.5
shows ten different methods used to determine the extent of lipid oxidation,
ranked in decreasing order of usefulness (Frankel 1998).
Gas chromatography’s results vary with different unsaturated oils, as well
as with different additives such as antioxidants and metal inactivators (Frankel
1998). Gas chromatography provides useful data on the origin of volatiles and
flavour precursors (Ranken, et al. 1997). However, their significance and impact
on flavour stability are not clearly established and are difficult to evaluate
(Frankel 1998).
Possibly the best non-human chemical method of determining lipid
oxidation is ultraviolet absorption, as it is sensitive, precise and simple to evaluate
(Frankel 1998). Ultraviolet absorption measurements involve the determination of
conjugated dienes by ultraviolet spectrophotometry, related to the contents of
polyunsaturated hydroperoxides that act as flavour precursors (Min and Smouse,
1985). These measurements are sensitive and reproducible, but the information is
only useful to determine precursors of volatiles formed from polyunsaturated oils
(Frankel 1998).

TABLE 2.5 Ranking of lipid oxidation methods.
Source: (Frankel 1998).
British Cellophane Limited’s (1985) study clearly stated that due to further
decomposition of the primary products of lipid oxidation, peroxide values (PV) of
the extracted oil from the snack food did not correlate as closely as moisture
absorption, with product deterioration. More recent studies state that the peroxide
value (PV) is accepted as an indication of the extent of oxidative rancidity (Man
2002; Cadwallader and Rouseff, 2001). Peroxide value is also widely used in the
food industry as an indication of oil stability (Anonymous 2007a, personal
correspondence).

Peroxide value methodology is precise, however the method is empirical
and the results may be misleading in samples that have been thermally abused or
subjected to light oxidation (Frankel 1998). Since the snack is packaged in opaque
metallised OPP, elevated oxidation due to light is not a factor. However, it is
important that samples are stored at a constant temperature (throughout
experimentation) (Man 2002).
2.12.2 Extraction of Lipids
In order to determine the extent of rancidity of the lipids, the lipid must be
extracted from the food samples (McDonald and Mossoba, 1997). There are three
basic types of procedures that are used to extract lipids from foods: reflux
extraction, acid or alkaline digestion prior to solvent extraction, and non-heating
methods (McDonald and Mossoba, 1997). There are numerous standard methods
of extraction that use a variety of solvents ranging from non-polar hydrocarbons,
to mixtures containing alcohols and water. The methods involve simple shaking,
refluxing or prolonged extraction in a Soxhlet apparatus, preliminary grinding,
homogenisation of a suspension in solvent and/or partitioning of the sample
between an organic and aqueous phase (King 1980).
The aim of all extraction procedures is to separate cellular or fluid lipids
from the other constituents, proteins, polysaccharides, small molecules, but also to
preserve these lipids for further analyses (King 1980). Removing the non-lipids

without losing some lipids is a complex challenge, while extracting some specific
lipids is not always reliable for other kinds of lipids. Therefore, knowledge of the
type of lipids being extracted is also critical in the determination of a suitable
method (Leray 2007).
The extraction of lipids from foodstuff and its subsequent storage before
analysis, must take into account that most lipids consist of hydrolysable esters
containing unsaturated centres vulnerable to further oxidation by air (King 1980).
The method chosen should depend on the nature of the foodstuff; whether its
lipids are free, physically entrapped or molecularly bound by non-lipid
components; whether the lipids are abnormally liable or volatile and the time
available for extraction (King 1980).
The Soxhlet method is the most common method used for lipid extraction
from foods (Leray 2007), and is recognised by the Association of Official
Analytical Chemists (AOAC) as the standard method (AOAC method 960.39) for
crude fat analysis (Food Science Australia). The oil and fat from solid material is
extracted by percolation with an organic solvent, usually hexane or petroleum
ether, under reflux in special glassware as seen in Figure 2.7. The solvent is held
in (1), the extraction chamber houses the sample in (2), some devices contain a
funnel to recover solvent as in (3), and (4) is the condenser which condenses
solvent vapours (Leray 2007).

FIGURE 2.7 Glassware used in Soxhlet extraction (Leray 2007).
Despite its popularity, there are several disadvantages in using this method
of extraction: poor extraction of polar lipids; long times involved; large volumes
of solvents; tendency for the lipids to oxidise; and the hazard of boiling solvents
(Leray 2007; Christie 2007). However, automated extraction instruments have
been developed that have improved on the original method. They are able to boil,
rinse and recover solvent automatically in a significantly shorter time period
(Leray 2007). An example of this equipment is the Lab Synergy: Fast Extraction
(Viscal 2006).

Accelerated Solvent Extraction (ASE) is a technique that was made to
replace Soxhlet and other extraction techniques required for numerous samples.
The automation and rapid extraction time of ASE overcome the shortcomings of
Soxhlet extraction. Common liquid solvents are used at an increased temperature
and pressure to accelerate the extraction process (Dionex Corporation, 2004).
Another method of extraction is the Folch method (Folch, et al. 1957).
This procedure remains one of the best described and most widely used by
lipidologists (Leray 2007). Folch et al. (1957) suggest the use of a chloroform-
methanol (2:1 by volume) solvent combination, as well as saline solution. This
method is a useful, multipurpose extraction technique for biological specimens,
mixed diets and all types of animal tissues, and recovers greater than 96% of
lipids (Spiller 1996). With this method, the proportions of chloroform, methanol
and water are critical in order to reduce lipid losses (Christie 2007). This method
is best suited for the extraction of lipids for peroxide value evaluation, as it avoids
over exposure to solvents and the possibility of oxidation of the extracted lipids
(Spiller 1996).
An alternative to the traditional Folch method was described by Marmer
and Maxwell (1981) using a dry-column method. This method was found to
produce similar results as Folch’s chloroform/methanol method in analysing
muscle and adipose tissues. However, this technique has been generally used in

the isolation of drugs, herbicides, pesticides and other pollutants from animal
tissues, fruits and vegetables (Leray 2007).
Other more complex solvent extraction methodologies, including
supercritical fluid extraction and microwave irradiation or ultrasonification to
improve yields, are widely researched (Leray 2007). These methods require
specialised equipment, and are also more specific, requiring testing on a wider
range of sample matrices (Christie 2007). Leray (2007) also suggested variations
of extraction methods, as suggested by Christie (2007) However, these methods
are specific for extraction of particular types of lipids.
2.12.3 Moisture Migration and its effect on Texture
There are three primary methods of measuring food texture – instrumental,
acoustic emission and sensory attributes (Rosenthal 1999). There are force,
distance, time, work energy and power, measuring instruments that are effective
in measuring texture of foods (Bourne 2002). Cumming, et al. (1971) stated that
the most widely used instruments for texture evaluation in quality control were
rotational viscometers (for liquids) and penetrometers (for solids).
Some food technologists use a compressive or tensile force and relate the
magnitude of the force to the deformation done in millimetres of the sample
(Rosenthal 1999). Katz and Labuz (1981) used the initial slope of the force-

deformation curve. The value of the initial slope was found to be a good indicator
of crispness for saltines (cracker) and potato chips. However, for extruded corn
snacks, the initial slope was nonlinear and unsatisfactory. The work done to
compress the corn snack by 75% was an acceptable indicator of crispness (Katz
and Labuz, 1981). The work done to compress the snack by 50% was used by
Azanha and Faria (2005).
Moisture absorption is determined by the difference between the weight of
the sample initially at the point of manufacture, and the weight after some time of
storage (Man and Jones, 2000). The weight gained is taken to be the moisture
uptake of the product (Man 2002). This is not accurate as the many chemical
reactions, including those of rancidity, may also be responsible for changes in
weight. Despite this limitation, this method is acceptable in determining moisture
absorption (Steele 2004).
2.12.4 Sensory Evaluation
Sensory evaluation is critical in the accurate determination of a product’s
shelf life (Man 2002; Steele 2004). However, dependable sensory analysis is
based on the skill of the sensory analyst (Meilgaard, et al. 1999). All humans are
afflicted by factors, physiological and psychological, which can influence sensory
perception such as adaptation, expectation, stimulus, logical and the halo effect
(Meilgaard, et al. 1999).

In addition to these factors, which can be minimised by proper procedure
of testing, humans by nature are quite variable over time; and very variable
among themselves (Meilgaard, et al. 1999). Meilgaard, et al. (1999) suggests that
measurements are repeated; enough subjects are used (≥ 20) so that verdicts are
representative. Literature reviewed revealed that panels with as few as 6 subjects
may be used effectively (Ucherek 2004; Freitas, et al. 2003). The sensory analysts
must either all be untrained, or all be properly trained and respect the many rules
and pitfalls that govern panel attitudes (Meilgaard, et al. 1999).
The type of sensory evaluation test that is used for the determination of the
shelf life of all products, is the difference from control test (Man 2000). An
example of an applicable scorecard can be seen in Appendix. The test evaluates
the key components of the extruded corn snack food, which are flavour and
texture, as well as the overall acceptability of the product (Man 2000).
Panellists who are experienced with this the extruded corn snack food,
determined when the sample is no longer acceptable. The development of a scale
used to measure panellists’ responses must be done with several reference points
(Meilgaard, et al. 1999). Statistical analysis is critical to accurately determine the
end of the product’s life (Meilgaard, et al. 1999; Freitas, et al. 2003).

Chapter 2: Literature Survey: Accelerated Shelf Life 59
2.13 ACCELERATED SHELF LIFE
Accelerated shelf life determination is used to shorten the time required to
estimate a shelf life which otherwise can take an unrealistically long time to
determine (from a number of months to a few years) (Kilcast and Subramaniam,
2000). Shelf life is accelerated by elevating a condition under which the product is
being stored (Man 2000). The effect of elevating the temperature on many
chemical reactions, as well as adverse changes in food during storage, have been
explored. The assumption is that by storing food at a higher temperature, any
adverse effect on its storage behaviour, and hence the end of the shelf, becomes
apparent in a shorter time (Kilcast and Subramaniam, 2000). Accelerated tests are
based on Arrhenius’ model see Fig 2.8, but this is only appropriate for simple
chemical systems and often fails for foods that are, in reality, more complex (Man
2000).
FIGURE 2.8 Arrhenius’ equation (Man 2000).
TR
Ea
eAk 

k = rate constant, A = constant, Ea = Activation Energy, R = Universal Gas Constant,
T = Temperature in Kelvin

Chapter 2: Literature Survey: Accelerated Shelf Life 60
Accelerated determinations are useful when the patterns of changes are
practically identical during normal and accelerated storage, so that shelf life under
normal storage can be predicted with a high degree of certainty, such as the
quality of orange juice made from frozen concentrate. Changes found after 6
months at 20o
C corresponded to the changes after 13 days at 40o
C, and after 5
days at 50o
C (Man 2000).
Accelerated determinations have 2 limitations – (i) temperature rises can cause a
change of physical state, which may in turn affect the rates of certain reactions
and (ii) storage at a constant elevated temperature, without a corresponding
elevated relative humidity, that may lead to unexpected results (Man 2000). The
second limitation has particular relevance, as moisture migration and texture
degradation have been found to be critical in consumer acceptability of the
extruded corn snack food (Anonymous 2007a, personal correspondence).

Chapter 3: Methodology 61
CHAPTER 3
METHODOLOGY
3.1 STORAGE OF SAMPLES
Two hundred and eighty eight (288), twenty-gram (20g) packets of the
snack food, from a production, run were randomly assigned to the following three
storage conditions:
 Frozen: Samples ‘C’ were kept frozen, in a deep freezer at approximately
-18o
C as recommended by Man (2000), and were used as the Control for
the duration of the evaluations.
 Ambient: Samples ‘A’ were stored in a cage at ambient temperature of
33o
C, and relative humidity 100%. This storage condition was chosen to
simulate the worst case scenario as recommended by Bryce, LCCP and
WFI (2007a).
 Retail: Samples ‘R’ were stored in an air-conditioned room of 18o
C with a
relative humidity 75%. This storage condition was chosen to simulate
the environment typical of a retail outlet.

3.2 SENSORY EVALUATION OF A LOCALLY MANUFACTURED CHEESE-
FLAVOURED EXTRUDED CORN SNACK BY TRAINED PANELLISTS
The aim of the experiment was to determine the shelf life of a popular
locally manufactured cheese-flavoured extruded corn snack food, stored under
two separate conditions.
Twenty (20) employees of the product’s company’s trained sensory panel,
who were therefore familiar with the product, formed the sensory panel. They
attended panel sessions weekly, every Tuesday, between the hours of 1:30pm –
2:30pm. The “difference from control” test was issued to the panellists to
compare Samples ‘C’, ‘R’ and ‘W’ as recommended by Meilgaard et al. (1999)
see Appendix. Panel members were all given a copy of the score sheet, a bottle of
still water, and a pencil.
Ten (10) packs of samples from each of the storage conditions were placed
in the panel room fifteen (15) minutes before panel sessions, so that they all
attained room temperature (20o
C). The panel room contained six (6) booths, and
was designed to facilitate the reduction of errors as recommended by Meilgaard et
al. (1999). Four (4) small Styrofoam containers were labelled, one (1) was
labelled control, while the other three were labelled with a random three-digit

number as recommended by Meilgaard et. al (1999). Each three-digit number for
that week was assigned to one of each of the three different sample conditions,
“blind control”, “retail” and “ambient”. The control sample was placed into the
container labelled, “control” and the three-digit number assigned to the “blind
control” for that week as seen in Figure 3.1.
FIGURE 3.1 Sensory evaluation test booth set up

Panellists were instructed to compare the coded samples with the control,
and to assign a score on an eleven-point scale (0 to 10) to each attribute of texture
and flavour. Panellists were also instructed to assign a score to the “overall
attribute” of the sample. The overall attribute was the total sensory experience of
the snack food experienced by the panellists. This included the appearance,
flavour, sound and texture experienced during consumption of the snack food.
Each attribute was scored separately, such that it would be possible to have a
sample fail in one attribute, but acceptable regarding another.
A score of “0” was equated with “no difference” between sample and
control, while a score of “10” signified “extreme” difference. A score of six “6”
was defined as unacceptable. Samples were evaluated weekly until sixty-five
percent (65%) of the panel rejected the “overall attribute” of the samples, as
recommended by Anonymous (2007a, personal correspondence).

3.3 DETERMINATION OF THE MOISTURE UPTAKE IN STORED SAMPLES
The aim of the experiment was to determine, over time, the mean increase
(in g) in weight per pack of snack food. This increase in weight was equivalent to
the moisture absorbed by the snack food (Man 2002).
All samples (packs) were numbered and weighed at week 0, prior to
randomly placing them into the three storage conditions. Each week, five (5)
samples from each storage condition, were removed and then weighed before
sensory evaluations. The difference from the initial weight of the pack was
recorded as recommended by Man (2002).
3.4 TEXTURE ANALYSIS OF SAMPLES USING THE PENETROMETER
The aim of the experiment was to evaluate the change in the texture of the
extruded corn snack food over time, with the aid of a digital penetrometer.
One (1) snack piece was taken from each of five separate packets from
Samples ‘A’, stored under ambient conditions. The five pieces were tested at the
identical position at the top and centre of each piece. An automatic penetrometer
was used with a 47.5g base and a 2.5g needle (50g total weight). The tip of the

needle was placed on the very edge of the snack piece, and then allowed to plunge
into the piece under its own weight for 5 seconds. The distance (in mm) which the
needle travelled into the snack piece was determined automatically by the
penetrometer. The mean of the five distance readings was calculated weekly for
each of the three storage conditions.
3.5 LIPID EXTRACTION FROM THE EXTRUDED CORN SNACK FOOD
The aim of the experiment was to extract the maximum quantity of lipid
(oil) possible from the extruded corn snack, in order to determine the peroxide
value of the oil.
Folch’s lipid extraction method (Folch, et al. 1957) was modified due to
the increase in sample size from 2g to 10g. The modified method was used
weekly in order to extract available lipids from the snack foods stored under the
different conditions. One (1) extraction was done initially (Week 0), as all
samples were the same. Subsequently, lipids were extracted weekly from samples
stored under the different conditions.
One (1) pack of the snack food (approximately 20g) was opened and the
contents emptied into a mortar. The packaging material was discarded. A pestle
was used to crush the sample. The crushed sample was divided into 2 – 10g

samples and added to a 250ml conical flask. Two hundred millilitres (200 ml) of a
homogenised mixture of two parts chloroform – one part methanol was added to
each of the 10g samples. The whole mixture was agitated at a speed of 600 rpm,
for 20 minutes, using a magnetic stirrer.
“Whatman #4” fast filtering speed, filter paper was used for filtration. The
filter paper was 11.0cm in diameter, and had a retention (in microns) of 20 – 25.
The filter paper was folded in half, and then into quarters. The folded filter paper
was then opened to form a cone. The homogenate was filtered with a glass funnel
and the folded filter paper. The filtrate was recovered in a 250 ml conical flask.
The residue and filter paper were discarded.
The filtrate was washed with 40ml of a 0.9% sodium chloride (NaCl)
solution. The mixture was manually swirled for a few seconds. The mixture was
placed into four (4) 50ml test tubes and sealed with fitting covers. The closed test
tubes were then each placed into four (4) centrifuge canisters which were also
sealed with a fitting cover. The canisters were placed into a centrifuge, where they
were centrifuged at 2000 rpm for 10 minutes. The mixture was centrifuged in
order to separate the two phases. The upper phase was siphoned, using a Mohr
pipette, and discarded.

The lower phase from both 10g samples were combined and added to a
pear-shaped glass flask. The solvent was evaporated using a rotary evaporator
under vacuum as seen in Fig 3.2. The oil that remained after evaporation was
weighed and then evaluated for peroxide value.
FIGURE 3.2 Rotary evaporator (Sigma-Aldrich, 2007).

3.6 PEROXIDE VALUE EVALUATION OF EXTRACTED OIL
The aim of the experiment was to determine the peroxide value of the
extracted oil. The peroxide value can be equated to off-flavour production (a
function of the flavour attribute) in the snack food as suggested by Man (2002)
and Cadwallader and Rouseff (2001).
After the oil was extracted from the samples, the oil was analysed for
peroxide value using the Association of Analytical Chemists (AOAC) procedure
for peroxide evaluation of oils and fats as recommended by Cunniff (1997) with
no modifications.
3.7 TREATMENT OF RESULTS
The blind control was used to evaluate the validity of the panellists’
results. In analysing the sensory data from the panel sessions, only panellists who
gave the “blind control” a score of “0-2” were accepted as recommended by
Freitas et al. (2003). The scores of all the panellists may be seen in the Appendix.
Several methods of regression (linear, exponential and polynomial) were
used to evaluate the sensory data, with the aid of Microsoft Excel. Microsoft

Excel is one type of software that is designed to accurately fit data to a variety of
different regression models. The regression model that was used for each test
condition was chosen to fit as many of the data points as possible as
recommended by Triola (2004).
Minitab® 15.1.1.0 is a statistical software that was used to perform
logistic regression. Logistic regression was used to compare objective testing
results (penetrometer, moisture increase and peroxide value) with sensory
evaluation results (texture and flavour attributes), in order to determine if any
correlation existed. The number of panellists with acceptable texture attribute
sensory scores (blind control ≤ 2), for each week for both ambient and retail
conditions, were placed together in one column. The corresponding number of
failures per sensory evaluation that occurred were placed in the second column,
and the objective moisture uptake value was placed in the third column. The same
was done for penetrometer values, however peroxide value results were paired
with flavour attribute sensory scores.
Minitab’s logistic regression was able to determine Pearson’s p-value,
which is and indication of statistical correlation. The p-value is the probability of
obtaining a result at least as extreme as a given data point, under the null
hypothesis. A p-value that is less than 0.05, suggests that one variable has a
statistical impact on another (Freund 2004; WFI 2007b).

Chapter 4: Results & Discussion 71
CHAPTER 4
RESULTS & DISCUSSION
4.1 RESULTS OF SENSORY EVALUATION OF THE EXTRUDED CORN SNACK FOOD
4.1.1 Sensory Evaluation of the Texture Attribute
Table 4.1 shows the results of the sensory evaluation of the texture of the
extruded corn snack food, stored under ambient (33o
C, 100% RH) and under retail
(18o
C, 75% RH) conditions.
TABLE 4.1 Sensory evaluation of the texture of an extruded corn snack food.
Sample
Storage Period in Weeks
1 2 3 4 5 6 7 8 9
A
(33o
C|
100%RH)
1. average
score
1.8±
1.6
2.5±
1.9
2.6±
1.9
3.3±
1.3
3.6±
2.0
5.1±
1.4
5.7±
1.3
8.8±
1.2
9.0±
1.2
2. %fail 6 8 7 7 18 27 44 94 100
R
(18o
C|
75%RH)
1. average
score
1.8±
1.5
2.3±
1.7
1.7±
1.3
2.4±
1.7
3.1±
1.5
4.2±
1.4
4.7±
1.3
5.8±
1.1
7.1±
1.8
2. %fail 0 0 0 7 12 13 33 59 83
The average score of the panel members is the mean score of the members
that gave the “blind control” a score less than 3 (acceptable score). While percent

(%) fail is the percentage of panellists, with acceptable scores, that gave Samples
‘A’ or ‘R’, where applicable, a failing grade (score greater than 5).
4.1.1.1 Evaluation of the Texture Attribute of Sample ‘A’
Between Weeks 1 – 5, the sensory panel detected a very gradual change in
the difference between the textures of the Control and Sample ‘A’. Texture scores
increased from 1.8 to 3.6 from Week 1 – 5. From Week 5 the difference in texture
between the Control and Sample ‘A’ increased more rapidly weekly; from 3.6 to
9.0 from Week 5 – 9. Thus shelf life of Sample ‘A’ as determined by subjective
texture evaluation ended within the seventh week of storage. At the start of Week
7, 44% of the panellists had failed Sample ‘A’, but by Week 8, 94% of the
panellists failed Sample ‘A’.
4.1.1.2 Evaluation of the Texture Attribute of Sample ‘R’
Average scores over time from the sensory evaluation of texture of
Sample ‘R’, were very similar to results obtained from Sample ‘A’. At Week 1,
the average score of Sample ‘R’ was the same as that for Sample ‘A’ (1.8).
However, from Week 2 onwards, Sample ‘R’ was consistently given a lower
average score than Sample ‘A’ by the sensory panel. Thus shelf life of Sample ‘R’
as determined by subjective texture evaluation ended a week later than with
Sample ‘A’. The shelf life of Sample ‘R’ ended within the eighth week of storage.

At the start of Week 8, 59% of the panellists had failed Sample ‘R’, and by Week
9, 83% of the panellists failed Sample ‘R’, again as determined by its texture.
According to Man (2002) and Kilcast and Subramaniam (2000), foods
stored at higher temperatures generally have a shorter shelf life than those stored
at lower temperatures. This could explain why at the higher ambient temperature
the shelf life of the extruded corn snack based on the deteriorations in texture,
ended a week earlier than at the lower retail temperature.
4.1.2 Sensory Evaluation of the Flavour Attribute
Table 4.2 shows the results for sensory evaluation of the flavour attribute
stored under ambient (33o
C, 100% RH) and under retail (18o
C, 75% RH)
conditions.
TABLE 4.2 Sensory evaluation of the flavour of an extruded corn snack food.
Sample
1 2 3 4 5 6 7 8 9
A
(33o
C|
100%RH)
1. average
score
2.2±
1.7
3.0±
2.1
2.2±
1.6
3.0±
1.2
3.4±
2.1
4.7±
1.8
5.8±
1.4
8.9±
1.5
8.9±
0.9
2. %fail 6 8 7 7 12 27 67 94 100
R
(18o
C|
75%RH)
1. average
score
2.3±
1.5
1.9±
1.5
1.7±
1.4
2.8±
1.5
3.5±
1.2
4.1±
1.5
5.1±
1.5
5.8±
1.1
7.4±
1.9
2. %fail 0 0 0 7 6 27 39 59 83

4.1.2.1 Evaluation of the Flavour Attribute of Sample ‘A’
The flavour attribute of Sample ‘A’ was given, by the panel, a consistent
average score between 2.2 and 3.0, within the first 4 weeks of storage, when
compared to the Control. Similar to the texture attribute, there was a sharp
increase in the average score after Week 4. The end of the shelf life of the snack
food as determined by the deterioration of the flavour, occurred near the end of
Week 6, as at the beginning of Week 6, 27% of panellists failed Sample ‘A’, but
by the beginning of Week 7, 67% of panellists failed Sample ‘A’.
4.1.2.2 Evaluation of the Flavour Attribute of Sample ‘R’
The average score of Sample ‘R’ at Week 1 was 2.3. The average score
given by the panel in the subsequent 2 weeks was lower, however from Week 3,
the panellists’ average score consistently increased. Samples ‘A’ and ‘R’
possessed very similar flavour attributes up until Week 6. As with Sample ‘A’ the
average score increased rapidly after Week 4. The end of the shelf life of the
snack food as determined by the deterioration of the flavour, occurred within the
eighth week of storage

4.1.3 Sensory Evaluation of the “Overall Attribute” (sound, texture, flavour,
appearance)
Table 4.3 shows the results of the sensory evaluation, of the “overall
attribute”, of the extruded corn snack food, stored under ambient (33o
, 100% RH)
and under retail (18o
C, 75% RH) conditions.
TABLE 4.3 Sensory evaluation results of a snack food showing “overall attribute”.
Sample
1 2 3 4 5 6 7 8 9
A
(33o
C|
100%RH)
1. average
score
2.1±
1.5
2.8±
1.9
2.9±
1.7
3.6±
1.4
3.8±
1.9
4.7±
1.5
5.8±
1.1
9.5±
0.6
9.2±
0.9
2. %fail 6 8 7 7 24 33 50 100 100
R
(18o
C|
75%RH)
1. average
score
2.1±
1.3
2.2±
1.5
2.1±
1.5
2.9±
1.5
3.7±
1.3
4.4±
1.2
5.2±
1.1
5.9±
1.2
7.6±
1.9
2. %fail 0 0 0 7 12 13 39 53 83
4.1.3.1 Evaluation of the “Overall Attribute” of Sample ‘A’
The average score of the overall attribute of Sample ‘A’ increased steadily
from Week 1 – 5, similar to previous attributes, a sharp rise in the average score
occurred from Week 5 – 9. The end of the shelf life of the snack food as based on
the sensory evaluation of the “overall attribute” was found to be within the
seventh week of storage.

4.1.3.2 Evaluation of the “Overall Attribute” of Sample ‘R’
The “overall attribute” average score of Sample ‘R’ remained constant at
approximately 2.1 from Week 1 – 3. Then a steady increase in average score from
2.1 to 7.6 from Week 3 – 9. The shelf life of the overall attribute for Sample ‘R’
ended within Week 8.
Sensory results of the “overall attribute” of the snack food were very
similar to sensory results of texture and flavour attributes.
4.2 STATISTICAL ANALYSIS OF THE SENSORY EVALUATION RESULTS OF THE
EXTRUDED CORN SNACK FOOD
4.2.1 Statistical Analysis of the Texture Attribute Sensory Results
Fig 4.1 shows the percentage of panellists that failed the texture attribute
of the snack food, stored under retail and ambient conditions. Third order
polynomial regression was the least complex fit that best fit both sets of data
(ambient and retail conditions) using Microsoft Excel. Equations 4.1 and 4.2 are
the equations of best fit for the texture attribute of the snack food, stored under
ambient conditions, and retail conditions respectively, where y represents the
probability of failure and x represents storage time in weeks (Triola 2004). This

was determined by trial and error using different regression models in Microsoft
Excel.
The end of the product’s shelf life was set to 65% of panel rejection of the
sample. The cubic equations relating failure probability and storage time may be
solved, for y = 0.65, in order to determine storage time in weeks x.
y = 0.002x3
- 0.005x2
+ 0.005x + 0.033 Equation 4.1
y = 0.002x3
- 0.009x2
+ 0.016x - 0.004 Equation 4.2
Using Equation 4.1, for the evaluation of the texture attribute of the snack
food stored under ambient conditions,
when y = 0.65, x = 7.7 weeks ≡ 53 days
Using Equation 4.2, for the evaluation of the texture attribute of the snack
food stored under retail conditions,
Therefore, panellists determined that under ambient conditions (33o
C,
100% RH) the time in days that the texture attributes of the extruded corn snack
food remained acceptable was 53 days. While the texture attributes of the
extruded corn snack food, stored under retail conditions (18o
C, 75% RH)
remained acceptable for 58 days.

Texture attribute results from both storage conditions obtained from
Weeks 1 – 4, could be fit using linear regression, while and an exponential fit
could be fit from Weeks 4 – 9. This does not correlate with Charalambous (2003)
and Man (2000). In those studies, a linear regression was the best fit for the
sensory evaluation of the texture attribute for the entire life of the snack food.
Panellists made increasingly negative comments about the texture of the snack
food as it aged over the weeks. Panellists found that Samples ‘A’ and ‘R’ became
harder and rubbery as they aged (Taub and Singh 1998). In addition to the
samples becoming harder and rubbery overall, the panellists commented on an
increase in the presence of small areas within the snack food of significant
roughness (tough spots). It is possible that the combination of the increase in
overall hardness and the increase in these ‘tough spots’, created the exponential
increase in the average score of the texture attribute from Weeks 4 – 9.
4.2.2 Statistical Analysis of the Flavour Attribute Sensory Results
Fig 4.2 shows the percentage of panellists that failed the flavour attribute
of the snack food, stored under retail and ambient conditions. For the flavour of
the snack food stored under ambient conditions the equation of best fit was a
quadratic expression (Equation 4.3). While, for the snack food stored under retail
conditions the equation of best fit was a third order polynomial expression
(Equation 4.4).

y = 0.020x2
- 0.073x + 0.066 Equation 4.3
y = 0.001x3
+ 0.003x2
- 0.020x + 0.007 Equation 4.4
Using Equation 4.3, for the evaluation of flavour attribute of the snack
food stored under ambient conditions,
Using Equation 4.4, for the evaluation of flavour attribute of the snack
food stored under retail conditions,
Therefore, panellists determined that under ambient conditions (33o
C,
100% RH) the time in days that the flavour attributes of the extruded corn snack
food remained acceptable was 51 days. While the flavour attributes of the
extruded corn snack food, stored under retail conditions (18o
C, 75% RH)
remained acceptable for 58 days.

Some panel members were able to detect slight off-flavours from as early
as Week 3. Those panellists detected a slight “plastic/chemical” odour and a
slightly sour aftertaste (Lusaas and Rooney 2001). Similarly to the sensory
evaluation of the texture attribute, the flavour attribute of the snack food stored
under both conditions, showed a linear best fit up to Week 4, and then an
exponential fit from Week 4 – 9. Charalambous (2003) found that there was a
linear change in the flavour attribute of the snack food within the first quarter of
storage. However, the rate of degradation declined for the final three-quarters of
the snack food’s shelf life. However, Charalambous (2003) studied a snack food
stored with a different packaging system. The snack food was stored in nitrogen-
flushed canisters. Modified atmosphere packaging systems generally reduce the
rate of degradation in foods (Man and Jones 2000).
4.2.3 Statistical Analysis of the “Overall Attribute” Sensory Results
Fig 4.3 shows the percentage of panellists that failed the “overall attribute”
of the snack food, stored under retail and ambient conditions. The equations of
best fit for sensory evaluation of the “overall attribute” of the snack food stored,
under ambient and retail conditions, were both third order polynomial expressions
see Equation 4.5 and 4.6.
y = 0.005x3
- 0.038x2
+ 0.098x - 0.003 Equation 4.5
y = 0.001x3
- 0.007x2
+ 0.011x - 0.003 Equation 4.6

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