The document summarizes a seminar presentation on refractance window drying in food processing. Refractance window drying is described as a novel thin film drying method that allows for low-temperature drying of heat-sensitive foods while better retaining quality attributes compared to other drying methods. Key aspects of refractance window drying covered include the principle, construction, process parameters, quality parameters, applications, advantages, and future prospects. An example case study is also presented comparing freeze drying and refractance window drying of acerola pulp.
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REFRACTANCE WINDOW DRYING
1. SEMINAR PRESENTATION (0+1)
KERALA UNIVERSITY OF FISHERIES AND OCEAN STUDIES
BTECH FOOD TECHNOLOGY
TOPIC: REFRACTANCE WINDOW DRYING IN FOOD PROCESSING
PRESENTED BY,
ATHULYA BABU
OET-2019-02-15
B. TECH 19
4. DRYING
• Drying is the process of removing moisture
from food materials.
• It is a complex process that involves
transfer of heat and mass as well as various
rate processes, chemical or physical
transformations that may cause changes in
the quality of product and mechanisms of
transfer of heat and mass.
• Energy intensive unit operation
• Advantages,
➢ more stable to microbialcontaminationand other
deteriorative chemicalreactions
➢ facilitate storage& minimize transportation costs
➢ Offer yeararound availability
➢ Reducewastage
4
5. • Cabinet and bed type dryers such as kiln, tray,
truck tray, rotary flow conveyor and tunnel
• Spray dryer
• drum dryer
• Freeze dehydration
• osmotic dehydration
• Microwave drying, infrared drying, heat pump
drying, fluidized bed, radio frequency and refractance
window drying
FOUR GENERATIONS OF
DRYING...
2
1
3
4
5
6. • Preserving the quality attributes- a
challenge in drying heat sensitive products
• Traditional drying methods-affects the
taste, colour, nutritional qualities and
preservation of bioactive compounds
• Thus arise the need for development of
improved and innovative drying methods
for preservation.
6
7. Drying of puree and juice products was achieved by convective tray dryer, drum, spray, and freeze
drying, each with certain limitations.
Drum dryer- drying temperaturebetween120-170°C,cause
severe qualityloss in the products
Freeze drying-
• High operationalcost
• Porous nature of products
• Longertime
Spray drying-
• Raw materialwith high moisture contentis required for
atomization
• High capitalinvestment
• Low thermalefficiency
Convective tray dryer-
• Changesin colourwhile drying
• Time consuming.
The Need
• Low drying time & temperature
• Low investing & operational cost
• High quality product
7
8. REFRACTANCE WINDOW DRYING
• It is a thin film drying system having high
heat and mass transfer rates that speed up the
rate of drying at a comparatively lower
temperature.
• Invented by Richard Magoon
Refractance window (RW) drying is a novel fourth-generation drying technology
that is employed to dry heat-sensitive products to retain the product colour,
aroma, antioxidant compounds, and nutritional properties.
8
9. • Water transmits heat energy to the
product
• involves all modes of heat transfer-
conduction, convection & radiation
• Product temperature- less than 70°C
• Drying time- 3-6 min
• Self limitingdrying process
9
10. INDUSTRIAL ORIGIN
• RW DRYING-invented and patented by Richard
Magoon (1986)
• He founded MCD Technologies (Tacoma,
Washington, USA) to trade dryers to other
companies for producing food powders with
desirable retention of flavour, colour and
nutrients
• RWD developed for drying of thin film fruit
pulp & similar materials and producing a dried
material such as fruit leather.
• RW drying process uses 50–70 % less cost and
more than 50 % less energy than the freeze-
drying process when the drying capacity is
kept the same.
10
11. All three heat transfer methods: conduction, convection and radiation occur in heating of water. The loss of heat or
energy from water to its surroundingsoccurs by either conduction or evaporation with convection and radiation being
internal processes.
REFRACTANCE
WINDOW
CONCEPT
Based on the refractive principle of
infrared radiations from the surface of
hot circulating water.
When water is heated
12. If a transparentlayer of
material is placed on the surface of water then the energy loss can only take place
by means of
conduction
When hot water is covered by a IR
transparent medium (polyester film)
12
If a transparentlayerof materialis placedon the surface of waterthen the energy loss
can only take place by meansof conduction.There’sa refractiveindex mismatch
betweenwaterand air, so the incidentradiantenergy atthe plastic-waterinterfaceis
mostly reflectedback and a very little amountis transmitted through the film into the air
13. When wet product is applied
uniformly on IR membrane
13
Then the mismatch of refractive index reduces to a greater extent between the
water and food (i.e water-plastic-food system).water in the material creates a
“window” that allows for the passage of infrared energy through the material
due to minimized refraction at the belt material interface
14. When dried product is obtained
14
As the product losses moisture and the
heat transfer decreases and the “window”
of infrared energy closes and “refracts”
back into the heated water source, no
longer exposing the material to heat.
15. WORKING
• The pureed products are uniformly spread over the polyester conveyor belt,
which passes over hot water trough.
• Temperature of water is maintained at 95-97℃
• drying takes place due to transfer of thermal energy from warm water to food
• Conduction, radiation, and convection are involved in the initial process of
drying;
• As the thin layer of material is positioned on the surface of the conveyor belt,
due to decreased refraction on the belt-material’s interface, the humidity in the
material produces a window allowing passage for direct transfer of infrared heat
energy through the sheet, from the surface of the hot water to the material to
be dehydrated.
• Moisture in the material absorbs electromagnetic radiation in a spectrum of
wavelengths ranging from 3.0 to 15.3 μm
15
16. 16
• This process is self-governing; as the drying progresses and the material is dried, the infrared
window gradually closes for less passage of infrared heat as little moisture is left in the
material.
• The conduction becomes the prevailing mode for heat transfer.
• Since plastic is a low heat conductor, small quantities of heat are transferred to nearly dried
materials and there is a significantly reduced chance of overheating than thin drying processes.
• Due to convective cooling provided by ambient airflow over the drying material the product
temperature is relatively low (below 70 °C) in RW drying and the moist air is exhaust-driven
out of the dryer
• As the dried product reaches the cold-water section, product gets harden. This allows easy
seperation of the product from belt by using a scraping instrument.
18. CONSTRUCTION
• The film used in RWD is
Mylar®, it is commonly
known as polyethylene
terephthalate (PET)
• can either be moving
(continuous) or stationary
(batch).moving belt has
velocity ranges from 0.6
to 3 m/mins.
18
19. In continuous RW dryer,
• The film is supported over a two
end pulley for moving the film.
• Below this film, a shallow water
bath is present in which hot
water is coming from the water
heating unit,
• a pump provided to pump the hot
water from the heating unit to
the hot water bath
• a valve, between the hot water
bath and heating unit, which
helps in pumping the hot water,
if the temperature of the water
bath decreases from the desired
value.
• cold water bath used for lowering
the product’s temperature.
• scraper blade removes the product
of the conveyor belt .
• At the top of the film, a stainless
steel hood is provided to exhaust the
vapour that is evaporated from the
product.
19
21. Water temperature
• The temperature of the water is normally
less than 100 °C.
• Drying rate mainly depends upon the
water bath temperature.
• Maximum water temperature results in a
higher drying rate and less drying time, it
further leads to a product with less water
activity and maximum quality retention.
21
22. Initial product temperature
• High initial temperature of the product requires less sensible heat to increase
the temperature of the product. Therefore, less drying time, less energy
requirements, and a high evaporation rate.
Product thickness
• The time required to move the moisture from the center to the product surface
increases with an increase in the thickness of the product, resulting in a lower
drying rate thereby the drying time increases.
• The lower thickness of the product offers
➢Better quality retention (high drying rate and lower drying time)
➢Improved shelf life (low water activity and MC)
22
23. Radiant source
• Application of RWD technology and various advanced drying systems improves the drying rate,
thus reducing the drying period and the main limitation with product thickness can be overcome.
• for ex- The usage of ultrasound and infrared radiation along with RWD decreases drying time
from 35.6 min to 26.2 min.
• The use of infrared radiation increased the drying rate during the early drying phase and the use
of ultrasound resulted in lower MC at the final stage of the drying process.
IR assistedRefractance window dryer 23
24. Residence time
• In RWD, the drying time is influenced by the temperature of hot water,
product thickness, and air velocity over the product.
• Prolonging exposure of products to high temperatures results in poor product
quality.
• The selection of optimum drying time is therefore of the utmost importance
in order to achieve better quality products.
24
25. Thickness of the film
• The thickness and type of film being used influences the amount of
energy radiated and conducted.
• The lesser the thickness higher the transmission of radiant energy
through the film, but limits the mechanical strength
• A high H&MT rate is achieved with a thin film, which results in rapid
drying.
• The choice of optimum film thickness
• is essential in order to meet both the requirements, the first is a high
heat transfer and the second is maximum mechanical strength.
• 0.25 mm is satisfactory
25
27. PHYSICAL PROPERTIES
❑ Bulk density & porosity
• The bulk density of RWD products was significantly higher than that of other advanced
drying technologies, such as FD and SD, due to the flat structure of the dried product
resulting in lower porosity.
• Due to the lower porosity and higher bulk density of the RWD product, less prone to the
oxidative degradation processes during storage due to its lower surface area, therefore it
can also be stored and transported easily.
❑Water activity
• The shelf-life of the RWD product would be higher due to its low aw and, furthermore,
would be less prone to a microbial attack during the storage period.
27
28. Chromatic properties
• The change in the color of the dried product depends on the drying
temperature, the drying time, the thickness of the product, and the drying
method.
• Due to its low drying temperature and less drying time, RWD is suitable for
drying all types of food and food products where color is the primary
quality parameter.
Carrotflakes producedby RW dryer(a)and Drum dryer(b) 28
29. Microstructure
• The smooth surface of the RWD product particles, the surface area
exposed to the surrounding environment, was less likely to be
susceptible to oxidation and the smooth surface was favorable for free
flow.
SEM images of individual particles(180-
250 µm; magnificationof 1000x, 20 kV)
[RWD (a); FD (b); DD (c); and SD (d)].
29
30. Microbial Properties
• RWD can significantly minimize the microbial load of its dried products
• The maximum temperature of product in RWD is less than 70 ºC
• The gram negative flora is inactivated but the gram positive bacteria survives
due to its heat resistance
• The microbial counts after RW drying greatly reduced for four
microorganisms
Microbialcount in cfu/ml as affected by
RW Drying in inoculatedpumpkinpurees
30
31. Functional Properties
• The functional properties of food samples are influenced by the
drying temperature, drying time, and MC of the dried products.
• Therefore, both the sample thickness and the hot water temperature
are crucial parameters for achieving the desired functional
properties. The functional properties of RWD food commodities
are superior to other conventional drying methods.
31
32. Biochemical properties
• The biochemical properties of food products are mainly influenced by
drying temperature, drying time, and type of drying
• Polyphenols
• Anthocyanin
• Carotenoids
• Ascorbic acid
• Antioxidant capacity
• Aroma composition
Better
retention
of
32
33. Textural properties
• The textural properties of the food depend mainly on the drying
temperature, the type of drying, and the MC of the dried product.
• RWD foods has good textural properties compared to other
conventional methods
• RWD food has less firmness due to its fragile nature.
33
34. Thermal Efficiency
• Water from the dryer flows back into the
reservoir and reheated instead of wasted
during the RW drying process.
• unutilized heat is recycled in hot water
• Thus, the RW drying system is comparatively
more efficient than other dryers
• pilot scale- 4-25%
• commercial scale-52 to 70% Comparisonof evaporativecapacity& thermal
capacityof RWD with selecteddryers 34
35. Energy efficiency
• RWD- energy efficient choice.
• overall energy used for a 150 g batch dehydration was 375–525 W for
RW dryers(5-7 min) that were quite lower than 70–84 kW in case of
freeze-drying (20-24 h).
Overall energyefficiency
Of RWD, FD & Spray dryer
35
37. • Short drying time
• High energy efficiency
• Low installation and operation cost
• processing under low temperature and
atmospheric pressure
• No chemical additives needed
• less complex process
• no cross contamination
• Retains complex, subtle flavors and aromas as
well as colors and nutrients
• Reduction of 4-6 decades of microbial load
• conservation of functional and sensory properties
• limited scope to oxidation and free radical
formation
ADVANTAGES OF RWD
37
38. • Scaling up of RW drying process.
• it is inconvenient in handling
powder with high sugar content
(stickiness)
• high cost as compared to the drum
and spray drying .
DISADVANTAGES
38
39. APPLICATIONS
• Fruits and Vegetables – purees,
juices, pulps, slices
• Meat, Fish, and Poultry
• Dairy and Egg products, including
dairy cultures
• Cereals, Grains, and Starches
• Flavors and Colors
• Beverages – concentrates and
mixes
• Nutraceuticals
• Pharmaceuticals
• Fine Chemicals
• Micro- algae and herbal
formulations
39
40. COMPANIES THAT USE
REFRACTANCE
DRYING TECHNOLOGY
• MegaFood New Hampshire
• NutraDry Australia
• Cerule Oregon
PRODUCTS
MANUFACTURED
Whole food suppliments from fruits &
vegetables
Whole food suppliments from fruits &
vegetables and meats
Algae
40
41. • RWD is an emerging drying technology that has a positive effect on product quality,cost &
energy efficiency
• extensive research work is needed to employ the RWD in the drying of completely solid foods
like fruits, meat, vegetables,nuts etc
• study on combining RWD with novel processing technologies like microwaves, ultrasound, or
osmotic dehydration as a pre-treatment and its affect on drying efficiency
• the relative contribution of conduction, convection, and radiation in the drying process must
be explored through the computational fluid dynamics (CFDs) approach.
• research is necessary for development and modification in the design of the RWD system for
effective use of limited industrial floor space
FUTURE PROSPECTIVE
41
42. CASE STUDY
Source: Vitor Augusto dos Santos Garcia, Josiane Gonçalves Borges (2023),
Production and Characterization of Dehydrated Acerola Pulp: A Comparative
Study of Freeze and Refractance Window Drying- food science and
engineering, Volume 4 Issue 1|2023| 21
Production and Characterization of Dehydrated Acerola Pulp: A
Comparative Study of Freeze and Refractance Window Drying
42
43. Production and Characterization of Dehydrated Acerola
Pulp: A Comparative Study of Freeze and Refractance
Window Drying
Freeze drying
Frozen (-18 °C for 24 h)
Dehydrationin FD under vacuum
( -55 °C for 54 h)
RW Drying
Productthickness- 1mm
watertemp - 65 ± 2 °C
Drying for 15 min
Acerola pulp
Method:
43
44. Water activity, moisture content, and hygroscopicity
Effect of the drying (freeze-dryingand refractancewindow)on wateractivity,moisture content
and hygroscopicityof acerolapulp.
RESULTS
44
45. Colour parameters
Effect of drying process (FD and RWD)on colour
parameterof acerolapulp
Scanning Electron Microscopy (SEM)
Micrographsof freeze-dried acerola pulp (a) and
obtainedby RW (b) At 100xmagnification
45
46. Ascorbic acid concentration and stability
Effect of storagetime under controlled conditions(30⁰C
and 75% RH) on ascorbicacid concentrationby FD and
RWD processes
Antioxidant activity
Effect of drying processesonantioxidantactivity
46
47. Conclusion(Case Study)
• Both freeze dried and RW dried products had similar microstructure characteristics.
• RW dehydrated pulps showed a higher concentration of ascorbic acid and stability
during storage time (10 days). The results may be related to the shorter drying time.
• The RWD is a more appropriate technology to dehydrate acerola pulp with high
vitamin C content, antioxidant activity, and ascorbic acid stability than freeze-drying.
Source: Vitor Augusto dos Santos Garcia, Josiane Gonçalves Borges (2023), Production and
Characterization of Dehydrated Acerola Pulp: A Comparative Study of Freeze and Refractance
Window Drying- food science and engineering, Volume 4 Issue 1|2023| 21
47
48. • RWD is a novel hydro thin-film drying technology alternative to the existing conventional
drying methods.
• The higher drying efficiency and heat and mass transfer rate of RWD with relatively lower
product temperature, less drying time,cost and retention of sensory, functional, and nutritional
properties make the RWD superior to other conventional drying technologies
• Additionally RW drying technology has potential for use in parts of the world where effective
drying methods such as freeze drying have been difficult to implement due to its high cost.
• No doubt, it is more economically viable, but its scalability is still a big challenge that must be
explored by researchers.
• Hence RW drying has a potentially bright future in achieving high standards of quality and
safety in drying heat sensitive materials.
CONCLUSION
48
49. REFERENCE
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of different process parameters on quality of foods: A comprehensive review of trends and technological developments-
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