1. Microbiology 3) Which modified atmosphere i'sthe best for storingmeat productsin films?
4) Give the list of microorganisms based on their oxygen requirement for
growth in foods?
9.4 PATTERNS OF MICROBIAL GROWTH IN
FOOD
Let us study what course the microorganisms in a food will follow during its
growth, and why? The pattern of microbial growth depends on internal
environment of a food (intrinsic factors) as well as storage conditions under
which it is stored (extrinsicfactors)sincethese may either supportor discourage
the growthof microorganisms.The growthofmicrobes in foodsoronanartificial
growth medium follows a definitecourse.This pattern of microbial growth in a
food or an artificiallymedium seeded (inoculated)with any microorganism can
be studied in the form of a 'microbial growth curve'. The growth curve can be
obtained by plotting the count of the bacterial cells against the corresponding
storagetime(Fig. 9.2).Undertheconditionsfavouringgrowthofmicroorganisms,
a microbial growth curve would show the following four phases:
a) Lag phase: Initially, there would not be any appreciable increase in the
number of microbes, though there may be an increase in their size. This
initial period is the time rzquired for adaptation of microbes to the new
environment(foodoramedium).Duringthisperiod ofmicrobialadaptation,
adequate quantitiesof necessary enzymes and metabolicintermediatesare
build up inside the microbial cells as these would be required for
subsequent multiplication. The duration of the lag phase varies with the
type of microbial species, size of inoculum,nature of the food or medium,
and environmentalfactors such as temperature (Fig. 9.2).
b) Log (logarithmic) or exponential phase: Following the lag phase, the
cells start dividing by binary fission (one cell is divided into two in each
cycle), thereby, doubling their numbers with each successivecycle of cell
division. In this manner, the microbes increase in their numbers
exponentially or by geometric progression with time, i.e., 1-2-4-8-16-32
and so on. Therefore, if the logarithmofthe viable countisplotted against
time, a straight line will be obtained (Fig. 9.2).
c) Stationary phase: After a varying period of exponential growth, cell
division stops due to depletion of nutrients and accumulation of toxic
products.Thenumberofprogeny cells formed isjust enoughto replacethe

2. number of cells that die. As an equilibrium exists between the dying cells
andthe newly formedcells,the viable countinthisphase remains stationary
(Fig9.2).
Microbial Growth in Fa
and its Cont
d) Phase of decline: his is the phase when microbial population decreases
due to cell death. Besides nutritional exhaustion and toxic accumulation,
autolytic enzymes may also cause cell death (Fig,
9.2).
Viable count
I
Number of
cells
Time
; ~
-
b 1
Pig. 9.2: Bacterial growth curve. Showing lag, log, stationary and the decline phases
The principal environmental factors influencing bacterial growth are food,
temperature, moisture, oxygen availability,hydrogen ion concentration and the
presence of inhibitory substances. The combined effect of these determines the
microbial flora.
Check Your Progress 3
1) List the four phases of bacterial growth curve.
2
) What are the causes of death of the cells after stationary phase of their
growth?
9.5 CONTROL OF MICROBIAL GROWTH IN
FOOD
The control of microbial growth as used here, means to prevent growth of
microorganisms. This control is affected in two basic ways: (A) by killing
microorganismsor (B) by inhibiting the growth of microorganisms. Control of
growth usually involves the use of physical or chemical agents which either kill
or prevent the growth of microorganisms. Agents which kill cells are called
'cidal' agents; agents which inhibit the growth of cells (without killing them)