Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL GROWTH CHARACTERISTICS 61
condition. The growth rate slows down on either side of the optimum growth
temperature until the growth stops. The area under the two points on both sides of
an optimum growth condition where minimum growth occurs is the growth temperature
range. When the cells of a microbial species are exposed to a factor (e.g.,
temperature) beyond the growth range, the cells not only stop growing, but, depending
on the situation, may be injured or lose viability. The growth range and optimum
growth of a microorganism under a specific parameter provide valuable information
for its inhibition, reduction, or stimulation of growth in a food.
E. Growth Curve
The growth rate and growth characteristics of a microbial population under a
given condition can be graphically represented by counting cell numbers, enumerating
CFUs, or measuring optical density in a spectrophotometer at a given
wavelength (above 300 nm, usually at 600 nm) of a cell suspension. Cell mass,
or specific cell components such as proteins, RNA, or DNA, can also be measured
to determine growth rate. Each method has several advantages and disadvantages.
If the CFU values are enumerated at different times of growth and a growth curve
is plotted using log 10 CFU vs. time (log 10 CFU is used because of high cell
numbers), a plot similar to the one presented in Figure 5.2 is obtained. The plot
has several features that represent the conditions of the cells at different times.
Initially, the population does not change (lag phase). During this time, the cells
assimilate nutrients and increase in size. Although the population remains
unchanged because of change in size, both cell mass and optical density show
some increase. Following this, the cell number starts increasing, first slowly and
then very rapidly. The cells in the population differ initially in metabolic rate and
only some multiply, and then almost all cells multiply. This is the exponential
phase (also called logarithmic phase). Growth rate at the exponential phase
follows first-order reaction kinetics and can be used to determine generation time.
Following this, the growth rate slows down and finally the population enters the
stationary phase. At this stage, because of nutrient shortage and accumulation of
waste products, a few cells die and a few cells multiply, keeping the living
population stable. However, if one counts the cells under a microscope or measures
cell mass, both may show an increase, as dead cells may remain intact.
After the stationary phase, the population enters the death phase, in which the
rate of cell death is higher than the rate of cell multiplication. Depending on the
strain and conditions of the environment, after a long period of time (may even
be a few years) some cells may still remain viable. This information is important
to determine some microbiological criteria of food, especially in controlling
spoilage and pathogenic microorganisms in food. It is important to note that by
changing some environmental parameters (e.g., refrigeration), the growth rate of
some microbial species can be slowed down, but after a long time, the population
can reach high numbers to cause problems in a food. Dead cells, by the action
of autolytic enzymes, may lyse and release the cellular enzymes in a food, which
then can act on food components.
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