the increase in number of microbes occurs at
an exponential rate until some environmen-
tal factor becomes limiting. The length of
this phase may vary from 2 to several hours.
The number of microorganisms and environ-
mental factors, such as nutrient availability
and temperature, affect the logarithmic
growth rate of the number of microorgan-
isms. Effective sanitation to reduce the micro-
bial load can limit the number of microbes
that can contribute to microbial proliferation
during this growth phase.
Stationary Growth Phase
When environmental factors such as nutri-
ent availability, temperature, and competition
from another microbial population become
limiting, the growth rate slows and reaches an
equilibrium point. Growth becomes relatively
constant, resulting in the stationary phase.
During this phase, the number of microor-
ganisms is frequently large enough that their
metabolic by-products and competition for
space and nourishment reduce proliferation
to the point that it is nearly stopped, is
stopped, or a slight decrease in the microbial
proliferation occurs. The length of this phase
usually ranges from 24 hours to more than
30 days but depends on both the availability
of energy sources for the maintenance of cell
viability and the degree of pollution in (hos-
tility of ) the environment.
Accelerated Death Phase
Lack of nutrients, metabolic waste prod-
ucts, and competition from other microbial
populations contribute to the death of
microbial cells at an exponential rate. Accel-
erated death rate is similar to logarithmic
growth rate and ranges from 24 hours to 30
days but depends on temperature, nutrient
supply, microbial genus and species, age of
the microorganisms, application of sanita-
tion techniques and sanitizers, and competi-
tion from other microbes.
Reduced Death Phase
This phase is nearly the opposite of the lag
phase. It is caused by a sustained accelerated
death phase, so that the microbial popula-
tion number is decreased to the extent that
the death rate decelerates. After this phase,
the organism has been degraded, steriliza-
tion has occurred, or another microbial pop-
ulation continues decomposition.What causes microorganisms to grow
Factors that affect the rate of proliferation
of microorganisms are categorized as extrin-
sicandintrinsic.Extrinsic Factors
Extrinsic factors relate to the environmen-
tal factors that affect the growth rate of
microorganisms.Temperature
Microbes have an optimum, minimum,
and maximum temperature for growth.
Therefore, the environmental temperature
determines not only the proliferation rate but
also the genera of microorganisms that will
thrive and the extent of microbial activity
that occurs. For example, a change of only a
few degrees in temperature may favor the
growth of entirely different organisms and
result in a different type of food spoilage and
foodborne illness. These characteristics have
been responsible for the use of temperature
as a method of controlling microbial activity.
The optimal temperature for the prolifera-
tion of most microorganisms is from 14ºC to
40ºC, although some microbes will thrive
below 0ºC, and other genera will grow at
temperatures up to and exceeding 100ºC.
Microbes classified according to tempera-
ture of optimal growth include: