Microbial growth control
Most methods used to kill microorganisms
may be applied in a milder treatment to inhibit
microbial growth. Sub-lethal heating, irradia-
tion, or treatment with toxic chemicals fre-
quently causes injury to microorganisms and
impaired growth without death. Injury is
reflected through an increased lag phase, less
resistance to environmental conditions, and
greater sensitivity to other inhibitory condi-
tions. Synergistic combinations of inhibitory
agents, such as irradiation plus heat and heat
plus chemicals, can increase microbial sensi-
tivity to inhibitory conditions. Injured cells
appear to require synthesis of some essential
cell materials (that is, ribonucleic acid or
enzymes) before recovery is accomplished.
Microbial growth is inhibited through mainte-
nance of hygienic conditions to reduce debris
available to support bacterial proliferation.
Refrigeration
The effect of temperature on microbial
proliferation has been discussed. Freezing
and subsequent thawing will kill some of the
microbes. Those that survive freezing will
not proliferate during frozen storage. Yet,
this method of reducing the microbial load
is not practical. Also, microorganisms that
survive frozen storage will grow on thawed
foods at a rate similar to those that have not
been frozen. Refrigerated storage can be
used with other methods of inhibition—
preservatives, heat, and irradiation.
Chemicals
Chemicals that increase osmotic pressure
with reduced Awbelow the level that permits
growth of most bacteria can be used as bac-
teriostats. Examples include salt and sugar.
Dehydration
Reduction of microbial growth by dehy-
dration is another method of reducing the
Awto a level that prevents microbial prolifer-
ation. Some dehydration techniques restrict
the types of microorganisms that may multi-
ply and cause spoilage. Dehydration is most
effective when combined with other methods
of controlling microbial growth, such as salt-
ing and refrigeration.Fermentation
In addition to producing desirable flavors,
fermentation can control microbial growth.
It functions through anaerobic metabolism
of sugars by acid-producing bacteria that
lower the pH of the substrate, the foodstuff.
A pH below 5.0 restricts growth of spoilage
microorganisms. Acid products that result
from fermentation contribute to a lower pH
and reduced action of microorganisms.
Foods that are acidified and heated may be
packed in hermetically sealed containers to
prevent spoilage by aerobic growth of yeasts
and molds.Microbial load determination
Various methods are available for deter-
mining microbial growth and activity in
foods. The choice of method depends on
the information required, food product
being tested, and the characteristics of the
microbe(s). One of the most important fac-
tors in obtaining accurate and precise
results is the collection of representative
samples. Because of the large numbers and
variability of microorganisms present,
microbial analyses are less accurate and
precise and, therefore, more subjective than
are chemical methods of analysis. However,
these results need to be interpreted. An
excellent information source for rapid test
kits is the AOAC Research Institute that
has certified a large number of test kits.
Technical knowledge and experience related
to microbiology and food products areThe Relationship of Microorganisms to Sanitation 55