during visits to developing nations where
hygienic practices may be substandard.
Microbial destruction..............................................................................
Microorganisms are considered dead
when they cannot multiply, even after being
in a suitable growth medium under favorable
environmental conditions. Death differs
from dormancy, especially among bacterial
spores, because dormant microbes have not
lost the ability to reproduce, as evidenced by
eventual multiplication after prolonged incu-
bation, transfer to a different growth medium,
or some form of activation.
Regardless of the cause of death, microor-
ganisms follow a logarithmic rate of death,
as in the accelerated death phase of Figure
3–1. This pattern suggests that the popula-
tion of microbial cells is dying at a relatively
constant rate. Deviations from this death
rate can occur due to accelerated effects from
a lethal agent, effects due to a population
mixture of sensitive and resistant cells, or
with chain- or clump-forming microbial flora
with uniform resistance to the environment.
Heat
Historically, heat application has been the
most widely used method of killing spoilage
and pathogenic bacteria in foods. Heat pro-
cessing has been considered a way to cook
food products and destroy spoilage and
pathogenic microorganisms. Therefore,
extensive studies have been conducted to
determine optimal heat treatment to destroy
microorganisms. A measurement of time
required to sterilize completely a suspension
of bacterial cells or spores at a given temper-
ature is the thermal death time(TDT). The
value of TDT will depend on the nature of
the microorganisms, its number of cells, and
factors related to the nature of the growth
medium.
Another measurement of microbial des-
truction is decimal reduction time(D value).
This value is the time in minutes required
to destroy 90% of the cells at a given tempera-
ture. The value depends on the nature of
the microorganism, characteristics of the
medium, and the calculation method for
determining the D value. This value is calcu-
lated for a period of exponential death of
microbial cells (following the logarithmic
order of death). The D value can be deter-
mined through an experimental survivor
curve.
Increased concern about pathogens of
fecal origin, such as E. coliO157:H7, has
been responsible for the investigation and
implementation of hot-water spray washing
of beef carcasses immediately after harvest-
ing as a method of cleaning and decontami-
nation. Smith (1994) identified the best
combination (and sequence) of interven-
tions reducing microbial load to be: use of
74ºC water in the first wash and 20 kg/cm^2
pressure and spray wash with hydrogen per-
oxide or ozone in the second wash (espe-
cially if 74ºC water temperature is not
incorporated in the first wash). Additional
investigations are evaluating the efficacy of
steam pasteurization/steam-vacuum as a
technique for microbial reduction of beef
carcasses.Chemicals
Many chemical compounds that destroy
microorganisms are not appropriate for
killing bacteria in or on a foodstuff. Those
that can be used are applied as sanitizing
agents for equipment and utensils that can
contaminate food. As the cost of energy for
thermal sanitizing has increased, the use of
chemical sanitizers has grown. It is hypothe-
sized that chlorine disinfection may result
from slow penetration into the cell or the
necessity of inactivating multiple sites
within the cell before death results. (Addi-The Relationship of Microorganisms to Sanitation 53