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Unformatted text preview: steurization Because the rates of enzymatic reactions are slowed as the temperature is lowered, the
growth of mesophiles will be slowed by refrigeration temperatures (4-8°C). Figure
9-5 shows an experiment evaluating the effect of temperature on the growth of aerobic,
facultatively (an)aerobic and aerotolerant heterotrophic bacteria normally present in
The growth of some mesophilic bacteria may be completely halted if the
refrigeration temperature is low enough to cause a fluid to gel state transition in the
bacterium’s membrane. Recall however that psychrotolerant mesophilic
bacteria are able to keep their cytoplasmic membrane fluid at low temperatures by
synthesizing lipids unsaturated hydrocarbon tails. For example the Gram-negative
facultatively anaerobic rod Flavobacterium sp. (flavo=red) are typically
psychrotolerant. This bacterium frequently appears as the “pink stuff” that
grows on the inside of refrigerators.
Even when growth is totally halted, short-term refrigeration is still a bacteriostatic
treatment because growth will resume when the food is warmed to room temperature.
Further, refrigeration and warming does not harm endospores or exotoxins already present
in the food.
Although refrigeration is always thought of in positive terms, consider the
following: Bacteria not only grow more slowly in cold, but also die more slowly.
Further refrigeration also selects for psychrotolerant mesophiles giving them a growth
advantage over other bacteria in the food.
If done properly, cooking temperatures will significantly reduce numbers of any
vegetative bacteria present. However, this may or may not render the product safe
because endospores are not harmed and some exotoxins are heat stable. For example,
the C. botulinum exotoxin is inactivated by normal cooking temperatures, whereas
the exotoxin produced by S. aureus is not. Further, one of the exotoxins produced by
ETEC is heat-labile (sensitive) while the other one is heat-stable.
Pasteurization was originally devised by Louis Pasteur to save products of the French
wine industry from devastating bacterial spoilage. Today it is used for foods that
cannot withstand sterilization conditions (see below) without "significant" loss of
texture and taste (eg. milk, "organic" juices, honey). In pasteurization, the objective is
to kill all "likely" food-poisoning vegetative bacteria and reduce numbers of other
microbes. It is therefore a disinfection not a sterilization method since many
nonpathogenic bacteria survive. This is why, for example, pasteurized milk rapidly
goes sour at room temperature.
Pasteurization is most applicable for extending the shelf life of liquid foods with
consistent, well-understood microbial flora. With respect to milk, pasteurization is
designed to reduce the overall numbers of bacteria in milk and to kill all cells of
Mycobacterium tuberculosis, the organism that causes tuberculosis. Many
different time and temperature combinations c...
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- Fall '12