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Unformatted text preview: bles such as temperature, reactive oxygen species, pH, solute concentration eg.
sugar and salt (NaCl). Temperature
Problem bacteria From the standpoint of temperature, the bacteria of most concern in the food industry are:
Mesophiles: these organisms show significant growth rates at room temperature and
human body temperature.
Psychrotolerant mesophiles: mesophilic bacteria show significant growth rates at
refrigeration temperatures i.e. they are tolerant to cold. Like psychrophiles, these
bacteria bacteria are able to keep their cytoplasmic membrane fluid at low temperatures
by synthesizing lipids with unsaturated hydrocarbon tails.
Endospore-forming mesophiles. FOOD-292 Freezing and
dehydration Freezing and
thawing Freeze-drying and
rehydration Because bacteria require liquid water in order to grow, freezing and dehydration can be
used to halt bacterial growth in a food product. These treatments are normally used to
preserve food against spoilage. Wet freezing can be bacteriocidal or bacteriostatic. The killing effect of wet freezing is
ice crystal formation which punctures membranes. The number of ice crystals formed
is affected by the freezing rate: fast freezing minimizes nucleation of ice crystal
formation while slow freezing has the opposite effect.
The chemical composition of the bacteria and the food product influence the
killing effect of freezing. Some chemicals act as “anti-freezes” inhibiting nucleation of
ice crystal formation. For this reason, the killing effect of freezing is variable and
vegetative bacteria are not always harmed by freezing and resume growth upon
thawing. This is why deep-frozen turkeys, for example, can still cause salmonellosis
(caused by the Gram-negative bacterium Salmonella).
In addition to halting growth, freezing stops exotoxin production and endospore
formation. This being said, freezing is no protection from preexisting exotoxins and
endospores in the food. Neither are harmed by freezing and thawing. In freeze drying, water is sublimated from frozen food under vacuum and the food is then
stored at room temperature. The treatment can be bacteriocidal or bacteriostatic, the effect
being largely dependent on how fast the food is frozen (flash-freezing minimizes ice
crystal formation). It is very common for bacteria to resume growth upon addition of
water to freeze-dried food. This is expected. Flash-freezing and flash freeze-drying are
normal ways of preserving bacterial cultures (When bacterial cultures are frozen
“cryoprotectant” chemicals [eg. glycerol] are added to inhibit ice crystal nucleation.
Freeze drying is no protection from preexisting exotoxins and endospores in the
food. Neither are harmed by freezing and rehydration. Figure 9-5: Effect of temperature on the growth of aerobic, facultatively anaerobic and aerotolerant heterotrophic
bacteria normally present in hamburger. Growth was followed using the plate count method. CFU =
colony forming unit. FOOD-293 Refrigeration
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- Fall '12