Cfu colony forming unit food 293 refrigeration

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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 hamburger. 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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