Low temperatures. Low temperatures are used to preserve food by lowering microbial activity through the reduction of microbial enzymes. However, psychrophilic bacteria are known to grow even at cold refrigerator temperatures. These bacteria include members of the genera Pseudomonas, Alcaligenes, Micrococcus , and Flavobacterium . Fungi also grow at refrigeration temperatures. Slow freezing and quick freezing are used for long-term preservation. Freezing reduces the number of microorganisms in foods but does not kill them all. In microorganisms, cell proteins undergo denaturation due to increasing concentrations of solutes in the unfrozen water in foods, and damage is caused by ice crystals. Chemicals. Several kinds of chemicals can be used for food preservation, including propionic acid, sorbic acid, benzoic acid , and sulfur dioxide. These acids are acceptable because they can be
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