Notes - Physical Methods of Food Preservation

Notes - Physical Methods of Food Preservation - PHYSICAL...

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PHYSICAL METHODS OF FOOD PRESERVATION Physical methods of food preservation are those that utilize physical treatments to inhibit, destroy or remove undersirable microorganisms without using additives or products of microbes. PHYSICAL DEHYDRATION PROCESSES Water is one of the most important factors controlling the rate of spoilage of food by microbial or non- microbial effects. The availability of water is measured by the water activity (a w ) of a food, defined as the ratio of the vapor pressure of water in a food, P, to the vapor pressure of pure water, P o at the same temperature Water activity A w = P/P o Water activity Most spore forming bacteria do not grow below A w 0.93. Germination and outgrowth of spores of Bacillus cereus are prevented at 0.97-0.93. Minimum a w for growth and spore germination C. perfringens is 0.97-0.95. Various serotypes of C. botulinum differ in their a w requirements (0.94-0.97). Water activity In general, among bacteria, gram negative species (Enterobacteriaceae 0.96) have the highest water activity requirement (Pseudomonas 0.93). Less sensitive to reduced water activity are gram-positive non- spore forming bacteria (Lactobacillus/0.94). Staphylococci are unique among nonhalophilic bacteria in being able to grow at high levels of NaCl. Water activity Several species of yeasts have a w requirements much lower than those of bacteia. Salt tolerant species such Debaryomyces and Hansenula and Candida may grow well on cured meats and pickles at salt concentrations of 11% (a w =0.93). Some osmophilic species (Zygosaccharomyces) are able to grow in foods of high sugar content (jams, honey syrups). Water activity In general, molds have a w requirements much lown than those for other groups of foodborne microorganisms. However, molds with sporangiospores have greater moisture requirements and germination of their spores may be inhibited at a w 0.93. The most common xerotolerate molds belong to Eurotium (asexual forms are Aspergillus glaucus group) .
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Water activity The minimal a w for their growth is in the range of 0.71-0.77. The relationship of a w and toxin production is complex . It is of great public health importance that the minimal a w is higher than that for growth. (when molds grow they do not produce toxin) Differences in a w limits for growth reflect differences in osomoregulatory capacities. DRYING Hot air drying of solid foods has two functions . It transfers heat to evaporate water from the food and carries away the vapor produced. In the process of drying microorganisms on the raw food are affected by the drying temperature and changes in the a w . The a w of dried foods us usually lower than the critical value for microbial growth.Dried products are stable if the relative humidity is less than 70%. FREEZE-DRYING
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Notes - Physical Methods of Food Preservation - PHYSICAL...

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