【正文】 probably the second greatest cause for food spoilage. B. Many of same principles for food preservation apply for enzymes as for microanisms. ? For example, when a food is pasteurized, enzymes are denatured and thus rendered inactive. ? When a food is refrigerated to slow down the growth of microanisms, so is the activity of enzymes slowed down. C. However, some food enzymes may be more resistant to preservation effects than microanisms。 occurs under the best of conditions. Factors adversely affecting food changes in temperature (heat and cold), light and other radiation, oxygen, changes in moisture content (water loss or uptake), detrimental enzymes of the food, microanisms and macroanisms, industrial contaminants (., packaging materials) and close proximity of other foods, and time. Some of the most important advances in food technology have occurred as a result of war. Nicolas Appert developed the technology of canning (1809) as a result of prize money offered by Napoleon for preservation of food for the French army and navy. Shelflife and dating of foods A. Definition of shelflife: The time it takes for a product to deteriorate to an unacceptable level (what is unacceptable is sometimes a matter of opinion). B. A better definition: Length of time a product remains salable. It is mon for a food manufacturer to define a minimum acceptable quality (MAQ) for a product. Actual length of shelflife dependent on Processing method, Packaging, Storage conditions. Dating system 1. Different code dates: Date of manufacture (pack date), Date the product was displayed (display date), Date by which the product should be sold (sell by date), Last date of maximal quality (use by date), Date beyond which the product is no longer acceptable (use by date or expiration date). 2. Use of these code dates requires a need to predict and monitor shelflife. Models for predicting shelflife are particularly useful for new products without a distribution history. V. Major causes of food deterioration A. Microanisms (bacteria, yeasts and molds) B. Insects and rodents C. Heat and cold can cause deterioration of food if not controlled. D. Moisture and dryness E. Oxygen F. Light G. Time A. Microanisms (bacteria, yeasts and molds) More types of microanisms can spoil food than cause foodborne disease. Sources of these microanisms: soil, water, air, food itself, humans, food equipment environment. Healthy living tissue (internally) is usually sterile, hence the presence of spoilage anisms is mostly the result of contamination. Bacterial endospores are most difficult to inactivate. Heat and moisture will increase growth and activities of microanisms. Molds as pared to bacteria can generally grow at: lower pH (more acid conditions), lower moisture contents (dryer conditions), higher salt concentrations, and lower temperatures (in refrigerated foods). Molds usually only a problem with spoilage, not safety, but Aspergillus flavus and Aspergillus parasiticus produce aflatoxins which are potent hepatocarcinogens. Molds require oxygen for growth. B. Insects and rodents Insects destroy 5 to 10% of . grain crop annually (in other parts of the world it can reach 50%)。 liquid foods are easiest to dry. ? Mashed potatoes and tomato puree processed by drum drying (drum heated from within, applied layer of food flashes off its moisture on contact with heated drum, and thin film of food scraped off drum with long knives). ? Peas and diced onions dried by moving through a long tunnel oven (subject of overheating and shrinkage), but a preferred method is, ? vacuum freezedrying (used for coffee), food frozen, dehydrated under vacuum from the frozen state. Forming foods made into specific shapes (., fish sticks) Application of pressure within an appropriate form. Range of pressures used, varies considerably dependent upon the product. For example: Extrusion {Breakfast cereals Extrusion cooking} Formulated dough or mash is extruded under high pressure with heat. Packaging (food containers) To protect food from microbial contamination, physical dirt, insect invasion, light, moisture pickup or loss, flavor pickup or loss, and p