Methods of food preservation
Food preservation aims (1) to destroy microorganisms as by heat, or (2) to retard their growth by removal of moisture or the use of cold temperatures. Chemical changes are minimized by avoiding exposure to air and light, by reducing the environmental temperature, and by destroying enzymes.
Dehydration, one of the oldest methods of preservation, eliminates the moist environment that microorganisms need for their growth.
Freezing inactivates bacteria and enzymes. Foods stored in the home freezer at -18° С (0° F) may be left for several weeks or months (depending on the product) with minimum loss of texture, color, flavor, or nutritive value. Once foods are thawed, bacteria and enzymes are reactivated and the foods should be used promptly before spoilage can occur.
Freeze-drying consists in rapidly freezing the product and then removing the moisture in a vacuum.
Cookery. The cooking or baking of food leads to destruction of microorganisms and enzymes. Lower temperatures, such as those attained in a double boiler, are not sufficient to destroy some organisms, such as Salmonella in eggs. Some spores of bacteria and some toxins are not destroyed by the heat used in ordinary cooking methods.
Pasteurization is the application of heat to destroy pathogenic bacteria, but it does not sterilize the product. In the high-temperature, short-time process now widely used, milk is held at 160° F for at least 15 seconds. Milk and cream for the manufacture of cheese, ice cream, and butter is usually pasteurized.
Canning is still the primary means used to preserve foods for long periods of time. Home canning is far less frequent than at one time. Meat, poultry, and nonacid vegetables, such as corn, peas, and green beans, should be canned only with a pressure cooker for specified times in order to ensure destruction of the spores of CI. botulinum.
Chemical preservation. Sugar has some preservative effect when used in high concentrations for jams, jellies, and preserves, but molds will grow on the surfaces unless they are protected from air. Brine is used for pickles, sauerkraut, and pickled fish. Sodium benzoate may be used in a limited number of products, including margarine. Sulfur dioxide prevents the darkening of apples and apricots during dehydration. Calcium propionate in bread and sorbic acid in cheese wrappings retard mold growth.
Nutritive values of processed foods
Many people have the mistaken notion that processed foods have been robbed of their nutrients.” Commercial processing techniques today ensure maximum retention of nutrients. In fact, fruits and vegetables that have been frozen or canned at the peak of their quality may be higher in nutritive value than those sold as fresh in markets where the temperatures were too high or the products were held for too long a period.
In canned foods the water-soluble vitamins and minerals distribute evenly between the solid and liquid. Suppose one fourth of the contents were liquid, then up to one fourth of the water-soluble nutrients would be lost if the liquid were discarded. Thus, the general rule: use liquids in which foods have been canned or cooked.
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