Food preservation methods are intended to keep microorganisms out of foods, remove microorganisms from contaminated foods, and hinder the growth and activity of microorganisms already in foods.
To keep microorganisms out of food, contamination is minimized during the entire food preparation process by sterilizing equipment, sanitizing it, and sealing products in wrapping materials. Microorganisms may be removed from liquid foods by filtering and sedimenting them or by washing and trimming them. Washing is particularly valuable for vegetables and fruits, and trimming is useful for meats and poultry products.
Heat. When heat is used to preserve foods, the number of microorganisms present, the microbial load, is an important consideration. Various types of microorganisms must also be considered because different levels of resistance exist. For example, bacterial spores are much more difficult to kill than vegetative bacilli. In addition, increasing acidity enhances the killing process in food preservation.
Three basic heat treatments are used in food preservation: pasteurization, in which foods are treated at about 62°C for 30 minutes or 72°C for 15 to 17 seconds; hot filling, in which liquid foods and juices are boiled before being placed into containers; and steam treatment under pressure, such as used in the canning method. Each food preserved must be studied to determine how long it takes to kill the most resistant organisms present. The heat resistance of microorganisms is usually expressed as the thermal death time, the time necessary at a certain temperature to kill a stated number of particular microorganisms under specified conditions.
In the canning process, the product is washed to remove soil. It is then blanched by a short period of exposure to hot water to deactivate enzymes in the food. Diseased sections in the food are removed, and the food is placed into cans by a filling machine. Sealed cans are then placed into a sterilizing machine called a retort, and the food is processed for a designated time and temperature.
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. Freezingreduces 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, includingpropionic acid, sorbic acid, benzoic acid, and sulfur dioxide. These acids are acceptable because they can be metabolized by the human body. Some antibioticscan also be used, depending upon local laws and ordinances. Tetracycline, for example, is often used to preserve meats. Storage and cooking normally eliminates the last remnants of antibiotic.
In many foods, the natural acids act as preservatives. In sauerkraut, for example, lactic acid and acetic acid prevent contamination, while in fermented milks (yogurt, sour cream), acids perform the same function. For centuries, foods were prepared in this manner as a way of preventing microbial spoilage.
Drying. Drying is used to preserve food by placing foods in the sun and permitting the water to evaporate. Belt, tunnel, and cabinet dryers are used in industry for such things as instant coffee and cocoa. Freeze-drying, a process calledlyophilization, is also valuable for producing a product free of moisture and very light.
Radiations. Ultraviolet radiation is valuable for reducing surface contamination on several foods. This short-wavelength light has been used in the cold storage units of meat processing plants. Ionizing radiations such as gamma rays can be used to preserve certain types of vegetables, fruits, and spices, according to state and U.S. federal regulations.