Relative humidity refers to the atmosphere surrounding a material or solution. The term water activity is related to relative humidity. One of the oldest methods of food preservation is by drying, which reduces water activity sufficiently to prevent or delay microbial growth. The slowing of microbial activity with decreased temperatures is the principal behind refrigeration and freezing preservation. At temperatures below 10 oC, growth is slow and becomes slower the colder it gets. Most bacteria, yeasts, and molds grow best in the temperature range 16-38 oC (except psychrotrophs). Cooler temperatures will prevent growth, but slow metabolic activity may continue. As the temperature drops from this optimal temperature toward the minimal, the rate of growth of the organism decreases and is slowest at the minimal temperature. Each microorganism present has an optimal temperature for growth and a minimal temperature below which it cannot multiply. The lower the temperature, the slower will be chemical reactions, enzyme action, and microbial growth. A process known as ultra high pasteurization uses even higher temperatures of the order of 194 to 266 ☏ (90 to 130☌) for periods of a second or more. In a more recent revision of that process, milk can also be "flash-pasteurized" by raising its temperature to about 160 ☏ (71 ☌) for a minimum of 15 seconds, with equally successful results. Conventional methods of pasteurization called for the heating of milk to a temperature between 145 and 149 ☏ (63 and 65 ☌) for a period of about 30 minutes, and then cooling it to room temperature. The most familiar example of the latter situation is pasteurization. In other cases, cooking is neither appropriate nor necessary. In many cases, foods are actually cooked prior to their being packaged and stored. A preliminary step in many other forms of food preservation, especially forms that make use of packaging, is to heat the foods to temperatures sufficiently high to destroy pathogens. In this respect, heating foods is a form of food preservation comparable to that of freezing but much superior to it in its effectiveness. Heating food is an effective way of preserving it because the great majority of harmful pathogens are killed at temperatures close to the boiling point of water. The term "thermal" refers to processes involving heat. Souring (fermentation) lactic and acetic acid, e.g. It involves application of chemical additives which act as antimicrobial agents.In such situation one has to opt for chemical methods of preservation.Quite often it is either impossible or undesirable to employ conventional physical methods of the preservation.The principles of various methods for food preservation are asġ) Prevention or delay of microbial decomposition Preservative for food may be defined as any chemical compound and/or process, when applied to food, retard alterations caused by the growth of microorganisms or enable the physical properties, chemical composition and nutritive value to remain unaffected by microbial growth. It also includes processes to inhibit natural ageing and discolouration that can occur during food preparation such as the enzymatic browning reaction in apples after they are cut. Preservation usually involves preventing the growth of bacteria, fungi, and other micro-organisms, as well as retarding the oxidation of fats which cause rancidity. There is a adverse effect on appearance, flavour, texture, colour, consistence and/or nutritional quality of food.įood preservation is the process of treating and handling food to stop or greatly slow down spoilage (loss of quality, edibility or nutritive value) caused or accelerated by micro-organisms. Chemical and/or biochemical reactions results in decomposition of food- due to microbial growth. The vast majority of instances of food spoilage can be attributed to one of two major causes: (1) the attack by microorganisms such as bacteria and molds, or (2) oxidation that causes the destruction of essential biochemical compounds and/or the destruction of plant and animal cells. The major factors affecting food spoilage areġ) Growth and activities of microorganisms (bacteria, yeasts, and molds)Ģ) Activities of food enzymes and other chemical reactions within food itselfĤ) Inappropriate temperatures for a given food Foods are subject to physical, chemical, and biological deterioration. In addition, minor constituents such as minerals, vitamins, enzymes, acids, antioxidants, pigments, flavours are present. Carbohydrates, proteins and fats are the major constituents of food. GENERAL PRINCIPLES OF FOOD PRESERVATION-PHYSICAL METHODSįoods are mainly composed of biochemical compounds which are derived from plants and animals.
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