By Shalini Asrani
•The greater the heat treatment the more organisms will be killed.
•Heating will produce sterility of the product.Various degree of heating used on food-1) Pasteurization – It is the heat treatment that kills the microorganisms involving the application of temperature below 100°C.2) Heating at about 100°C – A temp. of 100° C is obtained by boiling a liquid food.3) Heating above 100°C – ‘AUTOCLAVE’ a microbiological instrument may be used to kill all the pathogenic microbes at about 121°C and 15 lbs pressure for 15 minutes.
•Low temperature - Retard chemical reactions and actions of food enzymes. Slow down or stop the growth and activity of microorganism in food.
•Commercial refrigeration temperature is lower than 5 to 7.2 °C.
•At 5°C: - Food Remain unspoiled for longer duration.
•At -5° C: - Ice crystal form.
•At -60°C: - Smaller crystals are formed.Temperature Employed in Low Temperature Storage
1.Common, or Cellar, Storage –
•The temperature in this is lower than 15°C
•E.g.:- Root crops, potatoes, cabbage, celery, apples etc. 2) Chilling, or “Cold Storage” –
•Involve cooling by ice or by mechanical refrigeration.
•Used as main or temporary preservation method.
•E.g.:-Eggs, dairy products, meats etc.
•Enzymatic and microbial changes in the food are slowed down.3.Freezing or “Frozen Storage’’-
•Volume of food increases, ice crystal form and grow in size,
•Microbial growth is prevented entirely and action of food enzymes is greatly retarded.Blanching or scalding -Brief exposure of food to hot water or steam prior to freezing reduces the number of viable microbes on the food, inactivates plant enzymes and fixes the natural food color, and enhances the efficacy of freezing as a food preservation method.
2.DRYING/DESICCATION-•Drying refers to removal of water by reduction in the amount of available moisture and/or lowering of water activity that prevents microbial growth and survival.
•Removal or reduction of water content from the food prevents microbial growth. 1.Sun drying- This method is one of the most traditional methods of drying. It is slow and only practical in hot, dry climates. However, it is still used today e.g. sun dried chillies, raisins or tomatoes.2. Solar drying- It is a modification of sun drying in which the sun rays are collected inside a specially designed unit with adequate ventilation for removal of moist air. The temperature in unit is usually 20 to 30 degrees higher than in open sunlight, which results in a shorter drying time. Lack of control of weather is the main problem as factors such as temperature, relative humidity, and air velocity vary.3. Evaporation- Heat is artificially produced under controlled temperature, Relative Humidity, air flow.4. Condensation- Partial removal of some moisture from liquid food like milk. Sweetened condensed milk is a very thick, sweet product which when canned can last for years without refrigeration if unopened. 5. Smoking of food- Smoking is the process of flavouring, cooking, or preserving food by exposing it to smoke from burning or smouldering material, most often wood. Meats and fish are the most common smoked foods.
6. Freeze drying/ Lyophilization- Material is cooled below its preferable temperature firstly to be solidified completely, then dried in vacuum space at low temperature by sublimation drying and desorption drying till 95-99% of moisture is removed.CHEMICAL METHODS - Those food additives which are specifically added to prevent the spoilage of a food are referred to as chemical preservatives.1) Salt-
•Lower the water activity
•NaCl is used in brines and curing solution
•Slow or prevent growth of microbes or permit acid fermentation 2. Sugar-
•Such as sucrose or glucose make water unavailable to organism.
•E.g.:- Milk, Fruits in syrups, Jellies and Candies etc.BIOLOGICAL METHODS
1. Antibiotics –
•A chemical substance produced by microorganism to kill or inhibit the growth of related microorganisms.
•They are added to protein rich foods like meat, fish and poultry and enhance their shelf life.
•E.g. ‘Subtilin’ produced by Bacillus subtilis, inhibits gram positive bacterial growth as it is heat and acid resistant, used in canned foods as well to prevent spore formers growth.2.Bacteriocins-
•Proteinaceous toxins that are produced by bacteria to inhibit growth of other organisms.
•E.g. ‘Sakacins’ produced by Lactobacillus sakei, used in dry, cured sausage meat products, cheese and other lactic acid fermented products
•‘Pediocins’ effective against Listeria monocytogenes.It can be concluded that food preservation gives many benefits to us, with preservation processes, perishable products such as fruit and vegetables, meat and fish can be kept for a long time. Food can be marketed to far places overseas. This can increase income in the agricultural sector. Canned foods are much easier to store and distribute from one place to another compared to raw foods.Food wastage is reduced- Excess food can be kept in a refrigerator for the next meal. Without preservation, perishable foods like vegetables and fruits cannot be eaten a few days after harvesting. Food preservation is very important to fulfill the food supply needs in a country. It also ensures that food supply can be distributed to the people at all times. Furthermore, problems like food shortage or famine can be avoided.The above article was co-written by Purva Asrani and Shalini Asrani.
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The above writer, Dr. Shalini Asrani, serves as assistant professor at the Department of Home Science Extension and Communication Management of CSK Himachal Pradesh Agriculture University in Palampur, India. She contributes her research papers and articles to world's newspapers as well as to academic journals. She can be reached at email@example.com
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