Principles And Methods Of Food Preservation

Food is a source of energy for all of us. Unhealthy or spoiled food is the major cause of diseases in humans. This is known as food poisoning. To prevent this the food needs to be preserved in a variety of ways. Preservation usually involves preventing the growth of bacteria, fungi, and other micro-organisms, as well as retarding the oxidation of fats which cause unpleasant smell of food. It also includes processes to inhibit natural ageing and discoloration that can occur during food preparation such as the enzymatic browning reaction in apples after they are cut. 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. The main aim of food preservation is to prevent contamination and spoilage of food.

What Is Food Preservation ?

It is the technique to prevent food spoilage, food poisoning, and microbial contamination in food, thus allowing it to be stored in a fit condition for future use. It can also be defined as the state in which any food may be retained over a period of time without  being contaminated by pathogenic organisms or chemicals , losing optimum qualities of colour, texture, flavor and nutritive value.

Principles of Food Preservation

The principles of various methods of food preservation are as follows :

1. Prevention or delay of microbial decomposition

• By keeping out microorganisms ( asepsis )
• By removal of microorganisms ( filtration )
• By hindering the growth and activity of microorganisms ( by low temperature, drying, anaerobic conditions or chemicals )
• By killing the microorganisms ( by heating or applying radiation )

2. Prevention or delay of self decomposition of food

• By destruction or inactivation of food enzymes ( by blanching )
• By prevention or delay of chemical reactions ( by using antioxidants )

3. Prevention of damage caused by insects, animals and mechanical causes.

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Food Preservation Methods

Preservation By Dehydration

Drying

Drying food means the application of heat under controlled conditions to remove majority of water normally present in food by evaporation. It reduces water activity sufficiently to prevent or delay microbial growth. It is one of the earliest form of food preservation used by our ancestors

Freeze drying is another method of drying. It is a method of food preservation that makes the use of physical principle known as sublimation. Sublimation is the process by which a solid passes directly to the gaseous phase without melting. The food to be preserved by this method is first frozen and then placed into a vaccum chamber. Water in the food freezes first and sublimes, leaving water content in the food as low as 0.5%.

Preservation By High Osmotic Pressure

• Salting And Pickling

Salting

Adding salt to the food even at 20% of its concentration disturb the osmotic balance of microbial cells as a result they die. Salting is also called curing as it removes the moisture content from foods like meat.

Pickling

It means preserving the food product in brine ( salt solution ) or marinating in vinegar ( acetic acid ). In chemical pickling we add brine, vinegar ,alcohol and vegetable oil. These chemicals kills the various microorganisms there by increasing the shelf life of the food. In fermentation pickling, bacteria produce organic acids which act as a preserving agents.

• Sugaring

Sugar have a tendency to absorb water from microbial cells and leaving them dehydrated. These dehydarted microbial cells thus die as water is necessary for every living thing to survive.

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Preservation By High Temperature

• Pasteurization

It is a process applied to a food product with the objective of minimizing possible health hazards arising from pathogenic microorganisms associated with the product mainly milk products which is consistent with minimal chemical, physical and organoleptic changes in the product. It is also applicable to other products like cream, eggs, fruit juices, fermented products, soups and other beverages. Pasteurization is usually done for milk products. Nowadays three methods are used for pasteurization. They are :

Holding or Batch system (Holder method) – bringing the milk or cream to a temperature usually 65 o C and holding at that point for at least 30 minutes followed by rapid cooling.

High Temperature Short Time Method (HTST) (Flash method) – In this method temperature of the milk is raised to at least 72 o C for 15 seconds followed by quick cooling.

 Ultra High Temperature System (UHTS) – In this method milk is held for 3 seconds at 93.4 o C. After pasteurization the milk is cooled rapidly to 7 o C or lower.

• Canning

It is a method of food preservation in which the food products are sealed in containers which applies heat treatment for the prevention of spoilage. In conventional canning, food product is placed inside the container, air is removed by vaccum cans are hermetically sealed. The cans are then sterilized with steam.

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Preservation Using Preservatives

Preservatives can be classified into two types, class I preservatives and class II preservatives. Class I preservatives are salt, sugar spices, vinegar, honey and edible vegetable oils. Class II preservatives are benzoic acid and its sodium and potassium salts, sorbic acid and its sodium and potassium salts. The preservatives used in fruits and vegetables are again classified into organic and inorganic preservatives.  The organic preservatives are benzoic acid, chlorobenzoic acid or salicylic acid. Sulphur-di-oxide is the only permitted inorganic preservative  which is generally used in the form of sulphites. The preservatives permitted in fruit and vegetable products in India are sodium benzoate, sulphites and sorbic acid. Sulphur-di-oxide, sulphites and metabisulphites are used in confectionary, fruits and fruit juices and wines. Sodium or potassium metabisulphites are used in the preservation of fruit products. Sodium benzoate is used as a preservative in grape crushes and tomato ketchup.

Food Irradiation 

In this method of food preservation food is exposed to ionizing energy –  radio isotope cobalt – 60. Food irradiation are mainly used :

• To avoid harmful chemicals compounds like methyl bromide and ethylene oxide for insect disinfestation in stored products and microbial decontamination of spices.
• To extend shelf life of meat, poultry and sea foods by killing microorganisms causing their spoilage.
• To enhance safety of food by killing food borne pathogenic microorganism and parasites.

Irradiated foods does not change the flavor, taste, smell, texture and mineral contents of foods and they are safe for human consumption.

Importance Of Food Preservation

• Increases the shelf life of food products, so many perishable foods can be preserved for long time.
• Decreasing wastage of food by preventing decay or spoilage of food.
•  As the food has already been partially processed, it reduces the preparation time and energy.
• Stabilizing prices of food, as there is less scope of shortage of supply to demand.
•  Preserved foods help people to bring a variety in the diet, thereby decreasing dietary deficiencies.
• Seasonal foods are available throughout the year.
• Expands the supply of food.

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