Methods of Food Preservation

Methods of Food Preservation

Food preservation methods can be classified as follows

I. Physical Methods

A. Preservation by low temperatures

1. Refrigeration

2. Freezing

B. Preservation by high temperature

1. Pasteurization

2. Canning

C. Preservation by drying

1. Sun drying

2. Drying by mechanical driers

·        Spray drying

·        Foam mat drying

·        Drying by osmosis

·        Freeze drying

D. Preservation by irradiation

II. Chemical Methods

A. High concentration of salt

B. High concentration of sugar

C. Using chemical preservatives

III. Fermentation

1. Physical Methods

A. Preservation by Low Temperatures

1. Refrigeration: The temperature main-tained in the refrigerator is 0 to 5°C. Enzymatic and microbial changes in foods are slowed down considerably. Perishable foods like eggs, dairy products, meat, sea foods, fruits and vegetables are stored in refrigerators. Food can be stored safely for few days or a week.

2. Freezing: The temperature of freezer is –18 to –40°C. Microbial growth is pre-vented completely and the action of food enzymes greatly reduced. Frozen foods have better quality and needs uninter-rupted supply of electricity while storing. Foods like poultry, meat, fish, peas, vege-tables, juice concentrates can be preserved for several months by this method. In veg-etables, enzyme action may still produce undesirable effects on flavour and texture during freezing. Various methods of freez-ing include slow freezing, quick freezing, cryogenic freezing and dehydro freezing.

Quick freezing is an ideal method of preserving nearly all baked products. Bread frozen at –22oC or below retains its freshness for many months. Cakes, cook-ies, short cakes, waffles and pancakes are also frozen and marketed frozen.

B. Preservation by High Temperature

Food commodities in which pasteuriza-tion is employed include milk, beer, wines and fruit juices.

1. Pasteurization: Pasteurization is a mild heat treatment that kills a part but not all the microorganisms present and usually involves the application of temperature below 100°C.

There are three methods of pasteurization.

a.  Bottle or Holding Pasteurization: This method is commonly used for the preservation of fruit juices. The extracted juice is strained and filled in bottles. The bottles are then sealed air tight and pasteurized.

b. Over Flow Method: Juice is heated to a temperature about 2.5oC higher than the pasteurization temperature and filled in hot sterilized bottles.

The sealed bottles are sterilized at a temperature 2.5oC lower than filling sealing temperature and then cooled.

c.  Flash Pasteurization: The juice is heated rapidly to a temperature of about 5.5oC higher than the pasteur-ization temperature and kept at this temperature for about a minute. This method has been developed specially for canning of natural orange juice, grape and apple juice. It has the advan-tages of minimizing the flavor loss and preserving the vitamins.

2. Canning: Canning is the process in which the foods are heated in hermetically sealed (airtight) jars or cans to a temperature that destroys microorganisms and inactivates enzymes that can cause food spoilage.

The general steps to be adopted for canning foods are cleaning, blanching, filling, exhausting, sealing, sterilizing, cooling and labeling.

i. Cleaning: It is the first step in canning. Thorough cleaning of the product to be canned helps to remove most of the spoilage organisms.

ii. Blanching: This process serves as an additional hot water wash. It inactivates the food enzymes and fixes the natural colour of the product. It also softens the fibrous plant tissues and facilitates removal of skin. In this process the raw food material is immersed in hot water or exposed to live steam for 2–5 minutes and immediately dipped into cold water to prevent further exposure to heat.

iii. Filling: Either manually or by using machinery the contents can be filled in the cans. A headspace of 6–9mm depth above the level of food in the can must be left.

iv. Exhausting: Gases are expelled by passing the open can containing the food through an exhaust box in which hot water or steam is used. It expands the food and expels air and other gases from contents and also from the head space area of the can.

v. Sealing: The exhausted contain-ers are immediately sealed to avoid re-contamination.

vi. Sterilization: To bring about com-plete sterilization, thermal process is carried out. This ensures the destruc-tion of spoilage microorganisms. This is usually done by the application of steam under pressure.

vii. Cooling: The containers are cooled rapidly to check the action of heat and prevent unnecessary softening of the food or change in the colour of the contents. It can be done by means of air or water.

viii. Labelling: The containers are labelled with nutritional characteristics of the food inside.

C. Preservation by Drying

Microorganisms need moisture to grow. When exposed to sunlight or subjected to dehydration, the moisture in the food is removed and the concentration of water is brought below a certain level. This pre-vents the growth of microorganisms and thereby spoilage of food. Food preserva-tion by drying is one of the oldest meth-ods practiced from ancient times. This method consists of exposing food to sun-light and air until the product is dry. It is a very useful and economical process.

Both the terms ‘drying’ and ‘dehy-dration’ mean the removal of water. Drying is used to remove moisture by the application of unconventional energy sources like sun and wind. Dehydration means the process of removal of moisture by the application of artificial heat under controlled conditions of temperature, humidity and air flow.

Treatment of Foods Before Drying

a.     Selection and sorting for size, maturity and wholesomeness

b.     Washing, especially fruits and vegetables

c.      Peeling of fruits and vegetables by hand, machine or knife

d.     Subdivision into halves, slices, shreds or cubes

e.      Blanching or scalding of vegetables and some fruits like tomatoes and peaches

f.       Sulphuring of light coloured fruits and vegetables by exposure to sul-phur-di-oxide gas.

i. Sun Drying: It is limited to regions with hot climates and dry atmosphere and to certain fruits such as raisins, prunes, figs, apricots, pears and peaches. It is a slow process. Many Indian foods are preserved by sun drying. Papads, vadams­ and vathals are made using this principle. 

Vegetables like sundaikai, cluster beans, bitter gourd and green chillies are preserved by this method. Fish and meat are also sun dried. The common dried fish or karuvadu (local name) is a good example for sun drying.

ii. Drying by Mechanical Driers: Artificial drying involves the passage of hot air with controlled relative humidity over the food to be dried or the passage of the food through such air. Fruits, vegetables, nuts, fish and meat can be successfully preserved by this method. In the dehy-dration process, artificial drying methods like spray drying, vacuum drying, drum drying and freeze drying are used for dry-ing foods. 

Although it is expensive when compared to natural sun-drying process, it is very hygienic, rapid and the products obtained are uniform in colour because the temperature and relative humidity can be maintained.

i. Spray drying: Milk and eggs are dried to a powder in spray driers in which the liquid is atomized and sprayed into hot air steam or almost instant ­drying.

ii. Foam mat drying: Foam mat drying may be used commercially to preserve orange and tomato juice. In this pro-cess a small amount of edible foam sta-bilizer is used. The foam is spread in a thin layer and dried in a steam of hot air. The product gets separated easily into small particles on cooling.

iii. Drying by osmosis: Drying by osmo-sis results when fish is heavily salted. In this case, the moisture is drawn out from all the cell tissues. The water is then bound with the solute making it unavailable to the microorganisms. In osmotic dehydration of fruits, the method involves the partial dehydra tion of fruits by osmosis in a concen-trated sugar solution or syrup.

iv. Freeze drying: Removal of water from a frozen product by sublimation is called freeze drying. Freeze dried foods will be of superior quality with light and porous texture.

D. Preservation by Irradiation

Food irradiation also known as cold steri-lization is another preservation technique. The foods are bombarded by high energy rays called gamma rays or by fast mov-ing electrons to kill bacteria, fungi and insects and in some cases to delay fruit ripening or prevent sprouting in onions and potatoes. The goal of irradiation is to kill the microorganisms and inactivate the enzymes without altering the food.

2. Chemical Methods

A. Salt or Brine

It is an ancient preservation technique. Food is treated with salt or strong salt solution. Salt causes high osmotic pressure and shrinking of cell, dehydrates foods and microbes by drawing out moisture.

B. High Concentration of Sugar

Sugar has the ability to bind water and make it unavailable for microbial growth. It reduces the solubility of oxygen in mois-ture, which is essential for the growth and multiplication of microorganisms. Apples, oranges, guavas, grapes and pineapples are suitable for making jams and jellies. The fruits should be just ripe because the pectin content is high in such fruits.

C. Using chemical preservatives

The chemicals when added interfere with the cell membrane of the microorganisms, their enzyme activity or their genetic mechanisms. They also act as antioxi-dants. The common chemical preserva-tives permitted are

1. Benzoic acid (including benzoates) Sodium benzoate is a salt of benzoic acid and is used in preservation of coloured fruit juices and squashes.

2. Sulphur dioxide (including sulphites): Potassium meta-bi­-sulphite is used as a source of sulphur dioxide when it is added to the juice or squash. When used in fruits with deep colours like blue grapes, jamun, watermelon it bleaches the colour and hence in such cases benzoic acid is desirable.

3. Organic acids and their salts:

Foods can be preserved by adding lactic, acetic, propionic, citric acids and their salts. Nitrates and nitrite compounds are used to preserve meat and fish prod-ucts. It gives desirable colour, flavor and discourages the growth of micro-organ-isms. It also prevents toxin formation by the microorganisms in food.