Methods of preservation of fruits and vegetables

The important methods of preservation of fruits and vegetables are:

(1) Preservation by high temperature—

In this method, food material is exposed to a higher temperature and high temperature helps to kill the microorganisms due to the coagulation of protein. It helps in the inactivation of enzymes. Here moist heat is more effective than dry heat. At low pH high temperature is required than a higher pH. High temperature can be employed by the following methods:

(i) Pasteurization: below 100 degree C

(ii) Boiling/Cooking: at 100 degree C

(iii) Canning: above 100 degree C

(i) Pasteurization: It is a mild heat treatment. By pasteurization, milk is pasteurized by HTST at 720C for 15 sec. Fruit juices are pasteurized at such temperature and for such periods as would render them sterile, without impairing their flavor. Usually, the juices are pasteurized at about 850C for 25-30 min., according to the nature of the juice and the size of the container. Acid fruit juices require lower temperatures and less time for pasteurization than the less acid ones.

Juices can be pasteurized in two ways

(1) By heating the juice at a low temperature for a High time (LTHT) and

(2) By heating the juice at a high temperature for a short time only (HTST).

(ii) Boiling/Cooking :

The primary objective of cooking is to produce palatable food. Cooking results in:

1. Destruction or reduction of microorganisms and inactivation of undesirable enzymes.

2. Destruction of potential hazards in the foods which are present naturally through microorganisms.

3. Improvement of color, flavor, and texture of the food.

4. It improves the digestibility of food components.

5. Putting the temperature at about 100 degree C. By this method, food can be preserved for 10-24 hours at low temperatures.

(iii) Canning: Canning is done at or above 100 degree C. In the case of fruits that are acidic, they are canned at 1000C, while in the case of vegetables that are non-acidic; they are canned at above 100  degree C. Here, high temperature can be obtained by using steam pressure; time varies according to the type of food. Due to anaerobic conditions, any survivable organism would not grow.

(2) Preservation by low temperature-

Low temperature retards microbial growth and enzyme reaction because it retards chemical reactions. This is not a permanent method because some microorganisms can also grow at low temperatures.

1. Cellar storage : (Above 15 degree C)

2. Refrigerated storage : (0 to 5 degree C)

3. Freezing storage : (-18 to -40 degree C)

1) Cellar Storage: Cellar Storage (about 150C). The temperature in the cellar (underground rooms) where surplus food is stored in many villages is usually not much below that of the outside air and is seldom lower than 150C. Root crops, potatoes, cabbage, apples, onions and similar foods can be stored for limited periods during the winter months.

2) Refrigeration: Refrigerated (or) chilling (0 to 5 degree C). Chilling temperatures are obtained and maintained by means of ice or mechanical refrigeration. Most perishable foods, including eggs, dairy products, meats, seafood, vegetables and fruits may be held in chilling storage for a limited time with little change from their original condition. Enzymatic and microbial changes in the foods are not prevented but are slowed considerably. Fruits and vegetables can be stored for 2-7 days. Semi-perishable crops, such as potatoes, apples etc., can be stored, in commercial cold storage with proper ventilation, and automatically controlled temperature for one year.

3) Freezing: Freezing (-18 to 400C). At temperatures below the freezing point of H2O, the growth of microorganisms and enzyme activity are reduced to the minimum. But, sometimes enzymes are active even below the 00C. In this case before freezing, ‘Blanching’ is necessary for vegetable freezing. Most perishable foods can be preserved for several months. Fruits, vegetables, juices and fleshy foods (meat, poultry, fish and sea foods) can be preserved in this method.

4) Preservation by drying- 

Drying is just the removal of moisture from the food to a certain level at which micro organisms can not grow is called drying, It can be done by two methods:

(i) Application of heat :

(a) Sun drying

(b) Mechanical drying

(c) Vacuum drying

(d) Freeze drying

(ii) Binding the moisture in the food :

(a) Use of Sugar

(b) Use of Salt

(i) Application of heat :

(a) Sun drying: Sun drying is the method in which food is directly exposed to sunlight. It is generally done in the places where plenty of sunshine is available for long period e.g. Rajasthan. The dried product in this method is

inferior in quality.

(b) Mechanical drying:

This is a method of drying where application of heat is applied by a mechanical dryer under the controlled conditions of temperature, humidity and air flow.

(c) Vacuum drying: The temperature of the food and the rate of water removal are controlled by regulating the degree of vacuum and intensity of heat input.

(d) Freeze drying: In this method, the food is dried by sublimation process, i.e., just converting the food into ice without passing through the liquid form of water by means of vacuum plus heat applied in the drying chamber. In this method, the product is first frozen, then water is removed by vacuum and application of heat which occurs simultaneously in same chamber.

(4) Preservation by filtration -- 

Filtration is the only successful method for the complete removal of organisms and its use is limited to clear liquids. The liquid is filtered through a previously sterilized ‘bacteria proof’ filter made of sintered glass, diatomaceous earth, unglazed porcelain, membrane pads, (or) similar material and the liquid is forced through by positive or negative pressure. This method has been

used successfully with fruit juices, beer, soft drinks, wine and water.

(5)Preservation by use of chemical preservatives—

Chemical preservatives are substances that are added to food just to retard, inhibit or arrest the activity of microorganisms such as fermentation, pacification and decomposition of the food.

Chemical preservatives are of two types:

Class-1 preservatives: common salt, sugar, dextrose, spices, vinegar, Ascorbic acid etc.

Class-2 preservatives: Benzoic acid and its salt, SO2 and the salts of sulphuric acid, nitrates, ascorbic acid and its salts, propeonic acid and its salts, lactic acid and its salts.

Among the class-2 preservatives, only two chemical preservatives are used in fruits and vegetables preservation:

(i) KMS(Potassium Meta bisulphate) :

     (1) It releases SO2 and it is unstable.

     (2) It is used for fruit that has non-water solvent pigment (colourless).

     (3) It can not be used in naturally coloured juices such as phalsa, jamun because they have the

         Anthocyanin pigment.

    (4) It can not be used in the product which is packed in the container because it acts on the tin containers and oil, Hydrogen Sulphide (H2S) which has an unpleasant smell and also form a black compound with the baseplate of containers.

      (5) Best to control moulds than bacteria.

      (6) 350 ppm KMS is mostly used in fruit juice products.

(ii) Sodium Benzoate :

     (1) It is a salt of benzoic acid and soluble in water.

     (2) It delays the fermentation of the juices.

     (3) It is commonly used in products which are having natural colour such as anthocynin pigment.

     (4) It is more effective against yeast.

     (5) 750 ppm Sodium benzoate is mostly used in fruit juices, squashes and cordials.

(6) Preservation by use of food additives (Sugar, Salt, acids and vinegar):

Food additives are substances or mixtures of substances other than basic foodstuffs, which are present in the foods as reagents of any aspects of production, processing, storage, packaging, etc. Food additives are (i) sugar, (ii) salt, (iii) acids, (iv) spices.

In the case of sugar and salts, they exert osmotic pressure and water diffuses from the product through a semi-permeable membrane until the concentration reached equilibrium. They kill the microorganisms or do not allow them to multiply.

(i) Sugar: The concentration of 68-70% is used for the preparation of jam, jelly, marmalades etc. Sugar act as a preservative by osmosis and not as a true poison for microorganisms. It absorbs most of the available water, so little water is available for the growth of microorganisms.

(II)Salt:

A concentration of 15-20% is used for the preparation of such as pickles. Salt inhibits enzymatic browning and discoloration and also acts as an antioxidant. It exerts its preservative action by:

(i) Causing high osmotic pressure resulting in the plasmolysis of microbial cells.

(ii) Dehydrating food and microorganisms by tying up moisture.

(iii) Ionizing to yield the chloride ion which is harmful to microorganisms, and

(iv) Reducing the solubility of oxygen in water, sensitizing, and the cells against

CO2.

(III)Acids: Many processed foods and beverages need the addition of acids to impart their characteristic flavor and taste in the final product because acids provide desired flavour and taste. They adjust the sugar and acid ratio in the food to the proper balance of flavor of the food. They are also playing the role of controlling the pectin-gel formation.

The main acids are the following:

1. Acetic acid (Vinegar)

2. Citric acid (Lime juice)

3. Lactic acid (Lactose)

1. Acetic acid: It is commonly used for pickles, chutney, sauce and ketchup, just to inhibit the growth of microorganisms.

2. Citric acid: It is used for the preparation of jam, jelly, squash, nectar etc.

just to increase the acidity.

3. Lactic acid: It is used for the formation of curd from milk.

(7) Preservation by oil-

A layer of oil on the surface of any food produces anaerobic conditions which prevent the growth of moulds and yeasts. Eg., pickles

(8) Preservation by fermentation -

The decomposition of carbohydrates by microorganisms or enzymes is called fermentation. Foods are preserved by the alcohol or organic acid formed by microbial action. The keeping quality of alcoholic beverages, vinegars, and fermented pickles depends upon the presence of alcohol, acetic acid and lactic acid respectively. Wines, beers, vinegar, fermented drinks, fermented pickles, etc., are prepared by these processes. In wines – 14% alcohol acts as a preservative. About 2% acetic acid prevents spoilage in many products.

(9) Preservation by Carbonation –

Carbonation is the process of dissolving sufficient CO2 in water or beverage so that the product when served gives off the gas as fine bubbles and has a characteristic taste. Fruit juice beverages are generally bottled with CO2 content varying from 1 to 8 g/ l, which is sufficient for supplementing the effect of acidity on pathogenic bacteria. For complete inhibition of microbial activity (14.6 g CO2/ l) creating an anaerobic condition, which reduces the oxidation of ascorbic acid and prevents browning. Although carbonated beverages contain sugar much below 66%, the absence of air and the presence of CO2 in them help to prevent the growth of moulds and yeasts. The keeping quality of carbonated fruit beverages is enhanced by adding about 0.005% sodium benzoate. The level of carbonation required varies according to the type of fruit juice and type of flavor.

(10) Preservation by Antibiotics-

Certain metabolic products of microorganisms have been found to have germicidal effects and are termed as antibiotics.

Nisin is an antibiotic produced by Streptococcus lactis. Commonly found in milk, curd, cheese and other fermented milk products. It is non-toxic. Used in the food industry, especially for the preservation of acid foods in which it is more stable.Used in the canning of mushrooms, tomatoes and milk products. Nisin suppresses the growth of mainly the gas-producing spore-forming bacteria and toxin-producing Clostridium botulinum

Subtilin- an antibiotic obtained from certain strains of Bacillus subtilisis used in the preservation of asparagus, corn, and peas. It is most effective against gram-positive bacteria and spore-forming organisms. Canned peas and tomatoes contain 10 and 20 ppm of subtilin respectively.

Pimaricin- is an antifungal antibiotic that can be used for treating fruits and fruit juices. The use of antibiotics along with other sterilizing agents including heat and radiation offers good promise.

(11) Preservation by irradiation-

Sterilization of food by ionizing radiations is a recently developed method of preservation. The unacceptable flavor of some irradiated foods and the fear that radioactivity might be induced in such food has come in the way of its greater use. When gamma rays (or) electron beams pass through foods there are collisions between the ionizing radiation and food particles at atomic and molecular levels, resulting in the production of ion pairs and free radicals. The reactions of these products among themselves and with other molecules results in physical and chemical phenomena which inactivate microorganism in the food. Thus irradiation of food can be considered to be a method of “Cold Sterilization” i.e. food is free of microorganisms without high-temperature treatment. The radiation dose of up to 1 Mradis not hazardous. Ionizing radiation can be used For sterilization of foods in hermetically sealed packs, Reduction of the spoilage organisms in perishable foods, delay of ripening of fruits, Inhibits sprouting of root vegetables and control of infestation (insects) in stored cereals.

(12) Drying and dehydration of fruits and vegetables

Drying preserves foods by removing enough moisture from food to prevent decay and spoilage. The water content of properly dried food varies from 5 to 25 percent depending on the food.