Preservation of Fruits and Vegetables

Preservation is processing of foods which prevents it from spoilage and makes it possible to store for future use. Preservation slows down the process of normal decay of food and food lasts longer with minimum loss of natural colour, taste and value.

Factors for Food Spoilage

Food may be spoiled by a number of factors. These factors cause various physical and chemical changes that take place in foods, which contribute to its spoilage.

  • Microorganisms: These are the basic agents for spoilage. The different microorganisms are bacteria, moulds and yeasts. These thrive best in moist, warm climate and produce undesirable changes in appearance, texture and flavour of food.

  • Moisture: Most bacteria grow in moisture. As fruits and vegetables have high content of water, they facilitate microbial growth and tend to spoil fast.

  • Temperature: Generally food spoils fast at room temperature. Microorganisms thrive best in temperature 25-35°C. Fruits and vegetables if kept at these temperature spoil faster.

  • Enzymes: These are chemicals that are naturally present in fruits and vegetables. The enzymes help in ripening of fruits and vegetables, but if the changes continue they spoil food.

  • Insects and Rodents: Worms, birds, moths, rats cause damage to food. They either eat the kernel or spread microorganisms in the food.

Preservation is done to keep the food products safe and maintain them in their present state for a longer period of time. There are various techniques and methods adopted for preserving food products. The basic principle of preservation is to slow down or remove the favourable condition for spoilage through:

  • Moisture Removal - Which involves drying and concentration
  • Heat treatment - Which include blanching; pasteurization and sterilization
  • Low Temperature - Refrigeration and Freezing
  • Acidity control - Fermentation and Acidic Additives
  • Chemical preservatives
  • Removal of Air - Maintaining vacuum

Blanching: It is the process in which the food is dipped in hot boiling water for a short time. It inactivates the enzymes present in the food and also decreases the number of microorganisms.

Food Color

Nature has given each fruit and vegetable a distinct colour and flavour. The colour in these is due to various pigments contained in their tissues. Some of the major pigments are:

  • Chlorophyll: Green colour of spinach, coriander, mint and other green leafy vegetables is due to this pigment.
  • Carotenoids: Yellow and orange colour of carrots, papaya and peaches.
  • Flavonoid: Pale yellow colour of onion, asparagus, apple and cauliflower.
  • Anthocyanin: Red and purple colour of beetroot, cherries.

Whenever food is cooked, there are some changes that take place with respect to the colour of the fruit or vegetables being cooked with lemon juice or cooking soda. The colour change is because of the effect of the pH of the cooking medium. (lemon juice having an acidic pH and soda having an alkaline pH.)

Preservation with Sugar

Large variety of fruits and vegetables are preserved with the help of sugar. Besides giving taste, the sugar preserved food products provide more calories to meet additional and increased energy needs.

Jam: Jam is a semi-solid and soft product made of fruit pulp, sugar, acid and water. Mostly ripe or just ripe fruits are used for its preparation. Among Jam, Jelly and Marmalade, Jam is the simplest and easy to prepare.

Jelly: This is a semi-solid, soft and transparent product made of fruit extract. In the preparation of jelly, it is necessary that the pectin, sugar, acid and water are present in right proportions. Thus, pectin rich fruits and that too at the just ripe stage which contain higher levels of pectin are suitable of jelly making.

Marmalade: Like jelly, this is a semi-solid soft and transparent product made of citrus fruits. Marmalade is also transparent but it has thin shreds of prepared fruit peels added to it.

Sugar as Preservative

Sugar acts as a preservative because, the high concentration of sugar exerts a high osmotic pressure and withdraws water from the fruit mass. This lack of water prevents the growth of micro-organisms because micro-organisms cannot survive in absence of water. Equal quantity of sugar by weight is used to make jams, jellies and marmalade with sour fruits like oranges, apples, plums. Sweet fruits like apples, need only three-fourths (3/4) sugar by weight. Lemon juice is also used at 1.5-2 g/kg of fruit pulp.

For making jam, jelly or marmalade the following basic ingredients are required - Pectin, Acid and  Sugar. Only when the pectin, acid, sugar and water are in definite amount along with the fruit extract, gel is formed and this gel helps in setting the jam, jelly or marmalade.

Pectin: Presence of pectin is very essential for making jams, jellies and marmalade. Although all fruits do contain some amount of pectin, only a few of them have a high pectin content. Such fruits are more suitable for making jam, jellies and marmalade.

Pectin content of the fruit not only varies with its type but also with the stage of maturity of the fruit. Generally, the amount of pectin is the highest in fresh, just ripe fruits.

Acid: Apart from pectin, another important ingredient required for the preparation of jams, jellies and marmalade is acid. This acid may come from sour fruits or may be added in the form of citric acid. Fresh lemon juice may also be used. Acid has the following functions:

  • Removal of dirt from the sugar.
  • Prevents crystallization of sugar.
  • Helps in gel formation.
  • Imparts sour taste.

Sugar: Sugar helps in gel formation in the presence of pectin, acid and water. In jams, jellies and marmalade, sugar also helps in their preservation as its presence in large amounts hinders the growth of microorganisms.

Many fruits.can be preserved in sugar medium. Jams are made of apple, pineapple, mango, mixed fruits (papaya, strawberry, grapes, plums). Jelly is made with guava, apple, grapes. Marmalade is made by citrius fruits like keenu, malta, oranges.

Squashes and Syrups

Squashes: Squashes are made up of strained juices containing moderate amount of fruit pulp to which cane sugar is added for sweetening. Ready-made squash contains 25% fruit juice and 50% sugar.

Syrups: Syrups have higher concentration of sugar (60-70%) therefore do not require any addition of preservatives. Citric acid is added to clean the sugar and to prevent the crystallization of sugar. Colour and essence are also added.

Preservation with Salt

Pickling

The process of preservation with salt when applied to fruits and vegetables is called pickling. The principle of preservation in pickles is the addition of salt from 15 to 20%, followed by addition of edible oil. The high concentration of salt creates high osmotic pressure and prevents water from being available for bacterial growth.

The oil in the pickle acts as an air seal and prevents the microorganisms in the air from coming into contact with the food. Sometimes spices are also added as they have antimicrobial properties. These not only help in preservation of pickles but also improve its aroma and taste. Ingredients like lemon juice (citric acid) and vinegar (acetic acid) also act as preservatives.

Chutneys, Ketchup, and Sauce

Chutney, Ketchup and Sauce are preserved with the help of a combination of sugar, salt, vinegar and spices. The main ingredients for preservation are salt and vinegar. Acetic Acid is present in vinegar and does not allow microorganisms to be active.