The principle used in permanent preservation is to eliminate complete or to prevent the activities of the organisms capable for destroying the product. The main methods used for permanent preservation are as follows:
1) Sterilization or Processing:
The fundamental principle of preserving foods by heat is know as “Processing or Sterilization”
Sterilization by heat means the complete destruction of all forms of life in the product. To avoid spoilage, sterilized products must be sealed in such a manner that any of the live microorganism from outside cannot enter into it. In commercial practice, all the canned food are not sterile but even that they are not spoiled as the conditions of inside can are not favourable for growth of microorganisms i.e. lack of oxygen. This process of heating accomplished by lack of oxygen is termed as “Processing”. The temperature and time necessary to sterilize different product varies. The product that are difficult to sterilize are low in acid and often high in protein and contain spore bearing bacteria, requires high temperature and longer time for processing.
Fruits which are naturally acidic and the more acid vegetables like tomatoes could be satisfactory preserved at boiling water temperature 100 degree cellules for 30 minutes. Vegetable products require high temperature to kill the spore farming organisms being not acidic and containing more starch than sugar. Continuous heating for 30 – 90 minutes at 240 degree F is essential for their sterilization. Before using empty cans and bottles, should also be sterilized about 30 minutes by placing them in boiling water.
2. Preservation by Antiseptics:
Food may be preserved permanently by addition of antiseptics like sugar, salt and vinegar and chemicals in sufficient concentration to prevent the growth of microorganisms either by osmosis or by poison or by both.
If the concentration of sugar in the preserved material is increased about 66 %, the water content is decreased to such an extent that the multiplication of microorganisms is checked and the present ones die in due course.
Examples: Jellies, Preserves etc.
Strong salt solution never allows the microorganism to grow in the preserved products. It acts both by osmosis and as a poison and it is more effective than sugar. A brine solution of 10 to 15 % is sufficient for permanent preservation of most of the products.
Examples: Pickles and Canned Vegetables
Acetic acid of vinegar is most effective than sugar and salt and acts as poison for microorganisms. A solution of about 2 percent of acetic acid prevents most of the products from spoilage.
d) Preservation by Food Additives i.e. Chemical Preservatives:
A chemical additive or food additive can be defined as “ a chemical or mixture of chemicals , other than basic food stuff that is added intentionally either during production, processing, storage or packaging directly or indirectly to improve or maintain nutritional value, enhance quality and consumer acceptability , improve keeping quality and check spoilage caused by microbes and enzymes and facilitates preparation.”
Common types of intentional food additives are salt, sugars, acids spices, essential oils, buffers, bleaching agents, emulsifying and thickening agents, food flavour, colour, preservatives, antioxidants, clarifying agents and humectants. Use of food additive is another effective approach for preservating fruits and vegetables. Some of them in combination can check microbial activity, enzymatic and non-enzymatic browning of the preserved products during storage. Acids lower the PH and act as antimicrobial agents like chemical preservatives. They have bacteriostatic and bacteriocidal properties. The permitted preservatives used in our country for various products are benzoic acid including salts( sodium benzoate) and sulphurous acid including salts and sulphurous acid including salts or combination of these. The detail of these chemical preservatives is given in table given below. They are used either alone or in combination to check spoilage in fruits and vegetables and their products as well as to increase their shelf life by keeping proper nutritional value. Preservation by this method is cheap and easy to operate technology, best suited for its application in the developing countries to preserve perishable commodities.
Seasonal fruits pulp and juices from fruits and vegetables such as peaches, plums, bael, apricot, mango, guava, pineapple, litchi, citrus fruits, Phalsa, jamun, carrot, tomato, etc, can easily be preserved and stored after heating them to 80-85 0C and then by adding chemical preservative and acid is shown below:
1. Fruit pulps/ juices: 1 kg
2. Citric acid: 20 kg
3. Chemical preservatives (Potassium metabisulphite or Sodium benzoate) : 1kg
Potassium metabisulphite is used to preserve most of the pulps whereas sodium benzoate is used for coloured juices or pulps along with citric acid. The acid and preservative are added after dissolving them separately in a small quantity of water. These pulps or juices can be stored either in jars or glass bottles. Whenever , needed, one bottle of pulp or juice can be opened and made into 3-4 bottles of squash or about a kg of jam by standard recipes. Mixed fruit jam or marmalade can also be made by mixing pulps of various fruits and cooking them to thick consistency having 68-70 0 Brix. Preserved fruit and vegetables pulps or juices can also be used to prepare chutneys and sauces.
Preservative is any substances which is capable of inhibiting , retarding or arresting the process of fermentation acidification or decomposition of food. Chemical preservatives are used in very little quantities to check the microorganism as these are most effective than sugars, salt, and vinegar. Benzoic acid, Sulphorous acid and their salts. E.g. Potassium –meta-bisulphite , Sodium –meta-bi-sulphite and Sodium benzoate are permitted by law as chemical preservatives. Borax , Boric acid Salicylic acid and Formaldehyde have been prohibited to use them as preservative due to their toxic effect. Sodium Benzoate ( 0.2%) prevents spoiling of most acid food products.
i) For squashes 350 of SO2 or 600 ppm of Benzoic acid.
ii) For Jam and jellies 40 ppm SO2 or 200 ppm Benzoic acid are used.
There are two important preservatives viz. Benzoic acid and Sulphur dioxide.
i) Sodium Benzoate:
It is salt of which is tasteless, odourless and effective is small quantity. It is used in the preservation of fruits juices and squashes. It is 180 times more soluble than benzoic acid.
The quantity of addition of Sodium Benzoate depends on the nature of juice particularly its acidity and the extent the type of infection to be overcome.
For acidic fruit juices , 0.06 to 0.10 percent Sodium Benzoate is sufficient but in less acid juices the percentage used is 0.3.
The preservative action of Benzoic acid increases in the presence of Carbon-di-oxide resulting in inhibition of growth of microorganisms. Benzoic acid is more effective against yeasts than against moulds. It does not stop Lactic acid and Vinegar fermentations.
ii) Sulphur Dioxide:
Potassium meta bisulphate ( KMS ) is used as a source of sulphur dioxide. Dry powder is easy to use than liquid or gaseous form. When added to juices the crystals of KMS reacts with the acids present in it and the SO2 is released and forms sulphurous acid with the water of juice, which retards the growth of bacteria, yeast , fungi, and moulds.
The quantity to be added differs from product to product.
Citrus squashes of 45 0 Brix to 60 0 Brix.
It prevents discoloration and loss of flavour due to oxidation in fruit juices and squashes by its strong effect retarding oxidation. However, it can not be used in the case of naturally coloured juices, like Phalsa, Jamun and Pomegranate juices, strawberry pulp, etc , on account of its bleaching action. It cannot also be used in the case of those juices which are to be stored in tin containers , because not only does it act upon the tin of the can causing pinholes, but also forms hydrogen sulphide which has a disagreeable small and forms a black compound with tin plate. Both are highly objectionable defects.
The chemical preservatives should be added in the soild form to fruit juices and squashes. They should be dissolved in a small amount of juice or water and the solution then added to the bulk of juice. If this care is not taken, the solid preservative may settle at the bottom of the container with the result that fermentation may start before the preservative dissolves.
3. Preservation by Drying:
It is the most popular of preservation. Drying by dehydration is more rapid process as artificial heat of higher temperature is provided. The fruits and vegetables are dried to such as extent that the moisture content is reduced and microorganism fail to survive on them and it checks the action of enzymes as well.
Drying helps in preservation of foods. Microbes cannot grow and multiply in absence of sufficient water in their environment. Many of the enzymatic reactions are hydrolytic in nature, requiring water. Chemical reactions in food materials are slowed down when the reaction are in solid state. Hence by removing water from the commodity, it should be possible to preserve them by checking the important spoilage agents. The principle forms the basis for dehydrated food and for osmotic, dehydration where high sugar or salt acts as a preservative. Fruits and vegetables may be dried in air, super heated steam in vacuum, in inert gases or by direct application of heat.