The following points highlight the top six techniques used to preserve food. The techniques are: 1. Heat Treatment 2. Low Temperature Treatment 3. Dehydration 4. Osmotic Pressure Treatment 5. Chemical Treatment 6. Treatment by Radiations.
Technique # 1. Heat Treatment:
High temperature is one of the most reliable and safest method of food preservation.
This treatment can be summarized under following two heads:
Pasteurization is a process that employees relatively brief exposures to moderately high temperature to reduce the number of viable microbes and to eliminate human pathogenic microorganisms. Pasteurization is used specially when the aim is to kill pathogenic organisms and where the spoilage organisms are not very heat- resistant and the product cannot stand high-temperatures.
Since pasteurization does not kill all the microorganisms, it is necessary to store these products at low temperatures. Two methods of pasteurization are used: high temperature short time (HTST) method and low temperature long time (LTLT) method.
The minimal heat treatment for market milk is 62.8°C for 30 minutes in LTLT holding method and 71.7°C for about 15 seconds in the HTST method. Grape wines may be pasteurized for one minute at 81 to 85°C and grape juice at 76.7°C for 30 minutes.
(ii) Steam under Pressure (Canning):
Unlike pasteurization, canning of foods normally involves exposure for longer periods of time to higher temperatures in order to kill endospore-forming microorganisms.
Steam under pressure (e.g., a pressure cooker) is the most effective method since it kills all vegetative cells and spores. Such heating’s to high temperatures by steam injection is followed by rapid cooling. The whole process of canning includes cleaning, blanching, filling of cans or jars, sealing, autoclaving, and cooling.
Fresh vegetables, fruits or meat of high quality are selected and freed of all dirt and undesirable parts in order to reduce bacterial load and to preserve the best portion of the product.
The cleaned food is subjected to hot water or steam treatment in order to effect blanching or scalding. This treatment kills part of microorganisms, fixes the natural colour of the product, and removes air within the product by expansion which might otherwise help in decomposition. Il docs not allow effective autoclaving, and stops the activity of autolytic enzymes.
3. Filling of cans or jars:
The hot food is then filled into cans or jars which are also kept hot in order to expand the food and remove air or gas trapped within. In fact, steam occupies all the space that is not occupied by food. The containers are soon sealed.
The cans so filled and sealed are subjected to heat treatment in the form of steam under pressure. Acid foods may require only immersion of containers in boiling water. Non-acid foods, however, need to be subjected to 15 lb pressure (116°C-121°C) for one to two hours. Meats may take a longer time The amount of time will also depend upon the load of the autoclave. Heavily loaded autoclaves will take more time.
As soon as autoclaving is done, it is better to cool the container immediately. By so doing the undesirable changes in texture and flavour of the product due to overcooking may be checked. In addition, thermophilic bacteria surviving heat treatment will not be allowed to germinate. The only dangerous spore forming bacterium which survives the treatment is Clostridium botulmum.
Technique # 2. Low Temperature Treatment:
Temperature approaching 0°C and lower retard the growth and metabolic activities of microorganisms. Modern refrigeration and freezing equipment has made it possible to transport and store perishable foods for long periods of time. Refrigerated trucks and railway cars, ship’s storage vaults, and the home refrigerator and freezer have improved the quality of the human diet and increased the variety of food available.
Any food that needs to be preserved must be freed as best as possible from microorganisms before subjecting it to this method because low temperatures do not kill them but only inhibit their activity. Low temperature preservation includes two different methods viz., chilling and freezing.
This involves preservation of foods like meat, eggs, fish, vegetables, etc. only for few days at a temperature between 4°C to 7°C. If foods are kept for longer periods, undesirable changes due to active enzymes and psychrophilic microorganisms such as Pseudomonas fluorescence and some Micrococcus spp. take place causing spoilage.
This process is used for preserving perishable plant and animal products for long periods, from weeks to months. Before freezing the foods are stored, trimmed, washed and blanched. Blanching consists of immersing the food in boiling water or exposing it to live steam for a few minutes. Blanching destroys most of the microorganisms and inactivates enzymes that would alter the product even at low temperatures.
The food is then immediately packaged and frozen. Quick freezing, which is preferred to slow freezing, implies a freezing time of 30 minutes or less and the temperature between -18°C to -34°C. Quick freezing produces smaller ice crystals and less damage to the food tissues. Slow freezing produces large crystals of ice which rupture cell structures and cause extensive drip or loss of fluid upon thawing.
Frozen fruits may be stored between -1°C and -18°C with little further change. It should be emphasized that freezing cannot be relied upon to kill all microorganisms no matter what the temperature.
The number and types of viable and non-viable microorganisms present in frozen foods reflect the degree of contamination of the raw product, the sanitation in the processing plant, and the speed and care with which the product was processed.
The microbial count of most frozen foods decreases during storage; but many microorganisms, including pathogens, e.g., species of Salmonella survive for long periods of time at – 9 and -17°C.
However, frozen food should be immediately used after thawing because the surviving microorganisms begin to multiply as soon as they are warmed. Frozen foods are not expected to lose their nutritional value but. the flavour and aroma of fresh food is lost with the length of storage period.
Technique # 3. Dehydration:
Dried foods have been used for centuries, and they are more common throughout the world than frozen foods. The removal of water by drying in the sun and air, or by applied heat causes dehydration. Food products containing 10% or less of free moisture are not subjected to spoiling by microorganisms as their activity is suspended for want of moisture.
The preservative effect of dehydration is due mainly to microbistasis. The microorganisms are necessarily killed because their growth is prevented by reducing the moisture content of their environment below a critical level.
The critical level is determined by characteristics of the particular microorganisms and the capacity of the food item to bind water so that it is not available as free moisture.
Once dehydrated, the food should be kept in air-tight containers so that it is not exposed to fluctuations in humidity content of the atmosphere. Slight increase in moisture contents will permit growth of various microorganisms such as moulds and yeasts first, and bacteria later.
Technique # 4. Osmotic Pressure Treatment:
When high osmotic pressures are built in a food by increasing its solute concentration, microorganisms having osmotic pressure relatively lower are plasmolysed and eventually die.
Salt is widely used to preserve certain foods. The salting and bringing of fish, corning of beef, and bringing of green olives are examples of the use of high salt concentration. With the exception of halophiles, practically no multiplication of microorganisms occur in salt concentration of 25%.
Preservation of jellies jams, maple syrup, and honey is because of high sugar content. It is not uncommon to find mould growth on the surface of jelly which has been exposed to air. This may also be due to the condensation of evaporated water on the surface of the jelly to produce a layer of less concentrated sugar solution. Osmophilic yeasts occasionally grow in honey and produce sufficient carbon dioxide to burst the jar.
The food preserved by addition of salts or sugars should be kept in airtight containers and stored in cool locations to avoid contamination by high osmotic pressure tolerating microorganisms such as yeasts and moulds.
Technique # 5. Chemical Treatment:
Chemical preservatives are added to kill or inhibit microorganisms in food. They may be incorporated into the foods, or only on their surface. The wrappers used to cover them may be treated, or there may be the use of gas or vapours around the food.
Some chemicals may be effective on selected group of microorganisms while others on a wide variety of them. Chemical preservatives may be harmless if they are added during the storage period and are removed before the food is consumed. But, if they are consumed as such, they may be poisonous to man or animal as well as to microorganisms.
(i) Organic Acids and their Salts:
Several organic acids and their salts are common preservatives as they have marked micro-biostatic and microbicidal action. Benzoic acid and benzoate are used for the preservation of vegetables. Sodium benzoate is used in the preservation of jellies, jams, fruit juice and other acid foods.
Salicylic acid and salicylates are used as preservatives of fruits and vegetable in place of benzoate because the latter is considered to be deleterious to health of consumer.
Scorbic acid is recommended for foods susceptible to spoilage fungi, e.g., it inhibits mould growth in bread. Wrapping material for cheese may be treated with it. It is also used in sweet pickles and for control of lactic fermentations of olives and cucumbers.
Foods prepared by fermentation processes, e.g., milk products, etc. are preserved mainly by lactic, acetic and propionic acids. Flavouring extracts of vanilla lemons are preserved in 50-70% alcohol as it coagulates cell proteins.
(ii) Inorganic Acids and their Salts:
Most common among the inorganic acids and their salts are sodium chloride, hypo-chlorites, sulphurous acids and sulphites, sulphur dioxide, sodium nitrate and sodium nitrite.
1. Sodium chloride:
Sodium chloride produces high osmotic pressure and, therefore, causes destruction of many microorganisms by plasmolysis. It causes dehydration of food as well as microorganisms, releases disinfecting chlorine ion by ionization, reduces solubility of oxygen in the moisture, sensitizes microbial cells against carbon dioxide and interferes with the action of proteolytic enzymes. These are the reasons why this common salt is used widely for preservation cither directly or in brine or curing solutions.
The hypochlorous acid acts as an effective germicide provided the organic matter content of the medium is not high. It is oxidative in its action. The commonly used forms are sodium and calcium hypochlorites. Drinking water or water used for washing foods or icing them may be dissolved with hypochlorites.
3. Sulphurous acids and Sulphites:
Sulphurous acids and sulphites are added to wines as preservatives. Sulphurous acid is used especially in the preservation of dry fruits. It helps retention of original colour of the preserve and inhibition of moulds more than either yeasts or bacteria. Potassium metabisulphite is used in canning.
4. Sulphur dioxide:
Sulphur dioxide has a bleaching effect desired in some fruits and also suppresses the growth of yeast and moulds. It is used as a gas to treat drying fruits and is also used in molasses.
5. Nitrates and Nitrites:
Nitrates and nitrites produce an inhibitory effect on bacterial growth and are used usually together in meat and fish preservation and for retension of red-colour of the meat. Nitrate is changed to nitrous acid which reacts with myoglobin to give nitric oxide myoglobin. It is the latter which gives a bright red colour to the meat making it more attractive in appearance.
However, both nitrite and nitrate are poisonous if present in potable water or food products in more than minimal amounts. That is why the generous use of these chemicals as preservative in meat and fish products has been questioned.
Aureomycin (chlorotetracycline) is the most commonly used antibiotic for the preservation of animal products under chilling conditions. It is extensively used for the preservation of poultry, meat, and fish.
The antibiotic is applied to the surface of the fresh meat by dipping it in a solution of the antibiotic or it may be fed to the animal by mixing it with feed or water for one to several days before slaughter. Fish are treated by adding the antibiotic in the ice or water in which they are to be transported.
The indiscriminate use of antibiotics as preservatives, however, should be prevented or the antibiotics used should be such that it is demobilised on cooking so that the internal flora of man using such food is not constantly exposed to the effect of the antibiotic.
It is important because otherwise use of antibiotics would lead to the development of the antibiotic resistant strains of microorganisms in the body. Aside from this, some individuals sensitive to antibiotics become exposed constantly to allergy.
Technique # 6. Treatment by Radiations:
(i) Gamma Rays and High-energy Electron Beams:
Gamma rays and high-energy electron beams have been used for the preservation of fresh perishable, canned, and packaged foods. They have good penetration and are effective to a depth of about 15 cm in most foods. Food preservation by such radiation dosage is called ‘cold sterilization’ as it produces only a few degrees’ rise in temperature of the product.
(ii) Ultraviolet Rays:
Ultraviolet rays are short waves and are used to sterilize the surface of foods. These rays have been successfully used for the treatment of water for beverages, aging meats, packaging of sliced bacon, treatment of knives for slicing bread, for sterilizing utensils, for prevention of spoilage by microorganisms on the surface of preserved pickles, cheese and prevention of air contamination.
Cold-storage rooms of meat-processing plants are sometimes equipped with germicidal lamps which reduce the surface contamination and permit longer periods of spoilage-free storage.
Radiation pasteurization or sterilization represents a term which describes the killing of over 98% but not 100% of the microorganisms by intermediate dosage of radiation. This method increases the storage life of some meats, sea-foods, certain fruits, and vegetables when stored at low temperatures.
Radiation pasteurization provides the possibility of an entirely new approach to food preservation and could bring about a radical change in industrial methods of food processing. However, the effect of radiation on colour, flavour, nutritional quality of food, odour and texture needs to be more carefully understood.
Similarly, chemical changes in food products brought about by radiations may cause bad effects on animal and human beings and need to be more adequately investigated.