Methods of Food Preservation
Important methods of preservation of fruits and vegetables are:
(1) Preservation by high temperature-
The use of heat on food destroys microorganisms. Specific treatment varies with:
- The organisms to be killed.
- The nature of the food to be preserved and
- Other means of preservation are intended to be used in addition to high temperatures.
The higher temperatures used for preservation are usually:
(a) Pasteurization (temperatures below 1000C)
(b) At about 1000C temperatures and
(c) Sterilization (temperatures above 1000C)
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a) Pasteurization (less than 100oC)
Pasteurization is a heat treatment that kills some but not all of the microorganisms present in the food and the temperature is kept below 1000C. Heating is given by means of steam, hot water, dry heat or electric currents, and the products are cooled immediately after heat treatment. If spoilage is to be avoided, the remaining microorganisms are inhibited by low temperatures (or) by some other preservative method.
Preservative methods used to complement pasteurization include (1) refrigeration (of milk) (2) packaging the product usually in a sealed container to keep microorganisms out (3) emptying the containers under anaerobic conditions, maintaining them airtightly (4) Giving high concentration of sugar, such as sweetened condensed milk and (5) Giving chemical preservatives example use of organic acids on pickles.
HTST method – high temperature and short time (above 700C)
LTHT Method – Low Temperature and Higher Time (OR) Holding Method (60-700C)
b) At about 1000C temperature
A temperature of about 1000C is achieved by boiling a liquid food, immersing a container of food in boiling water, or by exposing it to flowing steam. Some very acidic foods, for example, sauerkraut are first preheated to a temperature slightly below 1000C, packaged hot, and do not need to be processed with more heat. Blanching fresh vegetables also involves heating them for some time at around 1000C before freezing or drying them.
c) Sterilization (above 1000C)
In this method, all the microorganisms are completely destroyed due to the high temperature. The time and temperature required for sterilization vary according to the type of food. Temperatures above 1000C can only be achieved by using steam pressure sterilizers such as pressure cookers and autoclaves.
Fruit and tomato products should be heated at 1000C for 30 minutes. So that the bacteria producing spores sensitive to high acidity can be completely killed. Vegetables such as green peas, okra, beans, etc., being alkaline or none acidic and containing more starch than sugar, require high temperatures to kill the spore-forming organisms. That is why continuous heating at 1160C for 30-90 minutes is necessary for their sterilization. Before use, empty cans and bottles should also be sterilized by placing them in boiling water for about 30 minutes.
Difference between pasteurization and sterilization
1. Partial destruction of microorganisms
Complete destruction of microorganisms
2. Temperature below 100oC
Temperature 1000C and above
3. Normally used for fruits
used for vegetables
(2) Preservation by low temperature
low temperature inhibits microbial growth and enzyme reaction as it inhibits chemical reactions. But this is not a permanent method as some microorganisms can grow even at low temperatures.
a) Refrigeration (Refrigerated or chilling 0 to 50C):
The chilling temperature is maintained through ice or mechanical refrigeration. Most perishable foods, including eggs, dairy products, meats, seafood, vegetables, and fruits, can be stored in chilling storage for a limited period of time with little change from their original condition. Enzymatic and microbial changes in foods cannot be stopped, but are significantly slowed down.
Fruits and vegetables can be stored for 2-7 days. Some less perishable crops, such as potatoes, apples, etc., can be stored for one year in commercial cold storage with proper ventilation, and automatic controlled temperature.
b) Freezing (-18 to -400C):
At temperatures below the freezing point of water, the growth of microorganisms and enzyme activity is reduced to a minimum. However, sometimes enzymes remain active below 00C. In such a situation, ‘blanching’ of vegetables before freezing is necessary. Most perishable foods can be preserved for several months. Fruits, vegetables, juices, and meaty foods (poultry, fish, and seafood) can be preserved in this method.
Types of freezing: On the basis of the rate of formation of the ice front, the freezers are classified into the following types:
a) Slow freezing: In this type, the rate of ice formation is very slow i.e. 0.2 cm/h-l (eg, fixed air freezer and cold store). Slow freezing also kills some microbes and forms large ice crystals that cause mechanical damage to cells
b) Quick Freezing: The rate of ice formation is very fast ie 0.5-100 cm/h-l (eg air blast and plate freezers, fluidized bed freezer, cryogenic freezer). Kills are microbes and maintains homogeneity in extra or intracellular water and forms smaller ice crystals causing less damage to cells and preserving natural juiciness and flavor
(3) Preservation by drying
Drying refers to the removal of moisture from food to a certain level at which microorganisms cannot grow, this is called drying; This can be done in two ways:
(i) Application of Heat:
(a) Sun Drying
(b) Mechanical drying
(c) Vacuum drying
(ii) Binding of moisture in 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 exposed to direct sunlight. It is usually performed in places where sufficient sunlight is available for a long period such as Rajasthan. The product dried in this method is low in quality.
(b) Mechanical Drying: It is a method of drying where food items are dried using heat by mechanical dryers under controlled conditions of temperature, humidity, and airflow.
(c) Vacuum drying: The temperature of the food and the rate of water removal are controlled by controlling the level of vacuum and the intensity of heat.
(d) Freeze-drying: In this method, the food is dried by sublimation, that is, the food is placed in a drying chamber through the heat with a vacuum, without passing through the liquid form of water. In this method, the product is first frozen, then the water is removed by vacuum, and heat is also used which is simultaneously in the same chamber.
(4) Preservation by filtration
Filtration is the only successful method for the complete removal of organisms and its use is clearly limited to liquids. The liquid is filtered through a pre-sterilized ‘bacteria proof’ filter made of sintered glass, diatomaceous earth, unglazed porcelain, membrane pads, or similar materials, and the liquid is seeped under pressure or without pressure. This method is successfully used for fruit juices, beer, soft drinks, wine, and water.
(5) Preservation by use of chemical preservatives
Chemical preservatives are substances that are added to food only to slow down, inhibit or stop the activity of microorganisms such as fermentation, pecification, and decomposition of food.
There are two types of chemical preservatives:
Class-1 Preservatives: Common salt, sugar, dextrose, spices, vinegar, ascorbic acid, etc.
Class-2 Preservatives: Benzoic acid and its salts, SO2 and salts of sulfuric acid, nitrates, ascorbic acid, and its salts, propionic acid and its salts, lactic acid and its salts.
Of the Class-2 preservatives, only two chemical preservatives are used in the preservation of fruits and vegetables:
(i) KMS (Potassium Meta bisulphate):
- It releases SO2 and it is unstable.
- It is used for fruits that contain water-insoluble pigment (colourless).
- It cannot be used in naturally colored juices like phalsa, and Jamun as they contain anthocyanin pigment.
- It cannot be used in a product packaged in a container because it reacts with the tin container and the oil, hydrogen sulfide (H2S), producing an unpleasant odor and the base plate (bottom) of the containers) also forms a black compound.
- Better at controlling moulds than bacteria.
- 350 ppm KMS is mostly used in fruit juice products.
(ii) Sodium Benzoate:
- It is a salt of benzoic acid and is soluble in water.
- It slows down the fermentation of the juice.
- It is commonly used in products that contain natural pigments such as anthocyanin pigments.
- It is more effective against yeast.
- 750 ppm sodium benzoate is mostly used in fruit juices, squash, and cordials.
(6) Preservation by use of food additives (sugar, salt, acid and vinegar)
Food additives are substances or mixtures of substances other than the original food substances, which are present in foods as reagents for any aspect of production, processing, storage, packaging, etc. Food additives are (i) sugar, (ii) salt, (iii) acid, and (iv) spices.
In the case of sugar and salt, they exert osmotic pressure and water diffuses from the product through a semi-permeable membrane until the concentration reaches equilibrium. They kill or prevent microorganisms from multiplying.
(i) Sugar: A sugar concentration of 68-70% is used for the preparation of jams, jellies, marmalade, etc. Sugar acts as a preservative by osmosis, not as a poison to microorganisms. It absorbs most of the available water, so there is very little water available for the growth of microorganisms.
(ii) Salt: 15-20% concentration of salt is used for making pickles. Salt prevents enzymatic browning and discoloration and also acts as an anti-oxidant. It performs its preservative action as follows:
- Plasmolysis of microbial cells occurs due to high osmotic pressure.
- Dehydrating food and microorganisms by binding moisture.
- generating chloride ions from ionization which is harmful to microorganisms, and
- Reducing the solubility of oxygen in water, sensitizing, and reducing cells to CO2.
(iii) Acids: Many processed foods and beverages require the addition of acids to impart their characteristic taste and flavour to the final product as acids provide the desired taste and flavour. Sugar and acid ratios are adjusted in proper balance for the taste of the food. They also play a role in controlling pectin-gel formation.
The main acids are the following:
- Acetic Acid (Vinegar): This is usually only used for pickles, chutneys, sauces, and ketchup to prevent the growth of microorganisms.
- Citric Acid (Lemon Juice): It is used in jams, jellies, squash, nectar, etc. to increase acidity.
- Lactic Acid (Lactose): It is used to make curd from milk.
(7) Preservation by oil
A layer of oil on the surface of any food item creates an anaerobic condition that prevents the growth of mold and yeast. eg, pickles.
8) Preservation by fermentation
The decomposition of carbohydrates by microorganisms or enzymes is called fermentation. Foods are preserved by alcohol or organic acids produced by microbial action.
The quality of alcoholic beverages, vinegar, and fermented pickles depends on the presence of alcohol, acetic acid, and lactic acid respectively. Wine, beer, vinegar, fermented drinks, fermented pickles, etc., are prepared by these processes.
In wine – 14% of alcohol acts as a preservative. About 2% of acetic acid prevents spoilage in many products.
(9) Preservation by Carbonation
Carbonation is the process of dissolving enough CO2 in water or a drink so that when the product is served, just before the gas is released as bubbles and has a distinctive taste. Beverages containing fruit juice are typically bottled with CO2 between 1 and 8 g/l, enough to counteract the effect of acidity on pathogenic bacteria. Creates an anaerobic condition for the complete inhibition of microbial activity (14.6 g CO2/l), which reduces the oxidation of ascorbic acid and also prevents browning.
Although the sugar content in carbonated drinks is much less than 66%, the absence of air and the presence of CO2 in them helps to inhibit the growth of molds and yeasts.
The quality of carbonated fruit drinks is enhanced by the addition of approximately 0.005% sodium benzoate. The level of carbonation required varies by the type of fruit juice and the type of flavor.
(10) Preservation by antibiotics
Some metabolic products of microorganisms have been found to have a bactericidal effect and are called 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, it is more stable, especially for the preservation of acidic foods. Used in the canning of mushrooms, tomatoes, and milk products. Nisin primarily inhibits the growth of gas-causing spore-forming bacteria and the toxin-producing Clostridium botulinum.
Subtilin – an antibiotic derived from certain strains of Bacillus subtilis, 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- This antifungal antibiotic is used for the treatment of fruits and fruit juices. The use of antibiotics in combination with other sterilizing agents, including heat and radiation, gives good results.
(11) Preservation by radiation
Sterilized preservation of food by ionizing radiation is a recently developed method. The unacceptable taste of some irradiated foods and the fear of inducing radioactivity in such food precludes its further use.
When gamma rays (or) electron rays pass through foods, there is a collision between ionizing radiation and food particles at the atomic and molecular levels, resulting in ion pairs and free radicals. The reaction of these products with each other and with other molecules results in physical and chemical events that inactivate the microorganisms in the food.
Thus irradiation of food can be considered a method of “cold sterilization” i.e. food is free from microorganisms without high-temperature treatment.
Radiation treatment up to 1 mrad is not dangerous. Ionizing radiations can be used to sterilize foods in hermetically sealed packs, reduce rotting organisms in perishable foods, delay fruit ripening, inhibit germination of root vegetables, and controls store grain infestations (insects).