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Freezing, canning and drying for preservation of foods
Friday, 26 February, 2021, 14 : 00 PM [IST]
Ganesh Gaikwad & Dr Vijaya Pawar
Food preservation
Food is the basic necessity of life and is invaluable for healthy existence of human beings. Most of our food consists of agricultural and livestock products, which are usually seasonal and spoil quickly. Foods gradually undergo deterioration or spoilage from the time they are harvested, caught, slaughtered or manufactured unless it is preserved. Deterioration of food in some cases is accompanied by production of toxic substances while in other cases it results in losses of nutritional value. With the rapid increase in the population of the country, an enormous problem of maintaining sufficient safe and nutritious food supplies always exists. There is need of more quantity of food and at the same time good quality food is required i.e. food should be safe, wholesome, nutritive and balanced. Therefore, it is essential to possess knowledge of processing and preservation methods that produce/preserve best possible foods over a long period of time with minimum loss of nutritive value.

Food preservation can be defined as the science which deals with the methods of prevention of decay or spoilage of food, thus allowing it to be stored in a fit condition for future use. In order to store the foodstuffs for a longer period without spoilage, proper preservation is important. To make food available throughout the year, humans have developed methods to prolong their storage life i.e. to preserve them.

Advantages of Food Preservation
Food preservation can result in several advantages. These include: increased shelf-life, decreased hazards from microbial pathogen, decreased spoilage (microbial, enzymatic) inactivation of anti-nutritional factors, ensured round the year availability of seasonal foods, perishable foods that can be transported to far-off distances from the site of production, increased availability of convenience foods (e.g. Ready-to-serve beverages, Instant mixes etc.), increased variety of foods, some with enhanced sensory properties and nutritional attributes, preservation in some cases produces a different form of the products which are of great importance in various cuisines. E.g. raisins, squash and wines made from grapes.

Food preservation, any of a number of methods by which food is kept from spoilage after harvest or slaughter. Such practices date to prehistoric times. Among the oldest methods of preservation are drying, refrigeration, and fermentation. Modern methods include canning, pasteurisation, freezing, irradiation, and the addition of chemicals.

Drying is one of the oldest and the simplest method of preserving food. It refers to removal of water from the food. Dried foods are preserved because the available moisture level is so low that the microorganisms cannot grow and the enzyme activity is also controlled. Drying can be accomplished by a number of methods viz. sun drying, mechanical/ artificial drying and freeze drying etc. Dried foods are compact and lightweight; do not require refrigeration and last much longer than the fresh foods. Dried foods should be stored in airtight containers to prevent moisture from rehydrating them and allowing microbial growth.

1.    Sun drying
Sun-drying takes heat from sun rays but it is a slow process involving risk of contamination and spoilage. The limitation for sun drying is availability of climate with a hot sun and a dry atmosphere.

2.    Mechanical/ artificial drying
Dehydration process usually implies the use of controlled conditions of heating, with the forced circulation of air or artificial drying (mechanical drier) in contrast to sun drying. Using mechanical driers, fruits, fruit leathers, banana chips, tea, coffee, milk, soups, fish, meat, eggs and vegetables can all be dried year-round.

3.    Freeze drying
Freeze-drying is a form of dehydration in which the product is first frozen and then water is removed under vacuum as vapour by sublimation. The principle behind freeze drying is that under certain conditions of low vapour pressure, water in the form ice evaporates as water vapour directly without turning into liquid phase. The advantage is that the food structure and nutritional properties are better conserved but the equipment and its maintenance is costly.

4.    Smoking
Smoking has been used as a method of food preservation from time immemorial. In this method, foods are exposed to smoke by burning some special kinds of wood, which has two main purposes, adding desired flavouring and preserving. Smoke contains chemicals like formaldehyde, which is bactericidal. And also the dehydration occurring due to smoking is responsible for its preservative action. The smoke is obtained by burning wood like oak, maple, walnut and mahogany under low breeze/wind. Most meat is smoked after curing to aid their preservation. Examples of smoke preserved foods are smoked beef, ham, bacon, fish and meat.

Preservation by Low Temperature
The metabolism of a living tissue is a function of the temperature of the environment. Low temperature is applied to retard chemical and enzymatic reactions in food. In addition, reducing temperature retards or stops growth and activity of microorganisms in the food. Lower the temperature, the slower will be the rate of above natural activities. Cooling thus slows down or stops the spoilage of foods. Freezing and refrigeration are among the oldest methods of preservation. Mechanical ammonia refrigeration systems invented during 1875 allowed development of commercial refrigerated warehousing and freezing.

1.    Cellar storage temperature (15°C)
It is usually used for the storage of surplus foods like root crops, potatoes, onions, apples, etc. for limited periods.

2.    Refrigeration/ chilling temperature (0 to 5°C)
Foods kept at this temperature slow down the microbial activities and chemical changes resulting in spoilage. Mechanical refrigerator or cold storage is used for this purpose. Examples of this include meats, poultry, eggs, fish, fresh milk and milk products, fruits, vegetables, etc. which can be preserved for 2-7 days by refrigeration.

3.    Freezing (-18 to -40°C)
In freezing, water in food turns into ice and hence, makes the water unavailable for reactions to occur and for microorganisms to grow. Most perishable foods like poultry, meats, fish, ice-creams, peas, vegetables, juice concentrates, etc. can be preserved for several months at this temperature. In vegetables, enzyme action may still produce undesirable effects on flavour and texture during freezing. Heating, like blanching, therefore, must destroy the enzymes before the vegetables are frozen.

Preservation by Use of High Temperatures
The process of heating was used centuries ago before its action was understood. Food is heated up or cooked. Heat is used to inactivate organisms or enzymes of spoilage significance in the foods. Microorganisms are killed by heat because the application of heat coagulates the food proteins and inactivates the microbial enzymes and thus results in death of microorganisms. The examples include all forms of cooked food, pasteurisation, milk sterilised by UHT (ultra high temperature), canning etc. One of the most important modern applications of the heat preservation is the pasteurisation of milk.

The process of sealing foodstuffs hermetically in containers and sterilising them by heat for long storage is known as canning. In 1804, Appert in Frarice invented a process of sealing foods hermetically in containers and sterilising them by heat. Appert is known as the 'Father of Canning'. This work formed the foundation for modern canning procedure. In honour of the inventor, canning is also known as appetising.

Canning Process
1.    Raw material selection/receiving:
For canning, fruits should be ripe but firm, evenly matured, free from blemishes, insect damage. Thus, harvesting at proper maturity is an important step in selection of raw material for canning. Over ripe fruits yield poor quality product, while under ripe/immature fruit generally shrivel or toughen on canning.

2.    Sorting & Grading:
Graded for uniform quality of size and colour. Sorting and grading ensures the removal of inferior or damaged commodity. For sorting, inspection belt can be used, in addition to trained personnel who detect poor quality produce unsuitable for canning.
•    Automatic colour sorters can be used for sorting to reduce labour cost.
•    The fruit and vegetables are graded to obtain uniform quality with respect to size, colour etc., after preliminary sorting.
•    For mechanical grading, the fruit and vegetables are passed over screens with holes of different diameter.
•    Different types of mechanical graders include screen grader, roller grader, rope or cable grader etc.

3.    Washing:
Fruit and vegetables are generally washed with water to remove dust, dirt and adhering surface micro-flora. Different methods of washing include soaking or agitating in water, washing with cold or hot water sprays etc.
•    Washing by using high pressure sprays is most satisfactory.
•    Detergents are frequently used in the wash or rinse water.
•    The water temperature should be kept low to keep the fruit firm and to reduce leaching losses.
•    High pressure sprays should not injure the fruits.

4.    Peeling:
 These are the primary unit operations for preparing fruit and vegetables for canning. Depending upon the type of commodity, peeling methods are selected such as (1) by hand or knife (2) by machine (3) by heat treatment (4) by using lye solution. Cores and pits in fruits like apple, peach, apricot etc., are removed by hand or by machine (de-corer).

5.    Cutting/halving/ slicing:
 After peeling, the fruits are halved or cored either manually or mechanically. However, peeled fruit should always be kept submerged in either water, containing 1-2 per cent salt solution or acid to avoid enzymatic browning. Peaches, apricot, pears, tomatoes etc., are peeled before canning.

6.    Blanching:
Treatment of fruit and vegetables with boiling water or steam for short periods followed by immediate cooling prior to canning is called blanching. The basic objectives of blanching are as under:
•    To inactivate enzymes
•    To clean the product initially to decrease the microbial load and to preheat the product before processing
•    To soften the tissue to facilitate compact packing in the can
•    To expel intracellular gases in the raw fruit to prevent excessive pressure built up in the container.

Blanching is carried out either by hot water or using live steam. Water blanching is generally of the immersion type or spray type as the product moves on a conveyer. Only soft water should be used for blanching as hard water toughens the tissue and destroys the natural texture.

7.    Cooling:
To keep fruits and vegetable into good condition, cooling is done after blanching.

8.    Filling in cans:
Tin cans are washed in hot water or in steam jet to remove any adhering dust or foreign matter. The cans are then sterilised by dipping in hot water tank or the cans are passed through a steam sterilising tunnel before use. Generally plain cans are used however, for coloured fruits like plums, black grapes; strawberries etc., lacquered cans are employed. The fruit and vegetable either slices, halves or whole are filled into the cans keeping in view the declared drain weight.

9.    Syruping or brining:
Solution of sugar in water, done only forfruits. improve the flavor, serve as a heat transfer medium, concentration 20 to 55° brix, filled at about 79 to 82°C, leaving head space of 0.3 to 0.5cmb or Brining: 1-3 per cent salt concentration used for vegetables.

10.    Exhausting:
Exhausting is a unit operation in which practically all air from the contents in the can is removed before sealing. The purpose of exhausting and creation of vacuum is to create an anaerobic environment in the can that would inhibit microbial spoilage. The removal of air from the contents also reduces the risk of corrosion and pin holing of the tin plate and discoloration of can contents. Exhausting helps in better retention of vitamin C. The vacuum in can prevents bulging of the can during storage at higher attitudes or in hot climate.

11.    Seaming/closing:
Immediately after exhausting, the cans are sealed by using a double seamer. Double seaming is a two-step operation. In the first operation, the can lid is inserted on the can body hook by holding and rotating the lid-in-position can between two rollers. This operation is called as clinching; during which first operation roller gently guides the lid in the body hook. The next step is to press the seam using the second operation roller, which results in an appropriate overlap of the body hook and cover hook which results in an appropriate countersink. Between the cover hook and body hook lies a layer of sealing compound which ensures the sealing process.

The critical parameters for an ideal hermetic seam are body hook, cover hook, seam thickness, seam width and overlap which need to be carefully controlled to prevent leakage in the can. Immediate closing of the cans is required to prevent excessive cooking of the surface of the product. Double seamers are of different designs and capacities like hand operated, semi-automatic and fully automatic. Modern double seamers operate at high speeds (300 cans per minute) while liquid products are sealed in cans at speed of up to 1600 per minute.

After exhausting can are sealed immediately with can sealer, temperature should not fall below 74°C during sealing.

12.    Coding/Embossing:
Coding of lid of the can is essential to identify the can, once it is closed. The code provides the necessary information about the product like name of canning unit, product packed in the can, date of packing; lot number etc. Coding is done on the second lid (end cover) of the can just before sealing.

13.    Heat processing:
The cans after sealing are immediately transferred to the heating retorts to achieve sterilization of contents. Heat processing consists of heating cans to a predetermined time and temperature of heating to eliminate all possibilities of microbial spoilage. Over cooking should be avoided as it spoils the texture, flavour and appearance of the product. Generally all fruits and acid vegetables can be processed satisfactorily in boiling water (100oC) as the presence of acid retards the growth of bacteria and their spores. While non-acidic vegetables (except tomato and rhubarb) are processed at higher temperatures of about 115-121oC under pressure. It needs to be ensured that required temperature reach the centre of the can. The temperature at the centre of the can should be maintained for sufficiently long period to destroy spores of more heat resistant bacteria.

14.    Cooling:
Cooled to 39°C to stop the process and to prevent the stack burning, done either by dipping or immersing can in cold water tank or by spraying jet of cold water.

15.    Storage:
After labelling cans are packed in strong wooden cases and stored in cool and dry place.

(The authors are Ph.D. research scholar, College of Food Technology, VNMKV, Parbhani and head, department of food process technology, College of Food Technology, VNMKV, Parbhani. They can be reached at
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