Principles of Food Preservation

Principles of Food Preservation

The principles of various methods for food preservation are as

1) Prevention or delay of microbial decomposition

ü  By keeping out microorganisms (asepsis)

ü   By removal of microorganisms (filtration)

ü  By hindering the growth and activity of microorganisms (e.g. by low temperatures, drying, anaerobic conditions, or chemicals)

ü  By killing the microorganisms (e.g. by heat or radiation)

2) Prevention or delay of self decomposition of the food

ü  By destruction or inactivation of food enzymes (by blanching)

ü  By prevention or delay of chemical reactions (By using antioxidant)

3) Prevention of damage caused by insects, animals, mechanical causes etc.

Aim or objectives of food preservation

1.      To preserve food stuffs in transit from the producer to the consumer by preventing undesirable changes by wholesomeness, nutritive value or sensory quality of foods.

2.      To smooth out irregularities in production, especially to overcome the hardness of seasons.

3.      To ensure as far as possible that local or seasonal surpluses are not wasted.

4.      To facilitate handling which is done by various forms of packaging

Method of food preservation:

        i.            Asepsis (keeping out of microorganisms)

     ii.            Removal of m.o.

   iii.            Maintenance of anaerobic condition

   iv.            Use of high temperature

      v.            Use of low temperature

   vi.            Drying

 vii.            Use of chemical preservatives

viii.            Use of radiation

   ix.            Mechanical destruction of m.o.

Asepsis (Absence of infection)

Asepsis means preventing the entry of m.o. maintaining of general cleanliness while pickling, grading, and packaging and transporting of fruits and vegetables increases their keeping quality and the products prepared from them will be superior quality. Washing or wiping of the fruits and vegetables before processing should be strictly followed as dust particles adhering to the raw material contain microorganisms and by doing so the number of organisms can be reduced considerably.

Filtration (Removal of m.o.):

In this method, the juice are clarified by settling or by using ordinary filters and then passed through special filters which are capable of retaining yeasts and bacteria. Various types of germ-proof filters are used for this purpose. This method is used for soft drinks, fruit juices and wines.

Preservation by high temperature

Use of high temperature food can preserve by the following methods:

        i.            Blanching

     ii.            Pasteurization

   iii.            Sterilization

   iv.            Canning

      v.            Bottling

Blanching

Blanching is generally applied to fruits and vegetables primarily to inactive natural food enzymes. Blanching also will destroy some microorganisms and less the volume of food. Normally, it is done at 70-80^o C for 5 minutes.

Objective of blanching:

        i.            To inactivate enzyme

     ii.            To less the food volume for further processing

   iii.            To remove the undesirable flavors

   iv.            To discolorise the raw foods.

Pasteurization

Pasteurization is a heat treatment by which pathogenic organism in food can be killed. In this method a part of the organism kills present in the foods that are to be further handled and stored under conditions, which minimize microbial growth.

It has the following advantages”

        i.            Loss of flavor is minimum

     ii.            Vitamins are not destroyed

   iii.            Effects economy of time and space

   iv.            Keeps the juice uniformly cloudy

      v.            Juice is heated uniformly and thus its cooked taste is minimum

There are three types of pasteurization:

1.      Batch pasteurization: in this case of milk it must be quickly brought to 62.8^oC (142^oF), held at this temperature for 30 minutes and rapidly cooled. In addition to being freed of common pathogens this heat treatment also inactivate the milk enzymes like lipase which otherwise would quickly cause the milk to become rancid.

2.      High temperature short time pasteurization (HTST): this type of pasteurization is conducted at least 71.7^oC for at least 15 sec for raw milk

3.      Low temperature long time (LTLT): this type of pasteurization is conducted at least 62.7^oC for at least 30 min. for raw milk.

Sterilization:

It’s a complete destruction of m.o. by heat treatment. It can be done at 121^oC temperature for 15 min. into pressure cooker or retort.

Difference between pasteurization and sterilization

Pasteurization	Sterilization
1.      Partial destruction of m.o.	1.      Complete destruction of m.o.
2.      Temperature below 100oC	2.      Temperature 100oC and above
3.      Normally used for fruits	3.      Normally used for vegetables

Canning

Principle: destruction of spoilage organisms within the sealed container by means of heat. pH of foods play an important role to preserve the food. In acidic food the microorganisms are low heat resistive, so they can be destroyed easily. But in low acid food the microorganisms are more heat resistive. So the low acid food needs high temperature during canning.

Flow sheet diagram for canning process:

Selection of fruits/vegetables

Grading

Washing

Peeling

Cutting

Blanching

Cooling

Filling and syruping or brining

Exhausting

Sealing

Processing

Cooling

Storage

Syruping: A syruping of sugar in water is called syrup. Syrup is added to improve the flavor and to serve as a heat transfer medium for facilitating processing. Syruping is done only for fruits. Strained, hot syrup of concentration 20-55^oBrix is poured on the fruit. Fruits rich in acid require more concentrated syrup than less acid ones. The syrup should be filled at about 79-82^oC leaving a head space of 0.30 to 0.50 cm.

Brining: A solution of salt in water is called brine. The objective of brining is similar to that of syruping. Only vegetable are brined. Hot brine of 1-3% concentration is used at about 79-82^o C leaving a headspace of 0.3 to 0.5 cm.

Exhausting: The process of removal of air from cans is known as exhausting. After liding before sealing exhausting is essential. The cans are passed through a tank of hot water at 82 to 87^oC.  Exhausting time varies from 6 to 10 minutes.

The major advantages of exhausting:

1.      Corrosion of tinplate during storage is avoided.

2.      Minimizes discoloration by preventing oxidation.

3.      Helps in better relation of vitamins particularly vitamin C.

4.      Reduces chemical reaction between the container and the contents.

5.      Prevents development of excessive pressure and strain during sterilization.

6.      Prevents building of cans when stored in hot climate or at high altitude.

Processing:

Heating of foods for preserving is known as processing. Processing time and temperature should be adequate to eliminate all bacterial growth, mostly it depend upon the pH value of food. Bacteria spore do not grow or germinate below pH 4.5. thus a caned product having pH less 4.5 can be processed in boiling water but a product having pH above 4.5 requires processing at 115 to 121^oC under a pressure of 10 to 15 lb/sp inch (0.70 to 1.05 kg/cm²) (to destroy the most heat resistive bacteria Clostridium botulinum if present)

Cooling: After processing the cans are cooled rapidly to about 39^oC to stop the cooking process and to prevent stack burning.

Bottling of fruits

Bottles have proved to be very good containers for home preservation of fruits. Although their initial cost is high they can be used several times and last for many years if carefully handled. The fruits look attractive through the glass and do not develop metallic flavor. Bottling does not need a sealing machine but is not suitable from the manufacturer point of views the initial capital required is high. Cans are cheaper, quite handy and lighter and loss due to breakage is less. Hence on commercial scale, tin cans are preferred to glass jars or bottles.

Preservation by low temperature

Low temperature is used to retard chemical reactions and action of food enzymes and to slow down or stop growth and activity of microorganisms in food.

By low temperature food can be preserved under following methods:

        i.            Chilling

     ii.            Freezing

   iii.            Cold storage

   iv.            Refrigerated gas storage

Chilling:

Storage of foods above freezing and below 15^oC is known as chilling storage. Chilling storage is widely used because it generally results in effective short-term preservation.

Method of chilling:

a)     Pre-cooling with moving air: The method is widely used because it is simple, economical and relatively non-corrosive to equipment. Major disadvantages of pre-cooling air are the dangers of excessive dehydration of products.

b)     Hydro cooling: The method is simple economic and rapid the product to be cooled in immersed in flooded with or sprayed with cool water

c)     Pre-cooling with ice: Pre-cooling with crushed ice is simple and effective if it is properly done. However, considerable discoloration is often involved when crushed ice is used product such as cabbage, peaches root crops etc. and frequently cooled by direct contact with crushed ice.

d)     Vacuum cooling: Vacuum cooling is extremely effective for product processing. It has two properties:

1.      A large surface to product ratio,

2.      An ability to readily release internal waters.

Freezing:

 The process of freezing is basically the transfer of heat from one substance to another. It is done at the temperature between 0^oC to -18^oC. In this method the frozen food can be preserved up to 2 years.

Freezing point:

The highest temperature at which ice crystals leave a stable existence in a food material in conventionally known as the freezing point

Classification of freezing system on the basis of time:

                    i.            Slow freezing or sharp freezing system

                 ii.            Quick freezing system

Slow freezing or sharp system: It is usually refers to freezing in air with only natural air circulation or at least with electric fans. The temperature may vary from -15 to -29^oC and freezing may taken from 3 to 72 hrs.

Quick freezing system: In this system food is frozen in a relatively short time. In this process the food attains the temperature of maximum ice crystal formation (0 to -1^oC) in 30 minutes or less. The main advantage of this system is the speedy freezing of foods results formation of very small ice crystals and hence minimum disturbance of cell structure. It is generally done at -17 to -45^oC.

Difference between chilling and freezing

Chilling	Freezing
1)     Chilling means lowering the temperature of food stuff near the freezing point but not below it.	1)     Freezing means lowering the temperature of food stuff near the freezing point or below it.
2)     It  is done for short-term preservation process	2)     It  is done for long-term preservation process
3)     Using ice generally does it	3)     Using refrigerant generally does it
4)     Water of foodstuff does not turn to ice	4)     Body water turn to ice
5)     Products processed by chilling are called chilled products.	5)     Products processed by freezing are called frozen products.

Cold storage:

Freshly harvested fruits and vegetable are perishable food items, due to its high content moisture. Some varieties of foods such as banana, potato, sweet potato. Apples etc. are not suitable for application of chilling or freezing preservation method. Here these products can be easily stored in cold storage for a certain period. The temperature vary from 0 to 10^oC

Site selection for a cold storage:

 The important considerations are:

                                i.            Sufficient good communication for the transport of goods

                             ii.            Good and stable sliding ground at a reasonable price

                           iii.            Ample space for outdoor storage

                           iv.            Availability of fresh water and electric power

                              v.            Availability of labor

                           vi.            Availability of technical and commercial services

Requirement of cold storage:

The useful storage life and quality of stored product depends on some factors. These are:

a)     Local condition

                                i.            Climate

                             ii.            Place of growth

                           iii.            Cultural practice

b)     Condition of the tuber:

                                i.            Variety

                             ii.            Effects of diseases and pests

                           iii.            Maturity

                           iv.            Effects of handling

c)     Storage condition:

                                i.            Temperature

                             ii.            Humidity

                           iii.            Ventilation

d)     Technical parameters of cold storage

                                i.            Operating temperature and relative humidity

                             ii.            Weight loss from the stored foods

                           iii.            Spoilage if any

                           iv.            Control system

                              v.            Cold storage layout

                           vi.            Stuff position and overall

Food preservation by Drying:

Microorganisms need moisture to grow so when the concentration of water in the food is brought down below a certain level, they are unable to grow. Moisture can be removed by the application of heat as in sun-drying or by mechanical drying (dehydration). Sun drying is the most popular and oldest method of preservation. In these days, mechanical drying has replaced sun drying. This is a more rapid process as artificial heat under controlled conditions of temperature, humidity and air flow is provided and fruits and vegetables e.g. green peas, cauliflower, mango, mahua, etc. are dried to such an extent that the microorganisms present in them fail to survive.

In this method, juices are preserved in the form of powder. The juice is sprayed as a very fine mist into an evaporation chamber through which hot air is passed. The temperature of  the chamber and the flow of air are so regulated that dried juice falls to the floor of the chamber in the form of a dry powder. The powder is collected and packed in dry containers which are then closed airtight. The powder when dissolved in water makes a fruit drink almost similar to the original fresh juice. Fruits juice powders are highly hygroscopic and require special care in packing. All juices cannot, however, be dried readily without special treatment. Mango juice powder is prepared by this technique but the method is very expensive.

Food preservation by preservatives:

Preservatives have been defined as chemical agents, which serve to retard, hinder or musk undesirable changes in food. The inhibition of the growth and activity of microorganisms is one of the main purposes of the uses of chemical preservatives. Preservatives may inhibit microorganisms by interfering with their cell membranes, their enzyme activity or their generic mechanisms.

Factors that influence the effectiveness of chemical preservatives

                    i.            Concentration of chemical

                 ii.            Kind, number, age  and previous history of organisms

               iii.            Temperature

               iv.            Time

                  v.            The chemical and physical characteristics of the substances in which the organisms are found (moisture content, pH, kinds and amounts of the solutes, surface tension etc.)

Characteristics of ideal antimicrobial preservatives:

1.      A chemical preservative should have wide range of antimicrobial activities

2.      Should be non-toxic to human beings or animals

3.      Should be economical

4.      Should not have an effect on the flavors, taste or aroma of the original foods

5.      Should not be inactivated by the food or any substances in the food

Some important chemical preservatives:

Sodium benzoate, potassium meta bisulfate (KMS), ascorbic acid, calcium carbonate, acetic acid, saccharin etc.

Food preservation by radiation:

Sterilization of food by ionizing radiation is a recently developed method of preservation which has not yet gained general acceptance. The unacceptable flavor of some irradiated foods and the fear that radioactivity might be induced in such food has come in the way of its greater use. The harmful effects on the human body of radiation from nuclear explosions have given rise to such apprehension in the mind of many people.

When gamma rays or electron beams pass through foods there are collisions between the ionizing radiation and food particles at atomic and molecular levels, resulting in the production of ion pairs and free radicals. The reactions of these products among themselves and with other molecules result in physical and chemical phenomena which inactive microorganisms in the food. Thus irradiation of food can be considered to be a method of “cold sterilization”, i.e. food is free of microorganisms without high temperature treatment.

Different organisms are sensitive to radiation to different extents e.g. a dose of 10³ to10⁷ rad  kill microorganisms, 10³ to10⁶ rad  kill insects and 10² to10³ rad are lethal to humans. Sprouting of potatoes, onions, carrots, etc. are inhibited by 10³ to10⁴ rad. In case of microorganisms, the approximate sterilizing dose for bacterial endospore is 3.0 ×10⁶ rad while that for yeasts and fungi is 5.0 ×10⁴ rad

Ionizing radiation can be used for sterilization of foods in hermetically sealed packs, reduction of the spoilage flora on perishable foods, elimination of pathogens in foods, control of infestation in stored cereals, prevention o sprouting of potatoes, onions etc.

Food preservation by carbonation

Carbonation is the process of dissolving sufficient carbon dioxide in water of beverage so that the product when served gives off the gas as fine bubbles and has a characteristic taste. Carbonation adds to the life of a beverage and contributes in some measure to its tang. Fruits juice beverages are generally bottled with carbon dioxide content varying from 1 to 8 g per liter. Though his concentration is much lower than that required for complete inhibition of microbial activity (14.6 g/litre), it is sufficient for supplementing the effect of acidity on pathogenic bacteria. Another advantage of carbonation is the removal of air thus creating an anaerobic condition, which reduces the oxidation of ascorbic acid and prevents browning.

Molds and yeasts require oxygen for their growth and become inactive in the presence of carbon dioxide. In ordinary carbonated drinks, the oxygen which is normally present in solution in water in sufficient amount to bring about fermentation is displaced by carbon dioxide. Although carbonated beverages contain much below 66%, the absence of air and the presence of carbon dioxide in them help to prevent the growth of moulds and yeasts.

High carbonation should, however be avoided as it usually destroy the flavor of the juice. The keeping quality of carbonated fruit beverages is enhanced by adding about 0.005 % sodium benzoate. The level of carbonation required varies according to the type of fruit juice and type of flavor.

Food preservation by sugar

Syrup containing 66% or more of sugar does not ferment. Sugar absorbs most of the available water with the result that there is very little water for the growth of microorganisms hence their multiplication is inhibited, and even those already present die out gradually. Dry sugar does not ferment. Thus sugar acts as a preservative by osmosis not as a true poison for microorganisms. Fruits syrup, jam, jelly, marmalade, preserve, candy, crystallized fruit and glazed fruit are preserved by sugar.

Food preservation by fermentation

Decomposition of carbohydrates by microorganisms or enzymes is called fermentation. This is one of the oldest methods of preservation. By this method, foods are preserved by the alcohol or organic acid formed by microbial action. The keeping quality of alcoholic beverages, vinegars and fermented pickles depends upon the presence of alcohol, acetic acid and lactic acid, respectively. Care should be taken to seal the fermented products from air to avoid further unwanted or secondary fermentation. Wines, beers, vinegar, fermented drinks, fermented pickles, etc. are prepared by these processes.

14% alcohol acts as a preservative in wines because yeasts, etc. cannot grow at that concentration. About 2% acetic acid prevents spoilage in many products.

Food preservation by acids

Low acid foods are spoilt rapidly. Highly acidic environment inhibits the growth of food spoilage organisms. Lowering the protein of certain foods by anaerobic fermentation, action on carbohydrates producing lactic acid is one of the methods of food preservation. The same spoilage inhibitory effects can be produced by acidic additives such as vinegar or citric acid. Nutrient losses through fermentation are small. In fact, in certain cases, the nutrient levels are increased particularly through microbial vitamin and protein synthesis. Acid conditions inhibit the growth of many microorganisms hence organic acids are added to or allowed to form in foods to preserve them. Acetic acid (vinegar), citric (lime juice) and lactic acids are commonly used for preservation. About 2% acetic acid prevents spoilage of many products. Onions are bottled in vinegar with a little salt. Vinegar is also added to pickles, chutneys, sauces and ketchups. Citric acid is added to many fruit squashes, jams and jellies to increase the acidity and prevent mold growth.

Food preservation by oil and spices

A layer of oil on the surface of any food produces anaerobic conditions which prevent the growth of mold and yeasts. Thus pickles in which enough oil is added to form a layer at the top can be preserved for long periods. Spices like turmeric, pepper, and asafoetida have little bacteriostatc effect and their ability to prevent growth of other microorganism is questionable. Their primary function is to impart their characteristic flavor to the food.

Food preservation by antibiotics

Certain metabolic products of microorganisms have been found to have germicidal effect and are termed as antibiotics. Their use in medicine for controlling certain diseases producing organisms in the body is well known. Some antibiotics are also used to preserve fruits, vegetables and their products.

Nisin is an antibiotic produced by Streptococcus lactis, an organism commonly found in milk, curd, cheese and other fermented milk products. It is non-toxic and has no adverse effect on the sensory qualities of food. It is widely used in the food industry especially for preservation of acid foods in which it is more stable. It is commonly used in canning of mushrooms, tomatoes, and milk products. Nisin suppress the growth of spoilage organisms, mainly the gas-producing, spore-forming bacteria and toxin-producing Clostridium botulinum.

Subtilin an antibiotic obtained from certain strains of bacillus subtilis, is used in preservation of asparagus, corn and peas. It is most effective against gram-positive bacteria and spore-forming organisms. Canned peas and tomatoes containing 10 to 20 ppm of subtilin respectively were found to be free of microorganisms. Subtilin and Nisin effectively reduce the thermal process requirements necessary to control the spoilage of several food products.

Pimaricin, an antifungal antibiotic can be used for treating fruits and fruit juices.

At present the above three antibiotics are permitted only in such foods as are cooked prior to use and in the process of cooking the residual antibiotic is expected to be destroyed. Use of antibiotic along with other sterilizing agents including heat and radiation offer good promise.