Principles of food preservation (part2)

Explanation Of Principles Of Food Preservation 

1). Prevention or Delay of Autolysis :

Enzymes require specific pH range to catalyze the life reactions. The enzymes are very sensitive to changes in pH of the environment.

This information is utilized to preserve several foods by mereiy altering their pH. For example, in biological preservation of vegetables (pickling), pH of raw materials is reduced through the activities of lactic acid bacteria that produce lactic acid. Some meat and milk products are likewise preserved or produced as a result of change in the pH. Sometimes preformed organic acids, such as acetic acid (vinegar), are added to foods for the purpose.                             Enzymes thrive best in a particular temperature range. Any deviation from this to the higher or lower side has an adverse effect on enzyme activity. The food can be subjected to a particular high temperature that will either destroy or inactivate the enzymes. In the food industry, some fruits and most vegetables are normally subjected to around 100°C for a few seconds to few minutes in an operation called blanching or scalding. As a result of this exposure, some damaging enzymes, e.g., the catalase system, are destroyed. Blanching is usually a pre-treatment in several food preservation methods such as canning dehydration and freezing.

                                  Another procedure employed to delay autolysis is the use of low temperature. The enzyme reactions are retarded at low temperatures that prevail in the refrigerator or cold storages. Thus, refrigeration delays ripening process in fruits. Freezing inactivates the enzymes and sometimes destroys them; hence frozen foods have a much longer shelf life than the fresh ones or those kept at above 0°C.

                                    Enzymes require moisture for the biochemical reactions which  they catalyse in foodstulfs Absence of water would mean that these reactions cannnt take place in the modum. This properiy ui enzyrnes is utilized, partly, in the preservation of foods by removal or binding of moisture. The moisture in foods is removed by sun drying, dehydration, concentration and evaporation. Available moisture may also be bound by

the use of chemicals as seen in the production of intermediate moisture foods.

                                      Chemically induced autolysis can be minimized by applying good

manufacturing practices (GMP) based on experience and scientific knowledge. Most chemical reactions are retarded by maintaining a low temperature profile, by the use of chemical inhibitors and by proper packaging. Non-enzymatic changes in foods may be prevented by the use of some chemicals, e.g. sulphites to inhibit colour changes or antioxidants to check rancidity. Further control can be achieved by

employing low temperature, controlling water activity in dehydrated foods, reduction of reducing sugars in potatoes by storage, reduction of amino-nitrogen content in juices by ion exchange and by packaging with oxygen scavengers.

2).Prevention or Delay of Microbial Activity :

While enzymes are part and parcel of fresh

foods, microorganisms are invaders and where

undesirable, they should be treated like any

other invader. Firstly, precautions may be taken

to prevent their entry into the food, but where

they have already gained access, these may be

thrown out. When such a strategy is not feasible,

the microbial activities may be controlled by

creating conditions that do not favour their

normal existence. As a final resort, they may be

destroyed by any suitable means.

A). Keeping Microorganisms Out :

Nature creates all foods free from

microorganisms and provides protective

covering to prevent their entry inside the tissues.

Orange, banana, mango, wheat, maize, rice and

groundnuts are all free from microorganisms

when produced. The skin, peel, husk and shell

provide hese commodities protection aqainst

infection from external sources. Similarly,

tissues of healthy animals are free from

microorganisms. The skin and fatty tissues qive

the animal protection from invading

microorganisms. As soon as the outer covering

of plant materials or animals is injured,

microorganisms invade the tissues and start

growing, thus causing infection or spoilage. To guard against the entry of microorganisms from an outside source protective covering is provided for the food in the industry. While fruits,

vegetables, meat and other foods may be

packed and protected in tin cans or glass

containers, some foods are given a coating of

wax or other inert material to protect them

from the attack of microorganisms. Examples

are coating of some cheese varieties and eggs

with wax or milar substances.

B). Removal of Microorganisms :

During food processing, it is difficult to keep

microorganisms away since these are

omnipresent. Even without obvious signs of

microbial attack, microorganisms gain entry

into. food during harvesting. storage and.

subsequent processing operations. Since

microorganisms are very small, it is not easy to

pick and throw them out Fruits and vegetables

carry microflora on their surface or in their

bruised, damaged and decaying tissues. The

food processor washes the raw material

thoroughly so that much of the contaminants,

including microorganisms, are removed from

the surface. The bruised, damaged and

decaved, portions are trimmed off in the operation designated as trimming. These

operations help to reduce the microbial load in

the food product and aid in its preservation.

However, from liquid foods such as

clearheverages (water, fruit juices, wines and

beer), microorganisms can be removed

with the help of filters-a technique called

filtration In the brewing industry special

centrifuges are used to separate yeast from-

beez In the dairy industry centrifugation is

applied to separate cream from milks

During this process quite a number of

microorganisms are also separated from the

milk fraction and are removed with the cream.

C). Creation of Unfavourable Condition :

Most food spoilage and pathogenic

microorganisms are aerobic in nature. The

spores of some of these organisms must have

oxygen to germinate.This information provides

the food technologist with-yet another tool to

control their activities. Creation of anaerobic

atmosphere will prevent growth of vegetative

cells and germination of spores. This is usually

accomplished in canning by the use of mechanical vacuuming or in an operation called exhausting. The air from the product is removed and the can sealed airtight. Thus, even if these microorganisms have gained entry, under the prevailing anaerobic conditions, they will not grow; hence will not be able to spoil the food. However anaerobic Organisms may pose a problem in such processes if their activities are not checked or these are not subsequently

destroyed.

                  Another technique used to create

unfavourable conditions and Drevent the

activities of microorganisms in foods, even after

they have gained entry, is the removal or binding  of moisture. All microorganisms require

available moisture for their activity.

Consequently, if this is less than their

requirements, the growth is inhibited. This fact

is utilized in preserving foods by removal or

binding of moisture. The available moisture

from foods may be removed by drying,

dehydration, or concentration or is made

unavailable by the use of certain chemicals

such as sugar, glycerin and other water-binding

substances generally known as humectants. In

this manner the available moisture in food is

either removed from it or made unavailable. The

microorganisms thus are unable to grow in their changed environment and the food does not deteriorate or spoil as a result of microbial activity.

             In order to prevent growth and activity

of microorganisms in foods, some chemical

substances known as preservatives are also

employed. The use of sulphur dioxide and benzoic acid (added in the form of potassium

metabisulphite and sodium benzoate, respectively) protect fruit juices from spoilage in

sealed containers. While some chemicals may

be intentionally added, others are developed or

produced. in the food. Lactic acid is a good

example of such a chemical. This organic acid is produced by the normal growth of lactic acid

bacteria. Thus, in the dairy and meat industries

some products are produced and preserved as a

result of the activities of çertain lactic acid bacteria. Sour cream, butter, cheeses and sausages are representative examples

depicting the beneficial activities of lactic acid

bacteria. In certain vegetable products like

pickles, the activities of lactic acid bacteria are

also encouraged. These grow in the juice of the

vegetable and produce lactic acid. In this

process, pH of the vegetable material is reduced

to a point that microorganisms, which normally

would grow in or on the vegetable material, are unable to do so.

                                    Temperature is yet another very effective tool in the hands of people engaged in food preservation to create unfavourable conditions. Use of low temperature (chilling or freezing) inhibits the gowth of microorganisms- their activities are slowed down at chiling and completely stopped at freezing temperature. Holding foods at freezing temperature or in to destroy or kill some frozen state also hel microorganisms.

D). Destruction of Microorganisms :

Spoilage in foods may also be prevented by des

troying microorganisms. High temperatures

(above the maximum growth temperature) have

adverse effects on microorganisms. Blaching is

employed in the food industry primarily to

destroy or inactivate enzymes.

                                                       However il also

helps to kill some vegetative forms of microorganisms. PasteunzatieOn lemperatures

(05 C to 88 C) are emnpioyed to kill pathegenin

nFOARISms incloding bacteria and yeasts in

somne liquid foods. Sterilization employs much

higher temperatures (100°C or above) to destroy all mierobial foms. In food processing, it

is neither possible nor practicable te0 destroy all

forms of microbial fife (sterilization), hence

Commereial sterlization or heat processing is

employed.some microoganisms or their spores

remain viable after this heat treatment However,

with the combinalion of other techniques

employed in the preservation method, these

remain dormant and are unable to cause trouble. The heat treatment necessary to kill

microorganisms of their spores will vary from

organism to organism, its state (vegetative or

spore) and the envirenment. Depending on the

effectiveness of the heat treatment empleyed,

only nart of the vegetative cells or all or most of the cells may be destroyed. Some bacterial spores are also killed at higher temperatures,

The heat treatment applied is a temperature/

time dependent phenomenon.

                                                      Some chemicats

are also very effective in controlling microbial

activity in foods. Use of sulphur dioxide and

benzoic acid is common in fruit and vegetable

preservation industry. In baking, where mould

growth is feared, propionates are employed.

Microorganisms in foods can also be destroyed by the use of radiations of various frequencies. The ultraviolet waves from the electromaanetic snectrum have germicidal properties. Gamma rays, emitted from by products of atomic fission are also germicidal and when applied to food, help preserve them.

3).Control of Pest Activities :

Insects, rodents and birds have attained pest status in most countries. However, only those are so designated that exhibit prolific reproductive capabilities and compete, often menacingly, with humans for food and space and may be associated with transfer of diseases to man. These are a constant threat to man's survival. The seemingly harmless species are, in fact, a necessary part of the balanced

ecological system in which we live.

A). Insects:

Insects alone account for the destruction and waste of almost fifty percent of the annual cereal grain crops in some developing countries. These small enemies of man attack food in the field, processing plants, warehouses, supermarkets, as well as in the home.                                                                    A typical preventive measure for insect control includes treatment with long-acting insecticides and insect repellants to dress seeds and the use of insect predators or other biological means. Curative insect control programme, on the other hand, involves the use of physical, chemical or biological methods to exterminate insects and

insect eggs from already infested food. A popular physical method for destroying insect eggs in wheat flour and other similar size-reduced dry foods involves the use of impact-based instrument, the entolater. Insect eggs that are thrown to the metal body of this revolving instrument are instantiy destroyed by impact. Heat disinfection is another physical method employed to destroy.insects in stored products.. In this .case, the temperature of food is raised to about 50°C since insects cannot survive such high temperature. Infrared devices are popular heat sources for heat disinfection of grains on account of their effectiveness and relative

low cost.

                The most desirable control measures are those that demonstrate effective insect control over long periods without creating intractable adverse effects on man and the environment. Experiences with the control of insects as with other animal pests, have shown that control which momentarily and indiscriminately reduce insect populations may introduce serious health hazards and environmental measures problems in the long run.

B). Rodents :

Gross rodent infestation is readily discovered by the presence of burrows, dark oil and dirty stains on rodent runaways, odour of mice, filth

deposited on food, as well as damaged food and other materials. Dry urine appears as darkened stain on food and containers. These stains will fluoresce under ultraviolet light. The presence of rodent filth in food processing and storage premises calls for prompt control measure.

                Complete eradication of rodents has, however, proved an almost impossible task. The situation is worse in developing countries where

rodent populations are known to overwhelm available control facilities due to poor environmental sanitation. Systematic control of mice and rats is practiced with a reasonable degree of success in most industrialized

Countries. A typical systematic control includes the use of poison baits, rat-traps and biological methods to kill mice and rats. Rodent harbourages are usually either destroyed or sealed off after treatment with a long-acting rodenticide. Foods are packaged and stored in

environments that make rodent invasion difficult. A sustained control effort can yield positive results only if backed by a reasonable measure of sanitation around food handling and storage sites.

C). Birds:

Birds consume substantial quantities of grains in the field prior to havest. In some tropical countries, birds, such as the weav been known to devastate entire corn crop in the field. This loss by bird in vasion has traditionally been checked by installing noise-making dummies (shaped like humans or animals) at strategic points on the farm scare birds away. Apart from the consumption of grains, birds can contaminate food and water. To prevent this, bird screens can be erected over important water supply sources and food handling areas.

4).Reduction in Physical Defects :

Added to the defects caused by autolysis, microorganisms and biological pests, additional quality deterioration in food can be caused by

physical and physico-chemical interactions. These interactions are induced by series of treatments that accompany food manufacture,

handling and storage. There is no simple rule of thumb that predicts the best processing conditions for all types of foods. However,

experimentation, experience and current exchange-of-scientifie-and technical information on the effects of various processing variables on food quality have helped in the development of optimized processes aimed at keeping physical defects in foods to minimum.

A). Surface Drying :

The desiccation or drying out observed on the surface of certain foods during frozen storage is an important physical defect that can be controlled by adequate packaging and the maintenance of a steady storage temperature. In refrigerated storage of flesh foods, moisture loss and surface drying can be minimized by maintaining higher than conventional. average relative humidity in the storage chamber. The increase in microbial activity induced by such high relative humidity is kept under check by the use of ultraviolet lamp as storage atmosphere sterilants.

B). Crystallisation :

Another common physical defect involves crystallizatioT of sugar from syrups and other sugar-based products. Such defective products

develop a distasteful sandy texture. Unwanted sugar crystallization in foods can be prevented by using a mixture instead of single sugar. A

solution containing above 60 percent sucrose at ambient temperature will develop. crystals, whereas a solution containing 75 percent total sugarS made up of a careful blend of sucrose, glucose and fructose visible signs of crystals. This knowledge is applied in the production of

high sugar products like jams and sugar-based jellies.

C). Other Defects :

Attempts at reducing physical and textural defects in - foods during processing and storage are seen in process handling and storage.

controls applied by the industry. The relative humidity and drying rates are controlled in dehydrators to minimize the incidence of case-

hardening; stabilizers are used to prevent the collapse of emuisions during handing and storage. Freeze-drying removes moisture from heat-sensitive foods and enhances the retention of natural nutritional and textural properties on rehydration.