Without preservation food becomes inedible and non-nutritious. Food is usually spoilt by biochemical changes and microorganisms, or contaminated by dirt and chemicals, or damaged by pests, dehydrated or becomes stale. Food is derived from living organic compounds. Even when the animal or plant is killed they proceed to differing degrees and lengths, but in an uncontrolled way, thus causing spoilage. Fruits and vegetables stay alive after harvesting, they continue to ripen, and then start to deteriorate.Enzyme action is needed to tenderize and add flavour to meat. The breakdown of proteolytic enzymes at room temperature causes putrefaction. Fatty meat e.g. lamb, pork, poultry and veal, also oxidise, and the more unsaturated the fat is the more susceptible to rancidity.
The enzymes in cold-blooded fish work at lower temperaturesthan those of land animals, and as such cause deterioration under refrigeration and down to freezing point. Their fats are more saturated than those of land animals. And therefore spoilage is more rapid than in mammalian meats.The most important microorganisms responsible for food spoilage are moulds, bacteria and yeast, all of which need water and nutrients to reproduce. Some are aerobic whilst others are anaerobes. Some produce toxins that lead to poisoning. The harmful ones are those that can heavily infest a food without producing alterations that can be seen, smelt or tasted.
Other than cooking the most important heat process is canning. Some food need some processing beforehand, which may include cleaning, peeling and the removal of inedible parts such as the stones of fruit or the bones of fish and meat. Fruits and vegetables may be blanched where there is shrinkage and the release of air so as to make them soft so that more can fit in. The can is then filled to an inch of the top with fluid. In the case of vegetables it is usually brine, and for fruit syrup and fish either brine or vegetable oil. Air is then removed from the top by placing a loose fitting lid and heating with water or steam to protect the contents from oxidative damage and to prevent the interior of the can from corrosion. It is now ready for heat processing which is at a minimum so that micro-organisms are inactivated whilst not affecting the taste or its nutrient content. If the time needed is too long not only does it become an ineffective procedure but the food also becomes over-processed. This can be circumvented to an extent by sterilizing food before canning by High Temperature Short Time ( H.T.S.T. ) / aseptic canning. This is suitable for large scale commercial catering for liquids and semi-solids.
Water is universally used to bring on convection of heat to the centre. In the pre-canning process there are heat liable vitamin losses, water-soluble vitamin and mineral losses, and losses in cooking liquid in blanching or H.T.S.T processing. The most susceptible vitamins are the B group, particularly thiamine, folic acid and vitamin C, the minerals potassium and magnesium are lost the most, whilst sodium is increased because of the addition of salt, calcium is variable increasing in some and decreasing in others, trace minerals are minute and variable depending on growing conditions. The greatest losses are through the leaching of nutrients into the medium in which the food is canned. Canning destroys the Life Force and this renders it an unviable source of food. There is little loss of bulk nutrients in comparison to other cooked foods, other then the loss of the bio-availability of methionine and lysine in protein.
The main forms of milk preservation are by pasteurization,
sterilization, concentration, sweetening and drying. Milk from healthy cows contains low levels of bacteria. The bacteria tubercle bacillus which causes tuberculosis, Brucella abortus which causes brucellosis, and Streptococcus pyogenes which causes scarlet fever and soar throats. Heat treatment preserves the milk for longer by destroying the lactobacillus that sour milk; but these are the bacteria that are favourable for the health of the colon.
( I ) Pasteurization does not change the taste, physical or chemical composition. 1020% of thiamine and Vitamin C is lost.
( ii ) Sterilization involves homogenizing to break the fat into droplets which are stabilized in a uniform distribution by them adsorbing protein onto the surface. This is done by heating and forcing out under pressure through a small outlet. It is then filtered, bottled and heated. There is loss of thiamine ( 30% ) and Vitamin C ( 50% ), and calcium through calcium phosphate. There is also a coagulation of lactalbumin and lactoglobulin.
U.H.T. ( ultra-high temperature ) sterilization of milk is similar but after homogenization the milk is passed over a heater. The product is the same but the taste is unaltered. Unlike traditional sterilized milk low fat versions of U.H.T. milk are available.
( iii ) Evaporated milk is concentrated by evaporating a proportion of the water content in sealed containers in which the pressure is reduced. The temperature is limited to prevent protein coagulation. Then it is homogenised, sealed in cans and sterilized. Condensed milk has been pasterized, concentrated and sterilised, but then it is sweetened by the addition of sugar, which is a preservative when adequately concentrated. This adds nutritional liability because of the constituents of sugar.
( iv ) Spray-drying develops a light, convenient granular product with a long shelf life. Micro-organisms will not attack it because it lacks water. Cheaper products are packed in air and some oxidative taste changes can be noticed after 9 months. The more expensive ones are sealed with nitrogen, giving a still longer shelf life, until opened. Instant milk powders are not lumpy when re-hydrated like spray dried milk powders. This happens when spray-dried skim milk is reconstituted in humid conditions and then dried again.
Cold storage does not kill micro-organisms but retards
their activities. If cold enough they will inactivate them. There is considerable depletion of the Life Force during freezing. Food is stored to increase its shelf life, the longer this is the less fresh it will be, and even in the absence of spoilage vitamins and minerals are lost. This is because the depletion of Life Force with time is unavoidable.
Drying utilises the advantage that micro-organisms are unable to grow and reproduce in food from which water has been adequately reduced. This is done by using a tray, tunnel and rotating drum driers in which air is simply passed over the food at a precisely regulated temperature and humidity. An advanced method is freeze drying in which the food is dried in a near vacuum from the frozen state. The ice within is sublimed into water vapour. This can also be done by 'accelerated freeze drying' ( A.F.D. ) in which the procedure is accelerated by supplying low heat at a precise rate so that the freezing point is not exceeded. Nevertheless with vegetables undergoing dehydration processes i.e. traditional freeze-drying, they first have to be blanched, which gives stability to the end product, better colour, destroys several micro-organisms and inactivates the oxidative enzymes. There is nutrient loss, and the use of sodium sulphate in blanching vegetables to improve the retention of vitamins has its nutritional disadvantages; there is an effect on the sodium potassium balance of foods, and the depletion of water-soluble vitamins, Vitamin C and Vitamin B1.
Foods preserved by salting disrupt the sodium/potassium balance. It is utilised because enzymes cannot function in high salt concentrations. The same applies to sugar. This happens naturally in dried fruit or honey and is utilised in jams, glazed fruit and sweetened condensed milk. Salting and Smoking is often used together. Smoke contains antiseptic agents and forms a protective outer shell by drying. These are undesirable additives. The products of smoking itself form polycyclic hydrocarbons e.g. benzpyrene and others that are carcinogens.
Separation is done when there are some sections of foods
that are more susceptible to spoilage, primarily the fats which oxidise quickly and become rancid. The Preservation of Food Regulations ( 1976 ) permits the use of 35 preservatives in the U.K. They fall in the following categories by chemical nature of their parent compound: Sorbic Acid; Benzoic Acid; Sulphur dioxide and its derivatives; Diphenyl and its derivatives; Nisin; Hexamine; Nitrites and Nitrates; Propionic and its salts; Antioxidants.
In irradiation food is passed in front of a radioactive isotope. Low dosage applications: ( I ) inhibit the sprouting of root vegetables; ( ii ) destroys insects; ( iii ) delays ripening of certain fruits and vegetables. Medium dosage applications: ( iv ) extend the shelf life of fruits and fish; ( v ) destroys spoilage and pathogenic micro-organisms; ( vi ) improves food quality. Other applications: ( vii ) sugar solutions irradiated can be used as preservatives instead of chemicals; ( viii ) tenderises meat; ( ix ) improves the flavour of carrots; ( x ) improves the quality of wheat; ( xi ) During the malting of hops the yield is 7% greater if the barley is irradiated. Spirits can be 'aged'. But there are several adverse effects.
In the case of bulk nutrients; carbohydrates and sugars are least affected; proteins that are affected are broken down in ways similar to digestion; a lot of vitamins are damaged; minerals should not be affected; the problem is severe with oils, fatty foods and dairy products- they are unsuitable for irradiation.
Packaging has its disadvantages because the food may become contaminated by the packaging material and substances inside. It is not usually the polymers that are toxic but the chemicals included to enhance the plastic.
Healthy eating includes eating less processed foods.
(R) thedailystar.net 2005