PRINCIPLES OF FOOD PRESERVATION

Food spoilage

From the moment foods are harvested or slaughtered they deteriorate and eventually become unfit or unsafe for consumption. One of the main purposes of food processing is to prevent or slow down this deterioration. Food spoilage is brought about by a combination of physical damage (e.g. bruising or cuts to the surface of fruits and vegetables), chemical and biochemical reactions (e.g. development of rancidity in fats or colour changes in fruits and vegetables due to enzyme activity), and changes caused by micro-organisms, such as the growth of mould or slime on the food surface and changes in texture and taste of foods.

Physical damage

Raw foods, especially fruit and vegetables, are very susceptible to physical damage. Poor handling at harvest or during transport leads to damage to the tissues, which are then more susceptible to biochemical changes and invasion and spoilage by microorganisms. Physical damage can be reduced by careful post-harvest handling and storage, and the use of containers instead of dumping foods in piles or in the backs of vehicles.

Enzyme activity

Enzymes are present in all living tissues and control the growth and development of the plant or animal. They continue to act after harvest or slaughter, causing undesirable changes in the appearance, taste and texture of foods. The ripening of fruits after harvest is also due to enzyme activity. When most plant materials are cut and exposed to air, enzymes present in the cells cause colourless chemicals to be converted into brown coloured compounds (known as enzymic browning). Enzymes that are produced by micro-organisms cause similar biochemical changes to foods to produce unpleasant and sometimes toxic products.

Damage caused by enzyme activity can be controlled by heating the food to denature the enzymes. Boiling, frying, pasteurization and canning are all heating methods that reduce enzyme activity. Blanching of vegetables is used before other forms of processing such as drying or freezing which do not heat the food sufficiently to destroy the enzymes. Alternatively, enzyme activity can be inhibited by changing the level of acidity, excluding air, or reducing the moisture content in some foods.

Chemical changes

Non-enzymic browning is a chemical reaction that takes place in foods, leading to browning of the material. In some instances, such as in the browning of bread crust and toasting of cereals, this produces desirable flavours and colours. However, in other foods such as dried milk it leads to the production of undesirable brown colours. It can be reduced by a number of methods: lowering the temperature of storage, optimizing the moisture content (the reaction rate is lowest at very high and very low moisture contents), increasing the acidity or using chemicals such as sulphur dioxide.

When foods are exposed to air, fats and oils are prone to oxidation which results in the development of off flavours and a reduction in nutritional value. This is a particular problem in oily fish and cooking oils. Oxidation is promoted by sunlight and some metals (e.g. copper and brass) which should not be used as containers for fatty foods. Oxidative changes can be prevented by excluding air with airtight packaging and keeping foods cool and away from light. At a more sophisticated level of technology, flushing foods with nitrogen or carbon dioxide gases before packaging in an airtight light-proof container can prevent oxidation.

Microbial spoilage and food poisoning

Micro-organisms live in abundance all around us in the soil, in water and air, and in the digestive tracts of animals. The three main types that are important in food processing are yeasts, moulds and bacteria, although viruses may also be important in particular foods (e.g. milk). Given the correct conditions for growth, micro-organisms multiply rapidly, causing undesirable changes to foods including changes to the taste, texture and appearance. Sometimes there is visible damage such as slime formation or the growth of moulds; often there is an offensive odour, particularly if proteins are broken down, and some foods may develop a bitter or acidic off flavour.

Microbial contamination of foods is wasteful but it can also be harmful, and in some cases life-threatening if food-poisoning bacteria (or 'pathogens') are present. They can be transmitted to foods by a number of routes: by animals, insects or birds that come into contact with foods, packaging materials or equipment; by poor personal hygiene of food handlers; or by cross-contamination from raw materials to a processed food via surfaces, utensils, equipment or human hands. Methods to control and prevent food poisoning by good hygiene and quality assurance are described in the chapter on Food safety, hygiene and quality assurance.

Food poisoning can be caused by eating the bacteria themselves, or by poisons (or 'toxins') that the micro-organisms release into the food. If this occurs and the microorganisms are then killed by processing, the poisons can remain in the food unseen (for example, aflatoxin, which is produced by a mould in poorly dried cereals and nuts – see Part II). Some types of micro-organisms also produce dormant spores which can regrow when conditions become favourable. This can be a source of contamination and potential food poisoning in dried foods which are subsequently rehydrated. The symptoms of an attack of food poisoning can include stomach pains, diarrhoea, vomiting, headache, fever and aching limbs. Sometimes the illness lasts for days, weeks or months, and in some cases it can kill.

However, not all microbial activity in foods is undesirable. Yeasts are used to leaven bread and produce alcohol by fermentation of fruits and grains. A number of foods are preserved by the action of micro-organisms, especially lactic acid bacteria which are used in vegetable, meat and dairy fermentations (see Part II). Moulds are used for the production of tempeh from soybeans, in some types of cheese and for development of flavours in a number of other fermented foods.

The factors that control microbial growth, and hence the risk of spoilage or food poisoning, are temperature, acidity, moisture content, presence of air and the concentration of salts or sugar. Control of these factors is the basis of most methods of food preservation.

Temperature

Micro-organisms have an optimum...