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 temperature range (20–30 °C) in which they can grow, which corresponds to daytime temperatures in many developing countries. Outside of this range either they die or growth is inhibited. Therefore heating and cooling are used as methods of controlling microbial activity. For example, freezing to minus 18 °C prevents microbial growth (but does not necessarily destroy the cells); chilling to below 5 °C or holding cooked foods above 65 °C prevents most microbial spoilage and food poisoning. Heating foods to above 90 °C for several minutes is the basis for destroying micro-organisms by blanching and pasteurization. Heat sterilization (canning or bottling) and concentration by boiling (e.g. jams and sugar confectionery) use higher temperatures (e.g. 105–120 °C) and/or longer heating times to destroy nearly all micro-organisms in a food.

Acidity

pH is a measure of the strength of acid or alkali using a scale from 1 to 14, where 1 is very strong acid, 7 is neutral and 14 is very strong alkali. The majority of foods have a pH of 7 or less and are classified into three groups:

* low acid foods: pH greater than 5.3 (e.g. meats, fish, milk, root crops, vegetables)

* medium acid foods: pH 4.5 to 5.3 (some fruits, e.g. banana, pumpkin, papaya, melon)

* acid foods: pH below 4.5 (e.g. pineapple, citrus fruits, tomatoes)

Foods that have low acidity (a high pH) are more susceptible to bacterial spoilage and food poisoning. Beneficial lactic acid bacteria are tolerant of more acidic conditions (pH as low as 3.8), and yeasts and moulds are the most tolerant to acidic conditions, being able to grow at a pH as low as 2.5. The pH of foods can be adjusted by the addition of acids such as citric or acetic acid to prevent the growth of food-poisoning bacteria (e.g. in pickles and yoghurt).

Moisture content

Water is essential for the growth of all animal, plant and microbial cells, and if it is removed, or made unavailable, cellular activity is decreased. For example, removal of water by drying or changing it to ice during freezing makes it unavailable to microbial cells and hence preserves the food. High salt or sugar concentrations have a similar effect.

Bacteria require more water than yeasts, which in turn require more water than moulds for cell growth. The amount of water in a food that is available to micro-organisms is referred to as the 'water activity' (aw). Pure water has an aw of 1.0, most bacteria are inhibited below aw of 0.9, most yeasts are inhibited below aw of 0.8 and most fungi below aw of 0.7. Almost all microbial activity is inhibited below aw 0.6. In practice, this means that foods are dried, or the concentration of salt or sugar is increased, to a point where the available moisture cannot support microbial growth. Table 1 illustrates the range of aw values and gives examples of the types of food in which they are found.

Methods of food preservation

Traditionally, the most widespread methods of food preservation have been dehydration, fermentation, salting, smoking and boiling. Other processes are controlled heating (pasteurization, canning etc.) and chilling and freezing. Table 2 describes the technologies that are commonly used worldwide. More recent advances, such as irradiation, ultra-high temperature (UHT) processing and extrusion, are generally not suitable for small-scale operations in developing countries and are not included in this book. Further information on these processes is available in food technology textbooks (Fellows, 2000).

Preservation by drying

Drying is used to remove water from foods to inhibit the growth of micro-organisms and to reduce the weight and bulk of food for cheaper transport and storage. When carried out correctly, the nutritional quality, colour, flavour and texture of rehydrated foods are slightly lower than fresh food. The colour of many fruits can be preserved by dipping them in a solution of 0.2–0.5 per cent sodium metabisulphite or by exposing them to burning sulphur in a sulphuring cabinet. However, these chemicals are not permitted in the United States and in some European countries, and if foods are intended for export, specialist advice should be sought. Blanching vegetables prior to drying preserves the colour and nutritional value by preventing enzyme activity.

At its most simple, drying foods can be carried out with a minimum of equipment using the heat from the sun. Although this is widely used to dry crops in the field, the lack of control and risk of contamination make it less suitable for food processing. More advanced dryers having various capacities and levels of complexity are available (Directory section 14.0) which have several advantages over sun-drying:

* the drying process is speeded up

* drying can be carried out in adverse weather conditions (rain, high humidity, no wind) and at night

* the temperature and rate of drying can be controlled, thus products of a consistent and higher quality are easier to produce.

However, if drying is carried out incorrectly there is a greater loss of nutritional and eating qualities, and more seriously a risk of microbial spoilage and possibly even food poisoning.

Principles of drying

Foods are dried when the water contained within them is removed into the surrounding air. It must first move to the surface of the food and then be evaporated as water vapour. For effective drying, the air should be hot, dry and moving. These factors are interrelated and it is important that each factor is correct (for example, cold moving air or hot, wet moving air are unsatisfactory).

The dryness of air is termed 'relative humidity' (RH), and the lower the humidity, the drier the air. Air with 0 per cent RH is completely dry air, whereas air at 100 per cent RH is fully saturated with water vapour. Air can only remove water from foods if it has the capacity to hold extra water vapour. If high RH (or wet) air is used, it quickly becomes saturated and cannot pick up further water vapour from the food. Humidity is affected by the temperature of the air. At higher temperatures the humidity is reduced and air can carry more water vapour. Normally the air in a dryer should be 10–15 °C above room temperature in solar dryers and at 60–70 °C in artificial dryers. The RH of air entering the dryer should ideally be below about 60 per cent. Further details of how to calculate air temperature and humidity are given in a number of textbooks (Fellows, 2000; Axtell, 2002).

When a food is to be dried it is necessary to carry out experiments to find the rate of drying. The information is used to find the time that food should stay in the dryer so that the moisture content is low enough to prevent spoilage. The rate of drying also has an important effect on the quality of the dried foods and (in fuel-fired dryers) on the fuel consumption. To find the drying rate, the food is weighed, placed in the dryer and left for 5–10 minutes. It is then reweighed and replaced in the dryer. This is continued until the weight of the food does not change. Typically, a drying rate of 0.25 kg/hour would be expected for solar dryers depending on the design and climate, and 10–15 kg/hour for artificial dryers. If the drying rate is lower than this, the air temperature or speed is too low and/or the RH is too high.

The sample of food is left in an airtight container for a day and checked to ensure that no further moisture has moved from the inside to the surface (if it has it is likely to be soft or even mouldy). Case hardening is the formation of a hard skin on the surface of fruits or fish which reduces the rate of drying and may allow mould growth. It is caused by drying too quickly during the initial period and can be prevented by using cooler drying air. Experiments with air temperature and speed can be used to select the best conditions for each food.

The moisture content of the food can be found using equipment (see Directory section 38.3) or by grinding the dried food to small pieces, weighing it and heating at 100 °C in an oven for four hours, then reweighing. The moisture content is found as follows:

Moisture content (%) = (initial weight - final weight) x 100/initial weight

The final moisture content of the dried food shows whether it will be stable during storage. When a satisfactory product is produced, the same temperature and time of drying are then used routinely in production.

To ensure safe storage of dried products, the final moisture content should be less than 20 per cent for fruits and meat, less than 10 per cent for vegetables and 10–15 per cent for grains. The stability of a dried food during storage also depends on its ability to pick up moisture from the air. Different foods pick up moisture to different extents, but the risk is greater in regions of high humidity. For hygroscopic foods which readily pick up moisture it is necessary to package them in a moisture-proof material.

Examples of moisture contents and aw values for selected foods and their packaging requirements are shown in Table 3.

Small-scale drying equipment

Solar dryers operate by raising the temperature of the air, which reduces the humidity and also causes the air to move through the dryer, increasing the rate of drying. Food is enclosed in the dryer and therefore protected from contamination by dust, insects, birds and animals. The higher drying rate also permits a higher throughput of food and hence a smaller drying area. The dryers are waterproof and therefore food does not have to be moved when it rains.