2.2 Types of deterioration
The section above indicated the types of food that can tee processed on a small scale. The reasons for their selection are mostly concerned with the demand from customers (the popularity of a food) and the feasibility of small-scale production. If it is decided that there is a good demand for a food and that production is feasible, it is then necessary to make sure that processing adequately preserves the food for its expected shelf-life. An understanding of the various factors that cause food to deteriorate is helpful to ensure that the correct processing and packaging is selected. In this section the causes of deterioration of foods are first described and then the different roles of processing and packaging in preservation of the food are described.
All fresh foods have micro-organisms on their surfaces, often in enormous numbers, and these can grow rapidly to spoil foods. A main purpose of processing foods is to destroy unwanted micro-organisms. Packaging prevents recontamination and together processing and packaging prevent micro-organisms from spoiling the product during distribution and storage (that is to give the food the shelf-life that is expected).
Micro-organisms may be divided into general groups such as bacteria, yeasts or moulds, each of which may be further divided into sub-groups. Of the many microorganisms, the main types of interest are those that cause food spoilage and those that cause food poisoning.
When food is processed correctly, the number of spoilage micro-organisms and their activity is reduced and controlled at known levels. It is when there is a failure in processing conditions or packaging materials that spoilage micro-organisms can have an effect on the food. They can result in the food going mouldy, developing an off-smell or fermenting. The different types of spoilage depend to a large extent on the nature of the food and in particular its acidity and its moisture content.
Food poisoning is mostly caused by bacteria. These are also controlled by the acidity and moisture content of a food as shown in Table 2-3.
Food poisoning micro-organisms may grow in low-acid foods. Heat processing to 121°C for 15-50 minutes in jars or cans is required to destroy these micro-organisms.
Such heating requires the use of pressure cooking and detailed technical knowledge. For these reasons it is strongly advised that these foods are not produced by small-scale processors, especially as a first venture.
Table 2-3: Microbial growth on different foods
Enzymes are naturally occurring proteins that act on foods to cause changes in flavour, colour or texture. There are many hundreds of different enzymes but some of the more important spoilage changes include softening of fruits, rancidity of oils, browning of cut fruit or root crops and loss of green colour in vegetables. In general enzymes are either destroyed by heating or prevented from acting by changing the acidity or water content of the food.
Enzymes and micro-organisms can only spoil foods if water is present. If the water is removed or made unavailable they cannot act. Different foods have different water contents (Table 2.4). Some, for example grains, are relatively dry when harvested and these can be easily preserved by removing the remaining water by drying. Other fresh foods such as fruits, vegetables and meat have a much higher water content and this should be made unavailable by either drying or concentration (water removed) or by freezing (water held as ice, which together with the low temperature, prevents micro-organisms and enzymes from acting).
However, removing water only prevents the action of enzymes and micro-organisms, it does not destroy them They can act again when water returns, for example during re-hydration of dried food, thawing of frozen food. Foods are therefore often heated (blanched) to destroy some of the enzymes and micro-organisms before drying or freezing.
Table 2-4: Water content of some foods
The normal moisture content of a processed food should be maintained during storage. This is a main function of packaging for some foods. If the moisture content falls below an acceptable level in moist foods, the food dries out, shrivels and is seen as spoiled by consumers. Similarly if the moisture content of a dry food is allowed to rise above an acceptable level it will first lose its crispness and become unacceptable to consumers, but it may also gain sufficient moisture to allow micro-organisms to grow and cause further spoilage. The acceptable range of moisture contents is different for each processed food and for some it is a critical factor. Table 2.5 shows some foods that require careful control of the moisture content by processing and packaging to maintain their quality.
Table 2-5: Foods that require careful control of the moisture content
Higher temperatures increase the rate of spoilage by micro-organisms and enzymes, up to a maximum above which they are destroyed (Figure 2-2).
When foods are heated above 60°C most enzymes and micro-organisms are destroyed - the higher the temperature the faster they are destroyed. This is one of the easiest methods of preservation and includes boiling, frying, baking and pasteurization. However, cooked food can be easily re-contaminated if it is not properly packaged.
Heat can also spoil packaged food. It can melt fats, cause a loss of texture or flavour and cause more rapid changes such as development of rancidity in oils and movement of moisture within a food which in turn leads to spoilage.
Sunlight contains ultra-violet (UV) rays which cause rapid deterioration of some foods (especially oils and fatty foods) or specific components of a food such as certain vitamins. UV can also cause packaging materials to fade or to become brittle and lose their properties (for example polythene in Section 3.2.2). In general all foods should be stored in the shade away from direct sunlight. Electric lights do not have the same effects on foods because they do not contain the UV component.
Some foods, especially those that have a high fat content or those with delicate flavours and aromas, are susceptible to oxidation (attack by air which causes off flavours to develop, known as rancidity). Biscuits, cooking oils, dried fish and other fatly foods are liable to spoilage by rancidity. These foods should therefore be protected from contact with air by storing them in airtight containers.
2.2. 7 Contamination
Foods are often contaminated when harvested or slaughtered and most processing involves a cleaning stage to remove these contaminants. However foods may also be contaminated after processing and this is more serious because the contamination will not be removed before the customer buys the food. The main types of post-processing contamination and possible sources are shown in Table 2-6.
Table 2-6: Types of post-processing contamination and possible sources
One of the main functions of packaging is to protect foods against contamination from the time it has been processed until it is consumed. It should be noted that the contamination described before is accidential contamination. Contamination that is done deliberately to increase profitability or from malice is named adulteration. Packaging that is intended to prevent or reveal adulteration is described briefly in Chapter 3.
2.2.8 Mechanical damage
Foods are frequently fragile and easily crushed, scratched, split or otherwise damaged during storage and distribution. Damage is caused in four ways:
- by pressure such as that caused by stacking or piling foods too high,
Another main function of packaging is to minimise these types of physical damage to foods during transport and storage. This type of packaging is often known as a shipping container and these are described in more detail in Chapter 4. A summary of the different roles of processing and packaging to prevent deterioration described above is shown in Table 2-7. The factors that affect the shelf-life of foods and the protection offered by different packaging materials are shown in Table 2-8.
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