Heat treatment is used to destroy enzymes, and spoilage and pathogenic organisms and so preserves food. However, heat-resistant bacteria, some toxins and spores may survive. Further more, unless very high temperature/time combinations are used, some normal bacteria will survive. The number depends on the initial loading, strain of organism, the acidity and the presence of protective substances such as proteins and fats.
To prevent re contamination of food after processing, suitable packing is used, for example, bottles or cans, which can also prevent the multiplication of surviving bacteria. Alternatively, or additionally, products are kept refrigerated.
Pasteurisation normally involves heating food at a relatively low temperature for a short time. The time and temperature combination chosen will depend on the particular food type and must be sufficient to destroy vegetative pathogens and most spoilage organisms, while keeping changes to the taste and texture of the food to a minimum. A slight reduction in vitamins and nutritional value will occur. The process is commonly applied to milk, which may be heated to 72°C for 15 seconds and immediately cooled to below 10°C. Other foods which are often pasteurised include milk, ice cream, eggs, wines, canned fruit and large cans of ham.
Spores, toxins and some spoilage organisms such as lacto bacillus, will survive the pasteurisation process and pasteurised products spoil more quickly than most other forms of heat treated products. Pasteurised foods need to be stored under refrigeration.
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Sterilisation involves the destruction of all micro-organisms. This is sometimes difficult to achieve, so processes are often designed only to destroy viable organisms, i.e. not spores. In this case food is considered ‘commercially sterile’. This means that organisms surviving treatment will be of no significant under normal methods of storage. Low-acid canned food is given such a treatment. Sterilisation temperatures normally exceed 100°C and are usually achieved by means of steam under pressure.
The main advantage of sterilisation is prolonged shelf life. The main objections are lowering of nutritional value, including loss of vitamins, and a marked difference in texture and flavour.
Ultra heat treatment (UHT)
The ultra heat treatment of milk is a technique used to extend its shelf life with out changes caused by sterilisation. Nutritional value is similar to that of pasteurised milk. Milk is heated to a temperature of not less than 135°C for one second before filling aseptically into sterile containers. This reduces the amount of caramelisation and also enhances keeping quality.
Cooking is a form of preservation but is essentially used to make food more palatable and safe for immediate consumption. Temperatures achieved during cooking are usually sufficient to ensure an effective reduction, or the elimination, of vegetative pathogens, although some preformed toxins and spores may be unaffected. In some cases, cooking activates spores and a significant multiplication of vegetative bacteria may occur during subsequent cooling. Internal temperatures of at least 75°C should be achieved to ensure bacteriological safety, although heating food to a lower temperature for longer periods of time may be equally as effective.
Cooked products, such as vegetables, will deteriorate more rapdily than raw products, and other methods of preservation must be used to prolong shelf life if stored, for example refrigeration, freezing or canning.
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Unlike most other forms of preservation the food inside the can remains an ideal medium for bacterial growth. It is therefore imperative that:
The most heat-resistant pathogens organism is Clostidium botulinum and this bacteria will not grow below pH of 4.5. Consequence, when determining the hear process, regard must be had for the pH of the can contents. All foods with a pH of less than 4.5 are known as acid foods and those with pH of more than 4.5 are termed low-acid foods.
Most fruits have a pH of less than 4.5 and consequently only receive a relatively low pasteurising heat process. Vegetables and meats have pHs much higher than 4.5 and they are given a process known as a ‘botulinum cook’ to render them commercially sterile.
Before canned food is heat processed it is normally prepared in some ways. The actual process of preparation will vary depending on the food type.
The minimum safe thermal process for a low-acid canned food is one which would reduce the chance of survival of one spore of Clostridium botulinum to less than one in 10. This is achieved by ensuring the core of the food reaches a minimum of 121°C for three minutes, or an equivalent time and temperature combination. After the heating process the cans must be cooled with chlorinated water. The greatest care must be taken after heat treatment, especially whilst the can is still warm and wet, and before the sealing compound has hardened. Bacteria are capable of being sucked into visually satisfactory cans through microscopic holes in the seams. If these bacteria are spoilage or pathogenic organisms, problems will occur. Warm, wet cans must not be handled.
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Pasteurised canned foods
As discussed earlier, most low-acid canned foods are processed to a minimum standard of the ‘botulinum cook’. However, some foods if fully processed would be inedible. One example of this is canned cured ham, This product only receives a pasteurisation process (a centre temperature of about 70°C). This means that chilled storage is essential to ensure its safety.
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