Food preservation is the treatment of food to prevent or delay spoilage and inhibit growth of pathogenic organisms otherwise render the food unfit for consumption or other purpose. Food preservation involves;
- The use of low temperature.
- The use of high temperature.
- The use of dehydration (moisture control technique).
- The use of chemicals
- Use of controlled atmosphere and the restriction of oxygen
- Use of physical methods including smoking and irradiation.
Food preservation by use of low temperature.
This form of preservation is based on the fact that all metabolic reactions of microorganisms are enzyme catalyzed.
The speed of enzyme reaction depends on temperature. And the colder it is, the slower the reaction.
Temperature applied may be
- Above freezing (refrigeration)
- At freezing
- Below freezing (freezer)
Temperature above freezing involves the commercial and domestic use of refrigerators, usually operating at 1-4°C and are suited to storing most perishable foods over a relatively short time.
Most common pathogenic organisms seize to multiply below 5°C, although increasing concern is being expressed about Yersinia enterocolitica, Aeromonas hydrophila and Listeria monocytogenes, all of which are capable of growth under refrigeration.
Temperature below freezing is when the effectiveness of foods for the purpose of preservation depends on it’s ability to inhibit enzyme reaction and reduce available moisture. Water activity (aw) is the amount of moisture present in a food material, capable of supporting the growth of microorganisms.
The freezing of food destroys some bacteria, including pathogens and a gradual reduction of survivors occurs during storage. Spores and toxins are practically unaffected by freezing or frozen storage.
Food preservation by use of high temperature
Heat energy is used to destroy both spoilage and pathogenic organisms and so preserves food. However, heat resistant bacteria, some toxins and spores may survive heat treatment.
The discussion of MCOs (microorganisms) depends on their D value which is the time required at a specific temperature to destroy 90% of the organisms present. Therefore if the D value of an organism is 1min. at 75°C and there are 10^6 organisms present, then after 1min, there will be 10^5 organisms present, after 2min, there will be 10^4 organisms etc. This log reduction of bacteria by heating clearly demonstrates the need to minimize the number of pathogenic bacteria in raw food.
Pasteurization
It involves the heating of food at a relatively low temperature for a short time. For example, milk may be heated at 72°C for 15sec. The time and temperature combination chosen depends on the particular type of food and must be sufficient to destroy vegetative pathogens and a considerable proportion of spoilage organism.
Toxins and spores generally survive pasteurization because most of them are heat resistant and are produced under unfavorable conditions. And to avoid the growth of heat resistant organisms and vegetative bacteria that may be present because of spore germination during heating, refrigerated storage is often essential.
Organisms preferring to multiply at temperatures above 45°C are known as thermophiles. And those capable of withstanding heat treatment or not necessarily multiplying at higher temperatures are known as thermoduric.
The main advantage of pasteurization is that food is rendered safe with the minimum effect on flavour and nutritional value.
Foods that are pasteurized include milk, ice cream, eggs, wine, canned fruits and large cans of ham.
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Sterilisation
This involves the destruction of all microorganisms and as this is sometimes difficult to achieve. Heat treatment adequate to destroy all viable organisms may be utilised. In this case, food may be considered commercially sterile. This means that any organism remaining after treatment will be of no significance under normal methods of storage.
Temperature required for sterilisation normally exceeds 100°C and are usually achieved by means of steam under pressure. There are several factors affecting the heat resistance of organisms;
1. Humidity
The lower the moisture, the more resistant the organism i.e dry heat is not as lethal as steam at the same temperature.
2. Fat, sugar and protein
The presence of these substances usually increase heat resistance.
3. pH
Neutral pH is preferred, acid and alkaline conditions increases heat sensitivity. For this reason, acid foods such as fruits, require a much lower temperature to render them commercially sterile.
4. Chemicals
The presence of certain chemicals such as nitrites may decrease heat resistance.
