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Pasteurisation derives its name from the French scientist Louis Pasteur who, found that heating of certain liquids to a high temperature improved their keeping quality. In general terms, it is the heating of milk to a temperature which destroys organisms responsible for tuberculosis and fever and nearly all the other micro-organisms present in that product without seriously affecting the composition or properties of the product. Pasteurisation should be followed by immediate cooling of the product to the temperature sufficiently low to check the growth of micro-organisms which are resistant to the temperature used. At present, pasteurisation is considered as an essential feature in the manufacture of butter, ice-cream and also in cheese industries. Pasteurisation also inactivates some of the natural enzymes like lipase.
Three general methods are in use now days.
- Holding or Batch System: The holding system consists in bringing the milk or cream to a temperature usually 65°C and holding at that point for at least 30 minutes followed by rapid cooling. A higher temperature is sometimes used in which case the time of holding may be shortened. For exampIe, holding at 68.3°C for 20 minutes . Careful control of both temperature and time of heating is important.
In more recent years it is found that the organism causing Q fever, coxiella bumetti, is slightly more resistant than the tuberculosis organism and required a treatment of 63°C for 30 minutes to ensure its destruction.
- High Temperature Short Time Method (HTST) or the Continuous System: The machines are so constructed for continuous operation and for this reason the system is called as "continuous flow or flash pasteuriser." The system depends upon the raising of the temperature of the milk to at least 72oC for 15 seconds as it passes through the machine. This is followed by cooling. This method does not impart cooked flavour and cream line is not affected.
- Ultra High Temperature System: This UHTS system results in a complete pasteurisation of milk. In this system, milk is held for 3 seconds at 93.4°C or for one second at 149.5°C. This system is also used extensively for the treatment of milk or cream in dairy industries. This product has a longer shelf life than milk pasteurised by other methods. After pasteurisation, the milk is cooled rapidly to 70C or lower.
Pasteurised milk is not sterile. It must be quickly cooled following pasteurisation to prevent multiplication of surviving bacteria.
- Role of Phosphatase in Pasteurisation: Raw milk contains alkaline phosphatase. This enzyme serves as a built-in indicator by which the adequacy of pasteurisation may be gauged. This enzyme has heat destruction characteristic that closely approximates the time, temperature, exposures of proper pasteurisation. Therefore, if alkaline phosphatase activity beyond a certain level is found in pasteurised milk it indicates inadequate processing. This enzyme has the ability to liberate phenol from phenol phosphoric acid compound. The liberated phenol gives a deep blue colour with certain organic compounds. If the milk is incubated it does not give blue colour with the indicator it means the pasteurisation is complete.
So sensitive is this test that it gives positive in the presence of 0.1 per cent raw milk added or if the pasteurisation temperature is less by 1 oF.
- Effect of Pasteurisation on nutritive value
- Nutritive value: Due to pasteurisation, there is no important change in nutritive value, except for the slight decrease in the heat-labile vitamins like thiamine and ascorbic acid. Whey proteins are denatured only slightly and minerals are not appreciably precipitated. Pasteurised milk produces finer curd when milk is digested.
- Flavour: Pasteurisation does not produce objectionable cooked flavour. The flavouring compounds present in milk are in no way destroyed by pasteurisation.
- Micro-organisms: Pasteurised products are not sterile. They contain vegetative organisms and spores still capable of growth. Pathogens like mycobacterium tuberculosis, salmonella typhi, corynebacterium diphtheria and brucella are destroyed but non-pathogens remain in the milk even after the pasteurisation is complete. Approximately 99 per cent of bacteria, yeast and moulds are destroyed by pasteurisation. The milk does not contain heat resistance moulds and yeast.
- Enzymes: Apart from phosphatase, lipase also gets inactivated which would have affected adversely the quality.
After pasteurisation, tests for temperature, fat, specific gravity, solids-non-fat, acidity and bacterial analysis are conducted. Pasteurised milk can be stored for a week or more in good condition. At room temperature it may spoilt in a day.
Schematic diagram shows the steps involved in pasteurising milk at Madhavaram Milk Dairy, Chennai.
Figure : Processing of milk at Madhavaram Milk Dairy, Chennai.
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