Milk is a highly perishable food so to enable it to be stored and distributed for consumption without spoilage, and without being a health risk through growth of pathogenic bacteria, it is heat treated. The most common type of heat treatment in many parts of the world is pasteurisation, which is performed at a minimum of 72°C for 15 seconds. This is the least heat treatment needed to destroy most pathogenic microorganisms and it also destroys most spoilage organisms. However, a small number of bacteria remain after pasteurisation and packaging, and can grow during storage. Such growth is slow at low temperature and consequently pasteurised milk is always kept refrigerated. Even under refrigeration, pasteurised milk only keeps for about two weeks.
There are two ways in which this can be carried out: in-container sterilisation and ultrahigh- temperature (UHT) processing. Both produce a ‘commercially sterile’ product which means the milk does not contain microorganisms which can grow under the normal conditions of storage which, in this case, is room temperature. In-container sterilisation, which uses canning technology, is a batch operation which involves heating the final containers of milk in an autoclave at 110-120°C for 10-20 minutes. By contrast, UHT processing involves heating the milk in a continuous flow system at about 140°C for a very short time – around five seconds. While the two procedures have the same microbiological effect they have very different chemical effects on the milk constituents.
Microbiological Aspects
The main microbiological aim of UHT processing is to inactivate spore-forming bacteria which could grow during storage and cause spoilage. The main targets are Bacillus species, particularly heat-resistant ones such as B. licheniformis and B.subtilus. Geobacillus stearothermophilus is an extremely heat-resistant sporeformer found in milk but because it only grows at temperatures above about 50°C, it does not cause problems in UHT milk unless the milk is severely temperature abused during storage. The thermal conditions used for UHT processing are designed to give a 9-log reduction in heat-resistant sporeformers. In practice, most UHT plants exceed this requirement by a reasonable margin.
The UHT Process
The major steps in a UHT process are as follows:
Change in Milk during UHT Processing
It is inevitable that heating a product such as milk at temperatures up to ~140°C will have some effect on its constituents, in addition to the intended bactericidal effects. Furthermore, storage at room temperature for long periods of time (up to 12 months) causes additional effects.
Modern UHT technology minimises the production of this flavour but most consumers can still detect it and it is one reason why many consumers prefer pasteurised milk. The typical flavour of UHT milk is due a combination of flavours, the chief of which are sulphurous flavours caused by volatile sulphur compounds released from the whey protein, and the proteins in membrane surrounding the milk fat globule.
The initial step in the Maillard reaction is the reaction between lactose and lysine in milk proteins, chiefly whey proteins. In fact, the extent of this reaction is an indication of the intensity of heat given to the milk. In practice, it is measured as furosine, a product formed when the lactose-containing protein is subjected to acid hydrolysis. Another indicator of the heat treatment is lactulose, an isomer of lactose. The instability of the whey proteins to heat has another consequence during UHT processing. Some whey protein denatures and attaches to the surfaces of the heat exchangers in proteinaceous deposits which obstruct the flow of milk and can eventually cause the plant to be closed down for cleaning. However, this is not the only type of deposit formed during UHT processing. The proteins, in fact, have been shown to be more digestible in UHT milk as a result of the heat treatment. UHT treatment may also reduce the allergenicity of the milk proteins.
Change in UHT Milk during Storage
The flavour changes through progress of the Maillard reaction and through oxidation by dissolved oxygen in the milk. The major flavour compounds produced are methyl ketones and aliphatic aldehydes but a large number of flavour compounds are generated. Other flavours which may develop during storage are due to the action of heat-resistant bacterial enzymes which may be present in the raw milk and survive the UHT heat treatment. These include lipases, which break down the fat and form free fatty acids, some of which have strong flavours, and proteases which break down proteins to produce peptides, some of which are bitter.
Another change which can be brought about by proteases is what is known as ‘age gelation’ where the milk thickens over storage and eventually turns into a gel akin to a yogurt. This undesirable defect can be caused by the heat-resistant bacterial enzymes but it can be also caused by plasmin, a naturally occurring protease in milk, which is quite heat stable and can remain active in UHT milk.
There are two ways in which this can be carried out: in-container sterilisation and ultrahigh- temperature (UHT) processing. Both produce a ‘commercially sterile’ product which means the milk does not contain microorganisms which can grow under the normal conditions of storage which, in this case, is room temperature. In-container sterilisation, which uses canning technology, is a batch operation which involves heating the final containers of milk in an autoclave at 110-120°C for 10-20 minutes. By contrast, UHT processing involves heating the milk in a continuous flow system at about 140°C for a very short time – around five seconds. While the two procedures have the same microbiological effect they have very different chemical effects on the milk constituents.
Microbiological Aspects
The main microbiological aim of UHT processing is to inactivate spore-forming bacteria which could grow during storage and cause spoilage. The main targets are Bacillus species, particularly heat-resistant ones such as B. licheniformis and B.subtilus. Geobacillus stearothermophilus is an extremely heat-resistant sporeformer found in milk but because it only grows at temperatures above about 50°C, it does not cause problems in UHT milk unless the milk is severely temperature abused during storage. The thermal conditions used for UHT processing are designed to give a 9-log reduction in heat-resistant sporeformers. In practice, most UHT plants exceed this requirement by a reasonable margin.
The UHT Process
The major steps in a UHT process are as follows:
- Preheating, with or without a holding time
- Homogenisation (for indirect systems)
- Heating to sterilisation temperature
- Holding at sterilisation temperature
- Initial cooling
- Homogenisation (alternative position for direct or indirect systems)
- Final cooling
- Aseptic packaging
Change in Milk during UHT Processing
It is inevitable that heating a product such as milk at temperatures up to ~140°C will have some effect on its constituents, in addition to the intended bactericidal effects. Furthermore, storage at room temperature for long periods of time (up to 12 months) causes additional effects.
Modern UHT technology minimises the production of this flavour but most consumers can still detect it and it is one reason why many consumers prefer pasteurised milk. The typical flavour of UHT milk is due a combination of flavours, the chief of which are sulphurous flavours caused by volatile sulphur compounds released from the whey protein, and the proteins in membrane surrounding the milk fat globule.
The initial step in the Maillard reaction is the reaction between lactose and lysine in milk proteins, chiefly whey proteins. In fact, the extent of this reaction is an indication of the intensity of heat given to the milk. In practice, it is measured as furosine, a product formed when the lactose-containing protein is subjected to acid hydrolysis. Another indicator of the heat treatment is lactulose, an isomer of lactose. The instability of the whey proteins to heat has another consequence during UHT processing. Some whey protein denatures and attaches to the surfaces of the heat exchangers in proteinaceous deposits which obstruct the flow of milk and can eventually cause the plant to be closed down for cleaning. However, this is not the only type of deposit formed during UHT processing. The proteins, in fact, have been shown to be more digestible in UHT milk as a result of the heat treatment. UHT treatment may also reduce the allergenicity of the milk proteins.
Change in UHT Milk during Storage
The flavour changes through progress of the Maillard reaction and through oxidation by dissolved oxygen in the milk. The major flavour compounds produced are methyl ketones and aliphatic aldehydes but a large number of flavour compounds are generated. Other flavours which may develop during storage are due to the action of heat-resistant bacterial enzymes which may be present in the raw milk and survive the UHT heat treatment. These include lipases, which break down the fat and form free fatty acids, some of which have strong flavours, and proteases which break down proteins to produce peptides, some of which are bitter.
Another change which can be brought about by proteases is what is known as ‘age gelation’ where the milk thickens over storage and eventually turns into a gel akin to a yogurt. This undesirable defect can be caused by the heat-resistant bacterial enzymes but it can be also caused by plasmin, a naturally occurring protease in milk, which is quite heat stable and can remain active in UHT milk.
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