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Pasteurisation, sterilisation, UHT
|Field of application|
|Techniques, methods and equipment|
|c) UHT treatment|
Pasteurisation is a controlled heating process used to eliminate any dangerous pathogens that may be present in milk, fruit-based beverages, some meat products, and other foods which are commonly subjected to this treatment.
A similar controlled heating process, referred to as blanching, is used in the processing of fruits and vegetables; its main purpose being deactivate the many enzymes present in the plant materials belonging to this food category. Both pasteurisation and blanching are based on the use of the minimum heat requirement needed to deactivate specific micro-organisms or enzymes, thus minimising any quality changes in the foods themselves. [87, Ullmann, 2001] In pasteurisation, generally a heating temperature below 100°C is applied.
Sterilisation is the removal of living micro-organisms, and can be achieved by moist heat, dry heat, filtration, irradiation, or by chemical methods. Compared to pasteurisation, a heat treatment of over 100°C is applied for a period long enough to lead to a stable product shelf-life.
UHT (Ultra-high temperature sterilisation) has a heat treatment of over 100°C during very short times; it is especially applicable to low viscous liquid products.
- batch wise pasteurisation: 62 – 65°C, up to 30 minutes
- high temperature short time (HTST) pasteurisation: 72 – 75°C, 15 - 240 seconds
- high heat short time pasteurisation (HHST): 85 – 90°C, 1 - 25 seconds
For continuous pasteurisation, flow-through heat exchangers (tubular, plate and frame) are applied, with heating, holding and cooling sections.
For killing bacterial endospores by dry heat, longer exposure times (e.g. up to 2 hours) and higher temperatures (e.g. 160 – 180˚C) are required than with moist heat.
Solutions containing thermolabile compounds can be sterilised by filtration through mediums such as nitrocellulose membranes, kieselguhr, porcelain, asbestos. UV irradiation is used to keep rooms partially sterile. Bacteria and their spores are killed quickly, but fungal spores are only moderately sensitive to radiation. Ionising radiation (X ray, gamma radiation) is used to sterilise food and other compact materials. Chemical means may also be applied. Ethylene oxide is used to sterilize food, plastics, glassware, and other equipment. [87, Ullmann, 2001]
Generally for sterilisation, the product is canned or bottled and then heat-treated in a steriliser with steam or hot (superheated) water. Sterilisers may be batch or continuous by operated.