The basis of microbiological laboratory methods and of the conservation of food and feed products is the killing of any micro-organisms present. Heat treatment of products is one of the main techniques in the food industry for food conservation. Heat treatment stops bacterial and enzyme activity; thus preventing a loss of quality and keeping food non-perishable. In heat treatment processes, various time/temperature combinations can be applied, depending on the product properties and shelf-life requirements.
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.
Field of application
Pasteurisation and sterilisation are used to treat all types of food products. These include milk, juices, beer and many others.
Techniques, methods and equipment
Pasteurisation temperatures commonly range from 62 to 90°C, and pasteurisation times vary from seconds to minutes. We can distinguish:
- 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
Batch wise pasteurisation is carried out in (agitated) vessels. Sometimes the product (i.e. beer, fruit juices) is pasteurised after bottling or canning, here the products in their containers are immersed in hot water or fed through a steam tunnel.
For continuous pasteurisation, flow-through heat exchangers (tubular, plate and frame) are applied, with heating, holding and cooling sections.
In sterilisation with moist heat, temperatures generally range from 110 to 120°C with sterilisation times being from 20 - 40 minutes. For example, canned foods are sterilised in an autoclave at about 121˚C for 20 min. Higher temperatures and shorter times may have similar effects (e.g., 134˚C for 3 min.). However, if conditions do not allow the germination of spores, lower temperatures and shorter times can also be applied. For example, with acid fruit juices, jam, or desserts, heating to 80 – 100˚C for 10 min is normally sufficient.
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.
c) UHT treatment
UHT treatment means a very short heat treatment at temperature of approximately 140°C (135 -150˚C) for only a few seconds. This results in a sterilised product with minimal heat damage to the product properties. UHT treatment is only possible in flow-through equipment. The product is thus sterilised before it is transferred to pre-sterilised containers in a sterile atmosphere. This requires aseptic processing. For UHT treatment, indirect heating in plate and frame or tubular heat exchangers is applied. However, direct steam injection or steam infusion may also be applied.