While pasteurization conditions effectively eliminate potential pathogenic microorganisms, it is not sufficient to inactivate the thermoresistant spores in milk. The term sterilization refers to the complete elimination of all microorganisms. The food industry uses the more realistic term "commercial sterilization"; a product is not necessarily free of all microorganisms, but those that survive the sterilization process are unlikely to grow during storage and cause product spoilage. Dairy treatment processes can be devided into 3 main types of processes:
Ultra High Temperature treatment
Dairy products can be made commercially sterile by subjecting them to temperatures in excess of 100° C, and packaging them in air-tight containers. The dairy may be packaged either before or after sterilization. The basis of UHT, or ultra-high temperature, is the sterilization of food before packaging, then filling into pre-sterilized containers in a sterile atmosphere. Dairy that is processed in this way using temperatures exceeding 135° C, permits a decrease in the necessary holding time (to 2-5 s) enabling a continuous flow operation.
Advantages of UHT
High quality: The reduction in process time due to higher temperature (UHTST) and the minimal come-up and cool-down time leads to a higher quality product.
Long shelf life: Greater than 6 months, without refrigeration, can be expected.
Packaging size: Processing conditions are independent of container size, thus allowing for the filling of large containers for food-service or sale to food manufacturers.
Cheaper packaging: Both cost of package and storage and transportation costs; laminated packaging allows for use of extensive graphics
Difficulties with UHT
Sterility: Complexity of equipment and plant are needed to maintain sterile atmosphere between processing and packaging (packaging materials, pipework, tanks, pumps); higher skilled operators; sterility must be maintained through aseptic packaging
Particle Size: With larger particulates there is a danger of overcooking of surfaces and need to transport material - both limits particle size
Cost: UHT is a significantly more capital intensive process to operate compared to pasteurization.
Keeping Quality: Heat stable lipases or proteases can lead to flavour deterioration, age gelation of the milk over time - nothing lasts forever! There is also a more pronounced cooked flavour to UHT milk.
There are two principal methods of UHT treatment:
- Direct Heating
- Indirect Heating
Direct heating systems
The product is heated by direct contact with steam of potable or culinary quality. The main advantage of direct heating is that the product is held at the elevated temperature for a shorter period of time. For a heat-sensitive product such as dairy, this means less damage.
There are two methods of direct heating (please see schematic diagrams of the equipment;
Injection: High pressure steam is injected into pre-heated liquid by a steam injector leading to a rapid rise in temperature. After holding, the product is flash-cooled in a vacuum to remove water equivalent to amount of condensed steam used. This method allows fast heating and cooling, and volatile removal, but is only suitable for some products. It is energy intensive and because the product comes in contact with hot equipment, there is potential for flavour damage.
Infusion: The liquid product stream is pumped through a distributing nozzle into a chamber of high pressure steam. This system is characterized by a large steam volume and a small product volume, distributed in a large surface area of product. Product temperature is accurately controlled via pressure. Additional holding time may be accomplished through the use of plate or tubular heat exchangers, followed by flash cooling in vacuum chamber. This method has several advantages:
- instantaneous heating and rapid cooling
- no localized overheating or burn-on
- suitable for low and higher viscosity products
Indirect heating systems
The heating medium and product are not in direct contact, but separated by equipment contact surfaces. Several types of heat exchangers are applicable:
- scraped surface
Plate Heat Exchangers: Similar to that used in HTST but operating pressures are limited by gaskets. Liquid velocities are low which could lead to uneven heating and burn-on. This method is economical in floor space, easily inspected, and allows for potential regeneration.
Tubular Heat Exchangers: There are several types:
- shell and tube
- shell and coil
- double tube
- triple tube
All of these tubular heat exchangers have fewer seals involved than with plates. This allows for higher pressures, thus higher flow rates and higher temperatures. The heating is more uniform but difficult to inspect.
Scraped Surface Heat Exchangers: The product flows through a jacketed tube, which contains the heating medium, and is scraped from the sides with a rotating knife. This method is suitable for viscous products and particulates (< 1 cm) such as fruit sauces, and can be adjusted for different products by changing configuration of rotor. There is a problem with larger particulates; the long process time for particulates would mean long holding sections which are impractical. This may lead to damaged solids and overprocessing of sauce.
Source: Professor Douglas Goff, Dairy Science and Technology Education, University of Guelph, Canada, www.foodsci.uoguelph.ca/dairyedu/home.html.