Pasteurisation is widely used within the dairy industry to ensure milk is safe to drink while also help to prolong its shelf life. Here, Tetra Pak highlights different methods of pasteurisation for food and liquid dairy applications.
Pasteurisation is a heat treatment which results in a particular level of bacteria removal, and the processing plant is monitored to ensure no unpasteurised product goes into the market. Various countries have different legal requirements on how to ensure that no unpasteurised products are distributed.
In powder production, there are heat treatments used to denature the protein in the front of the evaporator, this is known as the “pre-heat”. This provides different types of functionality due to protein denaturation. Generally, this is measure by a whey protein nitrogen index (WPNI), which is dependent on the raw milk level and heat treatment.
Ultra-pasteurisation can be utilised when a particular shelf life is required. For some manufacturers, two extra days might be sufficient, whereas others aim for a further 30-40 days in addition to the 2-16 days that are traditionally associated with pasteurised products. The fundamental principle is to reduce the main causes of reinfection of the product during processing and packaging in order to extend the shelf life of the product. This requires extremely high levels of production hygiene, and a distribution temperature of no more than 7oC. The lower the temperature, the longer the shelf life.
Heating milk to 125-138oC for 2-4 seconds and cooling it to no more than 7oC is the basis of extended shelf life (ESL), which is a term for heat-treated products that have been given improved keeping qualities. Nevertheless, ESL products must still be kept refrigerated during distribution and in retail stores.
On pre-heating, the desired processing temperatures are reached directly after pasteurisation, but sometimes it is necessary to cool and store the milk temporarily before the final processing is done. For instance:
- Cheese milk is pre-heated to 30-35oC prior to the vat, where a final temperature adjustment is made before the rennet is added. Hot water is used as the heating medium. Warm whey from a previous batch can also be utilised for a first pre-heating step in order to reduce heating costs.
- Yoghurt milk is pre-heated to 40-45oC prior to the fermentation tank, where the addition of culture takes place. Hot water is used as the heating medium.
- Milk can also be pre-heated before addition of other ingredients, such as chocolate powder, sugar or fats, in the manufacture of different milk-based food products.
All heat transfer in dairies takes place in the form of convection and conduction. Indirect heat transfer is the most commonly used method in dairies. In this method, a partition is placed between the product and the heating or cooling medium. Heat is then transferred from the medium through the partition into the product.
Assuming that the heating medium is hot water flowing on one side of the partition and cold milk on the other, the partition is consequently heated on the heating-medium side and cooled on the other product side. In a plate heat exchanger, the plate is the partition.
There is a boundary layer on each side of the partition. The velocity of the liquids is slowed down by friction to almost zero at the boundary layer in contact with the partition. The layer immediately outside the boundary layer is only slowed down by the liquid in the boundary layer, and therefore has a low velocity. The velocity increases progressively, and is highest at the centre of the channel. Similarly, the temperature of the hot water is highest in the middle of the channel. The closer the water is to the partition, the more it is cooled by the cold milk on the other side. Heat is transferred, by convection and conduction, to the boundary layer. Transfer from the boundary layer through the wall to the boundary layer on the other side is almost entirely by conduction, while further transfer to the milk in the central zone of the channel is accomplished by both conduction and convection.
This article is provided by Tetra Pak.