Within the food and pharmaceutical industries, heat treatment is used to kill bacteria and extend the shelf life of products. Throughout the heat treatment process, control of the product temperature as well as an understanding of the many variables within the process is of vital importance.
The primary purpose of heat treatment is to kill any harmful bacteria and thereby extend the shelf life of the end product in question. It is important to have a thorough knowledge of the correct temperature for each single type of heat treatment, as the process gradually changes the natural composition of the product. Heat treatment can take place both directly (through infusion or injection ) and indirectly (with a cross flow heat transfer such as a plate heat exchanger or a tubular heat exchanger . Depending on the product in question and the desired effect of the treatment, the following heat treatment methods are available:
Regenerative Effect
When working with heat treatment, it is very important to have an understanding - not just of all the technical aspects - but also of the environmental and business related aspects of the process. For many types of heat treatment, the product in question must first be heated and subsequently cooled. Pasteurisation of milk is an example of this, where chilled milk is heated to pasteurisation temperature and subsequently cooled to the original inlet temperature. By using the heat of the pasteurised milk to heat the incoming cold milk – which in turn then cools the outgoing hot milk - a more economical process can be achieved, thereby reducing energy consumption and consequently benefiting the environment. This process is also referred to as the regenerative effect.
Unit Running Time
The running time of a heat treatment unit depends on a number of factors:
- Product type
- Viscosity
- Raw material quality
- Temperature
- pH
- Protein and/or mineral content
- Regenerative effect
- Total solids content
- Type of selected heat treatment
- ΔT of product/heating medium
- Velocity
Holding Time
The individual holding times throughout the heat treatment process can vary from 0.5 to 600 seconds depending on the desired level of bacteria reduction or elimination and functional properties of the product. Holding cells and holding tanks can be installed either in the pipe bridge or in a separate tank depending on the process requirements.
Product Safety
To ensure that no contamination of the pasteurised product takes place, certain precautions must be taken in the design of the heat treatment equipment. This includes temperature surveillance by means of two or three temperature transmitters with short response time. One transmitter is to be installed for controlling the pasteurisation temperature, and another transmitter for controlling the flow diversion valve. Optionally, a third transmitter can be installed for controlling the functionality and calibration of the first two transmitters. The flow diversion valve is also part of the product safety program and can be either single or double seated. To increase safety conditions, two valves can be installed. The individual handling of the product after flow diversion is to be decided by the manufacturer.
Pressure difference over the heat exchanger means that the heat treatment equipment is designed to have a positive pressure on the pasteurised side of the product. This pressure can be enabled by installing a booster pump downstream of the equipment as well as a constant pressure valve on the outlet. Measuring of the pressure can be performed by means of a pressure transmitter or an indicator (manometer) for automatic or manual pressure surveillance. It is possible to measure on all sections of the heat exchanger - both medium and product sections.
Heating and Cooling
Heating can take place by means of steam, high-pressure hot water or electricity. Ice water, city water, cooling tower water, vacuum or glycol can be used to achieve a cooling effect. Both heating and cooling can take place either directly or indirectly, however, direct cooling can only be performed by means of vacuum which is consequently always combined with heating by means of steam as this is the only direct heating option.
Temperature Program
The purpose of heat transfer is to heat or cool the product in question from one temperature (inlet temperature) to another (outlet temperature) in each heat transfer section. The temperature program of a heat treatment process is determined by the end product as well as by any other types of treatment involved, e.g. filtration, homogenisation, separation etc. As an example, the pasteurisation temperature of milk depends on the desired shelf life of the end product in due consideration of the changes to the natural composition of the milk which takes place as a consequence of the heat treatment.
Our Competencies
At GEA Liquid Processing, we are – in cooperation with our sister companies in the GEA Group - able to supply all types of heat treatment and cooling options. This also includes skid-mounted heat treatment units with integrated process control system which can be installed together with existing processing equipment or as stand-alone plug and play units.