As with all food products, health and safety are paramount. For reasons of safeguarding public health, the dairy industry treats milk with heat, destroying pathogenic bacteria, which can cause illness in humans. The process has its origins in the 19th century, when milk was a common transmitter of tuberculosis and typhus. French scientist Louis Pasteur (1822 – 1895) discovered that heat treatment kills pathogenic bacteria, as well as bacteria causing spoilage. Since the mid 20th century it has been common practice to pasteurize milk in order to safeguard it for human consumption. Most countries employ legislation regulating the heat treatment of milk and dairy products to ensure that the milk products are safe from any pathogenic bacteria.
All heat treatment extends the shelf life of foods and beverages. Several methods of heat treatment are used to destroy microorganisms like bacteria, spores, yeast, mould and viruses. These methods are mainly distinguished by differences in time and temperature, which are the most crucial factors during heat treatment. Besides killing microbes, heating inactivates enzymes present in milk that can have a negative impact on taste and appearance during storage.
Shelf life refers to the length of time food products are stable and viable for consumption. There are four main stability concerns that influence the limits of a food product’s shelf life: microbiology and the growth of microorganisms; enzymatic reactions and changes; chemical degradation reactions like oxidation of food components; and finally, physical phenomena like separation and sedimentation occurring within the product.
The microbiological shelf life of milk is first determined by the number of microorganisms already present in the raw milk. Second, the efficiency in reducing microorganisms by the chosen preservation technique – in this case, heat – determines how many microbes will survive and be present after packaging. Finally, the storage and distribution temperature will govern how and if any surviving microbes grow and multiply.
If the cow the milk comes from is healthy, her milk is virtually bacteria free. Yet as soon as the milking process begins, it is vulnerable to contamination by milk-spoiling microorganisms in the milking area. Since milk is a conducive environment for microbial growth, it is important to keep the milking area and equipment very clean to maintain good raw-milk quality. It is also crucial to cool the raw milk as soon as possible and keep it cold before heat treatment can take place at the dairy. European standards for the bacterial content of raw milk are less than 100,000 colony-forming units (CFU) per ml. It is possible to process milk with higher bacteria counts, but the result will be milk of a poorer quality.
To facilitate large modern dairies and centralized, extended distribution networks that require more time, milk products with long shelf lives have become prevalent. Chilled distributed milk with an extended shelf life (ESL) offers retailers and consumers greater convenience and minimized waste during distribution and in the home, thanks to the fact that these products remain safe and retain sensory appeal for a longer time.
Milk that is stored and shipped at ambient temperatures provides an even more cost-efficient method of distribution. It also enables distribution to remote locations and parts of the world where cold distribution is not possible or a cold chain has not been developed.
To be able to distribute milk at ambient temperatures there is a need for the milk to be commercially sterile, meaning the absence of microorganisms capable of growing in under prevailing conditions. The most challenging microbial lifeforms to destroy during sterilization are spores, due to their thick, protective outer membranes that increase their resistance to heat, drying, freezing, chemical agents and radiation.
When determining the right kind of heat treatment for a specific food product such as milk, considerations include requirements for shelf-life, and conditions during storage and distribution. The achieved shelf life of processed milk depends on both the time and temperature of heat treatment. There are three principal categories of heating methods: pasteurization, extended shelf life (ESL) treatment and ultra-high temperature (UHT) treatment. Tetra Pak promotes continuous processing with quick heating and cooling, and a precisely calculated holding time, resulting in a much lower heat load applied to the milk compared with traditional batch methods.
Due to the potential impact of heat on taste, smell, colour and nutritional value, it is advantageous to process as little as possible, while achieving adequate food safety and desired shelf life. Though heat treatment can degrade nutrients in some products, this is not a significant issue when heat treating milk during continuous processing.
There are two basic principles for heating – indirect and direct. Indirect heating uses a partition to separate the product from the heating (or cooling) medium, with the partition transferring the heat from the medium to the product, avoiding any direct contact between the two. Indirect heating is done with different kinds of heat exchangers, where the choice depends on different factors, such as the physical properties of the product, flow rate, run time and cleaning requirements.
Direct heating puts the product into direct contact with hot steam under strictly controlled conditions, rapidly reaching the target temperature. After holding for a certain time at the target temperature, the temperature is also lowered rapidly by flash cooling in a vacuum vessel. This rapid heating and cooling minimizes the heat-load, resulting in a high-quality product.
Milk is commonly pasteurized in plate heat exchangers at 72-75°C for 15 seconds. This temperature is sufficient to destroy all pathogenic bacteria, while significantly reducing the number of organisms that can cause spoilage. Performing an alkaline phosphatase test with a negative result will confirm that the milk has been adequately pasteurized so that it is safe for human consumption. The shelf life of pasteurized milk differs depending on the quality of the raw milk and distribution temperature, with an average of 7-10 days in Europe.
High-temperature ESL treatment is performed to destroy all microorganisms that cause spoilage and can multiply during refrigeration. High-temperature treatment (HTT) encompasses heating milk at 120-135°C for 0.5-4 seconds. A common standard of ESL treatment is heating at 127°C for 2 seconds. In Europe, the average shelf life of milk treated at high temperatures is over 30 days.
While heat load is proportional to microbiological reduction, the same is true for sensory changes, such as taste and colour. ESL treatment can be achieved by both direct and indirect heating. Besides heat treatment, it is possible to produce ESL milk with microfiltration or bactofugation in combination with pasteurization.
The purpose of UHT (ultra-high temperature) treatment is to destroy all microorganisms that can grow and multiply under storage or shipping at ambient temperatures and destroy the product. UHT treatment sterilizes milk by treating it at a very high temperature for a short time. This process destroys all microorganisms, including spores, which are the most heat resistant and – if not eradicated – can germinate in packaged milk and spoil it.
UHT treatment normally consists of heating the product to 137-142°C for 2-4 seconds, with a typical example being 137°C for 4 seconds. After cooling, the product is packaged aseptically in an aseptic package, which achieves commercial sterility, meaning it can be stored at ambient temperatures for several months. Both direct and indirect heating (see above) are used in UHT treatment, depending on desired quality and production budget. If operating costs are the priority, indirect heating with plate or tubular heat exchangers is used. Due to the very low heat load resulting from direct heating via steam injection or infusion, this method is applied to ensure minimal sensory changes and the highest product quality.