Dairy Processing
Raw milk may contain harmful bacteria. Pasteurisation is a simple heating process that usually involves heating milk to 72 °C for 15 seconds. The milk is then cooled very rapidly to 4 °C to limit any loss of heat-sensitive nutrients. Pasteurisation is an important and well-established technique to destroy any harmful microorganisms in food and is used as a food safety measure all over the world.
Raw milk may contain harmful bacteria. Pasteurisation is a simple heating process that usually involves heating milk to 72 °C for 15 seconds. The milk is then cooled very rapidly to 4 °C to limit any loss of heat-sensitive nutrients. Pasteurisation is an important and well-established technique to destroy any harmful microorganisms in food and is used as a food safety measure all over the world.
No, not all milk is pasteurised. However, it is illegal to sell raw milk (i.e. unpasteurised) for direct use in South Africa unless the specific municipality where the sale is taking place is authorised to do so in terms of the law (R1555). A farm selling raw milk must be able to present a certificate stating the herd as being free of tuberculosis and brucellosis. Even then, raw milk should be dealt with cautiously and best be properly heated prior to use.
Appropriate heat treatment destroys harmful bacteria that may be present in food and ensures that the food is safe for consumption. There are four methods for heat treating milk:
- Pasteurisation involves heating milk to a high temperature (72 °C) for 15 seconds, followed by rapid cooling to 4 °
- Ultra-pasteurisation involves treatment at a higher temperature. Products treated in this way may stay fresh for more than 14 days if kept at 4 °C or lower.
- Ultra-high temperature treatment involves heating milk to between 135 °C and 150 °C for 2–4 seconds and then cooling it to 4 °C or lower. This treatment is used to produce long-life milk.
- Sterilisation involves heating filled bottles of milk to between 110 °C and 130 °C for approximately 10–30 minutes, after which the bottles are cooled with cold water. Unopened bottles of sterilised milk keep for a long time without having to be refrigerated. Once opened, it must be treated as fresh milk.
No, the nutrients in milk are not affected by heat. During pasteurisation, milk is heated to a fairly low temperature (72 °C) and only for a short period (2–4 seconds). This treatment destroys microorganisms, but not any nutrients. The rapid cooling limits the loss of any heat-sensitive nutrients in milk. Pasteurised milk is an excellent source of protein, calcium, potassium, vitamins A, B12 and B2, magnesium, phosphorus and zinc.
Fermented milk products are made from milk that has been treated with live bacterial cultures (usually lactic acid bacteria), yeast or mould. These products are also referred to as cultured milk products. Examples of fermented milk products include yoghurt, kefir, amasi, buttermilk, sour cream, cheese and cream cheese. During fermentation, some of the lactose in milk (milk sugar) is converted to lactic acid. This causes the milk to thicken. Fermented dairy products cause a relatively low glycaemic response, because the thickened milk and lower pH delay emptying of the stomach and so slow the release of milk sugar into the bloodstream. In addition, fermented milk products may also enhance gastric function and potentially increase the number of microbes in the gut.
Cream naturally separates from milk. This separation is due to cream (or milk fat) having a lower density than the water portion of milk, which makes it float to the surface when raw milk is left to stand. In the factory, this natural separation is hastened by a process called centrifugation, during which milk is spun in a special tank at high speed. The cream is used either in making butter, or put back in the milk, as per regulation, to prepare low-fat, full-cream or high-fat milk and other dairy products.
Raw milk is milk that has not been (e.g. pasteurised, sterilised or subjected to ultra-high temperature treatment). Drinking raw milk is generally considered to be risky as it may contain harmful bacteria, which would normally be destroyed during pasteurised milk.
Homogenisation
It is the process of reducing a substance, such as the fat globules in milk, to extremely small particles and distributing it uniformly throughout a fluid, such as milk. When milk is properly homogenised, the cream will not rise to the top. The process involves forcing the milk through small openings under high pressure, thus breaking up the fat globules
Homogenisation, is a physical process that helps to keep the cream mixed throughout the milk, so you get a more consistent glass of milk from the first sip to the last.
Homogenised milk refers to a specific mixing process where milk has been mechanically processed to spread out the milk fat/cream droplets into much smaller droplets. This process helps keep the fat/cream evenly distributed throughout the milk instead of allowing it to rise to the surface and form a layer of cream.
Homogenisation does not mean that anything has been added to the milk. It is a physical processing step that changes the size and distribution of the cream or ‘milk fat globules’.
To homogenise milk means to pass the milk through a very small opening under high pressure. This mixes the cream clumps or larger milk fat globules into many smaller ones throughout the milk.
The smaller fat globules are more evenly dispersed throughout the milk and are much less likely to rise to the top and form a cream layer.
The milk’s composition is not being replaced or changed. The cream is mixed using high pressure to keep the cream from separating from the rest of the milk.
Milk is homogenised to improve its stability and consistency
In milk that has not been homogenised the cream or ‘milk fat globules’ gradually rise to the surface. This is because milk fat is less dense than the watery part of milk. This can result in a visible thick cream layer to form on top in the neck of the bottle.
Homogenisation reduces the size of the fat globules, which greatly reduces their tendency to rise. This gives milk a more uniform appearance and texture and means that the fat is distributed more evenly throughout the container.
Importantly, homogenisation is not the same as pasteurisation. Homogenisation is a mechanical process that changes the physical distribution of milk fat, whereas pasteurisation is a heat treatment used for food safety
The main difference is how the milk fat is distributed.
In homogenised milk, the cream or ‘milk fat globules’ have been reduced in size so that they remain more evenly dispersed throughout the milk. This gives the milk a consistent appearance and reduces the formation of a cream layer.
In unhomogenised milk, the naturally larger milk fat globules can rise to the surface over time, allowing a layer of cream to form.
Both are milk.
Homogenisation only changes the physical characteristics of large milk fat globules and the stability of the milk emulsion. It does not remove nutrients from milk. The nutritional composition of milk depends on fat content of the milk.
Yes. Homogenisation itself is not a food-safety treatment. It is a mechanical processing step used mainly to improve the physical stability and consistency of milk. Milk safety depends on appropriate hygiene and, for commercially processed milk, the applicable heat-treatment and food-safety controls.