Milk is something many people take for granted as a cornerstone of their diet, although there are some people who cannot enjoy it due to intolerances, allergies or dietary choices (e.g. veganism). But generally, we consider it a safe and enjoyable source of protein, vitamins and minerals including calcium; which we can use in drinks and as part of cooking. What many probably are not aware of is that our milk today is far safer than it was 200 years ago, and that is all down to pasteurisation.
As discussed elsewhere, milk readily spoils and is an ideal environment for bacteria to grow, both the ‘beneficial’ ones that can help turn it into yoghurt as well as the pathogenic ones which can make us ill! It is perhaps also easy to forget that milk, like many foods, is not dead but is an ecosystem of bacteria, immunoglobulins and enzymes with the last two having evolved to help support the health and growth of the young animal it was originally destined to nourish. These factors, especially the bacteria, if given the opportunity can lead to the milk rapidly starting to spoil.
The dairy industry, has developed a number of ways to keep our milk from spoiling. Heating the milk can reduce the number of bacteria, refrigeration can slow their growth or removing water in the form of freeze-drying the milk to make milk powders. There are also filtering methods which use very fine filters to sieve out bacteria, increasing shelf life to a month or more. Finally there is also LP Ockham that helps extend the freshness, and protect nutrients.
When did we start pasteurising milk?
Many people know that pasteurisation is named after the French scientist Louis Pasteur. But before he could invent this process in the mid 19th century, he first discovered germ theory which linked bacteria to food spoilage and disease. Fortunately, it did not take long for pasteurisation of milk to become common practice, spreading across Europe in towards the end of the 19th century and in the early 20th century in the U.S.A with the first law being past in Chicago in 1908.
However, pasteurisation was not universally practised until later, with some farmers resisting it as it added extra costs to milk production. However, as food safety standards improved through the middle of the 20th century pasteurisation became commonplace, making milk safer for consumers. The use and benefits of pasteurisation are now so widely accepted that in a number of countries (including Scotland and Australia) it is now unlawful to sell milk that is unpasteurised for human consumption. In England, Wales and Northern Ireland it is lawful to sell unpasteurised or raw milk directly from the farmer to consumers e.g. at the farm gate and it needs to be produced under very strict hygiene standards.
Today most milk is pasteurised using the High-Temperature Short-Time method, where the milk is heated to 71.7oC for 15 seconds (no longer than 25 seconds) before rapidly cooling to 3oC. This can then be bottled and sold refrigerated. As well as pasteurisation, milk is also typically homogenised, meaning the fat in the milk is broken into tiny droplets, so the traditional layer of cream which used to be seen on the doorstep is a thing of the past, as the fat stays dispersed in the milk. The other common method is Ultra Heat Treatment (UHT), this uses much higher temperatures, at around 135oC for 2-3 seconds to ‘commercially sterilise’ the milk giving it a shelf life of 6 months or more. One problem of higher temperatures is that they can start to change the sugars and protein in the milk giving it a slightly cooked taste which some consumers do not like. Also depending on packaging, UHT pack is also susceptible to damaging light penetration if inadequate and not certified for light protection.
Which milk is healthiest?
Pasteurisation of milk has undoubtedly made it safer for many, meaning that we can buy fresh milk and keep it in our fridges at home for around a week. The temperatures used are selected to kill potential pathogenic bacteria without changing the taste and properties of the milk. It is also designed not to change the nutrition qualities of the milk, which, with proper storage and packaging, can also be retained for the shelf-life of the milk.
All views and opinions expressed belong exclusively to Dr Duane Mellor. Funding was provided to Dr Mellor for this time to write this post.