Milk processing 1,2
Raw milk is an excellent medium for the growth of microorganisms that can cause illness and so has a short shelf-life. Milk processing allows the preservation of milk for days, weeks or months and helps to reduce food-borne illness. Whole milk typically undergoes several processing steps including pasteurisation, homogenisation and further processing such as ultrafiltration.
Pasteurisation was first introduced in Australia in the late 1950s and is a legal requirement for milk produced for human consumption. All milk produced in Australia is required to undergo pasteurisation to destroy harmful microorganisms before it can be sold at retail. Pasteurisation involves heating milk for a short period of time (usually 72°C for 15 seconds).1
All animals used for milking have the potential to be carriers for pathogenic organisms capable of causing serious disease in humans, including listeriosis, typhoid fever, tuberculosis, diphtheria and brucellosis. Pathogens may be present in the farm environment, including soil, water, on pasture and in animal feeds. These pathogens can enter the milk during milking and, if such milk is consumed, can cause disease.
The most common pathogens found in association with dairy farms and milking animals include bacteria such as Escherichia coli (E. coli), Campylobacter and Salmonella, but other pathogens such as the parasite Cryptosporidium, which causes a type of gastroenteritis, may also be present.1 Campylobacter and Salmonella can cause severe diarrhoea and certain types of E. coli, particularly those known as Shiga toxin-producing E. coli (STEC), can cause very severe disease which can impair kidney function and may result in death.2
Pasteurisation is very effective at destroying almost all disease-causing microorganisms that may be present in raw milk, making the final product safer for human consumption.
These harmful bacteria have caused outbreaks and disease associated with the consumption of raw milk in many countries. Data from the United States indicates that over a 13-year period to 2011, there were 2,384 illnesses, 284 hospitalisations and two deaths associated with the consumption of raw milk.3 In Australia, raw milk contaminated by bacteria such as Campylobacter and Salmonella caused at least nine outbreaks of disease between 1997 and 2008, leading to 117 cases of illness.2
The pasteurisation process is very effective at destroying almost all disease-causing microorganisms that may be present in raw milk, making the final product safer for human consumption. It has been claimed that pasteurisation destroys some of the nutritional components of raw milk. There is no substantiated evidence to suggest there is any significant difference at all in the nutritional status of pasteurised or unpasteurised milk. There is also no credible scientific evidence to show raw milk provides additional health benefits.
Practically all milk sold at retail in Australia has undergone homogenisation, a process where the milk fat globules are physically reduced in size, so they remain suspended throughout the milk for long periods of time. Homogenisation prevents the cream from separating out and gives the milk a more uniform colour. Unhomogenised milk has a creamy layer on top where the fat globules have come together. There is no substantiated evidence to suggest there is any significant difference in the nutritional benefits of homogenised milk compared to unhomogenised milk.
Some brands of milk undergo a process of ultrafiltration to remove bacteria and further extend shelf life. Ultrafiltration is also a process used by some milk brands to filter nutrients and thereby modify the nutritional content.