Thermal Processing of Milk: Traditional to Modern Practice

Milk is everyone’s favourite. It is the only food for infants. It is highly nutritious and is needed by children for their growth both physical and mental. It is recommended for young and old and also the patients who are recovering. Milk proteins are popular among body builders and sports persons.

Milk is Nutritious

Milk has proteins needed for the growth of muscles and various other tissues and calcium for bones. It has carbs and fat that provide all the energy needed for various activities along with the vitamins to convert food into energy needed. Early lactation milk is called colostrum, which is contains many proteins that provide immunity to the child. For first six months, the infant is recommended only the breast milk and later other foods including solids and other dairy milk may be given.

Milk from cow or buffalo, is a very healthy and nutritious and is consumed even by older children, adults, convalescing people and seniors as part of healthy diet. It is a rich source of high-quality proteins, calcium, potassium, selenium, vitamins D, B2 and B12. It also provides vitamins A and B1 and minerals magnesium and zinc.

The breast-fed milk directly goes from the source to the child so there is very little possibility of it getting contaminated or spoiled. However, dairy milk from animals is usually collected and transported over large distances and may be stored for long before consumed, so there are chances of it getting contaminated by microbes that can cause spoilage and disease at times.

Why Heat Treatment?

The dairy farm environment has many bacteria present and when cows are milked these bacteria get into the milk. If the cow is diseased or there is contamination during handling or storage or milk, it may also contain pathogens that can cause all kinds of diseases in human if consumed without heat treatment.

In cooperative dairy systems that are common in India, small farmers collect milk from animals and is brought to collection centre, which processes the milk and packs before sending to consumers. This milk may get contaminated by various microorganisms, so in order to avoid spoilage or milk and to prevent any diseases the pasteurisation and other processes are carried out to keep milk safe and last longer.

Milk is a highly perishable food, with plenty of water, near neutral pH (neither acidic nor alkaline), good source of various nutrients, that makes it not just very nutritious to humans and animals but also highly growth promoting for various bacteria. So, if it is not protected then it very easily spoils. Fresh milk sours within a few hours without any treatment so various heat treatments are used to increase its shelf life and make it safe for consumption.

Heat not only can destroy the bacteria, it can also degrade the essential nutrients that are sensitive to heat, especially the vitamins and some proteins. At home, we used to boil the fresh raw milk to destroy the bacteria. However, this treatment is too harsh if attempted on a large scale. So, there are various pasteurisation processes developed that destroy the microbes but cause much less damage to nutrients.

Different Types of Heat Processing

Older process was Low Temperature Holding, where temperature was raised to 62.5℃ and held for 30 min and then cooled. Later it was found that is heated at about 71 to 73℃ for 15 sec and cooled. With such a short treatment, it was possible to heat milk continuously using heat exchangers which rapidly heat the milk to desired temperature and after holding for a short time cooling rapidly in another heat exchanger to excessive heating is avoided. Even more recent method of heating, Ultra-High Temperature (UHT) heating raises the temperature to about 130℃ and held for a second or two before cooling. These pasteurisation processes do not destroy all the bacteria but ensure destruction of all pathogens, the disease-causing bacteria along with a large number of other bacteria. This makes the milk safer to drink. If we need to sterilise the milk, we need to use either higher temperatures or hold the milk for longer time. The ones which we get in Tetra Pak packages are heated at UHT for between 130 and 150℃ for several seconds and sealed in sterile environment in absence of air, so no microbes contaminate milk and surviving ones cannot grow. This pack is called commercially sterile and can last for several months as the microbes surviving cannot grow in the conditions prevailing in the package.

Traditional Treatment

Such processes were not developed when traditional processing was done. So, people used to use different techniques besides boiling to prevent spoilage and making the milk last longer.

Microbes like a lot of water and near neutral pH. When milk was concentrated and also the available water was bound by substances like sugar microbes would find it difficult to grow. Thus, substances like khoa and various sweets were prepared by concentrating to remove water and tying the water by sugar. Various sweets like pedha, and burfi will last for some time.

Even allowing the bacteria to grow to some extent would produce lactic acid to make the conditions acidic. Since major milk protein, casein is sensitive to acid, when pH reaches 4.6 it precipitates. Thus, curd or dahi is formed. Under these acidic conditions bacteria are much slower to grow thus extending the shelf life. It can be further increased by draining out some water adding sugar to tie more of water. This is how shrikhand is made which has shelf life even longer. There is another advantage of making dahi namely, the bacteria used for this not just produce acid but produce many essential nutrients such as B vitamins.

Other Ways

Now, we have many other ingredients available such as condensed milk and milk powder that are prepared to prevent wastage especially when the milk production is much more than the demand. Such a situation exists during winter. Even some countries produce much more milk than their market, so they either pack in long lasting aseptic packs or convert to condensed or powder and export.

During summer, the milk production goes down so to extend the supplies, the powder could be reconstituted to milk as toned milk.

Thus, heating or thermal processing is a very useful tool to make milk, a very essential and nutritive food safe as well as keep it longer by avoiding its spoilage.

References

1. 5 Ways that Drinking Milk Can Improve Your Health, Kubala 2023, Healthline (https://www.healthline.com/nutrition/milk-benefits)
2. 12 Powerful Health Benefits of Milk, Florida Milk Blog, Florida Dairy Farmers (https://www.floridamilk.com/in-the-news/blog/nutrition/12-powerful-health-benefits-of-milk.stml)
3. Milk: the Nutrition Source, 2021, Harvard TH Chan School of Public Health (https://www.hsph.harvard.edu/nutritionsource/milk/)
4. Thermal Processing of Milk, Mandal et al. 2019, Thermal Processing of Milk, Recent Technologies in Dairy Sciences (2019): 39-66 (https://www.researchgate.net/publication/334326868_Thermal_Processing_of_Milk)
5. Dairy Ingredients for Food Processing, Chandan & Kilara, Wiley-Blackwell Publ. 2011.