Milk Fermentation: The Science Behind It

We know from history books that the nomadic tribes of the east used to keep milk in bags made from animal intestines. Owing to the presence of microorganisms and the effect of the sun as they walked throughout the day, it was often possible to filter off a cream from the milk which, in the course of time, the nomads learned to use and appreciate. This was the first form of yogurt.

We are accustomed to considering milk a sublime but perishable food that needs to be kept in the fridge, but what happens when we 'bombard' it with beneficial microorganisms? A process of fermentation is triggered.

Milk fermentation: what is it?

In the case of milk, when good healthy bacteria, such as Lactobacillus delbrueckii bulgaricus and Streptococcus salivarius thermophilus, are added and a particular temperature is reached, they start to nibble at the tasty molecules of lactose, which is a sugar, transforming them into pyruvic acid.

In the absence of oxygen, the latter eventually turns into a lactic acid. Now, seeing as there are also many proteins in milk, these react with the recently formed acid by breaking up into many pieces, only to recompose themselves in a different form.

Fermentation around the world: a menagerie of species and traditions

Cow milk has become so dominant in most markets that it’s usually just referred to as milk. But it hasn’t always been so, and still isn’t so in some places. For example, in India and other parts of South or Southeast Asia, buffalo milk is at least as common as its bovine cousin. Its fermented derivatives include dadiah, made by pouring the heated milk into a bamboo segment and sealing it with a leaf. Meanwhile, people across Central Asia sip chal, a fermented drink made from camel’s milk. Elsewhere people derive fermented dairy products from mammals as diverse as goats, sheep, yaks, reindeer, llamas, and donkeys.

Fermented milk products

As milk ferments, it transforms from a liquid into the dense solid consistency we are all so familiar with. What many people do not know, however, is that yogurt is not the only end product of this process: in actual fact, milk fermentation gives us a variety of tasty ingredients.

If we take all types into consideration, there are about a hundred that may be grouped into various categories: yogurt, buttermilk, crème fraîche, sour cream, ropy milk, koumiss and kefir milk.

Buttermilk is the beverage obtained when the fatty part of the cream has been skimmed off to produce butter. What remains is made to ferment and is served as a refreshing drink. Try our recipe of homemade buttermilk biscuits.

Crème fraîche, on the other hand, is obtained by fermenting cream with a fat content of at least 30% for 15-20 hours. It is excellent served with caviar, fruit and cakes.

Sour cream is its 'light' version with a 20% fat content, and therefore more suitable for use in cooking. Not surprisingly, it is used in soups and stews, especially in central-eastern Europe. Find out different ways to cook with sour cream.

Ropy milk, a typical product of Scandinavian countries, is a type of cream fermented with special bacteria which produce a semi-liquid and sticky cream ideal for eating with fruit.

Now we come to koumiss and kefir. The former is particularly widespread in Russia and is a fizzy, slightly alcoholic drink resulting from the fermentation of mare’s milk. Kefir is similar, but obtained by inoculating the milk with a special yeast bacterial starter called kefir 'grains'.

How to make homemade yogurt

So, learning how to produce home-made yogurt is only the gateway to a whole world of possible ingredients for making our recipes even more delicious.

To start, you need one litre of fresh full-fat milk and a mixture of bacteria you can buy at the chemist’s or in well stocked food stores. Alternatively, two spoonfuls of full-fat yogurt will do the trick, since it already contains bacteria.

Bring the milk to boiling point and when the cream starts to form, skim it off (you may use it for crème fraîche…). Cover the pan with its lid and immerse it in a basin filled with cold water until the temperature falls to between 38 and 40 °C. Put the spoonfuls of yogurt or the bacterial yeast mixture into a bowl and add one cup of the lukewarm milk you have just prepared, stirring thoroughly.

When you have eliminated all the lumps in the mixture, add the rest of the milk and place the bowl, covered with a cloth or blanket, in a warm place, ideally around 40 °C. You may heat the oven to 50 °C and then let it cool for a minute or two. Leave the mixture alone for at least eight hours.

Finally, remove the bowl, check that the milk has thickened and then pop it into the fridge. Your yogurt is ready for tasting.

More from the food science series

To satiate your craving for more nerdy food knowledge, check out the science of cooking vegetables, or read the science of express risotto to find out just how they make it so fast at restaurants. You can also read about another product that, like fermented milk derivatives, is preserved without cooking in the science of raw ham.