A spoonful of culture: Swetha Sivakumar on curd, yoghurt, kefir and skyr

Fermented milk not only tastes good and is good for one’s health, it also helps preserve the highly perishable milk. Cultures around the world, including in India, North Africa, parts of Europe and Central Asia, figured this out more than a thousand years ago.

The Indian dahi, morru, lassi etc; the Central Asian kefir; the European creme fraiche; the Icelandic skyr have been around much longer than the standard industrial “yoghurt” popularised first in the US in the 1970s.

All these, done right, make for a very healthy snack or meal accompaniment. Here’s why. Milk is rich in a form of sugar called lactose. A special group of bacteria called lactic acid bacteria are capable of breaking down the lactose, metabolising it as energy, and releasing lactic acid as a byproduct. As the lactic acid accumulates in the milk, the pH drops from about 6 to a more acidic range of about 4. The lactic acid is what gives yoghurt a sour taste. The acidic environment, meanwhile, makes it hostile to other microbes that might have spoiled the milk.

Although all milk-fermenting bacteria typically fall under the umbrella of lactic acid bacteria, there are different species and strains around the world. It is the type of bacteria (along with type of milk, fermentation times and temperatures) that determines whether milk will turn into, for instance, dahi or kefir or skyr, all of which differ slightly in taste and texture.

In order to qualify as yoghurt, fermentation must involve two specific strains: Lactobacillus bulgaricus and Streptococcus thermophilus.

The Indian dahi, the kind we culture at home with a spoon of curd from a previous batch or by borrowing from a neighbour, has many wild strains in it, including Lactobacillus acidophilus, Bifidobacterium lactis, Streptococcus thermophilus and Lactobacillus bulgaricus.

Kefir, which is milk fermented using a culture of bacteria and yeast, contains an even more complex microbial composition of close to 60 strains of lactic acid bacteria, acetic bacteria, yeasts and fungi. Exploring a wide variety of fermented milks from different parts of the world exposes the gut microbiome to many strains of good bacteria, so it’s a good idea to not restrict oneself to just one or two kinds.

When it comes to store-bought yoghurt, all varieties, so long as they have not been tinkered with post-fermentation (with preservatives or sterilisation), can be said to contain “Live / Active Cultures”. The really desirable tag is “probiotic”. This indicates that each gm of yoghurt contains very high amounts of very specific strains of live bacteria. (Watch the video posted alongside to learn more.)

It’s important to know that neither yoghurt nor dahi should contain any preservatives. When properly refrigerated, store-bought yoghurts have a shelf life of about 12 days, without preservatives. Strained yoghurts in particular will often list stabilisers. These are richer preparations such as shrikhand, skyr and Greek yoghurt, where the whey must be discarded. It typically takes up to four times as much milk to make them. One way to use a lot less milk and still get a creamy mouth-feel is to add stabilisers such as pectin, corn starch, etc.

Avoid sweetened yoghurts too. Yoghurt is so tart that it can take up to 5 tsp of sugar per cup to make it taste sweet. At which point it is no longer a protein-rich snack; it’s crossed over into dessert.

The Indian tradition of whisking yoghurt with water, salt, fresh herbs and spices to make a masala chaas or mint lassi is a far healthier way of getting one’s dose of live cultures.