Yoghurt (a word of Turkish origin meaning ‘thick’) originated in W. and C. Asia. It was something of a breakthrough for the times because it enabled milk to be kept longer, it had a refreshing tartness and it could be consumed by lactose intolerant individuals (i.e. most of W. and C. Asia). The list of yoghurt locales, types and preparations is now daunting.
Yogurt is made industrially by inoculating milk with the lactic acid-producing bacteria Streptococcus thermophilus and Lactobacillus bulgaricus. These bacteria break down milk sugar (lactose) into its simple-sugar components (glucose and galactose) that they use as an energy source, leaving lactic acid as a byproduct (i.e. fermentation). The slow, steady increase in acidity strips casein protein bundles (micelles) of their electrostatic repulsion and allows them to link up and thereby thicken the milk. It also disrupts the micelles, releasing casein proteins that also help gel the milk. The streptococci multiply first but their activity declines as acidity climbs to around 0.5% whereby the lactobacilli take over and increase acidity to ~1%.
Yoghurt is often marketed as a probiotic health product. Whether S. thermophilus or L. bulgaricus survive gastrointestinal conditions is not certain (it is known S. thermophilus suffers from exposure to gastric acids for example). However, there is evidence that they can remain somewhat viable, especially L. bulgaricus. Some manufacturers add other strains to their yoghurt to claim probiotic benefits. Examples are: L. acidophilus, L. casei, L. fermentum, L. rhamnosus and Bifidobacteria (B. bifidus and/or B. bifidum). These products are an attempt to approximate the diverse bacterial flora in naturally fermented milk. Their survival rate under gastrointestinal conditions is not clear to me. The bulk of supermarket yoghurt does not contain ‘live’ cultures at all.
S. thermophilus and L. bulgaricus are most active at 41-45C (quite high for bacteria). That’s where sous vide comes in. While yoghurt making devices exist, they are designed just for that purpose, whereas sous vide is muti-purpose and can maintain a steady temperature for many hours. Four steps:
1. Heat a quantity of milk to 82-85C and hold it there a few minutes. While this can also be done in a sous vide bath, precision is not necessary at this stage and a pot on the stove with a thermometer is more convenient (or just bring to a low simmer). This initial heating will denature (unfold) proteins (particularly the whey proteins) which contributes to the set of the yoghurt. The heat will also kill any bacteria that might have found its way into the milk.
2. Allow the milk cool to just below 43C (transfer pot to cold/iced water if you are in a hurry).
3. Meanwhile, heat the sous vide bath to 43C. If the bath has a water circulator, set it to minimum or direct it away from where the jars will be placed.
4. Add 5% store-bought ‘live’ yoghurt to the cooled milk (e.g. for 500g milk, add 25g of yoghurt), checking that the yoghurt lists S. thermophilus and L. bulgaricus. Alternatively, use a yoghurt starter powder according to manufacturers directions. Transfer to jars, seal and culture in the waterbath for ~5h. Cool and refrigerate.
Some other aspects:
In theory, each batch of yoghurt can be used to make the next batch. In my experience this can only be done a couple of times, presumably because industrial strains of bacteria are insufficiently diverse to form sustainable colonies. It might be possible to purchase a heirloom reusable starter culture online (e.g. in the US, or Australia here or perhaps eventually here). Look for a variation on Bulgarian/Greek strains. Sandor Katz is the guru.
There might be no point in using probiotic yoghurt – S. thermophilus and L. bulgaricus are particularly heat-loving (43C) whereas other cultures may not thrive under these conditions.
While the 5% ‘live‘ yoghurt need not be exact, adding more will not speed things up and the sudden increase in acidity might curdle the milk before the yoghurt can form.
It is the proteins that firm the yoghurt, and the fat gets in the way. Thus a low-fat milk will create a firmer yoghurt, but not a more tasty one. Similarly, skim milk foams better for a cappuccino, but the result tastes less interesting. The protein content of full cream milk can be increased by adding nonfat powdered milk.
If sweetening yoghurt with honey, add it after the yoghurt is made. Honey has anti-microbial properties that could slow culturing.
Although most yogurt is made from bovine milk (e.g., cow, sheep, goat, yak), it can also be made from soy milk, coconut milk and almond milk (and other nut milks).
Some specialised yoghurts are thickened by straining out any residual whey (you often see this floating on the top of a yoghurt curd). Greek yoghurt is an example. However, this also reduces yield and profit. Supermarket yoghurt marketed as Greek ‘Style’ has been thickened with gelatine and other stabilisers such as starch, rather than strained.
Frozen yoghurt is just milk to which about 25% yoghurt has been added before freezing.
If the yoghurt fails to set, try culturing a bit longer (~8h). If failure persists, it might be possible to use the acidified milk to make ricotta or as a buttermilk substitute. Maybe thicken it with some gelatine in the spirit of store-bought yoghurt.
Here’s a nice video from the chefsteps team.
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