How do you make yoghurt at home? The science of lactic acid bacteria

A winter evening. A pan of milk has been heated on the stove and taken off the heat just as it started to rise up the sides. After a while, Grandma dips her little finger into the milk and silently counts to ten. Then she puts two spoonfuls of yoghurt into a bowl, loosens them with a little of the warm milk and stirs the mixture into the pan. The pan is wrapped in a thick blanket and left next to the radiator. In the morning, when the blanket comes off, the pan holds a sweet-smelling yoghurt so thick that a spoon would stand up in it.

This scene has played out in homes all over Anatolia for generations. Yet few people know the science behind it. Those two spoonfuls of yoghurt are a starter culture carrying billions of live bacteria, and that night a small but very crowded microbiology experiment took place in the pan.

The team inside a spoonful of yoghurt

Two species of bacteria make yoghurt what it is: Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus for short) and Streptococcus thermophilus. Both the Turkish Food Codex and the international Codex Alimentarius standard define yoghurt as the product of fermenting milk with these two bacteria. L. bulgaricus was first identified in 1905 by the Bulgarian scientist Stamen Grigorov.

The two bacteria work as a team. S. thermophilus gets going faster in milk, makes it slightly acidic and produces small molecules that help its partner. L. bulgaricus, in turn, breaks down milk proteins and releases the amino acids that S. thermophilus needs. Together they ferment milk faster than either would on its own. Acetaldehyde, the compound behind yoghurt’s familiar aroma, also comes largely from L. bulgaricus.

From lactose to lactic acid, from milk to gel

Both bacteria feed on lactose, the sugar in milk, and turn it into lactic acid. This acid is what gives yoghurt its tang. But the really big job of lactic acid is to make the milk set.

Most of the protein in milk is casein. Casein is suspended in milk in tiny clusters called micelles. Fresh milk has a pH of about 6.6–6.7. At this pH the surfaces of the micelles carry a negative charge, so they push one another away. That is why milk flows.

As the bacteria produce lactic acid, the pH falls. As it approaches about 4.6, the casein micelles lose most of their charge, stop repelling one another and start to stick together. The clumped casein builds a three-dimensional network that traps the water and milk fat inside it: the milk turns into a gel. In other words, the yoghurt “sets”.

pH is a logarithmic scale: a drop of one unit means the concentration of hydrogen ions has risen tenfold. Going from 6.6 to 4.6 looks like a fall of only two units, but it means the hydrogen ion concentration has gone up about a hundredfold.

Heat, cool, culture: why each step matters

Why heat to 85–90 °C first? Heating the milk to this temperature and holding it there for a few minutes does two important jobs. First, it kills most of the unwanted microbes in the milk, so the yoghurt bacteria can take over without competition. Second, the whey proteins in the milk unfold in the heat and cling to the casein micelles. When these proteins join the gel network too, the yoghurt comes out thicker and weeps less liquid. This is what lies behind the old kitchen wisdom that “boiled milk makes better yoghurt”.

Why cool to 42–45 °C next? Yoghurt bacteria love warmth, and this is the range in which they work best. Milk hotter than about 50 °C harms them, and the yoghurt may not set. In much cooler milk, on the other hand, fermentation slows down and the milk stays lukewarm for hours, which can play into the hands of unwanted microbes. Grandma’s little-finger test tries to hit this range, but it varies from one person to the next. A kitchen thermometer is more reliable, and it is also safer than dipping your finger into hot milk.

Why wrap it up and wait? The bacteria need the milk to stay warm for several hours to do their work. The wrapping stops the temperature from dropping too quickly. It is also important not to jolt the pan during this time: if the gel network that is forming gets shaken, it breaks and the yoghurt turns watery.

Why straight into the fridge? The bacteria don’t stop once the yoghurt has set; they keep on making acid. Cold slows them down: the yoghurt doesn’t become too sour, its texture firms up and the risk of spoiling goes down.

Homemade yoghurt, step by step

1. Clean the pan, spoon and jars very thoroughly.
2. Heat 1 litre of pasteurised milk to about 85–90 °C (185–194 °F) and keep it at that temperature for a few minutes. If you are using raw milk, never skip this step.
3. Leave the milk to cool to about 42–45 °C (about 108–113 °F).
4. In a bowl, loosen one or two tablespoons of fresh, live yoghurt with a little of the warm milk, add it to the pan and stir gently.
5. Put the lid on, wrap the pan in a thick blanket or towel and leave it somewhere warm where it won’t be disturbed. How long it takes depends on the temperature and the starter; in most homes it is 4–8 hours.
6. As soon as the yoghurt has set, put it in the fridge.

Safety: hot milk can boil over and can cause serious burns. Children should make yoghurt with an adult, and the adult should be the one who takes the pan off the heat.

Hygiene: helping the right bacteria win

Making yoghurt means deliberately growing a culture of bacteria. Warm milk is a welcoming home not only for yoghurt bacteria but for other microbes too. So:

• Use pasteurised or thoroughly heated milk.
• For your starter, choose a yoghurt that you know is fresh and that looks and smells normal.
• Don’t leave the culturing milk out on the worktop for longer than necessary; once the yoghurt has set, chill it.
• If you see mould, pink, orange or black spots, bubbling or fizzing, or notice an unusual smell, throw the yoghurt away. Don’t taste it to “check”.
• Yoghurt is not safe for people with a milk allergy either.

Why won’t yoghurt set, and why does it go runny?

• The milk was too hot when the starter went in: most of the bacteria were harmed.
• The milk was too cool, or cooled down too fast: fermentation went very slowly.
• The starter was weak: a very old starter, or one kept going at home from batch to batch many times, can lose its strength over time.
• The pan was jolted: the gel that was forming broke and the liquid separated out.
• It was left too long: yoghurt that becomes too acidic shrinks and squeezes out liquid.

The yellowish liquid that collects on top of yoghurt, on the other hand, is usually nothing to worry about. It is whey that has separated from the casein network. If the yoghurt smells normal, you can stir it back in and eat it.

A small kitchen experiment: two jars

If you would like to see for yourself how heating affects thickness, you can try this experiment with an adult. Use fresh pasteurised milk. Long-life (UHT) milk has already been heated to a very high temperature at the factory, so it may not show the difference.

Heat half a litre of milk to about 85–90 °C, let it cool to around 43 °C and pour it into a clean jar. Warm another half litre of the same milk only to around 43 °C and pour it into a second jar. Stir a spoonful of the same yoghurt into each, wrap both in the same blanket, leave them for the same length of time, then put them in the fridge. The next day, compare the two yoghurts. In which one does a spoon stand up better? Which one has more liquid on top? Because the whey proteins in the heated milk have joined the gel network, you can expect the first jar to come out thicker. Before tasting, smell both yoghurts and look at them carefully; if anything seems odd, don’t eat them.

The journey of a word

Even the name is Turkish. From Turkish yoğurt the word has travelled into many languages, from English yoghurt (yogurt in American spelling) to French yaourt. Linguists usually trace it to the same root as yoğun, “dense, thick”, and yoğurmak, “to knead”. So the name of this kitchen experiment is spoken in Turkish in many of the world’s languages.

Summary

• Yoghurt is milk fermented by the bacteria L. bulgaricus and S. thermophilus. The bacteria turn lactose into lactic acid; when the pH falls to about 4.6, the casein clumps together and forms a gel that holds the water.
• The milk is first heated to about 85–90 °C (fewer microbes, thicker texture), then cooled to 42–45 °C and cultured, left undisturbed, and put in the fridge as soon as it has set.
• Clean containers, pasteurised or thoroughly heated milk and a careful check of smell and appearance are the safety rules for homemade yoghurt.