THE idea that we can improve our gut health by eating foods containing live 鈥渇riendly鈥� bacteria, or probiotics, dates back to the early 20th century. Ilya Mechnikov, a Russian biologist whose work on immunity led to a Nobel prize, postulated that consuming soured milk fostered beneficial bacteria in the intestines. He claimed that people in Bulgaria who ate yogurt lived longer as a result, and his ideas helped to popularise yogurt in western Europe and North America.
The main types of bacteria found in commercial yogurt are Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus thermophilus. Several studies have found that these bacteria can . It is difficult to isolate the health effects of individual foods, but studies have found that eating yogurt is linked to lower rates of , and , and also to .
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In the yogurt-making process, these bacteria metabolise the sugars in milk and produce lactic acid, which causes the milk to curdle. Casein, which makes up 80 per cent of the protein in cow鈥檚 milk, is usually bundled together in small particles called micelles. The proteins on the surface of the micelles have a negative charge, making them repel each other. But when the lactic acid causes the pH to drop below 5.5, the micelles start to fall apart, and some of the proteins lose their negative charge. This allows the casein proteins to join together to form a gel 鈥� aka yogurt.
To make your own yogurt at home, you will need some shop-bought yogurt to provide the bacteria for the fermentation. Make sure you choose one that contains live bacteria.
Heat up the milk in a saucepan to between 82 and 88掳C and keep it in that range for about 15 minutes. This kills off unwanted microbes and makes the whey protein beta-lactoglobulin lose its shape. Deformed beta-lactoglobulins help the caseins to bond more loosely, giving a smoother gel.
Then let the milk cool to a temperature of around 45掳C, which is ideal for the yogurt bacteria. Mix a little of the warm milk with the yogurt, then mix this in with the rest of the milk.
Next, you will need to keep the culture warm 鈥� between 30 and 45掳C 鈥� for several hours. Use a sous-vide circulator or a slow cooker if you have them. Or you can put the milk into insulated flasks, a clay Indian yogurt pot or glass jars wrapped in towels. Put the containers in a switched-off oven with the light on, or in a beer cooler surrounded by 45掳C water.
It could set in as little as 2 hours or may take as long as 18 hours, depending on the temperature. Once it sets, put the yogurt in the fridge to slow down the bacteria so they don鈥檛 over-acidify it. It should keep for up to two weeks.
What you need
Food thermometer
1 litre milk
1 tbsp yogurt
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