Yogurt

Ingredients

• 1 l milk
• 100 ml yogurt

Method

Heat the milk to 82°C, then cool it back to 45°C. The initial heat kills off competing bacteria and denatures whey proteins, which makes the final set firmer and less prone to weeping. Use a thermometer — guessing by touch will cost you. Once cooled, whisk the yogurt starter (the 100 ml) into the milk thoroughly; any clumps of starter will create pockets of different fermentation rates and an uneven curd.

Pour the inoculated milk into a preheated thermos. Rinse the thermos with boiling water beforehand and leave it empty for a moment so the walls are hot but not steaming — this stabilises the fermentation-temperature at the critical 40–45°C window where bacterial-fermentation proceeds fastest. The thermos acts as an insulator; temperature swing of more than a few degrees will slow or stall the cultures, and you'll end up with thin, sour yogurt instead of thick and mild.

Leave undisturbed for 8–12 hours. The exact time depends on ambient warmth and your starter's vigour. After 8 hours, tilt the thermos gently — if the yogurt coats the side with a slight wobble rather than sloshing freely, it's set. If it still pours like milk, leave it longer. The cultures are converting lactose into lactic acid, which lowers the pH and causes the casein proteins to coagulate; you're looking for a clean break when you insert a spoon, not a soupy consistency.

Chill immediately once set. Cold stops the fermentation dead and prevents the yogurt from becoming over-sour. Once cold, it will firm up a touch more as the gel tightens. Decant into a jar with a tight-fitting lid. Yogurt keeps for two weeks refrigerated; after that, the dairy-based cultures exhaust their food supply and the flavour turns sharply acidic and unpleasant.