Greek Yoghurt

Ingredients

• 1 l milk
• 100 ml yoghurt

Method

Heat the milk to 82°C, then cool it to 45°C. This pasteurisation step — brief and below true boiling — eliminates wild bacteria and lactobacilli that would compete with your starter culture, whilst leaving enough heat-stable proteins to set properly. Use a thermometer; eyeballing this is how batches fail.

Whisk the yoghurt starter into the cooled milk. The starter culture — Lactobacillus bulgaricus and Streptococcus thermophilus — needs milk at precisely this temperature to activate fermentation. Too hot and you kill the culture. Too cool and fermentation stalls. Whisk for two minutes to distribute the bacteria evenly; uneven distribution creates patches of bland, unfermented milk.

Preheat a thermos with boiling water, then empty it. Pour in the inoculated milk immediately — the thermos traps the heat, holding the milk at the incubation temperature required for bacterial-fermentation to proceed. The yoghurt will set in 12 hours; resist opening the thermos. You'll know it's ready when the surface has a slight jiggle and the smell is distinctly sour and tangy, with no yeasty or off notes.

Strain the yoghurt through muslin or cheesecloth. This is dairy-making at its simplest: gravity separates the whey from the curd. For thin yoghurt, strain for 30 minutes. For Greek yoghurt — thick, dense, and creamy — hang for 3–4 hours until you've removed roughly half the whey. The longer you hang, the more lactose drains away, concentrating flavour and raising the protein content. The whey itself is useful; don't discard it.

Transfer to a clean jar and refrigerate. The cold halts fermentation. Greek yoghurt keeps for two weeks refrigerated, though the flavour will sharpen slightly as residual bacteria work slowly in the cold. If the set yoghurt smells of acetone or looks slimy, the culture was contaminated or the thermos didn't hold temperature; start again.