Instructions

1. Use whole milk and avoid anything labelled long-life if you want a thick set. UHT milk has already had its proteins denatured by heat, and the culture has less left to build a structure from.
2. Heat the milk to 185 °F to 195 °F over a medium heat, stirring so it does not catch. This is not about killing anything — it is unfolding the whey proteins so they join the curd, and it is the difference between thick yoghurt and thin.
3. Take it off and let it fall to 100 °F to 115 °F. Any hotter and you kill the culture you are about to add; any cooler and it takes hours to get going.
4. Whisk the live yoghurt into a cupful of the warm milk first, then stir that back into the pot. A spoonful of cold yoghurt dropped into a whole pan sets into lumps and stays lumps.
5. Pour the mixture into clean glass jars and close them. Clean matters more than it sounds — anything else in the jar is being given the same ideal conditions for the rest of the day.
6. Pull all the trays out, stand the jars in the empty machine and hold them at 110 °F for 9 hours. Do not open the door and do not stir: the curd sets as a single structure, and breaking it now turns it back to liquid permanently.
7. It is ready when you can tilt a jar and the yoghurt moves as one mass away from the glass, with a little clear whey at the edge. A jar that still swirls like milk needs longer, and a sharply sour smell means it went too far.
8. Move the jars straight to the fridge and leave them overnight before opening. Most of the thickening happens as it chills, and yoghurt judged warm always looks like a failure.
9. It keeps about two weeks in the fridge. Save three spoonfuls to start the next batch, and buy a fresh culture after four or five generations — the mix of bacteria drifts and the set gets weaker each time.