Yoghurt not setting

Which of the four it is

No thickening at all, no sourness

The culture did not work. Dead starter, or milk still too hot when it went in.

Slightly sour but thin

Incubated too cool, or not long enough. Give it more time somewhere warmer.

Sour and thin

The milk was not heated long enough beforehand — this is a texture problem rather than a culture one.

Set, then broke into curds and whey

Over-incubated, or disturbed while setting.

Thin and slimy or stringy

A contaminating organism. Discard it and start again with fresh milk and a fresh starter.

Safety

How eggs and dairy should be stored, and which eggs are suitable raw or lightly cooked and for whom, are food-safety questions rather than culinary ones. Follow current Food Standards Agency guidance.

What is causing it

• Milk too hot when the culture went in. The commonest failure. Anything much above blood heat kills the bacteria outright. Cool it until it feels comfortably warm rather than hot.
• A dead or weak starter. Shop yoghurt must say live or active cultures, and must have no added thickeners, gelatine, flavours or sugar.
• Incubated too cool. The bacteria work slowly or not at all below their range. An airing cupboard that felt warm may not have been.
• The milk was not heated first, or not for long enough. Heating denatures the whey proteins so they join the set. Skipping it gives thin yoghurt even when the culture works perfectly.
• Disturbed while setting. Moving or stirring the container breaks the gel as it forms and it does not re-form.
• UHT or heavily processed milk, which sometimes sets poorly.
• Too much starter, which sours it fast and can give a weaker, grainier set rather than a firmer one.

Rescuing a batch that has not set

1. If it is barely sour, give it longer somewhere genuinely warm. Another four to eight hours often does it.
2. If it is sour but thin, the set will not improve — but straining will. Line a sieve with muslin and let it drip in the fridge for a few hours.
3. Strained thin yoghurt becomes thick yoghurt, and strained further becomes labneh. This turns a failure into a better product than you were aiming for.
4. Keep the whey. It is useful in bread, smoothies and for cooking grains.
5. If it never soured at all, the culture failed. Reheat the milk properly, cool it to blood heat, and start again with a fresh starter.
6. If it is slimy, stringy or smells wrong, discard it rather than trying to save it.

Getting it right next time

Heat the milk until it is steaming and just about to simmer, and hold it there for twenty minutes if you want a firm set. This denatures the whey proteins so they join the casein network rather than staying in the liquid, and it is the single biggest difference between thin yoghurt and thick.

Then cool it until it is comfortably warm to a clean finger — not hot. Use a thermometer and a reliable recipe if you want to be accurate about it.

Stir a few spoonfuls of live yoghurt into a little of the warm milk first, then whisk that back into the rest so it disperses evenly. Cover, keep it warm and undisturbed, and do not move the container for at least four hours.

Common questions