Why homemade yogurt fails and the temperature that prevents it
Most thin or grainy batches are ruined before the culture has had a chance. One number, 110 degrees Fahrenheit, decides most of it.
Most failed yogurt fails in the first twenty minutes, before the culture has done any work. The number that decides it is 110 degrees Fahrenheit.
The temperature that quietly ruins a batch
The bacteria that turn milk into yogurt work best between about 105 and 115 degrees. Stir the starter into milk that is still at 130 and a large share of it dies, which leaves you with warm milk and a long wait for nothing. Stir it into milk at 90 and the culture survives but crawls, and slow growth gives unwanted organisms room. Cool the milk to roughly 110 and confirm it with a thermometer. A fingertip cannot tell 110 from 125.
The hot end matters too. Heating milk to 180 degrees and holding it near there for about ten minutes changes the whey proteins so they bind into the set. Skip that step and the yogurt is usually thin, even when it tastes right.
Starter and time, the other two dials
More starter does not mean faster or better. One to two tablespoons of plain yogurt per quart is enough. Add a full cup and the bacteria crowd each other, and the result tends to be grainy or watery.
Incubation at 110 usually takes six to eight hours. Past ten or twelve, the yogurt turns sharply sour and starts to separate. Clear liquid on top is whey, and it is normal. Jostling the jar during the set breaks the fragile gel, so leave it alone, then chill it for at least four hours before judging the texture.
What to do instead
Hold a steady temperature rather than relying on a warm oven or a sunny counter, which can swing by 10 degrees or more over a day. A dedicated yogurt maker with an adjustable thermostat and timer exists for exactly this job. The useful question to ask of one is how closely it holds its setting, not how many probiotics the box mentions. A figure like trillions describes a whole batch at best, it is a marketing statement rather than a measurement, and strains differ in how many survive to your spoon. Check any machine once against a separate thermometer and write down the offset.
Then keep notes: which milk, how much starter, how many hours. Change one thing per batch. Yogurt rewards that kind of dull record keeping more than it rewards any setting on a dial.