I make yoghurt every week using just a jar, a few spoons of (any) yoghurt, and a thermometer. No oven, no blanket, no yoghurt-maker, no special starter needed. Here's how I do it:
1. Bring milk to the boil, then remove from the stove as soon as it starts to rise. I use ordinary supermarket milk.
2. As milk cools, a skin will form. I skim it off when I'm ready to add the yoghurt.
3. I wait for the temperature to drop to 49 degrees Celsius (140 F).
4. I add this much yoghurt to a jar. I used to measure how many spoons, but now I just eyeball it to be about this much.
5. I skim off the skin that formed in the cooled milk.
6. I scoop/pour some of the boiled milk to mix with the starter and mix it thoroughly. Don't skip this step or your starter might not mix properly.
7. Pour the mixed starter into the pot of milk and stir gently to blend the starter into the milk. Don't stir much or vigorously or the milk will cool down too far.
8. Pour it all back into the jar. Put on the lid, store out of sunlight for 8 hours.
9. Tilt the jar. If it's formed into yoghurt, there will be no sliding. If it's runny, then something has gone wrong (usually the milk was too hot or too cold when the starter was added). Refrigerate for about 8 hours before using, because this will thicken it up further and improve the taste.
10. Yoghurt has formed for eating.
SOME THINGS I'VE LEARNED:
- Homemade yoghurt tastes better because it's fresher and less sour (yoghurt sours with age). But the texture isn't as creamy or firm, and after 7 days it develops an off-smell and starts tasting worse. So it makes sense only if you regularly consume it because it lacks preservatives and additives of commercial yoghurt.
- I used to use an oven to keep the jar warm to incubate it, but later realised it didn't make a difference.
- I usually make it late at night and refrigerate it when I wake up.
- Once you've done it a few times it becomes a routine and second-nature.
- A thermometer isn't essential but it makes it far more convenient than the traditional finger-in-the-pot method to guess the temperature.