Update: these days I heat the milk in the saucepan, not the jar, as this allows me to quickly cool the milk by placing the saucepan in a sink full of cold water. If you heat and cool glass too quickly it will break; so it’s much safer to use the saucepan instead. In brief then: heat the milk to 85 degrees (not too fast or it’ll burn on the base). Cool it or let it cool to 49-44 degrees. Stir in a tablespoon or more of existing yogurt (commercial or home-made). Pour into jars or plastic containers, whatever you like. Put the lids on and keep warm for the next 4 or more hours. I put mine into a broccoli box (Styrofoam box with lid) and with a towel inside to isolate even further. I typically leave it for more than 5 hours to get an especially thick yogurt. I’m currently doing 3 litre batches every week and a half. I’ve also tried it with skim milk for a friend, and the result is apparently still very good.
Probably the easiest thing I’ve made so far.
You’ll need a large jar with a lid, a saucepan, a cooking thermometer, some commercial yogurt (preferably unflavoured and good quality), and a way to keep the new batch of yogurt warm. I have a nice purpose-built insulated container for the jar, but you could simply wrap the jar in a thick towel and put it in a bucket to keep it upright.
Heat milk to 85 degrees C, then allow it to cool to 44 degrees. You can heat it directly in the saucepan or put the milk in the jar and heat the whole thing in water as pictured. At 44 degrees add a heaped tablespoon of commercial yogurt and stir through, then pour it into the jar, put the lid on lightly, and wrap it in towels or use whatever method you have to keep it warm.
Let it sit for about 4 hours, then tighten the lid and put it in the fridge. The longer you leave it warm, the firmer and more tart the yogurt will be.
I’ve used this method three times in the last week and a bit, and last night I made an extra large batch, giving me three litres of yogurt. Fresh yogurt is pretty amazing. You can add it to fruit or blend favours with it after it’s chilled, but I haven’t tried that yet as the fresh plain yogurt is so good on its own.
When you near the end of the batch you can simply repeat the recipe, using your own remaining yogurt instead of commercial yogurt as the starting culture. If the yogurt doesn’t seem as effective at any point, you may need to buy more commercial yogurt to restart the process.
Milk here is about $1/litre, which is a very good price for an easily-replenished supply of fresh yogurt!
There are similar processes for making buttermilk, which can in turn be used to produce sour cream, creme fraiche, and cottage cheese. Will let you know if I get around to making those.