1. Fill a thermos flask with milk (you can choose any kind of milk - skimmed milk, full cream, goats milk - anything at all), then pour it into a microwave-safe container or a pan.
2. Bring the milk to the boil. I used a microwave, but you could heat it on a stove if you prefer. Just watch that it doesn't boil over. This step is to kill off any unwanted microbes that will cause the milk to spoil.
3. Allow the milk to cool to around 50°C (122°F). I've got a new whizzy electronic thermometer so I used that, but you could use any kind of thermometer, or you could stick your clean finger in it. 50°C isn't hot enough to burn you, but it's too hot to comfortably keep your finger in for long. This is the optimum temperature for the yogurt culture beasts to grow.
4. Add some live yogurt to the milk. You can use a little of your last batch, or you can buy some fresh yogurt to start your culture off. This contains some of the yogurt culture microbes that are going to multiply and turn the milk to yogurt. You don't need much. I usually use about a dessertspoon, but in extremis I have swilled the milk in an empty yogurt pot to rinse off the bits of yogurt clinging to the sides, and it worked fine.
5. Pour the proto-yogurt into the thermos flask. Fill it to the top. It won't change in volume at all. Now seal the flask and leave it somewhere it can be undisturbed for 8-14 hours. Overnight will be fine.
6. After 8-14 hours with any luck your flask will be full of yogurt. When you take the lid off, you'll think it has failed, because it has not changed in appearance at all. It gets me every time. But when you try to pour it out, you'll find it is no longer thin and liquid, but has thickened considerably*. But the chances are good that if you followed my instructions, you have made yogurt. Well done!
7. You can put your yogurt in another container and put it in the fridge (warm yogurt is kind of yuk) and eat it just as it is. But if you want to make Greek (strained) yogurt, there is another step to the process. Pour your yogurt into a muslin cloth, and suspend it over a bowl for two hours (I put the cloth in a colander over a bowl).
8. After two hours the colourless liquid whey will have strained into the bowl and the remaining yogurt will be thickened and creamier. You couldn't pour it now. You'll have to scrape it off the muslin cloth.
9. Congratulations, you've made Greek yogurt. It's deliciously thick and creamy, even when made with skimmed milk. It's more stable in cooking than normal yogurt (but did you know that if your yogurt separates when cooking, you can stir in a spoonful of cornflour and stabilise it?). I like it on its own with honey. It also makes wonderful raita.
10. If you leave it straining for longer than 2 hours it becomes thicker and thicker and eventually will have a consistency like cream cheese. Stir in garlic and herbs if you like, and use it like Philadelphia. Delicious.
*Well - sometimes batches fail. It happens to everyone. Here's a troubleshooting list:
- If the milk was too hot when you added the culture, all the beasts will have been killed. Warm it up to 50°C (122°F), add more culture and try again.
- If the milk was too cold, the beasts won't have reproduced. Warm it up to 50°C (122°F) but no hotter and try again
- If the starter was old, it may not have had any live beasts in it to begin with and the milk will still be runny. You'll need to get a fresh starter from the shops. It's up to you what you do with the warm milk. I suggest cocoa.
- If the milk was old it may have curdled. Pour it away, it's ruined.
- If the equipment wasn't clean, some spoilage microbes may have got in and spoiled the yogurt. Pour it away, it's ruined.
- If you didn't leave it long enough, it won't have had time to set (sometimes it seems sort of slimy but not really thick). Make sure it's still warm enough and leave it a bit longer.
- If you left it too long, it may have separated into yellow whey and white curds. It's fine. You can stir it back together, or use it as it is to make Greek yogurt.