Homemade Yogurt

There are lots of instructions for homemade yogurt out there. I developed one that I think is somewhere in between too-casual-I-can-tell-this-might-not-work and too-scientific-I'm-not-buying-a-heating-pad. Find the recipe that feels right to you and hopefully you'll have great success.
(p.s. This instructables version is better than the version on my blog, but I always include a link just in case you feel like visiting.)

You'll need:

- 1/2 gallon of whole or 2% milk that is not Ultra-Pasteurized (This has to do with the temperature it was pasteurized at. If milk has been Ultra-Pasteurized it must be labeled that way, so it's generally easy to avoid. The only organic milk I've found in my neighborhood that will work is from Trader Joe's)
- 1 small plain, all-natural yogurt that has live cultures (like Nancy's)
- thermometer
- slow cooker or double-boiler
- small bowl, stirring spoon, and ladle
- an oven or closed space that you aren't going to need for 8 hours
- towel
- tea-kettle or pot with lid
- butter muslin or cheese cloth if you like thick (Greek-style) yogurt
- optional fruit to stir in

Step 1: Sanitize Your Equipment

Clean all the equipment you'll use very well, being sure to get all the soap residue off. Just to be sure, I like to boil a pot of water and dump it in my slow cooker and the bowl. Then I swish a stirring spoon, ladle, and the thermometer in the hot water. You want to avoid adding any other bacteria to the milk so that you can just cultivate the good, yogurt bacteria.

Step 2: Heat the Milk to 180 Degrees

Heat the milk to 180 degrees in the slow cooker on high (or in a double-boiler). It takes my slow cooker about an hour and a half, but it needs very little attention through this stage.
I tried skipping this step once because it's basically just to kill whatever bacteria might be in the milk (my reasoning was, I just specially bought non-ultra-pasteurized milk, why would I heat it up so high now?), but the yogurt didn't work. It was really sticky and separated and not yogurt-sour, but strange sour.

Step 3: Cool the Milk to 110 Degrees & Add the Yogurt Starter

Turn off the heat and let it cool to between 100 and 115 degrees. This takes about 1/2 an hour to an hour. It goes faster if you stir.
In a clean bowl combine the store-bought yogurt with 1 cup of the warm milk and mix well. Add this yogurt mix back to the slow cooker.

If you add the yogurt starter when it's too hot you'll kill the bacteria, cook the yogurt, and it won't combine. If this happens (because maybe you were excited and trying to skip steps) you can strain out the cooked yogurt (I thought it tasted kind of good), let the milk cool, and add more starter yogurt.

Step 4: Keep the Yogurt Warm and Undisturbed

Wrap the slow cooker (lid on) in a thick towel and put it in the oven with the light on.
Boil some water in the tea kettle or pot and put that in the oven too. This keeps my oven at a perfect 100 degrees.
Let this sit undisturbed for 6 to 12 hours, depending on how tangy you like your yogurt. I do 8 hours.

Step 5: Now You Have Yogurt!

Ok, now you should have yogurt! Put it in the fridge to stop the fermentation process and help it firm up. If you have a lot of yellowish liquid floating around, don't worry, this is whey. You can stir it back in or, if you're going to strain your yogurt, just leave it.
If you think you might make another batch, save about 1/2 a cup for the next time instead of using a store-bought yogurt.

Step 6: Optional: Strain Yogurt to Thickens

You can be finished now, but I like thick yogurt so I line a colander with a square of butter muslin and scoop yogurt into it, then pick up the four corners and tie it into a little hobo bundle (this batch had to be divided into 3 bundles). I hang my yogurt over a bowl for about an hour before mixing in sugar and fruit.

Step 7: Optional: Add Fruit and Sugar

I make a quick "jam" while the yogurt is working in the oven. For a batch this size I cut up about 5 cups of fruit and simmered this on low for about an hour (until it was syrupy) with 1 cup of sugar and the juice of half a lemon. Be sure to let it cool before adding it to your yogurt.

ps - I read that it's not a good idea to add honey to yogurt if you're going to store it in the fridge because the honey bacterias can kill the yogurt bacterias. Add honey to each serving right before you eat it. If anyone knows more about this, let me know.