I followed directions from here.
--8 cups (half-gallon) of milk--pasteurized and homogenized is fine, but do NOT use ultra-pasteurized. (I use all nonfat and it works just great!)
--1/2 cup store-bought natural, live/active culture plain yogurt (you need to have a starter. Once you have made your own, you can use that as a starter) (You can also buy the dried starter packets. You can also freeze the yogurt in 1/2 c portions, which is what I do. You can only use starter from prior batches a few times before the bacteria lose their oomph.) (Note: the best starters are PLAIN YOGURT with live cultures. Dannon works well, and I've used Activia to start the last few batches and it turns out AWESOME. All generic plain is good as long as they don't add other junk like sugar, etc.)
--thick bath towel
I start in the evening about 5:00 pm and it's ready in the morning.
I used a 4 quart crockpot.
Plug in your crockpot and turn to low. Add an entire half gallon of milk. Cover and cook on low for 2 1/2 hours.
Unplug your crockpot. Leave the cover on, and let it sit for 3 hours.
When 3 hours have passed, scoop out 2 cups of the warmish milk and put it in a bowl. Whisk in 1/2 cup of store-bought live/active culture yogurt. Then dump the bowl contents back into the crockpot. Stir to combine.
Put the lid back on your crockpot. Keep it unplugged, and wrap a heavy bath towel all the way around the crock for insulation. (I put it in my gas oven with the pilot on, which seems to keep it at the perfect termperature.)
Let it sit for 8 hours. (Or a bit longer - the longer it sits, the tangier it gets. I think I've left it up to 10 hours or so.)
In the morning, the yogurt will have thickened---it's not as thick as store-bought yogurt. Put the crockpot directly in the refrigerator to let it chill and that will thicken it some. However, I find that the yogurt is still runny. If you want it thicker, strain it (I line a colander with a tea towel and put the yogurt in it with a bowl underneath. If you want it thick like supermarket yogurt, it only takes a few minutes. If you want it thick like yogurt cheese, it needs a few hours.) (Whey is the liquid you're draining out if you thicken it. Keep the whey and use it in soups, breads, or anything else calling for water or stock. It's pretty much flavorless and is packed with nutrients.)
I've tried other methods of thickening, such as adding in dry milk powder and I found that useless (and expensive). I could add in gelatin, but I like the texture better without that.
Note that you don't get the full amount of yogurt that you put in as milk if you strain it. I just now got (from 5 minute strain) about 5-6 cups of yogurt and about 3 cups of whey. (I don't know how much milk I put in because I just eyeball it - probably 8-9 cups.)
Your fresh yogurt will last 7-10 days (or longer ...).
Why make your own yogurt when you can just buy it from the store?
First and foremost, I find the taste far superior. Chop up some strawberries in it and it's heaven.
Plus, it's cheaper. Even buying cheap plain nonfat yogurt is more expensive than making it myself. And I can better control quality - I know exactly what's going in it.
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