You've read a lot of stories about making yogurt. I think there are enough innovations here to give you something new and interesting. If you need a gallon of yogurt every week, this is the fastest, most reliable, easiest, and cheapest home process by far. I'll have further musings at the end, but let's get right to it.
You will need a gallon of milk in a plastic jug, a pot bigger than that jug, 2 coffee filters, 2 oz. of plain yogurt in a cup-sized jar, a pair of gallon-sized shallow food containers, an insulated container to hold the jug, a drill or sharp knife, a cooking thermometer, and a kitchen with a stove, fridge, and sink.
The cheat sheet.
1. Sanitize everything.
2. Heat to 180 - 190F.
3. Cool to 100 - 110F.
5. Ferment for 6 - 24 hours.
6. Strain in refrigerator.
Step 1: Scald the Milk to 180º - 190º
Poke the temperature probe through the cap. Reserve a cup or two or three for pudding and place the jug in the pot. Add just enough tap water to barely float the jug. Cover the pot if you can and set the burner to high.
It takes my electric stove about 25 minutes to heat the milk to 175F. Then turn off the heat and let it coast up to 180+. I use a timer so I can turn my attention to other things, but depend on the thermometer.
It's hard to find the perfect vessel for this, but it really doesn't matter to the yogurt. Mine is a 12 quart aluminum pot, over 10 inches high inside, with a glass lid that has a vent hole. It saves time and heat. You can do this in an open 3 quart pot for a trial run, but it's more trouble.
Step 2: Cool the Milk to 100º - 110º.
You can take the jug out of the pot to cool with tap water in the sink, or just take the pot to the sink. Be careful, it's heavy and hot. I grab the jug with the hook end of a ladle. Once it's out of the water, the jug handle is cool enough for me to handle. Do whatever is safe for you. There is no urgency here.
Cooling time depends on the temperature of your tap water and kitchen air. Plan on a half hour in water to reach 110F, but depend on your thermometer. At 112F, dry off the jug and prepare to inoculate. But the exact temperature doesn't matter that much to the yogurt.
Step 3: Inoculate
Below 110F, your bacteria will be happy. Of course you want to use a nice clean starter culture, but it doesn't matter to the yogurt. I dilute some of my last batch with an equal volume of the scalded/cooled milk to let it pour easily into the jug. As little as a tablespoon of starter will work — they grow like crazy. I try to save about 2 oz for this because I'm going to eat it anyway.
Step 4: Ferment
Now the inoculated milk needs some quiet time in a cozy place. This is my 2nd generation Yogurtron, which I made specifically for this job. It is a carefully crafted foam box that holds the temperature loss to 3 degrees overnight. But the yogurt doesn't care.
You can use a cooler with foam peanuts or bubble wrap or whatever. Just don't use towels or they will smell a lot like yogurt. Which isn't as delightful as you might think. There is no need for a heater or worries about electrical failure. If your home is cold, build a nice thick foam box and preheat it with hot water.
I usually let the batch ferment overnight. Sometimes 6 hours if I'm in a hurry. Sometimes I forgot it for a whole day. The yogurt doesn't care.
Step 5: Strain the Yogurt
Now you should have a fairly solid jug of yogurt. And that may be all you need. Screw on an un-pierced cap, refrigerate, and serve directly from the jug.
I like thick yogurt, so I developed this strainer. It is made from a pair of gallon-sized food containers. Since they nest, only 1 cover is needed. I drilled about 50 1/8th inch holes in the bottom of the top container. You could do just as well by gouging out a dozen rough holes with a knife. The yogurt doesn't care.
I cover the holes with 2 coffee filters. The top container is set into the bottom container. Then I pour in the yogurt. But not quite all of it.
I put the lid on and leave the strainer on the counter for an hour or so to drain. Why shock my fridge when I can just pour off warm whey?
The strainer is showing no signs of wear from 16 months of constant use.
Step 6: Store and Serve
I slide the entire strainer assembly into the fridge. It keeps draining as I use the yogurt, so there are never any nasty puddles of whey. I hate those. And since I've gone this far without washing much of anything, why start now?
But there is one thing: I want to save some of each batch to inoculate the next. So the last few ounces out of the jug go into a jar. I leave this starter untouched for the week in the fridge.
Step 7: But Wait … There's More!
Now I have this valuable empty jug, so I give it a rinse. With the top cut off, it makes a handy container for edible garbage. Then my main garbage doesn't get stinky, so I can really pack those bags for weeks. One jug — three uses.
Step 8: The Circle of Life
I was thinking about these clever bacteria: They have evolved to trick us into using our bodies to promote their culture. So exactly who is the "superior" species?
This is a robust process. Over the past 2 years I have fermented about 80 gallons of yogurt. I've used different gear and different milk and made every mistake. For the past 60 batches, I have used only my own starter. Still *never* a failure. I am careful about sanitation; brush cleaning everything with a little bleach in my dish detergent. But my kitchen isn't a laboratory. This bacteria is so territorial, it's fighting off other microorganisms. It creates it's own environment for its safety and our delight. After 60 generations, it's growing like fresh culture.
I tried some batches with added dry milk. That's just more trouble and expense. Fluid milk is cheaper than the equivalent powdered milk. If I need more yogurt for a special culinary project, I'll buy a 2nd jug.
After losing a lot of weight and radically improving my health, I have switched from skim to whole milk. The resulting yogurt product is luscious. Like cream cheese (but with flavor), you can spread it on toast. Of course I use it for fruit smoothies every morning. With a dab of olive oil and spices, it makes wonderfully thick and creamy salad dressings. When loaded with Southwest spices or wasabi, it sauces sandwich meat. Creamy belly-filling vegetable soups too.
With foamed cream or pasteurized egg white, it makes a sweet whipped pie filling. Oh yes. I hope to spend years discovering new yogurt desserts. And living well.
Cheers from Sarasota.