This is essentially the yogurt making process I saw Alton Brown do on one of his shows. The thing I love about it is that it dispenses with those silly and expensive yogurt making devices you see in kitchen gadget shops. Everything you need for this you probably have between your kitchen cabinets and your bathroom closet. So, to make home-made yogurt you’ll need:
1 quart low fat milk
1/2 cup powdered milk
1/4 cup room-temperature yogurt (plain)
Optional: 2 – 4 tablespoons honey, maple syrup or refiner’s syrup
Optional: A teaspoon or so of flavoring like vanilla, lemon or coffee extract
To start simply pour the milk into a saucepan with the powdered milk and sweetener (if using). Whisk the mixture gently over medium heat until it registers 195 degrees, maintain the temperature there, taking the pan on and off the heat as needed, for ten minutes (you can skip this heating step if you wish and simply bring the mixture to 120, though you’ll get a better texture if you apply the higher heat). Then, pour it into a taller-than-it-is-long container, either a wide-mouthed jar or piece of miscellaneous tupperware. Allow it to cool to 120, then add the yogurt and stir until blended.
After that you simply need to keep the culture as close to 110 degrees as you can for the next 4-10 hours…which is easier said than done since most of our modern-day kitchen devices are designed to prevent microbial growth, not encourage it. You’ll need an instant-read or probe thermometer to take regular readings.
Some people like an electric oven for this job, assuming it can be set to 110 degrees or so, though most ovens do a very poor job of maintaining steady low temperatures and sudden spikes in temperature (when the oven turns on) can be disastrous. Other people like an unlit gas oven (assuming it has a pilot light) and a warm water bath, though the water needs to be changed periodically to keep the temperature up.
Me, I like the hi-tech approach that Mr. Brown suggested on one of his show, whereby you wrap your culture jar in one or two electric heating pads using rubber bands (or a larger container that you can tightly stuff the whole works in). Careful regulation of heat is critical, since the bacterial cultures that make yogurt are extremely fussy. They’ll die if they’re heated much over 120, and slow down considerably if they cool much below 105. More on this in the tutorial.