As long as we're getting deliveries of fresh, whole milk and cream, I thought we should start making our own yogurt on a regular basis.
On my first attempt, I made the mistake of following instructions from a dairy cookbook, heating our unhomogenized whole milk to just 115 degrees (Fahrenheit) before inoculating is with some store-bought yogurt. I left it to incubate overnight in a cooler packed with jars of hot water. What I got was a runny, fermented sort of yogurt wanna-be. Why hadn't it firmed up?
Then I recalled our yogurt lessons from "food appreciation" classes when we had cooked the milk at 185 degrees. I went back and read the pertinent section in Harold McGee's "On Food and Cooking" and kicked myself for failing to remember that it isn't the bacteria that make the yogurt thick, it's the temperature.
Bacteria ferment the milk by noshing on lactose and making acid. That gives yogurt it's pleasant tang. The thickness occurs when the proteins in the milk are heated to a fairly high temperature. Heat concentrates the proteins and forces them to gather in thick chains. Commercial yogurt makers often add powdered milk and gelatin to achieve the thickness consumers are used to.
For this most recent batch, I mixed 3 1/2 cups whole, unhomogenized milk with 1/2 cup heavy cream and heated it slowly in a heavy saucepan to 195 degrees. I maintained that temperature for 10 minutes (according to McGee's instructions) before moving the pan to a cold water bath and lowering the temperature to 115 degrees, at which point I mixed in 3 tablespoons of our favorite commercial yogurt from Seven Stars Farm. I poured the mix into a warm quart jar and placed the jar in a cooler with a heavy pot full of hot water.
I placed an instant-read thermometer in the cooler, aiming to maintain a temperature of about 115 degrees. Turned out I didn't need to do anything further. The yogurt had firmed up after just a few hours, but I left it in the cooler overnight. The next morning it was thick and delicious. I couldn't wait to slice up a peach and smoother it in our fresh, whole-milk (with some cream) yogurt.
Without a doubt, this is the best yogurt I have ever tasted.
Note: take care not to boil or burn your milk. Use a very heavy saucepan and don't try to rush the milk to 195 degrees. Be gentle with the heat and stir frequently with a wooden spoon or heat-proof spatula to prevent the milk at the bottom of the pan from scalding.