(Major update, 4/2/17: revised the instructions, added a diagram showing the basic steps.)
My plastic container bin used to be loaded with 1-quart yogurt containers, and I felt packaging guilt whenever buying a new tub, even when it was local and organic. I knew that it wasn’t too hard to make yogurt at home — I had mental images of electric yogurt makers from the 1970s — but a few years ago a “coincidence of information” (i.e., a series of web articles) showed me that I didn’t need an electric device or any fancy equipment. All I needed was some containers, some yogurt, and a warm place.
Making yogurt at home is not as easy as grabbing a tub at the market, but there is something satisfying about doing it yourself, and it’s a chance to reduce your use of plastic.
Warm, Heat, Cool, Mix, Wait
Making yogurt at home is relatively easy but requires some organization. The process can be summarized as warm, heat, cool, mix and wait.
Warm: The first step is to wake-up the cultures. This will make them more active when it’s fermentation time. A few hours before yogurt-making-time, I put 1-2 tablespoons (15-30 mL) of yogurt into a coffee cup and leave it out at room temperature. This yogurt is sometimes called the ‘seed’ yogurt. The warm-up step is optional, but it reduces the chance of a fermentation failure. For the first few months of my yogurt making, I skipped this step and had pretty good results. These days I do the warming out of habit and as insurance.
Heat: Heating the milk modifies the whey proteins, which gives you a finer, more compact curd. A good target is 190 °F (88 °C), but if the milk gets a little hotter, it’s no big deal. You can heat the milk in a saucepan over gentle heat (ideally with a flame tamer underneath the saucepan to provide even heating), or you can use the microwave. I use my microwave, heating the milk in 5 minute bursts, checking the temperature after each cycle (you’d think I might know exactly how long to run the microwave, but you’d be wrong — I haven’t bothered to keep track). Or, if you want to be high tech about it, you could use an Arduino-based temperature monitor and alarm. (More on the science of yogurt from Harold McGee in the New York Times)
Cool: The bacteria that convert milk to yogurt have a preferred temperature range, so the next step is to cool the milk to that range, about 120 °F (49 °C). Set some timers so you don’t forget, and check periodically. For me, with the milk in a Pyrex measuring bowl on the counter, it takes more than 30 minutes (the exact time depends on the room temperature, air currents, etc.). If you are in a hurry, you could transfer the hot milk into a metal bowl and set that bowl over cold water.
This is also the time to start prepping a warm spot for the fermentation. I use a flexible cooler, and add a jar of hot water to help keep the contents warm (it’s a cheap cooler, so not great at heat retention). If you have a piloted oven or have a heating device (like the hacked appliance described by Jennifer Jeffrey), skip this step.
Near the end of the cooling process, I also heat some water and pour it into the jars I’ll be using to make the yogurt to take their chill off.
Mix: Once the milk has cooled to the target temperature, pour a small amount of it into the container with the ‘seed’ yogurt, whisking as you pour to mix it thoroughly (“tempering”). Then pour the yogurt-milk mixture into the main milk container, and whisk to combine. Pour into clean containers.
I originally used 1-quart canning jars to hold the yogurt — they were OK, but a little bit too large (hard to get the last bits from the bottom of the jar) and the necks of the jars were hard to clean. In the last year or so, I have switched to the Working Glass from Crate and Barrel (my 10-year old glasses have a Luminarc mark on the bottom)
Put the lids on the containers and set them into warm place. I use a flexible portable cooler with good success.
Wait: Let the container sit in a warm place for a few hours until it sets. There are no fixed rules for this — I typically let the yogurt ferment for 4 hours.
The drawings below outline the process.
Many months after getting into a routine, I noticed that Paula Wolfert’s “The Cooking of the Eastern Mediterranean” has an appendix about yogurt and a recipe for making it at home. Wolfert writes that the most prized yogurt in the Eastern Mediterranean is made from buffalo’s or sheep’s milk because of the high butterfat content. She relates an old saying about the culturing process: “the quicker the sweeter.” In other words, the faster you can make the yogurt set, the sweeter it will taste, so some yogurt makers pour the warm milk-yogurt mixture into small jars for culturing. At 100 F, a batch of yogurt will jell in about 6 hours. At a lower temperature, it could take as long as 24 hours and give you a much tarter result. (Some day I should measure the temperature of my portable cooler during fermentation.)
Writer and editor Jennifer Jeffrey wrote about a clever way of managing yogurt culturing temperature with a hacked slow-cooker. By installing a dimmer switch in the power cord, she could control the amount of energy reaching the cooker’s heating elements, thus controlling the temperature of the water bath. (Important note: If you are going to hack commercial appliances, be sure you know what you are doing and follow good safety practices!)
After all of these words, remember that there are many ways to make yogurt at home — the yogurt cultures are somewhat forgiving about time and temperature — and what works for me might not work for you. Other authors might have methods that fit your schedule and taste preferences.