Easy Soy Yogurt
I used to buy soy yogurt out of the refrigerator cases at the local coops until WholeSoy botched up their production process which resulted in a soy yogurt shortage all over everywhere. So I decided to try making my own. It turned out to be so easy, I'll never buy it off the shelf again.
I started with this excellent guide to making yogurt. It's for making dairy yogurt, but the same principles apply to soy. I've refined the process even further and this, then, is that.
One quart of the simplest soy milk
I start with a quart of the simplest soy milk I can find. At the moment, it's WestSoy's super plain and boring soy milk ("Unsweetened Plain"). I've chosen this one because its ingredient list is nothing more than what I want: water, soybeans. Any soy milk should work, the added sweeteners, flavorings and whatnot shouldn't inhibit the subsequent enzymatic reactions. You can even use Eden Soy products if you want, but you should really consider not.
Decant it into a mason jar
I pour this plain soy milk into a 1-quart mason jar. It never quite fits. I always end up with an ounce or two or soy milk that would overflow the mason jar. I just drink it. Or dump it into my coffee. Or whatever. It's not a big deal.
Heat it to 110 °F
Into the microwave it goes for 90 seconds. At the end of the cycle it's at about 110 °F. I don't bother pasteurizing it because I've found it makes no difference. I used to bring it up to 180 °F on the stove and then wait for it to cool but all this did was make the process longer. Presumably, WestSoy does a good job of keeping unwanted microbial agents out of the soy milk when it's packaged. Whatever you do, you want your soy milk to be at about 110 °F before proceeding to the next step.
Inoculate with yogurt bacteria
To turn your soy milk into soy yogurt you need billions and billions of tiny yogurt making machines. Fortunately, you don't have to count them out by hand. In addition to making yogurt, these little guys also make more little guys. Their names are Bifidobacterium lactis, Lactobacillus acidophilus, Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, and so on (different yogurt types use different strains). The easiest way to find these guys is to scoop out a tablespoon of yogurt from a previous batch (or, for your first batch, from a store bought yogurt). You literally only need a tablespoon's worth to inoculate an entire quart of soy milk. I pour a cup's worth of heated soy milk into the nominally empty but unwashed jar of my previous batch, swirl it around a bit to dissolve the scrapings, and then pour it back into the new jar. No need for a fancy yogurt starter or anything more complex.
Warm it for eight hours
It takes about eight hours for the bacteria to do its thing. And because these bacterium are mesophilic they need to be kept a little bit warm. If you've got an oven with an incandescent light bulb you can do what many people do and leave the sealed jar in the oven with the light on overnight. I don't have such an oven, so what I've found that works for me is an electric heating pad in a small cooler. It took some trial and error to find the right settings and configuration but once I did I just use the same technique and it works perfectly every time.
Finally, you're done. This homemade yogurt doesn't have the same consistency or sweetness as store-bought because you haven't added any thickeners or sweeteners. You're free to do that, of course, but I've found it prefer it without. Furthermore, without the additives, it has more general uses (like vegan cheese-making, blending into smoothies or lassis, mixing with granola).
We do all this because, as we know: