Sometimes I wonder if cooking is an art or science. I guess it's a bit of both. Some types of cooking though are almost pure science. Bread baking for example, especially when dealing with natural leavening  or sourdough breads. Making a pie crust or a delicate cake is rather scientific too.
Making soy milk (to-nyuu 豆乳) and tofu（豆腐) feels more like a Mister Wizard  experiment than cooking. But it's a lot of fun. I have been making my own tofu and soy milk off and on for a while now, and each time it feels like a wonderfully geeky thing to be doing. It is a bit of a production number, but if you like soy milk or tofu and strange liquids that foam up like you wouldn't believe, give it a try!
Most of the time I make soy milk for the purpose of turning it into tofu, but on occasion I do scoop off some of that milky, beany liquid to consume as-is. Unlike the majority of East Asian people, I am luckily not lactose intolerant. But soy milk is a healthy, vegan alternative to milk. Freshly made soy milk smells and tastes so much better than the store bought kind, is preservative-free, and altogether nicer. You can add just the amount of sweeteners or other stuff to your totally pure soy milk and feel awfully good about yourself. Plus, you can select the soy beans you use.
1. Soy beans
Naturally you do need soy beans. You need the white dried ones, not the green fresh ones (aka edamame). Nowadays you can get dried soy beans from many supermarkets and most health food stores. Since one reason for making our own soy milk is because it's (we hope) healthier, it makes sense to use organic, non-genetically-engineered soy beans; a tad more expensive than ordinary soy beans, but worth it just for the ease of mind. Be sure to get beans that aren't too old; look at the sell by dates or production dates, and get your beans from a store with a high turnover.
2. 2 large pots, or one humongous pot
You will need a huge stockpot, or two normal-kitchen sized soup or pasta pots, for cooking the soy bean paste. If you are using two pots, each pot must have at least a 16-cup capacity, if not bigger. You'll see why later.
3. A muslin, cotton, fine-mesh cheesecloth, or other fairly porous cloth bag
You need a bag made of a finely woven but still fairly porous fabric. It should also be sturdy enough to stand up to washing, since you will want to use it over and over. Cotton muslin works great for this. To make the bag, take an approximately 40cm x 50cm or so (16 inches x 18 inches) piece of cloth, fold it in half, and sew up the side and bottom. If you want to make it neater you can hem the opening.
4. A food processor or blender
This is to grind up the beans.
That's all you need. No need for special soy milk makers and such!
You will need:
Yes! That little bag of soy beans is going to give you a big potful of soy milk. Here we go.
Wash the soy beans, pick out any discolored bits, stones and broken beans.
Cover with plenty of cold water, and leave to soak for at least 8 hours, a maximum of 24 hours. I find that oversoaked beans result in rather flavorless soy milk. Change the water a few times during the soaking period, especially during the warmer months. Sufficiently soaked soy beans should be soft enough to bite through easily.
Drain the soy beans and rinse. In the meantime, put your two big pots on the range with 8 cups of water each in them, and start heating up.
Put about half of the soy beans in your food processor with the steel cutting blade, and add enough water to barely cover the beans. Process for about 2 to 3 minutes, until the beans are very finely ground. You may need to stop the processor halfway through to scrape down the sides. Do the same with the remaining half of the soy beans.
At the end of this stage you'll end up with a creamy,foamy goo looking like this.
Divide the mixture evenly between the two pots you prepared previously (or into your one humongous stock pot). The mixture should not come up more than halfway up the side of each pot. Lower the heat to medium, and stir and watch the pots.
As the mixture heats, it will foam up a lot, and may threaten to boil over. If the foam threatens to rise to the rim, sprinkle about a half cup of cold water over it and stir rapidly. This should make the foam subside enough (though I have to admit I've been too late several times and the foam has spread all over the stovetop. This is when I am glad I have a ceramic top range, though otherwise I wish I had a gas range. But I digress.) Here you see two pots of foamy yet under control soy liquid.
Continue to cook this for about 20 minutes, stirring frequently. It will stop foaming as it cooks. At the end, the liquid should look rather grainy, as in the picture below. This means that the milk has separated from the fibrous part of the ground up soy.
Line a bowl with the cloth bag, and ladle in the cooked soy mixture a bit at a time.
Squeeze out the liquid, and put into another, clean pot. It helps to have a very sturdy spatula to press down on the bag to extract as much of the liquid as possible. Or, you can wear heat-resistant rubber gloves and squeeze the bag with all your might. Dump out the fibrous stuff into yet another bowl. You may need to dump it out a couple of times.
The filtered liquid is pure soy milk. I like it just plain, but many people have a problem with the slightly grassy taste. The easiest way to mask this is to add a bit of fruit and to whiz it up into a soy milk shake. Just half a banana, or a few fresh or frozen strawberries, will do the trick. You can also add about a tablespoon of honey, fruit juice, or even sugar. The point is that you can choose what to put in rather than whatever commercial soy milk is sweetened with. The glass below is a soy milk shake made with half a banana and a handful of frozen raspberries from last summer's crop.
Now what about that fibrous stuff? Don't just throw it in the garbage. It's called okara, and is maybe even more nutrious than the soy milk. It's very high fiber as you might expect, and full of protein. At the very least, it makes a terrific addition to your compost pile.
You can keep fresh soy milk for up to 3 days in the refrigerator, but you shouldn't try to keep it too long. Remember this is totally preservative-free. Okara can be frozen, or spread out onto two baking sheets and dried in a low-temperature oven. Dried okara will keep in airtight containers or bags for a while.
[Update:] And here are the links to Parts 2 and 3 of this series: