How can I prepare soy milk at home? Is it as simple as: soak soy, then blend and strain the milk, or is some centrifuge technique used?
It is basically as simple as you say (soak, blend, strain) ... just that you should also boil it (for long enough - cca. 10 minutes) to get rid of the fresh bean (grassy) flavor (mainly caused by oxidation of polyunsaturated oils in the beans), together with some other unpleasant side effects of eating uncooked beans (like the infamous gas-producing effect of some oligosaccharides).
McGee, On Food & Cooking says:
The key to minimizing the development of beany flavor is to inactivate the beans' enzymes quickly, before they have a chance to attack the oils. This is done by soaking the beans to speed subsequent cooking, and then immersing them in boiling water or pressure-cooking them.
There is a vast number of soy milk preparation instructions online ... this one has always worked really well for me: http://justhungry.com/milking-soy-bean-part-1-soy-milk with all steps and techniques described in detail, and you will also find some continuing posts on how to further turn it into tofu, if you wish so.
Depending on further processing steps and your personal preferences you might choose one recipe/preparation method over another, so I will not recommend any specifics, but in general this should set you on the right track.
p.s. I found that straining/squeezing through cheesecloth/cotton muslin/nut milk bag over a colander is a winning technique ... no need to go any fancier.
This is the procedure passed down to me thro my mom.
Let's say we are using two quarts of beans.
Ensuring the purity of texture and taste.
Compulsively ensure the beans are as clean as possible from impurities.
Such compulsion is especially necessary if you wish to coagulate the soy milk into soy custard. We need the texture of the milk or custard to be as smooth as possible.
My recommendation: Use whatever compulsive method you are comfortable with to rinse the beans.
I would rinse the beans first, once or twice in tap water. Then soak it for another half an hour before rinsing them again.
Double layers of muslin cloth used as strainer. Muslin cloth must be soaked in mineral/purified water to compulsively remove detergents and tap water chlorine. I would microwave the muslin cloth being soaked to about 100F, uncovered to let whatever chlorine escape.
You might even wish to use mineral water rather than chlorinated water for any of the water used in the next sections. Ensure the taste of the bottled mineral water you use has the taste that you like. Otherwise use bottled purified water.
Containers - if you don't mind plastic particles, steel flavour, aluminium flavour or bpa leaching into the beans/pulp, otherwise, be prepared to invest in large corning/pyrex ware that are totally glass/porcelain. Don't even think of using the earthen pot which you frequently used for pot roast unless you wish to have a hint of bacon flavour in the milk.
I think you should just use the cheapest container and not bother with the mineral/purified water for your first time. Or until you have gained sufficient experience to make premium soy milk.
Producing the pulp
I should soak soy beans for 24 hours. The water level should cover the beans in excess of at least half the volume of the beans. i.e., if we use two quarts of beans, the excess water level above the beans should be at least a quart. Cover the pot to keep the heat in.
But: I would soak it for 24 hours in water kept lukewarm with lowest possible heat on a stove. The idea is to keep it warm to encourage water absorption but not too warm to encourage soybeans nutrients from leaching out to the water.
OK, confession, I can't remember if soak for 24 hours or 8 hours.
You would see the beans having absorbed much of the water. Add water to ensure water level half an inch above the beans. Raise the heat to bring the water to boil. Turn the heat down to keep the water simmering. Cover the pot. Simmer the pot of beans for four hours. Check the level of the beans regularly to ensure sufficient water level above the beans.
I am thinking the next time, I would use a slow cooker for this step.
After the 4 hour simmering, drain the water. Grind the beans into a mush with a blender. Be compulsive in ensuring the mush particles are as fine as possible. I would grind no more than a third of the volume of the blender container each time.
Boil the soy pulp/mush with sufficient water. If you wish to have thick soy milk, especially if you wish coagulate into soy custard, ensure water level is no more than a quarter inch above the beans. Because we are now boiling pulp, the heat transfer of the pulp is impeded by its thick viscosity. We need to ensure the pulp at the bottom is not burnt and ensure heat is transferred to the top of the pulp by stirring. You would need to constantly add water to the pulp to balance between how thick you wish the soy milk to be vs how easy you prefer your stirring job would be.
When the pulp comes to a boil, simmer it for another half hour, by repeating the balancing act of stirring and adding sufficient water. I guess after a few times, I should be experienced enough to know how much water the pulp would absorb so that I would add all the water the pulp needed at the beginning to make my stirring job easier at first. The last time I made soy milk/custard was four years ago - so, I don't remember.
Squeezing out the milk
If you plan to make soy milk production your hobby, you would buy soy squeezing equipment. Squeezing the hot pulp by hand using the muslin cloth is the least enjoyable part of the process.
Each time scoop the pulp onto the doubled-up muslin cloth just enough so that you could fold the cloth into a cone to squeeze it.
You have to balance between how hot the pulp you could handle and how inefficient the fluid transfer is, from cold pulp - in your consideration in how cool you wish to wait for the hot pulp to cool down. You should consider using latex gloves that have been compulsively cleaned to handle the hot soy lava. Be prepared to take precautions to prevent hot fluids from being squeezed onto your face and especially the eyes or any body parts.
Two passes of squeezing
The first pass of squeezing is for thick milk you wish to make into custard.
Boil the dry pulp again under the same previous procedure, to repeat the muslin squeezing. This 2nd pass is for soy milk.
Making soy custard is another art altogether, techniques and timing you need to develop yourself to make firm and smooth custard.
Basically you pour the thickest fresh soy milk you have produced into a wooden container (e.g., wooden salad bowl) or corning ware and stir the right amount of coagulating salt.
Let it sit for hours. I can't remember the salt compound, but I'm sure I could google it he next time I plan on making soy custard+milk+chicken.
Do not throw the residual pulp away. It is for making soy chicken.
Make a paste from tapioca flour. Blend the tapioca paste into the pulp. The amount of tapioca paste must be sufficient to make it a binding agent for the pulp.
I have poor experience in this area. The last time I made this soy custard+milk+chicken, I actually used the residue to make real chicken meat balls by blending real chicken meat and cilantro into the tapioca and soy pulp mix (what my mom would do). I guess there wasn't enough tapioca and the chicken balls were falling apart.
What I need to perfect is having tiny bits of fresh vegetable and meat content in the soy chicken balls without the balls falling apart.
Roll the blended mixture into small spheres, cigars or patties with your fingers and palms constantly covered with tapioca flour and brine (to prevent the patties from sticking to your skin).
Dump the soy balls, cigars, patties into an already hot boiling pot of water. Fish them out after a minute or two.