How to make soy yogurt
It's easy to make soy yogurt at home. The soy yogurt is not identical to dairy yogurt but it works well in cooking. To make 1 liter soy yogurt you need 140 gram soybeans and a starter. As a starter you can use good quality plain soy yoghurt, or dairy yogurt (if you don't mind dairy products), or a commercial powdered yogurt starter (also dairy based).
Making the soymilk
This step is not necessary if you start from bought soy milk. Put the soybeans (ideally split soybeans) in a pan and poor 3 liter boiling water over them. Let it cool down and wait 6 to 12 hours.
Drain the beans, add 1 liter cold water and mix it in a blender for 3 minutes. Remove the soymilk from the solids by squeezing the mixture through a cheese cloth. Bring the soymilk to boiling point and continue to boil for 10 minutes.
Culturing the soy yogurt
Cool the soymilk down to 42-45°C (either by cooling the boiled soymilk or by heating the bought soymilk). The culture will only thrive in a narrow temperature range, too cool and it won't be active, too hot and it will die. Measure the temperature with a thermometer. Add 4 tablespoons of starter to the soymilk and mix well with a sterile spoon. Put the yogurt in a yogurt maker cups and follow the same directions as for dairy yogurt. If you don't have a yogurt machine you can put the yogurt in an oven at 42-45°C. After about 5 - 6 hours, when the yogurt gets firm, chill the yogurt.
Making the soy yogurt thicker
You will notice that home made yogurt will be a little runnier than dairy yogurt. To improve the thickness of soy yogurt you can add one level teaspoon of agar powder or two teaspoon starch, premixed in 50 ml water, to the soymilk when it starts boiling. The best option is to add extra soy protein, at least if you can get hold of it. One tablespoon soy protein should be added to one liter soy milk before boiling.