Heat 2 quarts of soy milk on medium heat in a saucepan until the temperature of the milk reaches 110˚F. Note: I measure out the milk in the jars before heating it, because a 1/2 gallon box of soy milk usually has a bit more milk in it than what fits in the jars, and I hate wasting anything! If the temperature of milk goes up above 110˚F, let it cool back down to 110˚F before adding starter culture.
Once soy milk reaches 110˚F, remove from heat and whisk in the contents of the packet of starter culture. Quickly and carefully dump out the hot water from the jars and fill them with the cultured soy milk. You may want to use a ladle so you don't lose any milk when pouring.
Seal jars with the sterilized plastic lids, but not too tightly.
Incubate jars for anywhere from 8-24 hours until cultured soy milk is thickened to a yogurt consistency. Soy yogurt will look a bit clumpy and watery compared to store-bought yogurt, hence why I strain it to produce labneh.
At this point you can refrigerate the yogurt or continue on to make labneh. See below...
Place a sieve over a bowl and line it with a lightly dampened coffee filter or several layers of cheese cloth (or possibly a nut bag would work too). Note: Chemex style coffee filters are perfect for this because they unfold into a large square.
Pour 1 quart of the yogurt into the coffee filter and you will immediately see the whey being sieved from the yogurt. I sieve 1 quart at a time but you could set up another sieving apparatus and do them simultaneously if you wish.
Fold the edges of the coffee filter or layered cheesecloth over the top of yogurt and return it to the fridge to complete sieving. Pour off whey if it fills bowl enough to touch the sieve. Save whey for other uses if you wish (google it).
Let it sieve for anywhere from 2-12 hours until desired consistency is reached.
Transfer labneh to a bowl, using a rubber spatula to scrape off any labneh that sticks to the coffee filter. Labneh will still look a little clumpy at this point. Whisk labneh until it is smooth if you wish to eliminate clumps. The clumps melt right out in your mouth but may not look as appealing as when whisked smooth. Enjoy!