My soymilk can spoil in a mere two days. Dairy milk is disgusting when it spoils. Mold spores in my living space is the cause. Borax is the solution. A quarter teaspoon of borax and a light sprinkle of nigari solves the problem.

Find nigari on Amazon.
Find fragrance free, granular, pentahydrate borax (bottled in plastic bottles to prevent absorption of fragrance from being stored in a warehouse with other borax products) at Soap Goods.