Soy milk does spoil just like cows milk. The first sign that soy milk is going bad will be the presence of curds in the milk, just like with regular milk. After that, bad soy milk will begin to expand inside the packaging and you will notice that the container becomes bloated or hardened. Those are the easiest ways to tell if your soy milk is spoiled.
One of the benefits of soy milk, however, is that it does enjoy a much longer shelf life than regular milk. Many brands of soy milk are stored unrefrigerated even though they do not contain any preservatives due to unique packaging that enables the product keep fresh.
Unrefrigerated soy milk often has expiration dates of about one year after manufacture, which is quite an impressive length of time. Soy milk that is sold in your grocers refrigerator usually have much shorter shelf lives and go bad about one month after manufacture. Of course, always refer to the expiration date printed on the top of the container to verify how long that particular batch is expected to stay fresh.
Once a container of soy milk is opened, it should be consumed within a couple of days, and should not be expected to stay fresh longer than that. Once opened, a container of soy milk should also always be refrigerated otherwise it will go bad in a matter of hours.
Soy milk can be judged for freshness by its smooth consistency and lack of lumps which become present when it starts to spoil, just like cows milk. Also, you can give it the “sniff test” because as soy milk starts to go bad it will give off a sour odor.