Although it might seem as if soy milk, as a plant-derived product, shouldn't go bad as quickly as cow's milk, it does spoil eventually. Soy milk is derived from soybeans, which, like all plants, eventually decompose. Always follow package instructions -- and your own senses of smell and taste -- to determine whether a product is safe to consume.
Expiration Date
Your soy milk carton should have an expiration date stamped on it. The product should be good until then if unopened; once the carton is opened, the soy milk may only last for seven to 10 days. Signs of spoilage include a lumpy texture and foul odor, much as with regular milk.
Shelf Stable
If you want your soy milk to last a little longer, buy the "shelf-stable" variety. Shelf-stable soy milk is found in the aisles of a grocery or health food store rather than in the refrigerated section. These cartons do not need to be refrigerated until they're opened. Shelf-stable soy milk does have an expiration date, but it could be good for up to one year after you buy it if it remains unopened. Once opened, it must be refrigerated and consumed within a week or two.
Soy Benefits
Most commercially sold soy milks are fortified with nutrients that people who don't drink regular milk may be missing, such as calcium and vitamin D. They may also contain added ingredients that supplement a vegan diet, such as vitamin B12, which only occurs naturally in animal products. Soy milk also contains soy protein, a useful nutrient for vegans and vegetarians who may worry about adequate protein intake.
Soy Drawbacks
Take caution when feeding soy products to infants, as soy may affect their thyroid hormone levels and could contribute to eczema. Soy milk is not an acceptable substitute for baby formula. If you have breast, uterine or ovarian cancer, ask your doctor about drinking soy milk or eating other soy protein products; soy has been linked to estrogen-mimicking effects, which could stimulate tumor growth in these hormone-sensitive cancers.
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