I buy 8th Continent Vanilla soy milk. The expiration date is usually several months after the date of purchase. But on the side of the bottle, it says (paraphrased) “Finish Bottle Within 10 Days of Opening”. Now, I’ve gone past this 10 day period in the past with no ill effects. But I wouldn’t go beyond 15 or so days. What does opening the bottle do to make it expire so quickly? Or is this just a trick to make me purchase at a faster cycle?
Usually (always?) the bottles are sterilized after they are sealed. So there are no organisms to cause it to spoil until you open it.
Once you open the packaging bacterial contamination is assumed to occur. Ten days is the amount of time the seller feels the product is still “safe” (not spoiled) after contamination.
Of course, you can judge by odor, color, etc. For me, all milk smells bad. I buy a quart or smaller amount at a time, use it for cooking, and throw out what is left in a day or two.
Then, pray tell, what does bad soy milk smell like?
The last time I read the ingredient list on a bottle of 8th Continent, it had so many additives I stopped reading. I assume at least one of those was a preservative.
FWIW, the soy milk I’m drinking at work right now, WestSoy plain, says “Best if used by 05 May 04”. (?! When did I buy this?! Is that May 5th, 2004 or May 4th, 2005?) It’s been open since 12 days ago (I usually just drink water at work since it’s free) and it tastes just fine; I suspect yours should last at least as long.