Why did my soy (Silk) milk suddenly become as viscous as rubber cement?

Soy milk can spoil, just like ordinary cow's milk. That seems to be what happened.

While spoilage in cow's milk is usually souring, and the smell is unmistakable at fifty paces, soy milk spoils by turning gooey. I'm not sure about the details, but it's something in how the proteins react to oxygen. In soy milk, if I'm not mistaken, they turn into longer, stickier chains.

As for why it spoiled, it was probably just open for too long. Even in the fridge, soy milk has a life of about 4-6 days once opened. The seal may have been damaged somehow even before you actually opened it, allowing air into the carton to cause the spoilage.

Do you have soy yogurt in the same fridge? Soymilk based yogurt with live cultures, if somehow able to cross contaminate your milk, could cause fermentation to occur. I've never used soy milk, but I know the same process for making regular yogurt is the same as the process for making soy yogurt, just with a different set of cultures. Short answer - it sounds like some buggies may have gotten into it. Give it a taste and see if it's pleasant or not.

I've used soy, oat and other milk alternatives for many decades. One reason is how milk turns 'sour' often within two or three days in the fridge - quicker after the bottle or container is opened.

Different brands of soy milk can behave differently, some brands are too much like real milk and sour nearly as quickly after opening, but others seem good for up to a week. I don't drink as much as I did - so I need milk alternates that last a little longer than cow's milk.

However, some brands just get a little thicker or gluggy within three of four days - as the solid matter settles onto the bottom of the container. I just add some water if it is just thicker without a change in taste. I practice giving the container a shake before using, but now wonder if I'm 'churning' it by doing this. Sometimes the gluggy matter will blend with the watery part, but sometimes it just stays in huge lumps.

I rarely had this problem, but these days, with the brands in Aldi, I end up throwing about half a litre out and starting a new carton. It seems like keeping it on the shelf for a few weeks now, also brings clumping on quicker than I remember over the last few decades, even though I immediately put the carton back in the fridge after pouring.

So, I'm wondering if I can add a yoghurt culture powder into the clumpy soy [or oat] milk, and get a yoghurt from it? I don't like wasting half of my soy milk.

Spoken to generally here, (the wiki on Tofu), and stated more specifically here

Tofu is manufactured by coagulating proteins in soymilk with magnesium sulfate. As bonding occurs between the positively charged magnesium ions and negatively charged anionic groups of the protein molecules, the proteins coagulate.

Since magnesium sulfate, also known as Epsom salt, is a common household item, anyone with a cursory knowledge on the matter can turn soy milk into the base for tofu with little effort. Perhaps a prankish roommate?