My title is, admittedly, a little misleading. You and carrageenan have almost certainly met before -- probably hundreds, if not thousands, of times.
That's because carrageenan is one of the many mystery ingredients you see (or choose not to see, in some cases) on ingredient lists everywhere. It's a polysaccharide (complex sugar) which is used as a commercial thickener, extracted from red seaweed.
Although you'll find it in just about everything, carrageenan is frequently used in beverages to mimic a creamy texture where one does not actually exist.
Like, I don't know, non-dairy milks and low-fat milks? As it turns out, yes.
Silk, the big brand name in soy milk, uses the thickener in their products. So too does Starbucks use it in their Silk knock-off.
Now you might think milk is creamy and thick enough. Apparently, you would be wrong. Skim milk frequently uses carrageenan. Which one might expect, given that skim milk has the texture of chalkly water otherwise. But even some brands of half & half and heavy cream (seriously) aren't thick enough in their natural state, apparently.
Carrageenan's ubiquity wouldn't be worth writing about, if not for two things: First, many people (myself included) prefer their foods to be simply what they are. Second, a rat study suggests that carreegeenan may cause cancer.
Wait, hold the phones. Cancer? Seriously? What doesn't cause that now days?
This might be cause for concern. Might be, because humans are not rats, the amount of carrageenan which was shown to be potentially dangerous was way more than one could reasonably ingest, and the product was degraded. This strikes me as similar to studies which show that OXIDIZED cholesterol (not the kind you eat) caused heart disease in rabbits (not capable carnivores like humans).
In this case, vague correlation does not imply, much less guarantee, causation.
Which means, at the end of the day, don't worry about it. Do I prefer my soy milk (Westsoy) to be only soybeans and water? Sure. But I don't think that a few glasses of Silk will give me cancer, either. Nor should you fret over whether your milk is just milk, or contains other stuff.