Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
KitchenMage asks via twitter:
"Disappointed to find that Instant Pudding doesn't thicken with soy milk. Why not?"
Instant pudding is a miracle of late-1940s food technology. Gone are the days of heating up milk to a 140° or so, along with all of the associated headaches: preventing the milk from scorching, stopping that nasty skin from forming, and the sheer amount of time and effort it took. Instead, you just mix stuff together and pudding happens. Woohoo!
Thickening all happens in pretty much the same way: you're trying to get some sort of solid evenly dispersed within a liquid medium in a particular concentration. With pudding, you're generally using corn starch, which absorbs water at the appropriate temperature. Like one of those novelty sponges that turn into dinosaurs when placed in water, the cornstarch granules expand like crazy while just floating around. In addition to just becoming bigger, strands of starches begin to break free from the granules and stretch between the granules, forming a mesh that holds the water and the starch in place. That gives you thick pudding.
Instant pudding, naturally, doesn't work like cornstarch does, because you never heat the instant pudding. Hence the appeal. So clearly some other method is at work. The other method of thickening in this case, according to the ingredient list on Kraft's Jell-O Instant Pudding, is the combination of Tetrasodium Pyrophosphate and Disodium Phosphate. And the one that seems to cause the trouble is tetrasodium pyrophosphate.
With these sort of industrial food chemicals, there are generally many different uses from emulsification to controlling pH to who-knows-what. Tetrasodium pyrophosphate, among its other uses, encourages casein proteins to group together. In milk, the casein protein also known as the nasty skin that forms on regular pudding when you cook it and cool it. The tetrasodium pyrophosphate is the pied piper of the protein world. It plays its magical tune, and gathers most of the casein and calcium that were floating in the water. The tetrasodium pyrophosphate joins the casein and the calcium together and they form into long chains. Those chains bind together and trap water between, just like any good thickening agent.
Soy milk contains no casein proteins, and really doesn't have all that much calcium compared to cow's milk. Consequently, there's not much there for the tetrasodium pyrophosphate to work with. Adding some milk solids, or mixing half soy milk and half cow milk, will take care of the thickening, but that doesn't particularly help out vegetarians. Vegetarians will either want to go with an instant pudding made to work with soy milk, or they'll want to go the traditional route with their pudding making.