There are several competing theories when comes to heating milk for yogurt. One holds that you needn’t heat the milk much at all. Another, that it must be heated to boiling and kept that way for hours. Still another, that the milk should get very hot but only for a little while. To help us evaluate which of these methods is the best, we’ll employ the handy tool of science.
Long-time readers of joepastry.com will know that proteins are long chain-like molecules. They’re made of amino acids, and are generally found balled up in little clumps, especially in things like milk. Heat them (or expose them to acid) and they “denature”, which is to say the chemical bonds that keep them clumped break. The molecules then uncoil into long strings that get tangled up with one another, forming a mesh. This mesh traps and holds other types of molecules like fish in a net, forming what is known in the world of high-end basketball shoe commercials as a gel.
It stands to reason then that the more protein you have the firmer your gel. Which is why yogurt makers of old would boil milk for hours. The long boiling evaporated some of the water and concentrated the proteins. The milk was cooled, the culture added and bingo: yogurt. Nowadays we don’t need to do that since we have protein-rich powdered milk available to us. This lets the modern yogurt maker add all the extra protein he or she wants without having to endure the laborious step of long-term boiling (then cooling).
This opens the door to the low-heat approach to preparing your milk, since if you’ve got plenty of protein, the only thing you really need to do is warm the milk to the point that it promotes microbial growth, about 120 degrees or so. The bugs start digesting the lactose and creating acid, which denatures the proteins and there you go: gel.
The only trouble is that not all gels are created equal, which is where the middle approach comes in: heating the milk to 195 for ten minutes, then letting it cool down 120 before introducing the culture. This produces a finer gel that won’t break as easily into curds and liquid whey when you insert a spoon. The operative question is: why?
The reason is because milk contains many types of proteins (caseins and wheys), not all of which behave the same way. Some denature when they’re exposed acid, others will only uncoil when heat is applied. One of them in particular, a protein called lactoglobulin, is especially useful in a gel as it prevents other proteins (notably caseins) from clumping too tightly together as the gel forms. Unlock its potential and you get a smoother textured yogurt with more stability that gives up less free water when the gel is disturbed.
But guess what it takes to get lactoglobulin into the milk gel game? That’s right: heat. So we raise the temperature of the milk to just below boiling, the temperature at which it uncoils. I should emphasize that it’s important to be careful here, because much more heat than that and the sensitive lactoglobulin will clench back up irrevocably and its stabilizing abilities will be lost.
You may well ask: do you really need denatured lactoglobulin to make a good yogurt? If people used to boil their milk, they clearly destroyed their lactoglobulin, but got along just fine anyway. Can’t I? You certainly can. I recommend that heating step because I think it yields a better product. Feel free to skip it if you want, I wanted to include it in the discussion so you had another arrow in your yogurt-making quiver, as it were.