This is where the fun happens, at least for me. The milk is 120 or so degrees and the starter (room-temperature commercial yogurt) goes in. The cooler starter brings the temperature of the milk down to about 115 degrees Fahrenheit, at which point the lactic acid bacteria start gorging themselves. Seizing the closest available lactose molecules, the bacteria hit them with enzymes that break the lactose into its constituent parts (galactose and glucose), which the bacteria can then convert into energy.
First to the buffet are the Streptococcus salivarius subsp. thermophilus (that last part of their name literally means “heat loving”) which thrive in warm, low-acid environments. But here’s the rub: they produce lactic acid as a by-product of their metabolism. So as they chow down and reproduce the acid level in their environment rises, until it reaches roughly half a percent of the volume of the milk. At that point the Streptococcus essentially begin to poison themselves with their own waste. And that, as it happens, is just the cue that the Lactobacillus delbrueckii are waiting for. They thrive in higher-acid environments, and so pick up where the Streptococcus left off: eating, reproducing, and eating some more until they too succumb to a combination of acidity (which by now is about 1%) and lack of food.
While this tiny drama is playing itself out, something else that’s very interesting is happening. The rising acidity causes the proteins in the mixture not only to uncoil, but to become increasingly attracted to one another. This has the effect of creating the tangly mesh I spoke about earlier, and a gel forms.
The whole process can take as little as two hours or as many as 18 depending on the kind of microbes involved and the temperature. The standard Streptococcus/Lactobacillus tag team will get the job done in just a few hours when the temperature is at the top of their preferred range, about 115 degrees. At 90 it’ll take a heck of a lot longer, though interestingly, the longer gelation will produce a different texture. Whereas as rapid gelling will yield the much coveted Jell-O-like consistency, the protein network will be coarser and more prone to breaking (commercial yogurt makers compensate for this by adding starch or other “stabilizers” to prevent this from happening after their products have been shipped). The long-gelling yogurt by contrast will be runnier, but since its protein network is finer, it will be a smoother and more uniform.