This website is conservative, what surprised me was their expectation to heat the milk to no more than 165 degrees. Until I read it, I had also heated my milk to 180 degrees.
The 165°F step is for pasteurization and is omitted when using commercial pasteurized milk. The NCHFP website recommends heating at 200°F (93°C) for protein denaturization, somewhat higher
than usually suggested.
It's the holding-at-170 that seems to thicken the yogurt up so well, but next time I'll try 165 and see how it does. I don't want to muck about with adding milk powder, but I suspect that holding it at the high temp for a while before dropping to 110 for inoculation and the longer ferment results in some evaporation; essentially, it does the exact same thing as adding the dried milk... increases the ratio of milk solids to liquid.
Evaporation shouldn't be too significant if the pot is covered during heating. A major effect of this step is the thermal denaturation of whey proteins. This results in more stable gel formation as the pH decreases during the 110°F (43°C) incubation. There are other effects too.
Before addition of the starter culture, the milk is heated to 80-90°C [176-194°F] for about 30 min. Being well in excess of the normal pasteurization requirements for safety, this has a substantial lethal effect on the microflora. All but heat-resistant spores are eliminated so that the starter culture encounters little by way of competition. The heat process also improves the milk as a growth medium for the starter by inactivating immunoglobulins, expulsion of oxygen to produce a micro-aerophilic environment, and through the release of stimulatory levels of sulfhydryl groups. Excessive heating can however lead to the production of inhibitory levels of these compounds. Heating also promotes interactions between whey or serum proteins and casein which increase the yoghurt viscosity, stabilize the gel and limit syneresis (separation of whey).
Bring 1/2 gallon whole milk to 180 degrees F (I use a thermometer with an alarm to alert me when it reaches the right temperature).
Turn flame to as low as it goes and put a lid on the pot, and try to hold it at about 170-180F for about two hours.
I'm curious why you heat your milk for two hours, significantly longer than usually recommended (though your temperature is slightly lower). If you're happy with your procedure, that's all that matters and you ought to stick with it but to some dairy scientists longer isn't better.
A further important effect of heating is the increase in hydrophilicity [tendency to interact with water] of the proteins which reduces syneresis [expulsion of whey] and increases gel firmness. This is a consequence of the covalent attachment between κ-casein and β-lactoglobulin which results in a new surface structure with fewer exposed hydrophobic groups. Maximal hydration is obtained by heating milk to 85°C [185°F] for 30 min, the yoghurt produced after this treatment exhibiting true thixotropic [gel-like] behavior. Increasing the severity of heat treatment further increases hydrophobicity and leads to syneresis and a poor quality yoghurt.
Who knew making yogurt was so complex?