Runny yogurt again

Once again I wound up with a batch of yogurt that did not set well. It still tastes alright, like it is supposed to, but it is very runny. This happened once before when I was using whole milk and cut the milk powder. Back then I speculated that the milk powder was indeed necessary for well set yogurt, and when the next batch (which included milk powder) was set again I felt my speculation had been confirmed.

The problem is that I did include milk powder in this batch and it still did not set. So I have to look at alternative explanations. The two most obvious are the ones I had already considered the last time: That my starter has somehow gone bad, and that my milk/starter mixture was already too much cooled down when I transferred it into the oven.

I don't think and I don't want to think it is the starter. That there was some thickening and souring suggests that the starter was definitely alive. Apart, I am growing emotionally attached to my starters as "mine", and they just wouldn't feel like mine if I was forced to replace them with new starter cultures from the store every so often.

So I am stuck with the theory that incubation temperature was too low for optimum result. Which actually makes some sense; the temperature of the mixture did seem a bit lower this time than it usually is. As it was with my last runny yogurt, as I recall. Following this approach I will make the following changes for my next batch:

• Stay next to the pot as it cools down so I can catch it before it cools too much. Don't rely on the thermometer but use the pinkie finger test.
• Keep the oven lamp on overnight while the yogurt is incubating. Hopefully, that will keep the incubation temperature higher for longer.

Also following the incubation temperature theory these variables will stay the same:

• The yogurt base will once again be one gallon of 2% milk thickened with one cup of non-fat milk powder.
• I will continue to use my own starter which will be a small sample of this runny yogurt that I just made.

I don't think there is a problem using runny yogurt for a starter because I used a runny starter successfully after the last time I had a problem. Besides the souring and partial setting of this batch suggest to me the  yogurt and hence the starter are indeed alive and hence biologically active.