I just made two batches of yogurt, one using a cooler and another one using sous vide. As a starter, I used yogotherm cultures. All the milk was heated and cooled to the same temperature, at the same time, in the same pot (82C and then 42C). The sous vide was set to 42C and the yogurt in the cooler had some towels and hot water bottles around it, and was placed near the fire place. I let both batches of yogurt stay at this temperature for the same amount of time (7 hours or so).
The result was interesting... both batches came out with the same consistency, but the sous vide yogurt is a lot more sour than the cooler one. The only difference between the two batches is really the temperature at which they were held.
Can anyone explain to me what causes sourness in the yogurt, and why the sous vide batch came out so much more sour?