The Fermentation

Once the milk has been heated, it’s cooled down to the desired fermentation temperature, the bacteria are added (often in a portion of the previous batch), and the milk kept warm until it sets. The fermentation temperature has a strong influence on yogurt consistency. At the maximum temperature well tolerated by the bacteria, 104–113°F/40–45°C, the bacteria grow and produce lactic acid rapidly, and the milk proteins gel in just two or three hours; at 86°F/30°C, the bacteria work far more slowly, and the milk takes up to 18 hours to set. Rapid gelling produces a relatively coarse protein network whose few thick strands give it firmness but also readily leak whey; slow gelling produces a finer, more delicate, more intricately branched network whose individual strands are weaker but whose smaller pores are better at retaining the whey.