Yeast gets most of the credit in sourdough — it is what makes the dough rise, after all. But the bacteria are what make sourdough taste like sourdough. Without lactic acid bacteria, you have leavened bread. With them, you have something considerably more interesting.

The distinction between homofermentative and heterofermentative LAB is the key to understanding why the same recipe tastes different when the temperature changes. It is not the recipe that changes — it is which bacteria are winning.

Acidity level also affects keeping quality. The organic acids produced by LAB inhibit mould growth, which is why a well-made sourdough loaf keeps for several days at room temperature where a commercial yeast loaf would not.