the ratio of fermented starter to fresh flour and water is critical in the development and maintenance of a starter. this ratio is called the "refreshment ratio". higher refreshment ratios are associated with greater microbial stability in the sourdough. in san francisco sourdough, the ratio is 40% of the total weight, which is roughly equivalent to 67% of the new-dough's weight. a high refreshment ratio keeps acidity of the refreshed dough relatively low. acidity levels of below ph 4.0 inhibit lactobacilli and favour acid-tolerant yeasts.