Until Doubled

I had a friend inquire about how I bake bread (I've documented some of the bread I bake at panemography.tumblr.com). I explained that what is essential to good bread, in my opinion, is a long ferment time, easily 10-24 hours, as the yeast provides both the leavening and flavor. He reasoned, though, that he could cut that time in half by doubling the amount of yeast.

This, of course, will not work. And it stems from lack of understanding about using a living microbe as an ingredient.

A single celled fungus, baker's yeast (Saccharomyces cerevisiae) grows by budding off little daughter cells. In general, though, we measure the growth of yeast by the doubling time - that is, the average amount of time for the growing population to double, and this is about 1.5-2 hours for yeast. But this is dependent on the temperature, the type of food available, and the availability of oxygen. So, even at the simplest and most ideal case, doubling the amount of input yeast at the beginning of the ferment will only shave 2 hours off the bread making schedule.

But as in most things in science, it's more complicated than that. If you are using dried yeast powder (that stuff most people get in the stores in little packets), most of that is actually dead. The ratio of live cells in that mix goes down as time goes by (and it's worse if the powder is stored at a warm place - cold storage extends viability), so it can be pretty variable. And though the measured doubling time of yeast is 2 hours, that's usually measured at the logarithmic phase of growth, when the cells are already happily chugging along.

The growth curve of yeast. From biotek.com.

But as we begin from inoculation, there's a lag phase when the cells come out of sleep and need to get metabolically active. Some bread baking techniques try to make the process more predictable by doing a pre-ferment (sometimes called a sponge or a biga), where a loose batter is prepared with the yeast and allowed to ferment overnight. The actual bread baking then uses the sponge as the inoculum and part of the liquid to get a big charge of live cells, shortening the ferment time for the baking phase. But it doesn't really do away with the preferment period, it just takes it out of the equation.