Monika Bartyzel on Slashfood did an interesting article recently on altering the amount of yeast that you use for cold-fermenting bread. The idea behind cold-fermentation is that that you keep the dough cold so that the yeast aren't particularly active. This allows the various enzymatic activities with the dough to happen on their own over time, increasing the flavor of the bread. That works especially well with non-enriched breads. There was a post that Monkia refers to that discusses a specific recipe someone is developing for a cinnamon bread that slow rises. In the comments of that post, someone suggests the baker's formula:
Original Amount of Yeast * Original Fermentation Time
Now, the commenter didn't say explicitly that this formula was for cold-fermenting breads. Also, I have to say that I'm a little suspicious of the simplicity of the formula. It could be that everything just works out fine with it, because there are a lot of close-enoughs that make it work out. But yeast don't reproduce in a linear fashion, they reproduce exponentially. Under ideal conditions, yeast will double in size every generation.
So instead of starting with 2 yeast, then having 4 the next generation, the 6 the next, then 8, 10, 12, and 14, we start with 2, then 4,8,16,32,64,128,256. After a while, the yeast by-products, alcohol in particular, will kill off the yeast, so they can only go so far before they all die off.
However, given their exponential growth beforehand, you can see that the amount of time that passes should eventually have a much greater effect on yeast reproduction than the amount that you reduce the initial batch by. So if I started with 30 yeasts instead of 60 yeasts, according to the formula I would be able to double the amount of time that it takes the bread to rise. But let's assume our target is 6000 yeasts,
With the 30 yeasts it would take: 30, 60, 120, 240, 480, 960, 1920, 3840, and over 6000 the next generation, or about 9 generations.
With the 60 yeasts, it would take: 60, 120, 240, 480, 960, 1920, 3840, and over 6000 the next generation, or about 8 generations.
That's not a huge time difference, and it gets smaller the longer you let it go (to a point). Of course, there are other factors. There's the amount of food available (the sugars and the potential sugars), the temperature of the environment, and if there are any wild yeasts ready to jump on the bandwagon.
With the cold-storage method, you control the temperature and the ability for wild yeasts to interfere, so that may help settle things down into what is, for all intents and purposes, a linear scale.
So, while I'm not saying that the formula is wrong, I am saying that it looks suspicious. A little too easy. Quiet… too quiet. I've got a bad feeling about this. I do not think it means what you think it means.
It's probably a good starting point, but I will do some experimentation in my own kitchen before I decide that I can put this dough in my fridge for almost exactly 16 hours and be ready to have perfect bread in time for my dinner party that night.
New Fermentation Time
New Fermentation Time