When scaling up yeasted bread doughs, it is logical to reduce the proportion of yeast. But does the same apply to sourdough breads, where the Production Sourdough (PS) is the leavening agent or ‘yeast’ of the main dough?
No. Keep the ratios of PS to final dough the same whatever the size of dough. This isn’t to say, however, that all recipes have the same percentage of PS – far from it. But here we’re talking about one particular recipe: you want to make a batch that’s, say, four times the size of your normal batch. If it were a yeasted dough, it would be desirable to reduce the percentage of yeast for the following reasons.
The key issue here is actually temperature. Very small doughs, i.e. below 3 kg in weight, tend to lose heat in an average British kitchen unless placed deliberately in a very warm place. However, large doughs cannot so easily dissipate the heat generated by both the energy of mixing and the process of fermentation because they have relatively less surface area to weight. Commercial bakers often used cold water or even ice in summer to keep large doughs cool enough after mixing. Since most bakery books tend to ignore the benefits of long fermentation, it has become customary to increase the amount of baker’s yeast recommended for small dough quantities in order to compensate for the gradual loss of heat that usually occurs in domestic baking. The dough would rise perfectly well with less yeast if given time, but people have been encouraged to be impatient and to assume that if the dough hasn’t ‘doubled in size’ after an hour or so, something is wrong.
The yeasts in a natural (sourdough) fermentation will, like any yeast, work slower at lower temperatures. But because they are fewer in number (by orders of magnitude) than the yeasts added from a highly concentrated source (i.e. a packet), the speed difference when the dough gets a bit warmer is fairly modest. There is therefore no real need to vary the amount of production leaven or sourdough as you scale up your doughs. But do keep an eye out for the temperature issue.
In fact the proportion of production leaven or sourdough in the final dough can be varied considerably with only modest effects on the character of the bread. But here is an interesting (apparent) paradox. You’d have thought that the more PS in your final dough, the sourer the bread would be. But the opposite appears to be true, certainly if your final dough has around 60% PS in it as opposed to the more normal 30% or thereabouts. The reason appears to be this: it is the lactic acid bacteria (LAB) that produce the acids (acetic and lactic, mainly) that create the sour flavour. LAB work more slowly than yeasts. More PS means more yeasts to raise the rest of the dough. The dough is therefore ready for the oven a bit sooner, and before the LAB have had time to produce as much acid as they would have over a longer period. Result: milder flavoured bread.