You make bread regularly using a rye Production Sourdough as specified in the Bread Matters book. Normally this ferments quite quickly but sometimes, for no apparent reason, it doesn’t and the resulting bread takes ages to rise and tastes very acidic. What’s going on?
The main variables are temperature and flour quality. It is unlikely to be the latter unless your flour is very old, in which case some degradation of the bioactive components may have taken place. Or it is sometimes the case that a new batch of rye flour has a different enzyme activity: there were some cases of this after the bad harvest of 2012 and several people reported that their rye sourdoughs were fermenting unusually quickly.
So that leaves temperature. If your sourdough gets very warm (say 30°C or more), it will ferment quickly and might be overfermenting. When this happens, the yeasts in the flour use up the available nutrients quickly and stop working – so you see no froth or bubbles. At the same time, the elevated temperature makes the lactic acid bacteria work faster. They produce acids which lower the pH of the dough (i.e. make it more acid), further inhibiting the action of the yeasts. If you use a production sourdough in this condition, the yeasts will be less active than normal, which means that they will take longer to produce enough gas to raise the bread. But, at the same time, the increased acidity of the production sourdough will make the final dough more acidic and this inhibits the action of the yeasts, allowing more time for the final dough to become more acidic…and so on, in a vicious circle.
What can you do? First, try to maintain some consistency in the temperature of your production sourdough. If the weather is very warm, put it in a cooler place. Monitor the temperature if you can and keep it below 30°C. Second, if in the morning you notice that the production sourdough appears to be rather ‘dead’, i.e. there are few bubbles and perhaps even a layer of liquid on the top, try using only half of the normal amount in the final dough. You will need to readjust your fresh flour and water quantities to achieve the same yield of loaves.
The point of this is to dilute the production sourdough to a larger extent than normal so that there is less acidity to inhibit the yeasts.
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