How to know when to refresh a liquid sourdough starter, based on its signs
Maintaining a liquid sourdough starter in good condition requires knowing how to read its signals: when it is ripe to refresh it and when it has started to become too acidic.
The key to achieving maximum fermentation is observation. The sourdough starter goes through several phases each day, although the intervals vary depending on the temperature, the flour, and the time between feedings: if the fermentation is slow, the signs appear more frequently, and if it's fast, at shorter intervals. Every sourdough starter is unique, so each baker ends up adapting the method to their kitchen and schedule.
As an example, at 10:00 the mature sourdough starter is refreshed with 70 g of white flour, 30 g of rye flour, 20 g of mature sourdough starter and 100 g of water at room temperature, with the kitchen at about 23 °C. At 12:00 there is slight activity and a very sweet aroma, practically that of the flour and water.
At 2:00 PM, four hours after being refreshed, the dough had risen by just over 100%, with a domed surface and a few bubbles appearing. At 3:00 PM, the expansion was even greater, with many more bubbles on the surface and sides, indicating that fermentation was progressing, although the dough wasn't quite ready yet. The aroma remained sweet, with a hint of acidity beginning to emerge.
At 5 pm, there are many more bubbles on the sides and surface, and the acidity intensifies. At 7 pm, the dome flattens, a sign that the upward growth has slowed. By 8 pm, the starter is mature: it has more holes and bubbles and a slightly acidic and vinegary aroma with hints of sweetness. It can then be used to prepare a preferment or to knead bread, and afterwards, it's advisable to refresh the starter again.
Around 10 p.m., it begins to show signs of maturity: streaks on the top and a center that starts to collapse. What matters are the cumulative signs: a general breakdown of the mixture, a more intense acidic aroma, and significant carbonation. That's the ideal time to refresh it.
If left to ferment overnight, the dough will sink and the center will drop noticeably, with longer streaks appearing on the sides. By 6 a.m. the next day, the sides will be stained and the surface covered with small bubbles; by 8 a.m., the aroma will be very acidic and vinegary. If left even longer, a clear, alcoholic, and bitter-tasting liquid may form on the surface. To revive the dough, this liquid is removed, the remaining dough is mixed in, and it is refrigerated as usual.
To avoid excessive acidity, which will alter the bread's flavor and make it more acidic, don't let the sourdough starter collapse and rest for extended periods. If it ferments too quickly for your schedule, you can prepare an intermediate batch, discarding the first portion and adding fresh flour and water. Use your sense of smell at each stage and stir thoroughly until no lumps or traces of dry flour remain.
If your water has high chlorine levels, use filtered or distilled water, or let it sit in a bottle overnight before using it. To speed up fermentation, increase the water temperature, the amount of mature sourdough starter in the jar, or use more whole wheat flour, which accelerates fermentation; to slow it down, reduce these factors.
