Maia

Two kinds of microbes share the flour. Lactic acid bacteria outnumber the yeasts about a hundred to one. The bacteria make the sourness, the yeasts make most of the lift, and both make CO₂. Each has its own temperature curve: the bacteria like it around 32°C, the yeasts nearer 27°C, and at 4°C both nearly stop.

Food runs out, acid builds up. Flour brings a little sugar, and its own enzymes keep cutting starch into maltose. As acid builds, the bacteria stop growing first; the yeasts tolerate lactic acid, though not much acetic. When the sugar runs out, the gas leaks faster than it's made, and the dome falls, leaving streaks on the glass.

Cool and stiff is sharper. Heterofermentative bacteria split their sugar between lactic and acetic acid. Cooler, stiffer, wholegrain starters lean acetic (vinegar); warm, liquid ones lean lactic (yoghurt). Left hungry, the alcohol separates out as hooch, and esters bring the nail-polish smell.

It's hard to kill. Acid keeps moulds away for weeks. A neglected starter loses yeasts faster than bacteria and wakes up slowly (a longer lag), so the first feed after a long sleep barely rises. Two or three feeds bring it back.