When the yeasts are aerobic, they are at their most active. Of course, when the yeast ferments in the absence of oxygen (anaerobic fermentation) it produces alcohol and slows down, which is why when you see a sourdough culture that has been left to ferment for a while without being aerated or refreshed it develops a thin layer of water and alcohol (called hooch) on the surface as explained above.
This alcohol provides a catalyst for additional flavour dimension to mature starters in the form of iso-alcohols which contribute to the esterification of organic acids with alcohols. In other words you get esters forming, which makes your starter smell lovely. It smells of fruit – ripe apples, pears and pineapple. The reality is though that when your sourdough is at this point, it is delicious smelling but way too acidic. It needs refreshing ASAP.
