There is a new ‘trick’ making the rounds on You Tube. Whipping air into the natural leaven.
A few years back the You Tube Fashion was ‘the importance of folding air into the dough’.
Spoiler alert, research has shown that any oxygen mixed into the dough, or natural leaven is used up by the yeast within an hour and it gives no substantial advantage. However, the oxygen is not used up quite quickly enough to stop the flavour molecules in the leaven from being oxidised and destroyed in the process.
Yeast has two forms of metabolism. One using oxygen (aerobic), like ourselves and the other which does not use oxygen (anaerobic). In commercial yeast manufacturing air is bubbled through the yeast brewing medium because yeast multiplies faster when oxygen is available to it.
So, it would seem to make sense to whisk air into the starter to increase the yeast population? That way when the leaven is added to the dough there is a better yeast population to ferment it. Well, no the yeast will multiply enough anyway. It will just take a little longer. That same oxygen, in the air, will oxidise the flavour molecules destroying them.
When we ferment the leaven we are building the yeast population to ferment our dough and building the flavour foundation for our loaf. The yeast will build a decent sized population without doing this and that small extra time needed is time building up flavours.
The great French Baker and Scientist Raymond Calvel revolutionised French Bread Baking with his book ‘The taste of Bread.’ French bakers had been using high speed mixing to knead their doughs and this was introducing air which destroyed the bread’s flavour. The resulting bread was quite poor. His book heralded in a new mixing system which was shorter, slower and incorporated less air. This is so important that modern industrial manufacturers knead their dough under vacuum.
So, if flavour matters to you, do not whisk your leaven and make no especial attempts to fold air into your dough during the bulk fermentation.
Having said all of this: As carbon dioxide builds up in the leaven some of it goes into solution, in the water, as Carbonic Acid. Stirring the leaven, to allow the CO2 to escape, lowers the CO2 levels and then some of the carbonic acid then reforms into CO2. This will lower the acidity of the leaven. Too high an acidity inhibits yeast activity. So, stirring the leaven theoretically might be an advantage. However the acidity won't be so very high with a new leaven and Yeast is tolerant of quite acidic environments. Stirring in a feed is more than enough to deal with CO2 build up.
There is more on managing Natural Leavens, or Sourdough Leavens here:
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