Lactic And Acetic Acid Taste Different And Behave Differently

Sour bread is not simply more or less sour. Two different acids are produced in varying proportion, and they contribute distinct flavours and distinct effects on the dough.

Two acids from the same bacteria

The lactic acid bacteria in a starter produce lactic acid, and many strains also produce acetic acid along with carbon dioxide and ethanol.

Lactic acid is mild, rounded and slightly creamy, the same acid found in yoghurt and other cultured dairy products.

Acetic acid is the acid of vinegar, sharp and volatile, and it dominates the perception of sourness even at low concentrations.

Hydration pushes the balance

A wet, slack starter favours lactic acid production, giving a milder, sweeter culture and a bread with gentle acidity.

A stiff, dry starter shifts production toward acetic acid, which is why traditional firm starters produce noticeably sharper bread.

Bakers who want to move flavour in one direction often change the starter's consistency before changing anything else, because the effect is reliable.

Temperature moves it too

Warm fermentation favours lactic production and fast gas generation, giving a rounder flavour and a quicker rise.

Cool fermentation slows the yeast more than the bacteria and favours acetic production, which is much of why long cold retards produce sharper loaves.

The two variables interact, so a stiff starter kept cool sits at the extreme sour end and a wet warm one at the mild end.

Acid affects the dough as well as the taste

Falling pH tightens gluten initially, which is why a moderately acidic dough feels stronger and holds shape better than a neutral one.

Beyond a point the same acidity, along with the enzymes it activates, degrades gluten and the dough becomes slack, sticky and prone to tearing.

This is why an over-fermented sourdough loses structure suddenly rather than gradually, and why acidity is a variable to manage rather than maximise.

Acidity does more than flavour

Low pH inhibits many spoilage organisms, which is why sourdough breads resist mould longer than commercially yeasted ones of similar composition.

It also affects starch and enzyme behaviour during the bake, which contributes to the different crumb texture of a sourdough loaf.

Rye in particular depends on this, since acidity is what restrains the enzymes that would otherwise leave a rye loaf gummy and unsliceable.