Sourdough Bread Day

What the day is

Sourdough Bread Day is observed on April 1. What makes sourdough sourdough is not a recipe but a living culture, and the culture is more interesting than the bread.

Origin and background

A starter is flour and water hosting two kinds of organism at once. Wild yeasts produce most of the carbon dioxide that raises the loaf, and lactic acid bacteria, which typically outnumber the yeasts by a wide margin, produce the acids that give it flavor. They are not competitors but partners: the bacteria tolerate the acidity they create, and the yeasts found in mature starters tolerate it too, while almost nothing else does.

That acidity is the whole defense. A starter fed on a regular schedule sits at a pH low enough to exclude the organisms that would spoil it, which is why a jar of flour and water left on a counter becomes a stable culture rather than a health hazard, and why starters kept for decades are entirely ordinary rather than remarkable.

The romantic claim that a starter captures the character of its place is mostly not supported. A large study that gathered starters from bakers across many countries found the dominant influences were the flour used and the way the starter was fed and kept, rather than anything local to the air or the baker's hands. A starter moved across the world and fed differently becomes a different starter.

Long fermentation does change the bread in ways that matter. It breaks down a portion of the phytic acid in flour, which otherwise binds minerals and limits their absorption, and it reduces some of the short-chain carbohydrates that trouble people with sensitive digestion. That last point is often overstated, and sourdough is not gluten free in any sense.

Much bread sold as sourdough is made with commercial yeast and an added souring agent, since the label is not protected in most places.

How to take part

• Start a starter. Flour, water, a warm spot and about a week is all it takes.
• Feed on a schedule. Consistency matters more than any particular ratio.
• Check the ingredient list on bread sold as sourdough.
• Keep it in the refrigerator if you bake weekly rather than daily.
• Give some away. That is how most starters travel.