Sourdough Starter: How it Works

Baking sourdough bread has become an increasingly popular pastime and source of comfort for many people. There are countless guides about how to create a sourdough starter and nurture it, but what actually happens in that mixture of flour and water? How does it become the source of such a rich taste, and why do you need to feed it regularly?

Humans have been making sourdough for ages, but it wasn’t until the late 20th century that scientists began to understand the ecosystem and biochemistry behind sourdoughs. A 1971 paper identified Lactobacillus bacteria and several wild yeast strains as the core players in sourdough fermentation.^1,2 So, what is the key to the cooperation between yeast and bacteria? 

Sourdough starter fermentation process: the chemistry

The thing is, the sourdough starter yeasts can’t digest the sugar maltose, which is formed by enzymes breaking down the starch in flour.  However, the Lactobacillus bacteria thrive on maltose.^1,3 Lactobacillus, as they grow on maltose, produce excess glucose and excrete it in a form accessible for the yeast to feed on.⁴

Read about the history of sourdough

Creating a sourdough starter is a deceptively simple process. It’s a mixture of flour and water, which is left to sit at room temperature. Over the next few days, half the mixture is regularly discarded and topped up with more flour and water. That’s all there is to it! The real work is being done by the wild yeast and bacteria which colonize the flour-water mixture over about a week, building up the microbial community characteristic of sourdough starters. This happens in 3 stages:⁵

Stage 1. The first to move in are the more generalist bacteria, such as Enterococcus and Lactococcus. 

Stage 2. The starter becomes more acidic by the end of the second day, and that’s when sourdough-specific groups like Lactobacillus make their appearance. 

Stage 3. There’s a tussle from Day 2 to Day 5 as different bacteria find their place in the community. By the fifth day, well-adapted sourdough strains become dominant, and a stable community is in place. 

The three phases are marked by changes in the bacterial community, not in their yeast partner. Though many people associate sourdough starters with wild yeast, the bacteria are doing much of the work. They outnumber the yeast cells about a hundredfold, and the bread’s characteristic tang comes from the lactic acid the bacteria produce.⁶

Keeping out bad microbes

An established sourdough starter can be a very robust community, keeping other microbial species out. For example, Danish industrial sourdoughs were monitored for their bacteria activity, and there was only a minor shift in the sourdough’s bacterial community over the course of seven months. The Böcker–Reinzucht–Sauerteig rye sourdough held together even longer, with the microbial community still stable after two decades.⁷ 

The stability comes from the same source as sourdough’s rich, complex taste: the lactic acid and other chemicals produced by the microbial community.⁸ The acidity keeps many microbes from growing, while the wild yeasts are able to tolerate it. There’s even speculation that the bacteria might also make antibiotics to keep dangerous outsiders from crashing into their cosy microbial community.¹

What impacts sourdough flavour?

The microbial diversity is a large part of the distinctive flavour of individual sourdoughs. The community in a sourdough starter is usually simple, often consisting of a single yeast and one or two dominant bacterial species.⁸ Despite this simplicity, starters can vary quite significantly as the environment and its microbes hugely impact the starter. For example, the microbial community of a bakery has a strong effect on the microbial community in a starter.⁸ The yeasts are generally the same, but the make-up of the bacterial community varies depending on the geography, as the ratio between the species differs from place to place.⁶ 

Sourdough fermentations also produce a more diverse cocktail of volatile compounds (flavour molecules) through wild yeast than fermentations using baker’s yeast.⁹  Researchers have identified over 60 bacteria species and 30 yeast species in sourdough cultures from around the world.⁸

The complex interactions in these microbial communities give birth to the flavours in a sourdough loaf. Many people are rediscovering the rewards of careful husbandry of these ecosystems, which scientists are still trying to understand. Like fine wines and craft beers, sourdoughs are part science and part art: the product of ecology, evolution, and a nurturing hand.