Unravelling the mysteries of sourdough bread

A delicious home-baked science experiment

During this pandemic a whole lot of people have been spending a whole lot more time in the kitchen. My social media feeds have been inundated with photos of cakes, cookies and pastries. But among them all, one stands out. The humble sourdough loaf, which has become almost a symbol of the pandemic.

Although I myself haven't dived into the world of sourdough baking (and may have told my friend she was a pandemic cliché when presenting me with photos of her sourdough loaves - please consider this a public apology), I have to admit that it's a pretty cool exercise in microbiology and chemistry.

Unlike "regular" bread, sourdough isn't made with commercial yeast. Instead it relies on a "starter" - a fermented mix of flour and water containing a mix of natural yeast and bacteria. These microbial mixes are the key to the unique flavours that we find in sourdough.

Flour is made mainly of starch. Mixed with water, enzymes within the flour go to work, breaking that starch apart, yielding simple sugars. And this is where the microbes come in. To bacteria and yeast those sugars are an all-you-can-eat buffet - and as they digest them they produce carbon dioxide (hence the bubbles in your starter), and a range of acids (hence the sour in sourdough).

Different combination of microbes will produce different types of acids, which lend different flavours to the starter. Lactic acid (as found in yoghurt) gives a milder flavour, while acetic acid (think vinegar) makes the starter more pungent and sour.

When we bake with commercial yeast, we're using just one strain of yeast. I can cook a loaf, my neighbour can cook a loaf, and the resulting flavours will be pretty consistent. The microbial melting pot that is a sourdough starter means that no two will ever be exactly the same, and the breads they yield will never taste the same.

The microbial composition depends on a number of different factors - the flour used, the temperature and humidity where the starter is stored, how much water is added, whether any extras, like yoghurt or grapes have been added - the list goes on. Even the microbes found on the hands of the baker will become added to a sourdough starter, influencing the microbial community and therefore the flavour of the bread.

That sourdough starter sitting on your bench is a living, breathing science experiment. And science never tasted so good.

Dr Mary McMillan is a lecturer at the School of Science and Technology, University of New England

This story A delicious home-baked science experiment first appeared on The Canberra Times.