Since 2013, every September has been known as Sourdough September and has gained popularity every year. It was set up by the Real Bread Campaign who are trying to educate households on ‘real’ bread so that fewer people have to rely on factory made, processed loaves from supermarkets.

Sourdough is any bread made using a sourdough starter (flour and water mixed together and allowed to ferment to nurture the yeasts and bacteria naturally present in flour) and this year, due to the pandemic lockdown, many people have turned their hand to creating a sourdough starter and sourdough products at home.

In order to make your own sourdough starter, mix equal amounts of water and flour together, cover loosely (place a lid on top, don’t screw it on, place a piece of plastic wrap on the top or as I like to do, cover it with a large disposable shower cap collected from hotel stays) and leave in a warm place. Every day, dispose about a quarter and ‘feed’ it (add equal quantities of water and flour and combine). After a few days your sourdough starter should have risen and be full of bubbles. You can now use it to bake bread. There are many different types of bread and recipes for sourdough loaves but my favourite is Vanessa Kimbell’s Basic Sourdough Boule recipe. It is a slight labour of love but although it’s time intensive, it isn’t particularly effort intensive or difficult and you are highly likely to have everything you need to make it at home already). Once you’re confident, you can adapt any bread recipe into a sourdough bread recipe and can even use your starter in other baked goods (I’ve made delicious sourdough brownies before).

How does a sourdough starter work?

A sourdough starter is a collection of microorganisms which come from the flour used to make the starter, the surrounding air and even the skin microbiome from the baker. When you mix the flour and water together, the amylase enzymes in the flour convert the starch molecules in the flour into maltose which maltase enzymes then convert into glucose (the perfect fuel for microbial reproduction). The two most important microbes in the world of sourdough are yeasts and lactic acid bacteria.

Yeasts are a diverse set of single-celled fungi. There are more than 1500 known species of yeasts. The most commonly known species is Saccharomyces cerevisiae (common baker’s yeast) which is used in both baking and the production of alcoholic beverages like beer.

Yeasts mainly contribute to the leavening power of dough as well as contributing to the flavour and aroma slightly. In order to reproduce, most yeasts convert simple sugars to carbon dioxide and ethanol. This process is known as alcoholic fermentation. As the yeasts continue to eat the available sugars, they multiply (the warmer it is, the faster the yeasts multiply). The production of carbon dioxide creates gas bubbles in dough, which, when trapped in a well-developed gluten matrix, cause the dough to expand. When baked at a high temperature, these bubbles expand further as more carbon dioxide is produced until the yeasts die (at about 60C), resulting in an airy, spongy loaf of bread. 

S. cerevisiae isn’t the only species living in a sourdough starter. The reality is much more complex. In studies of starters from around the world, DNA sequencing from varying samples has revealed the presence of a wide array of wild yeasts: Saccharomyces servazzii, a prolific producer of carbon dioxide with a very strong profound leavening power ( it has caused packaging to explode in factories); Pichia anomala, which produces isoamyl acetate, which smells like artificial banana as well as many other yeast species, all with varying characteristics and functions.

The differing ratios of these yeast populations is one of the things which make every sourdough starter unique.

Lactic acid bacteria are rod-shaped or spherical, and primarily produce lactic acid. Much smaller than yeasts, they are found in decomposing plants, dairy products, on the skins of vegetables, fruits, and even on your human skin. In a typical starter, they outnumber yeasts by as much as 100 to 1. Like certain yeasts, they digest simple sugars, but instead of the alcohol created by yeast, they mostly produce sour lactic acid as a by product. 

The production of lactic acid (as well as acetic acid) lowers the pH of your starter to around 3.5. This results in that characteristic sour flavour of sourdough. A low pH also eliminates unwanted pathogens such as Staphylococcus as they can’t survive in an acidic environment. A low pH also gives sourdough a longer shelf life than other breads by inhibiting mold growth. Lactic acid bacteria also release protease enzymes that break down gluten over time, resulting in a softer, lighter texture. There are two main types of lactic acid bacteria (LAB)

Homofermentative (or homolactic) LAB only produce lactic acid. They prefer temperatures between 30–35°C although they can grow at lower temperatures as well. They produce flavors characterised by dairy notes.

Heterofermentative LAB produce lactic acid, but also acetic acid, ethanol, and even carbon dioxide (therefore, providing some leavening power). These bacteria thrive at temperatures between 15–22°C. They impart a sharper, more vinegar-like tang to foods, likely due to the extra production of acetic acid.

As with yeasts, a single sourdough starter will likely contain several species of LAB over the course of its lifetime. For example, L. sanfranciscensis, the bacteria for which the San Francisco-style sourdough is named produces a distinctly tangy flavour. Early on in development, species like homofermentative Pediococcus, Enterococcus and Streptococcus bacteria have been shown to predominate but evidence suggests that over time, stable sourdough cultures contain mostly heterofermentative LAB which outcompete less adaptable homofermentative lactobacilli. (In other words, a stable starter tends to have a more sour aroma, and imparts more sourness to breads, than a young starter due to the additional production of acetic acid from heterofermentative LAB.)

Happy sourdough baking!

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