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The chemistry and biology of sourdough

Updated: Sep 3, 2020

As many others who work in the field of microbiology, I have always been keen on my home baking. My baking breakthrough happened 2.5 years ago, when I decided to make my own sourdough starter.

What is a sourdough starter?

Fermentation is an anaerobic biological process that converts carbohydrates into simpler substances. In baking, fermentation employs yeasts and bacteria to convert sugars into carbon dioxide, among other things, causing the dough to rise. Commonly, pure cultures of baker's yeast, Saccharomyces cerevisiae, are used as raising agents to make bread dough and other yeasted goods. Alternatively, a sourdough starter can be used, which consists of a mixture of yeasts and bacteria.

A recent study published in mSphere [1] led by Rob Dunn at North Carolina State University, analysed the microbiome of 18 sourdough starters made by bakers based in 14 different countries. Each baker was given the same starting material (flour) and instructions to make the starter, which was then used to make bread. What turned out is that the microbial composition of the sourdough starters was determined not only by the flour but also by what was in the baker's hands and the environment in which they made the starter, such as the kitchen and the air. In turn, the sourdough microbiome caused differences in taste of the resulting breads.

What microorganisms are present in a sourdough starter?

These can be divided into two main groups: yeasts and bacteria [1,2]. Among the yeasts, the most common species may belong to the orders Saccharomycetales (such as S. cerevisiae and Candida humilis) and Pleosporales. The main contribution of the yeasts in sourdough preparations is the production of carbon dioxide, which during the kneading of the dough is trapped amongst gluten chains and is ultimately responsible for the airy texture of the baked bread. Among the bacteria, those of the order Lactobacillaceae seem to be predominant, followed by Enterobacteriales. Lactobacillus bacteria produce acids through fermentation, such as lactic acid and acetic acid [3], which confer the characteristic acidic taste to the baked bread.

How did I make my own sourdough starter?

  • Mix together 100 g flour and 100 ml water and let it rest in a mason jar for 24 hours at room temperature.

  • Take 100 g of the starter (discard the rest) and mix it with 100 g flour and 50 g water, then let it rest in a mason jar for 24 hours at room temperature.

  • Repeat the step above every 48 hours for 30 days.

  • Now the starter is ready to be used to make bread/pizza dough, and it can also be stored in the fridge for up to two weeks in between feedings.

To maintain the starter

  • Take 1 part of the starter (preferably the core), discard the rest, and add 1 part of flour and ½ part of room temperature water (e.g. 120 g of starter + 120 g flour + 60 g water, will give you 300 g of new starter).

  • Mix it all together, knead for a few minutes until smooth.

  • Split it into 2 parts: 100 g (which will be the starter stock that can be kept in the fridge) and 200 g (which can be used to make bread or pizza).

  • Move the smaller portion into a mason jar and leave at room temperature for 24 hours, then store it in the fridge for up to 2 weeks.

  • Use the larger portion of the starter to make bread or pizza dough.

To make a large loaf of bread/two trays of pizza

  • Take 200 g of starter and add 350 ml water, 50 g strong bread flour, 1 teaspoon of honey and mix all together trying to dissolve the starter as much as possible.

  • Let it rest at room temperature covered in cling film for 30 minutes.

  • Add 550 g strong bread flour (if using wholegrain flour, use up to 200 g of it with 350 g white flour).

  • Add 10 g salt and 1 tablespoon olive oil.

  • Mix everything well and knead for 5-10 minutes until smooth.

  • Move into a large bowl and cover with cling film (divide into two dough balls if using it to make pizza or if making two small loaves of bread).

  • Let it rise for about 12 hours at room temperature.

  • Flip the bowl upside down and drop the dough on a floured surface.

  • Stretch and knead the dough as little as possible to give it the desired shape then place on a floured and oiled baking parchment on a baking tray (loaf shape for bread/flat for pizza).

  • Dust some flour on top and cover the dough with cling film.

  • Let it rise for about 12 hours at room temperature.

  • Remove the cling film and bake at 220°C for 30 minutes (for bread) or add the toppings and bake at 250°C for 20 minutes (for pizza).

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