Sourdough
Sourdough has become an increasingly popular trend recently, but what is actually special about it?
Differences in taste
Commercial yeast gives a sweet, beer-like aroma, whereas sourdough has a slightly sour, more complex and fragrant aroma.
How commercial yeast is made v. sourdough
Commercial yeast is made by growing specific strain(s) of yeast under controlled industrial conditions, then it is either:
Compressed to form fresh yeast
Dried to form active dried yeast
More carefully dried to form instant yeast (less damaged yeast cells so work more quickly)
However, sourdough is made by:
Mixing equal amount of flour and water in a jar/cup first. Then, discard some of the original mixture and mix in new fresh flour and water - Repeat this for about a week to make the sourdough ‘starter’.
Enzymes (amylases) in flour convert starch into simple sugar, so that the wild yeast and lactic acid bacteria found in the flour (mostly), air, jar, on your hands… can feed on the sugar and grow.
This means that the yeast convert sugar into carbon dioxide and ethanol, whilst lactic acid bacteria convert sugar into mostly lactic acid - also known as fermentation!
Lactic acid lower the pH hence creates the unique sourness and aroma of sourdough. This also makes harmful microbes unable to survive in acidic environment, that’s why sourdough generally doesn’t contain preservative and can last longer than homemade bread made with commercial yeast.
As you mix the ‘starter’ into the fresh main dough (flour, water, salt), the yeast and bacteria are able to feed on more flour and produce more carbon dioxide (which is trapped in network of gluten to make the dough rise), ethanol and lactic acid.
The longer fermentation carried out by wider range of yeast and bacteria allows more complex flavour to develop over time.
Making sourdough bread needs longer time compared to using commercial yeast(I usually need at least 1-2 days, time varies depending on the type of bread and temperature) because commercial yeast is designed to be more effective and in higher concentration, whereas sourdough contains different types of wild yeast and bacteria, so…
May compete for food
Need to work in balance to grow
Have narrower optimal temperature range and temperature change may slow down yeast activity more significantly
Acids produced by bacteria could slow down yeast activity
Sourdough is harder to control in baking because different wild yeast and bacteria interact with each other and can be affected more easily by temperature, hydration, flour types, ‘health’ of the star etc.
Mixing some wholegrain flour such as rye into the sourdough starter can make it more ‘healthy’ (active) as wholegrain flour without having their outer layer of grains removed contain more wild microbes needed for fermentation.
The sourdough starter either needs to be ‘fed’ consistently (discard and add new flour and water) OR it can be stored in the fridge to slow down fermentation and be fed once or twice every week.
Health benefits
More nutritious - Fermentation and low pH activate enzymes to breakdown antinutrients such as phytic acid, therefore make minerals to be absorbed by the body more easily; Increase availability of some vitamin Bs; Help the body to make certain micronutrients.
Easier to digest - Bacteria release protease enzymes which help to break down gluten and other proteins, therefore it’s more friendly to people who have mild gluten sensitivity.
Cause less bloating - Longer fermentation breaks down more FODMAPs, substances that gut microbes feed on and produce gases.
Lower GI - Fermentation produces organic acids which slow down digestion of starch into simple sugar, so sugar is absorbed into the bloodstream more slowly, which means slower increase in blood glucose levels. (Main reason - there are several other reasons as well)
Reference
Serious Eats (n.d.) Sourdough starter science. Available at: https://www.seriouseats.com/sourdough-starter-science (Accessed: 7 June 2026).
Sourdough School (2021) Is sourdough good for you? by Dr Vanessa Kimbell. Available at: https://www.sourdough.co.uk/is-sourdough-good-for-you-by-dr-kimbell/ (Accessed: 7 June 2026).
BroBread (n.d.) Sourdough bread glycemic index. Available at: https://brobread.com/sourdough-bread-glycemic-index/ (Accessed: 7 June 2026).
