Marvellous microbes: How to make a sourdough starter culture at home

Recently, many of us have been baking up a storm and sourdough bread has become the poster child of social isolation. However, this is not a new trend. In fact, humans have been baking bread for tens of thousands of years.

Baking bread has really taken off amidst this global pandemic, but what has caused the recent rise (no pun intended) in the popularity of sourdough? Perhaps necessity due to barren supermarket shelves or limiting unnecessary travel for food, curiosity or even boredom? Whatever the reason, when we bake sourdough we are actually performing science experiments in our kitchens.

Whether you want to give Sourdough baking a try, you’re curious about the science behind your bread, or there is room for improvement in your sourdough crafting skills, Microbiologist Dipon Sarkar from the University of Tasmania is here to tell us about the science of sourdough, the health benefits of eating it and how you can bake your own. The truth is, sourdough contains all kinds of fascinating chemistry and biology.

“Fermentation is the oldest preservation technique that we humans know. It is the way that bacteria breed. They break down glucose and other carbohydrates and produce acids in some cases and in other cases alcohol but in all cases, the energy that the bacteria use to live” Dipon explains

Baking bread harnesses the fermentation capabilities of ‘baker’s yeast’, Saccharomyces cerevisiae. Fermentation is where the yeasts or bacteria break down the carbohydrates (long sugar chains) like starch into single sugars and produce CO2 gas. These gasses (or yeast farts, as Dipon likes to call them) get trapped in the dough, forming bubbles which cause the bread to rise. Sourdough is different from regular white bread due to the “starter culture” which you add to the dough. This “culture” is a scientific term for a group of specific microscopic organisms (like bacteria and fungi) grown in a controlled way. The sourdough starter culture contains a mixture of yeasts and lactic acid bacteria (Lactobacilli) which occur naturally in the flour. The lactic acid bacteria in the culture break down the starch into simple sugars that the yeasts can use, while producing lactic and acetic acid. These acids give sourdough bread its tangy flavour and chewy texture as they modify proteins such as gluten in the flour. These acids also lower the pH of the bread which gives the bread a longer microbial shelf life, as the lower pH prevents the growth of bread mould fungi (1).

Sourdough is not a new trend.

“Sourdoughs have existed for a long long time” says Dipon

Humans have been making bread for tens of thousands of years before instant baker’s yeast was cultured and sold for baking purposes. In fact, Aboriginal Australians were some of the earliest known bakers – harvesting native seeds and grinding them into flour on grindstones to make bread. Natural yeasts on these seeds would ferment the sugars to produce bread that was then cooked over hot coals (2).

Sourdough is healthier than regular white breads, as the lactic acid bacteria are able to release important minerals in flour, such as iron, magnesium, calcium and zinc, which are otherwise bound to phytic acid in the flour (3). The bacteria’s break down of phytic acid means these minerals are available for our bodies to absorb. Thank you, good bacteria!

So, how do you create a starter culture?
You mix water and flour and leave it to ferment for about a week, though if your kitchen is particularly cold (ahem.. Tasmania..) it may take up to two weeks.

“As long as the flour is un-bleached, you can use it to make sourdough culture” advises Dipon.

Some bakers prefer to use stone ground or wholemeal flour as it often contains more of the microbes that create a good fermentation culture.
After about a week you should notice bubbles being produced from the culture – a good sign that the yeasts are alive and fermenting! The culture should double in size every 5-7 days, which is also approximately the length of time that you need to feed the culture for once it is established. Yes, because the starter is a living culture, you need to keep it alive, and like any pet, it will need food (flour), water, and love. Think of it as a modern day tomagotchi for any 90s kids reading this.

You will need:

  • Flour
  • Water (tap is fine)
  • A glass jar (around 750mL)
  • Kitchen scales

Day 1: Start your culture!

Add 100g of flour (whole wheat, or white) and 100g of water to the jar and mix with a fork. The mixture should be the consistency of thick pancake batter. You may need to add some extra water if it seems too thick.

Cover loosely and leave in a warm (~24°C) spot for 48 hrs. If you live in a cold house (and let’s be honest, we’re posting this how to blog right at the beginning of winter in Tasmania, so it is cold) you can find a warm spot perhaps on top of your fridge, in the hot water cylinder cupboard or you could set up a lamp pointed at the culture, so the light is giving off a small amount of heat. I took my culture into the office with me, so it wouldn’t sit in an unheated house all day. True love.

Day 2: Keep an eye out

You do not need to do anything today but look at your culture.
You may see some bubbles already, or you may not. Either is okay.
If you notice a black watery layer on the top of your culture at any point, do not be alarmed, this is called “hooch” and is a sign your culture needs to be fed. Hooch is the yeasts producing alcohol because they have not been fed fresh flour and have not been recently mixed leading to a low oxygen environment (this is how alcohol is made, feeding yeasts sugar to ferment in a low oxygen environment). You just need to pour off the liquid and feed the sourdough (below).

Day 3: Feeding time

By now you should see some bubbles, and the culture should have a fruity aroma, if not, feed it anyway:

Remove half the culture from the jar with a spoon and dispose of it. The culture will double in size with each feeding so you need to dispose of some.
Add 100g of flour and 100g of water and mix into the starter culture. Leave in the warm spot for 24 hrs.

Days 4, 5, 6: Repeat

Repeat the feeding process every day, removing half the culture and adding flour and water.

You will notice the culture will rise between feedings and should have bubbles throughout.

Day 7: IT’S ALIVE!

By day 7 you should notice that the culture doubles in size when left for 24 hrs. You can place a rubber band on the outside of the jar to mark the level when you feed it to see the change in size over time.

The texture should be thick and spongey with lots of bubbles and should have a pleasant smell. You can also try the “float test” by placing a spoonful of starter in a cup of water. If it floats on the surface, it is now ready to be used in your sourdough recipe!

Once your starter is properly established and has this spongey, marshmallow texture before each feeding consistently, you can begin to neglect it a little more. You can keep the culture somewhere cooler – perhaps on the kitchen bench or even in the refrigerator to really slow its growth – The lactic acid bacteria grows better in cooler temperatures, so keeping your culture cool can increase the sour flavours of the bread. This slower growth means you can feed your culture whenever you take some out to make bread, weekly works best.

Now go ahead and marvel at some microbes, get some loaves in your life and bake away the boredom! Keep us updated on your progress by tagging @ThatsScienceTas on social media.

Dipon Sarkar

Special thanks to our expert guest, Dipon Sarkar! Dipon is a PhD student at the Tasmanian Institute of Agriculture, University of Tasmania, specialising in food science and food safety.

This article was written by Kelsey Picard with thanks to Kate Johnson and Niamh Chapman for editing assistance, and to Lachlan Tegart for supplying photos.


  1. CORSETTI, A. & SETTANNI, L. 2007. Lactobacilli in sourdough fermentation. Food Research International, 40, 539-558.
  2. PASCOE, B. 2014. Dark Emu: Black Seeds: Agriculture or Accident? Magabala Books. Broome, W.A.
  3. LEENHARDT, F., LEVRAT-VERNY, M.-A., CHANLIAUD, E. & RÉMÉSY, C. 2005. Moderate Decrease of pH by Sourdough Fermentation Is Sufficient To Reduce Phytate Content of Whole Wheat Flour through Endogenous Phytase Activity. Journal of Agricultural and Food Chemistry, 53, 98-102.

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