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This article will be Oliver Kuhn’s last ‘Science Break’. We, the editors, would like to thank Oliver Kuhn for his long contribution to the Recorder magazine. Oliver has produced around 70 Science Break columns over the last 20 years. Science Break has been an enjoyable part of the Recorder, as Oliver has shared his passion for science with us all. This will be the last Science Break written by Oliver. On a personal note, I’ve known Oliver for decades; we went to school together at UWO, now Western University, in the early 1980s and worked together briefly at Geo-X in 1985-86. Oliver has had this passion for science as long as I have known him, and we have been lucky that we got to share in it through the Recorder. Oliver has been an important part of our Geoscience community from the get-go. Oliver received the CSEG Meritorious Service Award in 2003 for his many volunteer activities, and he served as President of CSEG in 2006.

This does not have to be the end of Science Break. If you have a passion for science and would like to share that among the geoscientific community, perhaps you would consider continuing this fine tradition that Oliver started. Also, perhaps you have a favourite Science Break column that you’d like to see highlighted in our Editor’s Picks. In either case, let us know at submissions@csegrecorder.com.



There were winners and losers created by Covid, and we tend to focus on the latter – movie theatres, the airline industry, and so on. But there were many surprising winners, and this article will focus on sourdough bread making, which enjoyed a resurgence in popularity among house-bound people, including my wife and me. Along with all the poodle hybrids added to households, there must have been thousands, if not millions, of sourdough starter cultures that found new homes in refrigerators around the world during Covid. So much Covid-induced home baking happened that there was a shortage of baker’s yeast, steering people towards the sourdough method, which does not require store-bought yeast. It also helped that all these amateur bakers had the 24+ hours needed to make sourdough bread. The history and science behind this ancient bread-making method are very interesting, but before getting into the details, here is a summary of how sourdough bread is made:

1. Four simple ingredients are mixed into a dough – flour, water, salt, and starter. The starter is a colony of naturally occurring yeasts and bacteria. Only some of the starter is used each time, and the rest is kept for future use (more on that later).

2. When the flour is mixed with the water, an enzyme called amylase breaks down the starches in the flour into the sugars maltose and glucose.

3. The yeasts and bacteria in the starter break down the sugars to create these by-products: carbon dioxide (which causes the dough to expand), acids, and alcohol.

4. The leavened dough is then baked. The term leaven refers to the change of the raw dough into something softer and fluffier.


Stretching back far into the past, well before human history was recorded, the sourdough method was the only way to make bread (Figure 1). Cultured yeasts were unknown, and bread makers kept colonies of naturally occurring yeasts and bacteria with which to leaven their bread. In regions where beer making was popular, at some point in the Middle Ages, barm, the foamy scum produced by beer fermentation, began to be used as the leavening agent for bread making. Obviously, barm is full of desirable yeasts. Examination of medieval records reveals these regional differences, with sourdough persisting in wine-drinking regions such as Italy and France but eclipsed in beer-drinking areas such as England and Germany by barm bread. Exactly when or how quickly barm bread methods became popular is unknown, and at least in medieval Europe, both types of bread were produced.

Figure 1. Depiction of bread making from the tomb of Rameses III, Valley of the Kings, 1155 BC. Public domain image originally from The Oxford encyclopedia of ancient Egypt ISBN 0-19-510234-7. CITATION Wik23 l 4105 (Wikimedia Foundation, Inc., 2022)

In the late 1700s, beer brewers had identified two types of yeast, Saccharomyces cerevisiae (top-fermenting yeast used in ales) and S. carlsbergensis (bottom-fermenting yeast used in lagers, and now more correctly named S. pastorianus in honour of Louis Pasteur). S. cerevisiae was sold in Holland starting in 1780 as a bread leavener in liquid form, and in 1825 a method to form it into solid blocks was developed. In the 1860s and ‘70s, the Pasteurs, Louis and Marie, opened up the microscopic world of bacteria, and this broader scientific understanding led to enhanced identification and isolation of the most effective yeasts in barm (typically S. cerevisiae), ancestors of today’s commercially available yeasts. In 1872 Baron Max de Springer brought a granular cultured yeast to market. All these developments created a revolution in bread making, making it possible to mass produce bread quickly and reliably, critical to feeding the growing numbers of industrial revolution city dwellers. This means that bread as we know it is only about 150 years old, but when you make sourdough bread, you are taking a step back in time and continuing a method that goes back thousands of years.

Bread has played an important role throughout human history, as it is a tasty, portable food that can be easily combined with other items to create an interesting and nutritious meal. The study of the ancient human shift from a nomadic existence to a settled one focuses on the farming of grains but often leaves out the critical role bread played, providing a way for humans to create a practical diet based on these grains. Leaving aside all the biblical references to bread and its use in sacred rituals, I particularly enjoyed reading about sourdough and its role in the 1849 gold rush. French bakers and their sourdough expertise were drawn to San Francisco by the boom. Their sourdough methods were adopted by the gold panners, who jealously guarded their starter pouches as they toughed it out in Alaska and the Klondike, eventually becoming known by the nickname “sourdoughs”.


The role of the enzyme amylase is often forgotten in discussions about bread. Most flowering plants store energy in the form of starch – photosynthesis uses sunlight to produce glucose, which the plant converts to starch. Starch is a complex carbohydrate made of either a long linear chain (amylose) or a branched chain (amylopectin) of glucose molecules. Many animal species have evolved to ingest and break down plant material to unlock this stored energy as a source of nutrition, including humans. In such mammals, amylase is produced in the pancreas and secreted in saliva, where it can take the first step to break down plant starches via hydrolysis. And, of course, amylase is present in the stomach’s digestive juices.

The reason we love starchy foods, such as potatoes, is that when chewed, the water and amylase in our saliva immediately start breaking down the starch, giving us tiny bursts of sugary flavour. The job started by our saliva and teeth is finished in our stomachs, producing glucose ready for use. This glucose is our most important source of nutrition and is circulated throughout our bodies by our blood, feeding all our cells.

Amylase is also naturally found in yeasts, as they, too, need it to perform that initial breakdown of starch into sugars which they can then metabolize. If you check the ingredients of most flours, you will see that the producers often add amylase to help bakers create dough, which is a bit disappointing for people who view sourdough making as “all-natural”.


Figure 2. Saccharomyces cerevisiae budding as seen by scanning electron microscopy. Used under Creative Commons 3.0 CITATION Das16 l 4105 (Das Murtey & Ramasamy, 2016)

Yeasts are a single-celled type of fungus. There are approximately 1500 known species, and they are pervasive, floating around in the billions, waiting to alight on some type of organic compound which they can break down as a source of nutrition – typically some kind of sugar, but also things such as alcohols and organic acids. While baker’s yeasts tend to be monocultures of a yeast such as Saccharomyces cerevisiae, the sourdough starter may have any number of different wild yeasts and will change over time as new yeasts alight in the starter, and others die off.

The yeasts in bread-making metabolize the sugars to produce carbon dioxide gas, which increases the dough’s porosity. Ethanol is also generated and serves two important purposes. First, it has a sterilizing effect, killing off unwanted bacteria in the raw dough. Second, when the dough is baked, the alcohol evaporates, further increasing the dough’s porosity and volume. Note that the size of the pores thus created is a function of the elasticity of the gluten, which in turn is a function of the bacterial action on the sugars, which is covered below.

The ideal amount of baker’s yeast used in non-sourdough bread is in the range of 1.5%-2.2% of the weight of the flour (interesting note, a “baker’s percentage” is always expressed as a percentage of the flour used in the recipe). With sourdough, it is impossible to be this accurate as the starter contains many other ingredients besides yeast, so typically sourdough recipes call for a baker’s percentage of ~25% starter. Any more than that, and too much CO2 and alcohol are created, and the bread won’t form properly and will also not taste right. This is because most of the glucose in the dough needs to be metabolized by the bacteria to produce the flavour and structure. Too little yeast is not as disastrous, as it will create a bread more at the unleavened (flat) end of the spectrum, something which is often done by choice, for example with pumpernickel. On a Germanic sidenote, pumpernickel uses sourdough starter and rye flour to produce a dark, heavy, moist flat bread. The New High German roots of this compound noun translate to something like “farting devil”. But regardless, creating a perfect loaf of sourdough bread requires that the starter is a healthy stable culture, so that the percentage of yeast in the dough is in that ideal range.


The bacteria in the sourdough starter are critical to the method. They outnumber the yeast by about a 100:1 ratio, and almost all of them are from the Lactobacillus genus. There are approximately 260 species in this genus, and the last couple of decades have seen them leap into prominence, as research has shown how important they are to a healthily functioning human body. They are critically represented in the microbiota of our stomachs and help us digest food, and they protect us against pathogens by forming acidic films in the mucous membranes of our orifices. I covered much of this in my article The Human Microbiota (Kuhn, 2016).

In the formation of bread, the Lactobacilli bacteria ferment the sugars, and this action proportionally outweighs that of the yeasts, roughly by that 100:1 ratio in the starter. Some bacteria have evolved to metabolize wheat-based flours and generate lactic acid, producing a soft, nutty/malty flavour, especially with whole wheat flour, while others prefer rye-based flours, and produce acetic acid and its sour, fruity flavour. The yeasts then further break down some of the bacterial fermentation by-products. Each starter has a unique mix of yeasts, a product of its heritage, the baking environment, the baker’s hands, and of course, the flour itself also brings its own mix of yeasts with it, more so with unprocessed flours. The organic chemistry that goes on within this interplay is extremely complex, meaning that each starter and baker will produce a unique bread, each batch, with its own flavour and texture.

Sourdough starter

When people first hear about sourdough bread making, it is the starter that catches the most attention. The idea that people keep these active, mouldering, putrid-looking bacterial cultures in their fridges seems odd, bordering on disgusting. However, speaking from personal experience, once you try it, the regular feeding and visual growth of the starter becomes very enjoyable, and an attachment starts to develop, something similar to keeping a pet.

Admission to the sourdough club is usually via a friend, who will give you a little container with some starter and some instructions on how to feed it and take care of it. In my case, I was told to take 50g of the existing starter, add 100g of room-temperature water, and 100g of all-purpose flour. My wife’s starter (yes, we had a sourdough duel going on for a while) regime was 50g starter, 100g water, 100g all-purpose flour, and 30g whole wheat flour. Apparently, feeding the starter with some or all rye flour is really good for it, and can restore an ailing starter to full health. You are probably starting to get the picture – it’s a folk art, with lots of opinions, but the rough method is similar to what I’ve described. The remaining original starter is thrown out, and the new mixture is allowed to rise to approximately double in size. Within a few days, the process is repeated, and the starter is refreshed. Storing the starter in the fridge can prolong the interval between feedings, but if you leave it too long, it becomes too acidic, and gruesome-looking moulds can develop as well. Each starter is a stable, self-sustaining bacterial culture that, if fed and maintained properly, can be used over and over to help bake an unlimited number of loaves of bread.


Gluten is an oft-mentioned scapegoat for a variety of digestive disorders. Some, such as celiac disease, are clinically proven and understood, while others are a medley of intolerances and sensitivities, perhaps sometimes with a dash of hypochondria and pinches of faddism and paranoia thrown in. But there is no doubt that the gluten which is a central component of most baked food can be tough for the digestive system to handle. Some digestive problems are actually due to fructans, sugars also found in bread that the body has difficulty digesting.

There is actually no gluten in raw bread ingredients; it is formed during the bread-making process. Raw flour contains two proteins, glutenin and gliadin. When they are mixed with water, they combine to form gluten. Proteins are chains of amino acids; glutenin chains tend to fold back upon themselves, resulting in a spherical shape; gliadins, on the other hand, adopt a linear, rod-like structure. When the two are mixed with water, hydrogen bonds form disulphide bridges between the protein rods and spheres, which creates the gluten, an amorphous mesh-like structure which is very elastic (Figure 3). In the case of bread dough, the more it is mixed and kneaded, the more elastic the gluten becomes.

Figure 3. Depiction of gluten formation. CITATION Pen22 l 4105

Gluten gives bread its attractive texture – indeed, it forms the structure of many of the foods we eat – but presents a bit of a challenge for our digestive systems. If undigested gluten passes from the stomach, it can irritate the intestinal walls. There is a misconception that sourdough bread is low in gluten. The extended period that sourdough sits as the Lactobacilli generate the flavourful acids does tend to result in somewhat lower gluten levels than with other bread-making methods, but not enough to make sourdough low gluten. Its delightful chewy texture is evidence of all the gluten that still remains.

Sourdough baking

My sourdough recipe, as derived from Internet sourdough doyenne Alexandra Stafford (alexandracooks.com), is as follows:

1. Mix the following in a bowl

a. 375g of room temperature water

b. 11g of salt

c. 50g-100g of starter

d. 500g of bread flour (sometimes I make this partly whole wheat and/or rye)

2. Transfer to a steep-sided container, cover, and let sit until it is 150% – 200% of its original volume (usually 3 or 4 hours, depending on the room temperature)

3. Fold the corners over to the middle every half hour, a total of 4 times

4. Let sit for 30 to 40 minutes

5. Cover and put in the refrigerator for 12-24 hours (the longer left, the more acid and flavour are produced)

6. Preheat oven and Dutch oven to 450F

7. Place dough on parchment, score it with a razor blade in some nice pattern

8. Bake in a Dutch oven for 30 minutes

9. Remove the top of the Dutch oven and bake for another 15 minutes at 400F.

So far, I have not ventured from this recipe (and it’s turned out perfectly every time), but my wife has made all sorts of delicious sourdoughs – rye bread, focaccia, pizza dough, etc. Now that winter is approaching, I’ll be spending more time indoors, and I plan to be a bit more adventurous.

Figure 4. Crumb shot of my most recent loaf.


Das Murtey, M., & Ramasamy, P. (2016, October 14). Saccharomyces_cerevisiae. Retrieved from Wikipedia: https://commons.wikimedia.org/wiki/File:Saccharomyces_cerevisiae_SEM.jpg

Holmes, B. (2020, August 13). Scientific American. Retrieved from The Science of Sourdough: How Microbes Enabled a Pandemic Pastime: https://www.scientificamerican.com/article/the-science-of-sourdough-how-microbes-enabled-a-pandemic-pastime/

Kuhn, O. (2016, April). The Human Microbiota. The CSEG RECORDER, 41(4). Retrieved from https://csegrecorder.com/columns/view/science-break-201604

Pendergrass, K. (2022). Gluten. Retrieved from Paleofoundation: https://paleofoundation.com/gluten/

Wikimedia Foundation, Inc. (2022, September 4). Ancient Egyptian cuisine. Retrieved October 24, 2022, from Wikipedia: https://en.wikipedia.org/wiki/Ancient_Egyptian_cuisine#/media/File:Ramses_III_bakery.jpg

Wikimedia Foundation, Inc. (2022, October 23). Sourdough. Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Sourdough