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Appliance Science: The uplifting biology of baking

We owe a lot to an invisible organism that makes foods like bread possible. In the latest installment of our Appliance Science column, we look at our favorite fungus: yeast.

An electron microscope image of Saccharomyces cerevisae in the process of budding.UK National Collection of Yeast Cultures

It is amazing how much of our lives we owe to things we can't see. Take bread, for instance. Most of us eat it without thinking, not realizing that this foodstuff is the result of the complex chemical actions of a simple organism: yeast.

This fungus has probably had more impact on human civilization than any other, shaping the way that we cook, make and eat food from prehistoric times up to the present day. But that doesn't just go one way: we have had as much (and probably more) impact on yeast as it has had on us, shaping it to our own purposes.

In prehistoric times when modern appliances were no more than a shaman's fever dream, we were changing yeast, taking the type that worked best and saving it for future use. This, perhaps, made yeast the first appliance, the first device that humans used to help make their food better and more digestible. This led us to Saccharomyces cerevisiae, the species most commonly used today in baking and brewing. This is only one of thousands of species of yeast, but it is the one that we found did the best job of helping us both bake bread and brew beer.

Saccharomyces cerevisiae is a eukaryotic, unicellular organism, meaning that it is a complex, single-celled organism that can eat and reproduce on its own. Unlike multicellular organisms like us, any individual yeast cell can grow and reproduce, creating new yeast without all of that tedious (and complicated) sex business. It can live aerobically (with oxygen) or anaerobically (without oxygen), using different ways to break down a range of organic chemicals that it can consume to harvest energy. It can thrive in a huge range of conditions and can survive as spores with tough outer shells in an even wider range, from boiling water to the cold of outer space. This ability to thrive and survive means that it is pretty much everywhere, including on the food we harvest.

Many thousands of years ago, prehistoric humans realized that this could be harnessed. If you ground up grains and mixed them with water, the naturally occurring yeast on the grains started eating the grains, breaking down some of the chemicals inside it into simpler ones. One way to use this is to add more water and store the mixture in a cool, dry place to make beer and wine. We'll discuss that route in another column.

A section of the bakery scene from the tomb of Ti at Saqqara.Dayr-Al-Barsha Project

The other way is to keep the mixture warm, with plenty of oxygen. If you left it for a bit and then baked this mix, you got bread, which was better tasting and more easily digested than the original grains. By allowing the yeast to digest the flour for you, it became more nutritious for us humans. Eventually, humans figured out what was doing the hard work here and harvested this yeast, finding specific types that could be stored and used for baking when needed.

It isn't clear where or when this first happened, but archaeologists have found evidence that baking was commonplace in ancient Egypt, because the process is recorded in hieroglyphs on an Old Kingdom tomb that dates from the 5th dynasty (between 2514 to 2374 BC) Recently, archaeologists have excavated the bakeries used to feed the workers who built the great pyramids and have reproduced their baking methods. It is humbling to look at these inscriptions from over 3,000 years ago and see a process that a modern baker would still recognize.

How yeast works

Grains store energy mostly in carbohydrates, long chains of sugar molecules. If the grain was to sprout, this would provide the energy for the plant to grow. When grain is harvested and prepared to make flour, most of the outside of the grain is removed (called the chaff), leaving just the rich, starchy interior. If you grind this up and add yeast and water to make dough, then the yeast starts chopping the long-chain carbohydrates down into shorter chains using an enzyme called alpha-amylase. This enzyme chops carbohydrates chains up into smaller pieces, and eventually into individual sugar molecules (a sugar called maltose), which are then converted down to the simplest sugar: glucose. The yeast can then metabolize (eat) the sugar and use the energy in it to grow and reproduce.

Colin McDonald/CNET

It also digests a number of proteins from the grains to create an elastic substance called gluten that gives the bread its physical structure, so it will hold together as a loaf. Incidentally, this is why gluten-free bread is hard to make: you have to use a different process to make the bread hold together without the gluten.

These two processes are what makes baking possible, because they make the flour easier to digest and give it the strength to hold together in a loaf. When the yeast has digested the sugar, it releases carbon dioxide (CO2) and a small amount of ethanol. The CO2 forms bubbles in the dough mixture, which grow as the yeast eats more and more of the sugar. Eventually, the dough rises to form the familiar shape, ready for baking. During baking, the ethanol evaporates away.

As any baker knows, you have to watch this process carefully. If the process fails to start, your dough isn't properly digested, and you get no sugar and gluten. If you let the process run too long, the yeast eats all of the sugar, and your bread doesn't taste right. A good baker knows when to strike the balance between the two, warming the dough to start it and interrupting this process by baking the dough to kill the yeast. This baking also solidifies the gluten to create a loaf, giving you your daily bread. And we should all be thankful for that.