From morning toast to evening wine, yeast is mankind’s daily blessing and our communion with this little fungus is deepening year by year. It has taken us from hunting and gathering to more settled lives as farmers, provided our daily bread, and fed our thirst for wine and beer. Without yeast, the earth would be an alcohol-free planet and every loaf of bread would be unleavened. In present times, yeast has become a crucial part of biotechnology, generating a catalogue of life-saving medicines, and billions of gallons of bio fuels in the quest to slow climate change.
How does yeast work?
Yeast, the sugar fungus has been civilization’s invisible partner from the get-go. Yeast cells were among the first microorganisms seen after the invention of the microscope in the seventeenth century. Anton van Leeuwenhoek examined them in drops of beer in 1680, although he did not consider that these tiny globules were alive. Chemists, including the French scientist Antoine Lavoisier, studied the fermentation process in wine making in the next century. Recognition that yeast was a living thing that produced alcohol came in the nineteenth century.
Alcohol is a rare molecule in nature. Additionally, its formation by yeast alcohol synthesis is limited to germinating seeds and a few kinds of bacteria. Yeast uses glucose and other sugars to fuel the dynamism of its cells. Energy is harvested from these molecules by splitting them into smaller parts and stripping energized electrons from their component atoms. Stripping electrons is called oxidation. Where there is sufficient oxygen around, yeast has the option of breaking down glucose through two sets of reactions. Capturing energy along the way, leaving nothing in its wake but water and carbon dioxide. Stage one is called glycolysis and stage two is called a citric acid cycle. The fungus is proficient in adjusting to the loss of oxygen and can keep running differently. This does not get as much as energy out of the sugar molecules but succeeds in meeting the immediate needs of a growing population of yeast cells. The residual energy is exhausted by the cell in the form of alcohol.
Yeast produces alcohol until the level rises to between 10 and 15 per cent, which kills the fungus. This process limits the alcohol content of beer and wine. Things work a little differently in nature, where yeast has the potential to keep growing after a period of fermentation by devouring its own alcohol. This dexterity allows the fungus to create, concentrate and consume alcohol.
The discovery of yeast’s effectiveness in raising bread dough was more serendipitous than its adoption for brewing. It would have escaped our attention if we had not made beer and wine first. Bread will never rise without a big shot of fresh yeast. The most likely source was an accidental splash of aromatic froth from the top of a beet vat. Finding itself in moistened cereal flour, the yeast began with the two-stage process of aerobic respiration, digesting sugars in the dough as it was kneaded, releasing water and CO2. Alcoholic fermentation would have kicked in too, as oxygen level dropped, releasing alcohol and more bubble of CO2 into the dough. The sight of rising dough must have seemed magical to the early bakers and it remains so today.
Besides its efforts in making wine, beer and bread, yeast has become indispensable in the modern science of molecular genetics. Schizosaccharomyces is one of several microorganisms that ferments a popular tea drink called kombucha. The supposed health benefits of Kombucha, which includes improved digestion, mood elevation and weight loss have turned the bottle drink into a popular product worldwide. The specific role of fission yeast in flavouring kombucha is unknown.
Most biologists have little knowledge of the yeast. Microbiologists who concentrate on bacteria and viruses offers scant coverage of these fungi, even mycologists would rather look at mushrooms. Brewer’s and baker’s yeast are taken for granted, but the larger lives of yeast are easy to ignore. It takes scientific knowledge as well as a considerable imagination to appreciate the skills that have enabled these fungi to sustain the planet, 24/7, for hundreds of millions of years.
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