How Scientists Solved The Mystery Of Rising Bread

From bleeding polenta and a corrupt grain trade, author Eric Pallant traces how scientists figured out that sourdough’s fermentation comes from living microbes.

The following is an excerpt from Sourdough Culture: A History Of Bread Making From Ancient To Modern Bakers by Eric Pallant.

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In the first decade of the 1800s, French chemist J. L. Gay-Lussac took one step closer to understanding yeast. Gay-Lussac filled flasks with grape juice and placed them on ring stands. Beneath each one he lit a Bunsen burner and boiled the liquid until the aroma of Chardonnay permeated his lab. As soon as he turned off the flame, he stoppered each flask and let the liquor sit for a year. He was aware, as all good Frenchmen were, that unboiled grape juice left for a year became wine, or at least vinegar.

Only once he opened his flasks and exposed them to air did his grape juices begin to acquire the aromatic and chemical characteristics of fermentation. His conclusion, dead wrong, was that heat from his Bunsen burner had deactivated his globules and that inrushing air contained the chemicals necessary for fermentation. In truth, once his flasks were unstoppered, bacteria and yeast charged in, landed happily in his sterilized liquids, and began at once to consume grape sugars.

Another piece of the puzzle was placed in August 1819, when so-called blood burst from a batch of polenta in Padua, Italy. The peasant who had cooked it tossed the batch, but the next day it reappeared in his bowl of fresh polenta. A priest was called to pray over the peasant’s polenta, to no avail. The following day, the epidemic of bloodstained polenta spread to other households. The press went wild. Soon everyone knew of the terrifying signals appearing in Padua.

The local explanation was that the Almighty was prepping his revenge upon Paduans who had engaged in excessive speculation in the grain trade. Gambling and manipulating prices were renowned sins. Now that the outbreak of discharging polenta had outed the community, doom was sure to follow.

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Bartolomeo Bizio, a Venetian chemist, took a scientific approach to unravel the mystery of the bleeding polenta of Padua. Exactly eighteen days after the outbreak, Bizio moistened some bread and some polenta and left them in a warm, damp atmosphere, the kind of environment, incidentally, you might expect inside a medieval church. Twenty-four hours later, they were bleeding. After five years of additional experimentation, he was able to say with confidence that microscopic blobs called bacteria were discharging the red liquid. He named them Serratia marcescens. Serratia appeared on bread or polenta that was warm and moist. It could be passed from one bowl of polenta—or from one loaf—to another by residue left in a bowl or by the hands of a baker or polenta maker.

In 1827, Jean Baptiste Henri Joseph Desmazières, an editor of scientific journals and amateur mycologist, drew pictures of everything he could see in a magnified sample of brewing beer. He named his microbes Mycoderma cerevisiae. Cerevisiae is the Latin word for beer. He did the same for wine, drawing Mycoderma vini. Desmazières drew figures that sure looked like yeast cells. He even figured out that they were simple living organisms. Regrettably, he failed to recognize them as the creatures causing fermentation.

In the 1830s, a trio of scientists led by Frenchman Charles Cagniard-Latour almost figured it out. Cagniard-Latour possessed an excellent microscope with a magnification of five hundred diameter, with which he observed yeast cells throughout the process of fermentation. At that level of enlargement, he recognized Leeuwenhoek’s globules “to be organized beings, which are probably of the vegetable kingdom.” As additional proof that these globules were in fact living organisms, he determined that their numbers increased during fermentation. He was able to describe for the first time the act of a yeast cell budding, and he was even able to see two yeast cells parting as they grew older. He pointed out that they broke down sugar only when they were alive, consumed sugar during fermentation, and reproduced like all other living beings. At last, yeast moved from chemistry to biology.

There was still one problem. Spontaneous generation made more sense than infection. Grape must kept in a vat fermented extemporaneously; a slab of meat left unattended generated maggots where none before existed; grain produced weevils even inside closed bags; and a slurry of wheat flour and water began to bubble if left in the open air for three days. How could a scientist prove that living organisms visible only under a microscope were responsible for infecting foods by coming into contact with them? In other words, how does a cupful of today’s sourdough starter make tomorrow’s bread rise?


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In the late 1830s, German physiologist Theodor Schwann filled four flasks with cane sugar and brewer’s yeast. Everyone knew that if he left them undisturbed, he’d have a beer-like substance within a month. Following Gay-Lussac’s earlier experiment, Schwann boiled all four for about ten minutes. While the flasks cooled, he covered them with mercury, a substance so dense that no air, yeast, or bacterium could penetrate it.

Next, he introduced air to all four flasks, but not the same air. Into two of the four flasks, he introduced air that he had first roasted by passing it through a red-hot glass tube. The heat killed anything floating into his intake. After four to six weeks of incubation, the sugared water in the flasks with heated air was as sterile as the day he sealed them. But the two flasks into which he had poured raw air were covered with a film of living organisms, bubbling away and reeking of fermentation.

Here was proof to counter Lavoisier’s earlier analysis, which led him to conclude that his equation depended solely on the correct mixture of chemicals in his substrate and in the air. Schwann demonstrated that whatever was in the air could be killed by heat. When Schwann looked at microscope slides that had been dotted with sterilized laboratory grape juice left open to the air, yeast appeared. It multiplied, and he watched as it produced bubbles of carbon dioxide, the very compound that effervesced beer and raised bread.

Nearly all the pieces were in place. Antonie van Leeuwenhoek had discovered a universe of cells but did not have the tools to understand what a cell really was. Some of Leeuwenhoek’s microscopic cells were yeast, and over time, observers concluded that yeast cells were living organisms capable of growth and reproduction.

It took until the 1850s, nearly two centuries after Leeuwenhoek drew a rough picture of a yeast cell, before French biologist Louis Pasteur finally placed the last pieces in the puzzle. Fermentation was possible only in the presence of microorganisms and sugar, he said. The microorganisms consumed sugar, multiplied, and produced alcohol and carbon dioxide. In complete refutation of spontaneous generation and one thousand years of church dogma, Pasteur concluded that yeast, like other microscopic fungi, bacteria, molds, and their ilk, was as omnipresent as God. It floated about us on currents of air, covering every surface in an invisible but unmitigated film.


Reprinted with permission from Sourdough Culture by Eric Pallant, Agate, September 2021.

Meet the Writer

About Eric Pallant

Eric Pallant is a professor of Environmental Science and Sustainability at Allegheny College and author of Sourdough Culture: A History of Bread Making from Ancient to Modern Bakers. He’s based in Meadville, Pennsylvania.

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