Our Mutual Friend

What species has been domesticated for thousands of years, lives in our homes, helps to feed us, comes in different strains (‘pedigrees’), was one of the first to have its genome sequenced, and has limited alcohol tolerance—like us? I’m not thinking about one of our furry friends, but we would soon miss it.

I was musing about yeast while the breadmaker pounded dough to a low drumbeat in the kitchen. We don’t normally regard yeast as a companion species, but it has been with us longer than dogs or cats. It has had a much greater impact on human culture than either of them, and all because of the peculiar way it generates energy for life—by fermentation.

This is one of the first processes we learnt about in biochemistry class, and one of the simplest. Yeast cells possess an enzyme (zymase) that converts 1 molecule of glucose into 2 molecules of ethyl alcohol and 2 of carbon dioxide. Alcohol and gas are just waste products to these cells, but gifts of the gods to us. We have been harnessing fermentation to make bread and drinks for thousands of years before we figured out how it worked. The transformation of a heavy lump of dough into a light loaf of bread or a cloudy ‘must’ of grape juice into wine seemed miraculous to our ancestors.

And wine that makes glad the heart of man…and bread that strengthens man’s heart (Psalm 104)

The man or woman who first discovered fermentation has gone unrecorded, although archeological relics suggest it was in Egypt and the Middle East thousands of years ago, perhaps in Neolithic times. Like a number of great discoveries, the breakthrough probably came not to someone searching for a better bread or drink, but to a quick-witted person whose curiosity was aroused by changes in a lump of uncooked dough and a jug of fruit juice left out in the warm. Instead of discarding them, as most of would, he or she watched the natural experiment develop and tested the product. It was good. They had no notion about the underlying biochemistry, and it took a long time to realize that the process that raised dough and fermented juice were the same.

France’s greatest 19^th Century scientists were engrossed with the process because the wine industry struggled for vintages of consistent quality and that didn’t spoil. Their names come straight out of textbooks—Lavoisier, Gay-Lussac, Cagniard de la Tour, and above all Louis Pasteur.

It is hard to put ourselves in the mind of people who until fairly recent had no clue that fevers, ‘fluxes,’ and biological decay are caused by microscopic organisms. Yeast, bacteria, and sperm (‘animalcules’) first came into view under the simple microscopes invented by the draper of Delft, Antoni van Leeuwenhoek. That was two centuries before Louis, the genius who unveiled the world of tiny things and announced a ground-breaking germ theory of disease.

He had an uphill struggle. Many fellow scientists were unconvinced that fermentation is caused by living cells, and they don’t need oxygen to ‘breathe.’ It was thought to be a purely chemical process, and the debate grew bitter with Justus von Liebig, a German chemist who can be claimed to be the Father of Marmite (concentrated yeast, see Post March 23, 2013). When knowledge is familiar and deeply-rooted it’s hard to understand there was ever a contrary view. It seems obvious that yeast is a living thing. Poured into a lukewarm sugar solution you can see them generating gas bubbles from ‘breathing.” Bake them in bread and they die. Transfer a few into a vat of nutritious fruit juice and the broth soon ‘boils’ (Latin fervere, hence fermentation).

The domestic and industrial applications of these energetic little friends are endless. As a dietary supplement yeast is unrivaled as a source of B complex vitamins. Yeast fermentation is used to make other liquors: kumis from milk/ kombucha from tea/ kvass from rye/ soy sauce, tauco and doenjang from soybeans/ root beer from sassafras (until it was declared carcinogenic and replaced). It didn’t matter if the alcohol content was low so long as there were bubbles. Fermentation of cereals looms large for manufacturing biofuel so our automobiles can run on 10% alcohol. Yeasts are used as low cost bioremediators to mop up pollutants like, including copper, zinc, nickel, and arsenic in groundwater and ponds. They even render safe the explosive TNT!

But our friends, domesticated baker’s and brewer’s yeast, have shadier relatives that also like to find a sweet place to call home. Yeasts spread by air or contact can spoil foods and on our bodies they have a particular fondness for orifices. If our immune systems did not protect us from pathogenic species we would soon succumb to them, and patients with weakened immunity need treatment with fungicides to protect them.

But when home bread makers get together the talk is positive, and only about the staff of

life, exchanging formulas, and recommended suppliers of flour and yeast. Not all are loafers like me, happy to let a bread making machine do the hard work. Some of them love the exercise of beating dough, and a few use such violence that I wonder what is in their minds.

When two cycles of kneading and resting in the machine are complete I haul the dough out. I love how a sloppy mess of ingredients changes into an elastic, living ‘organism’ at blood temperature. I punch it down for a final rise (‘proofing’) in a baking tin before sliding it in a hot oven.

Bread making brings out the experimenter. My current favorite recipe for making a 2 pound loaf is:

It takes barely 15 minutes hands-on to make a loaf, but an eternity to cool enough for cutting the first slice.

Alas, home wine making from berries (even grapes!) takes so much longer. I don’t have the patience. It was disheartening to open a vintage that was tenderly stored for a couple of years only to find it was contaminated with something that made it sour. Louis might have chuckled that I should have washed my feet before trampling fruit.

But if a recipe goes wrong I blame myself, not the yeast. Humans have shared life with yeast cells for thousands of years in a cooperative relationship that ecologists might call mutualism. We give them sugars to grow and they give us alcohol and bubbles in return. We are mutual friends.

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