While I was pondering the nature of flour in my cogitations about bread machines (I’m still debating which model, by the way – suggestions welcome, but local stores have few options), I turned my grey matter to the business of yeast. Yeast is, of course, important in bread making because it makes bread rise.
Why? You ask. Well, dear reader, the answer is simple but not pretty: farts.
Yep. Yeast flatulence makes bread rise. Yeast out-gases carbon dioxide after it eats. The gas gets trapped in the gooey dough and, since it can’t escape, the dough expands. Food writer Linda Stradley puts it more genteelly:
The main purpose of yeast is to serve as a catalyst in the process of fermentation, which is essential in the making of bread. The purpose of any leavener is to produce the gas that makes bread rise. Yeast does this by feeding on the sugars in flour, and expelling carbon dioxide in the process. As the yeast feeds on the sugar, it produces carbon dioxide. With no place to go but up, this gas slowly fills the balloon. A very similar process happens as bread rises. Carbon dioxide from yeast fills thousands of balloon-like bubbles in the dough. Once the bread has baked, this is what gives the loaf its airy texture.
You’d think something as simple as a single-celled plant would be pretty easy, but it’s a rich topic with a lot of considerations, opinions and options.
Not all yeasts are the same, although all have characteristics in common. Yeast is one of many species of a mono-cellular plant – a eukaryote – which Wikipedia somewhat stuffily describes as:
Yeasts are eukaryotic microorganisms classified in the kingdom Fungi, with 1,500 species currently described (estimated to be 1% of all fungal species). Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of strings of connected budding cells known as pseudohyphae, or false hyphae, as seen in most molds.
Yeast size can vary greatly depending on the species, typically measuring 3–4 µm in diameter, although some yeasts can reach over 40 µm. Most yeasts reproduce asexually by mitosis, and many do so by an asymmetric division process called budding.
By fermentation, the yeast species Saccharomyces cerevisiae converts carbohydrates to carbon dioxide and alcohols – for thousands of years the carbon dioxide has been used in baking and the alcohol in alcoholic beverages. It is also a centrally important model organism in modern cell biology research, and is one of the most thoroughly researched eukaryotic microorganisms. Researchers have used it to gather information about the biology of the eukaryotic cell and ultimately human biology.
Other species of yeasts, such as Candida albicans, are opportunistic pathogens and can cause infections in humans. Yeasts have recently been used to generate electricity in microbial fuel cells, and produce ethanol for the biofuel industry.
Yeasts do not form a single taxonomic or phylogenetic grouping. The term “yeast” is often taken as a synonym for Saccharomyces cerevisiae, but the phylogenetic diversity of yeasts is shown by their placement in two separate phyla: the Ascomycota and the Basidiomycota. The budding yeasts (“true yeasts”) are classified in the order Saccharomycetales.
Got that? Good yeasts, bad yeasts and not all are helpful. In fact, many are downright harmful. Right now, you’re covered in yeast. It’s on your skin, growing merrily in your navel and every crevice and fold. It’s in the air you breathe. On your pets and children. Much of it is benign, but some is troublesome and can cause food to spoil. Hint: wash your hands before handling foods.
Airborne yeast is what makes sourdough breads work (the wild yeast settles on the starter and colonizes it, reproducing merrily). Sourdoughs are always regional, even local breads because they depend on the yeasts in the air in the room where the starter is laid out. You can buy sourdough starters with embedded yeasts, but first try making your own. Instructions are on many sites.
And then there’s the poolish – a pre-fermented mix (I used to prepare something like this when I made my own bread, 20+ years ago):
A pre-ferment is a fermentation starter used in bread making, and is referred to as an indirect method. It may also be called mother dough.
A pre-ferment and a longer fermentation in the bread-making process have several benefits: there is more time for yeast, enzyme and, if sourdough, bacterial actions on the starch and proteins in the dough; this in turn improves the keeping time of the baked bread, and it creates greater complexities of flavor. Though pre-ferments have declined in popularity as direct additions of yeast in bread recipes have streamlined the process on a commercial level, pre-ferments of various forms are widely used in artisanal bread recipes and formulas.
For us the main type of yeast we want to discuss here is the minuscule powerhouse, Saccharomyces cerevisiae. It’s the mainstay of both baking and brewing. But in each it has a different role to play. You might note the Latin connection through the Spanish word for beer – cerveza.
What’s really remarkable is that humans have put these little plants to work for our benefit. Without domesticating yeast, we would have no bread, no beer, no wine, no tequila. Unthinkable! yet they toil at our pleasure for not cost, just the food they eat.
Yeast is our oldest domesticated organism, going back at least 8,000 years in our service. And more recently it has served us well in the lab when trying to work through human genetic issues:
…yeast and humans share about 40 % of highly conserved gene products. Human genes (cDNA), once transferred into yeast, can fully replace the functions of their counterparts.
Several facts underline the importance of yeast research: (i) within the last fifty years, seven Nobel Prize winners (at minimum) have worked with this system; (ii) the yeast genome harbors hundreds of genes that are highly related to ‘disease genes’ in humans: in many cases, the human genes were detected only by comparisons with the yeast genome; and (iii) yeast is successfully used in the research of neurodegenerative diseases, even though yeast has no nervous system whatsoever.
Modern, commercially available yeasts are like laboratory rats: created to serve a purpose and not natural:
Few people realize that the yeast in grocery stores is not a naturally-occurring substance. Laboratory created in 1984, the yeast sold today is so foreign to our digestive systems that some people develop allergies to the yeast itself. This quick-rising yeast appears increasingly connected to the nutritional and digestive disorders that plague so many today, including Celiac’s disease, gluten-intolerance, acid-reflux disease, wheat allergies and even diabetes. Both modern science and traditional wisdom tell us that natural yeast has health benefits that simply cannot be matched by modern yeast.
Without some technical, scientific research to back that up, it’s hard to digest those claims. And I have yet to find authentication of the 1984 date. It’s a complex history:
It appears that bread making dates back at least 6000 years, but use of leavening, which required the development of suitable cereal grains with easily removable hulls, gluten, and the introduction of yeast cells, did not appear until around 500 BC. With the development of agriculture, it was probably found that addition of some of the fermenting wine to dough resulted in a lighter, more pleasant bread. Alternatively, insects may have landed on the dough and inoculated it with yeast.
Yeast has a really fascinating history in human company. Science Daily reports it was a stowaway on European ships travelling to North America 500 years ago:
In the 15th century, when Europeans first began moving people and goods across the Atlantic, a microscopic stowaway somehow made its way to the caves and monasteries of Bavaria.
The stowaway, a yeast that may have been transported from a distant shore on a piece of wood or in the stomach of a fruit fly, was destined for great things. In the dank caves and monastery cellars where 15th century brewmeisters stored their product, the newly arrived yeast fused with a distant relative, the domesticated yeast used for millennia to make leavened bread and ferment wine and ale. The resulting hybrid — representing a marriage of species as evolutionarily separated as humans and chickens — would give us lager, the clear, cold-fermented beer first brewed by 15th century Bavarians and that today is among the most popular — if not the most popular — alcoholic beverage in the world.
And while scientists and brewers have long known that the yeast that gives beer the capacity to ferment at cold temperatures was a hybrid, only one player was known: Saccharomyces cerevisiae, the yeast used to make leavened bread and ferment wine and ale. Its partner, which conferred on beer the ability to ferment in the cold, remained a puzzle, as scientists were unable to find it among the 1,000 or so species of yeast known to science.
Now, an international team of researchers believes it has identified the wild yeast that, in the age of sail, apparently traveled more than 7,000 miles to those Bavarian caves to make a fortuitous microbial match that today underpins the $250 billion a year lager beer industry.