This post has already been read 10793 times!
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.
Domestic yeast is a remarkable workaholic on our behalf. Bakers’ yeast, Wikipedia says,
Baker’s yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and bakery products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. Baker’s yeast is of the species Saccharomyces cerevisiae, which is the same species (but a different strain) commonly used in alcoholic fermentation which is called brewer’s yeast. Baker’s yeast is also a single-celled microorganism found on and around the human body.
Conversely, brewer’s yeast – its cousin – is described as:
Brewing yeasts may be classed as “top-cropping” (or “top-fermenting”) and “bottom-cropping” (or “bottom-fermenting”). Top-cropping yeasts are so called because they form a foam at the top of the wort during fermentation. An example of a top-cropping yeast is Saccharomyces cerevisiae, sometimes called an “ale yeast”. Bottom-cropping yeasts are typically used to produce lager-type beers, though they can also produce ale-type beers. These yeasts ferment well at low temperatures. An example of bottom-cropping yeast is Saccharomyces pastorianus, formerly known as S. carlsbergensis.
Decades ago, taxonomists reclassified S. carlsbergensis (uvarum) as a member of S. cerevisiae, noting that the only distinct difference between the two is metabolic. Lager strains of S. cerevisiae secrete an enzyme called melibiase, allowing them to hydrolyse melibiose, a disaccharide, into more fermentable monosaccharides. Top- and bottom-cropping and cold- and warm-fermenting distinctions are largely generalizations used by laypersons to communicate to the general public.
Just a sidebar note here: spirits like tequila are fermented the same way beer is, then the result distilled to concentrate the alcohol and disperse the water. And another note: lambic beers are a specialty beer that uses re-fermentation based on wild yeasts (in breweries) and if you’ve never tried one, go to the LCBO and buy a bottle (I recommend the kriek beer if you can find it) or go to a good beer pub and ask for one. It’s quite different and exotic (and nothing like the fermented cardboard most big breweries try to pass off as “beer” to gullible consumers).
Yeasts simply convert sugars and starches to other stuff. Fun stuff, like alcohol:
Yeast eats sugar, glucose to be specific. If there is no glucose around but there are other sugars, starches or alcohols, yeast creates machines (enzymes) to convert these into glucose. The yeast carries information in its DNA for dozens of machines specific to many food sources.
Flour has a lot of starch in it, which is made of long chains of sugar molecules. Flour carries its own enzymes that work on the starches and chop them into simple sugars. This happens after the flour has been rehydrated with water or other liquids. Then the yeast uses the sugars for energy.
The alcohol made when baking gets burned off when cooking. In brewing, it’s retained in the wort. Have you ever stood above a vat of fermenting malt or agave mash? The released carbon dioxide is heavy, and doesn’t rise. It forms a protective shield that holds the alcohol in. If you sniff too closely to the surface, you can faint – lack of oxygen!
Shirley Corriher, of Fine Cooking, describes what happens with bread:
When you stir together flour and water, two proteins in the flour—glutenin and gliadin—grab water and each other to form a bubblegum-like, elastic mass of molecules that we call gluten. In bread making, we want to develop as much gluten as we can because it strengthens the dough and holds in gases that will make the bread rise.
Once flour and water are mixed together, any further working of the dough encourages more gluten to form. Manipulating the dough in any way allows more proteins and water to find each other and link together. If you’ve ever made homemade pasta, you know that each time you roll the dough through the machine, the dough becomes more elastic; in other words, more gluten is developed. And with puff pastry dough, every time you fold, turn, and roll the dough, it becomes more elastic.
Yeast, like kneading, helps develop the gluten network. With every burst of carbon dioxide that the yeast releases into an air bubble, protein and water molecules move about and have another chance to connect and form more gluten. In this way, a dough’s rising is an almost molecule-by-molecule kneading. Next time you punch down bread dough after its first rise, notice how smooth and strong the gluten has become, in part from the rise.
Bakers have a choice when cooking: baking powder, baking soda or yeast. All are leavening – rising – agents. You need about three times as much baking powder as baking soda to get the same effect. Baking powder is really baking soda (a base compound) with an acidic compound – like cream of tartar – and a filler (corn starch) mixed in. The interaction between base and acid (when heated and water is added) creates the bubbles.
I’m not a chemist, but I still remember some of it from my years as a student. Here’s how it works:
Baking soda, also known as sodium bicarbonate, has the chemical formula NaHCO3. Cream of tartar, also known as tartrate salt, has the formula KHC4H4O6. The reaction is:
NaHCO3 + KHC4H4O6 —-> KNaC4H4O6 + H2O + CO2
NaHCO3 + KHC4H4O6 —-> KNaC4H4O6 + H2O + CO2
Some baking powders contain sodium aluminum sulfate: NaAl(SO4)2. The reaction there is:
NaAl(SO4)2 + 3 NaHCO3 —-> Al(OH)3 + 2 Na2SO4 + 3 CO2
NaAl(SO4)2 + 3 NaHCO3 —-> Al(OH)3 + 2 Na2SO4 + 3 CO2
Many recipes call simply for baking soda rather than baking powder. Usually these recipes use some kind of liquid acid like buttermilk or yogurt to react with the baking soda to produce the bubbles.
Both yeast and baking soda/powder will flavour the result in different ways than yeast. Personally, I prefer the yeast flavour in breads, but soda/tea biscuits are usually made with baking powder, and they’re pretty good, too. It depends on what you’re baking. The big difference is that yeast converts sugars in the flour, which means they get used up, not retained as in soda biscuits.
As Shirley Corriher, of Fine Cooking points out, the difference in flavours is in part because of the fermentation. Yeast breaks the big molecular chains in the flour into smaller ones. And humans find smaller molecules tastier:
As Harold McGee, the author of On Food & Cooking, has pointed out, big molecules in proteins, starches, and fats don’t have much flavor, but when they break down into their building blocks—proteins into amino acids, starches into sugars, or fats into free fatty acids—they all have marvelous flavors. Fermentation, whether it’s acting on fruit juices to make wine or on flour to make bread, does exactly that—it breaks down large molecules into smaller, flavorful ones.
At the beginning of fermentation, enzymes in the yeast start breaking down starch into more flavorful sugars. The yeast uses these sugars, as well as sugars already present in the dough, and produces not only carbon dioxide and alcohol but also a host of flavorful byproducts such as organic acids and amino acids. A multitude of enzymes encourages all kinds of reactions that break big chains of molecules into smaller ones—amylose and maltose into glucose, proteins into amino acids.
As fermentation proceeds, the dough becomes more acidic. This is due in part to rising levels of carbon dioxide, but there are also more flavorful organic acids like acetic acid (vinegar) and lactic acid being formed from the alcohol in the dough. (This is similar to what happens to a bottle of wine that has been left uncorked for a while: the alcohol combines with oxygen to make vinegar.) The acidity of the dough causes more molecules to break down. The dough becomes a veritable ferment of reactions. Eventually, the amount of alcohol formed starts to inhibit the yeast’s activity.
But, as she concludes, yeast isn’t the only player here. Other single-celled creatures are competing for this food: bacteria. And they add their own flavour to the mix:
Yeast has help in producing flavorful compounds. Bacteria are important flavor builders as well. There are bacteria in the dough from the beginning, but as long as the yeast is very active, it consumes sugars as quickly as they’re produced, leaving no food for the bacteria, which also like sugar. But when bakers chill a dough and slow down its rise, the cold dramatically reduces yeast activity. The bacteria, on the other hand, function well even in cold temperatures, so they now have an opportunity to thrive, producing many more marvelously flavorful acids.
Which reminds me of the lambic beers, I mentioned above. the Prepared Panty tells us:
The biological and chemical actions taking place as the bread ages and rises are called fermentation. Generally, a long, slow fermentation makes for better flavor, texture, and moisture retention. Many fine breads call for “retarding” or slowing down the growth of the yeast with refrigeration. If dough is refrigerated, the yeast grows more slowly. Fermentation still takes place as the amylase enzymes work within the dough and sugar is released albeit at a slower rate. When the dough is warmed and the growth of the yeast takes off, there is plenty of sugar present for the yeast and an excess of sugar to sweeten the bread.
When yeast grows more slowly, we find the richer, fuller flavor of breads made with retarded dough. In the previous article, we discussed a focaccia that uses refrigeration to slow down the growth of the yeast and create the desired crumb and flavor. Is it a good bread without retarding? Yes, but retarding does give it desirable flavor overtones and a more open crumb.
Yes, you can use use wild yeasts for bread – that’s what real sourdough bread does. But you’re also open to allowing bacteria and other yeasts to pollute the starter with chemicals that make it taste bad. Any home brewer knows about a sour wort when domesticated yeast was overwhelmed by airborne yeasts and bacteria.
Okay, so you’re a neophyte like me and you go to the grocery store. What kind of yeast do you buy for bread? I look on the shelves and see several varieties:
Cake yeast, or compressed yeast, is fresh yeast. It is used by many professional bakers and can be found in the refrigerated section of some supermarkets. It has a short shelf-life of one to two weeks. Some pastry recipes call for fresh yeast, which comes in 0.6-oz squares.
Active dry yeast is the most commonly available form for home bakers. It is available in 1/4-oz packets or jars. Store jars in the refrigerator after opening. Be sure to check the expiration date before baking.
Instant yeast is a dry yeast developed in the past thirty years. It comes in smaller granules than active dry yeast, absorbs liquid rapidly, and doesn’t need to be hydrated or “proofed” before being mixed into flour.
“Bread Machine Yeast” is instant yeast that may include ascorbic acid, a dough conditioner.
Okay, so I pick up some dry yeast. What next? It needs to be prepared, or “proofed” before use:
Yeast makes carbon dioxide gas that acts as a leavening agent. Start by “proofing” or growing the yeast: this ensures it is active and re-hydrated (this step is not required for fresh or instant yeast):
Sprinkle the yeast onto warm (110 degrees F/45 degrees C) water and stir to dissolve. The water should feel warm, not hot, to the touch. Yeast feeds on sugars–honey, molasses or refined sugar–by breaking down the flour’s starches into sugar molecules.
Set the yeast aside until the mixture resembles a creamy foam. This should take between three to eight minutes.
If nothing happens, discard the mixture and try again with different yeast.
You may have to vary the amount of yeast called for in the recipe depending on the type you use. Most of this is trial-and-error. And making brick-dense loaves of bread. The HuffPost has a little guide to the types, and The Fresh Loaf has a good FAQ as well as many discussions about yeast types.
And the conclusion? Yeasts are both friend and foe, but essential to two basic human functions: baking and brewing. How they affect my own baking will be posted later, after I choose a bread machine (or choose to do it by hand).
Like I said earlier, suggestions are welcome.
- 3656 words
- 22346 characters
- Reading time: 1192 s
- Speaking time: 1828s