When the term yeast is used, most people think of freshly baked bread. Many people will also think of a cold, foamy headed beer. Both are made possible by yeast, but there are many more applications.
Yeast has been used to raise bread and make beer and wine since prehistory, and the work is very ancient. It comes to us in modern English via the Old English gyst, which in tern derived from the Indo-European word yes, meaning quite literally to bubble. Thus the word is very much older than our understanding that yeasts are living things, dating from the 1850s due to the work of Louis Pasteur.
When we think of yeast, we normally are referring to a single species (out of around 1500, give or take), Saccharomyces cerevisiae. This single species is responsible for raising bread, making wine and much of the beer that is drunk, as well as alcohol for beverage and industrial purposes. Unless I qualify, when I use the term "yeast" this is the species to which I refer.
There are hundreds of strains of S. cerevisiae, each developed for specific purposes. For example, strains that raise bread to give that wonderful lightness and unmatched flavor will ferment grape juice to wine, but the wine is not very good. Specific strains of this species are used to produce different types of wine (and beer, and spirits), and these strains are carefully propagated and kept viable using the pure culture technique.
There is another species, S. pastorianus, that is used to produce more beer in the United States than the other species. (This is sort of controversial, as some botanists consider S. pastorianus just to be a variant of S. cerevisiae). For the purposes at hand, we shall consider it to be a separate species, but it really does not matter. To make this more clear, we first must consider that there are two basic types of beer: top fermenting and bottom fermenting.
Top fermenting beers, often called ales, are produced using strains of S. cerevisiae that clump together as fermentation happens, trapping some carbon dioxide and rising to the top of the fermentation vessel. These are popular in the UK, but not so much here. Bottom fermenting beers, often called lagers, are brewed with strains of S. pastorianus that do not clump together and finally fall to the bottom of the vessel when fermentation is complete. Almost all beer sold in the US is lager beer. Typically, ales are fermented at a higher temperature than lagers, and have more complex flavors due to "wild" yeasts that are introduced as the rising yeast cap is skimmed from a batch of ale.
If the yeast that rises is not skimmed on a regular basis, it forms "rocky heads" that sink and contaminate the batch. Since this does not happen with lagers, skimming is not required and no wild yeasts are introduced if the fermentation process is done correctly.
No one knows exactly when yeast was first used, but I think that I could argue successfully that yeast fermentation is the oldest industrial process known. Since yeast is extremely common and widespread, it is not possible to prepare flour or fruit juice without it being present, unless some sort of sterilization is performed. Thus, dough that sits around in the warm for a while will start to rise on its own, sort of like sourdough. Fruit juice will also start to ferment and thus convert to wine the same way.
Yeast gets its energy by taking sugar and converting it to carbon dioxide and alcohol. In beer and wine, the carbon dioxide vents to the atmosphere (except for a small amount that is intentionally trapped to from the bubbles), but the alcohol for the most part remains in solution. When beer and wine is made by using native, wild yeasts, several species are present but usually are suppressed by rising alcohol levels. S. cerevisiae tends to be more alcohol tolerant than other species, so it dominates in the later stages of fermentation. By the way, except for very primitive affairs, no beer or wine is made using wild yeasts any more.
In bread, both the carbon dioxide and the alcohol are lost during baking, so unlike beer and wine, none of them are left in the final product. Most people are aware that it is the carbon dioxide that the yeast produces that makes the bread rise, but yeast also produces other byproducts during metabolism and those byproducts are largely responsible for the wonderful flavors that develop in yeast bread. Here is a thought experiment to consider. Think about yeast bread baking, and then think of biscuits baking. They are pretty much made of the exact same ingredients, except that biscuits are leavened by chemical agents (baking soda and some kind of organic acid) that produce carbon dioxide. I am not being critical of biscuits, but they do not have the complexity in aroma that yeast breads have.
Up until around the mid 1800s all bread was essentially sourdough, although bakers had become pretty good at keeping bad strains of yeast out, the the bread was pretty good. This all changed in 1868 (just 11 years after Pastuer demonstrated that yeasts were living organisms and were responsible for fermentation) when Charles and Maximilian Fleishmann formed a company that grew and sold pure culture yeast. Now, except for specialty sourdough breads, all yeast bread is made with pure culture yeast.
Originally, the only form of yeast available was what is now called cake or compressed yeast. This a cream cheese textured product of living, metabolizing yeast cells that has to be kept cold and has a very short (days or weeks) shelf life. Recipes in old cook books call for it, and usually tell you to proof the yeast by mixing it with some sugar dissolved in water. If bubbles form after a few minutes, it is OK and the rest of the ingredients can be added. If not, the yeast is dead and must be replaced.
During World War II, Fleismann's developed active dry yeast, a product familiar to most of us in the little foil packs. Active dry yeast has a much longer shelf life and does not require refrigeration (although it will last longer if it is). It still needs to be proofed, but not so much to see if it is alive, but rather to remove the debris coating the cells so that they can rehydrate. This is done by dispersing it in warm water, with or without sugar, for several minutes before adding it to the rest of the ingredients.
A more recent development is instant dry yeast, (also called bread machine yeast) and it is a superior product. It is also available in the little foil packs, larger glass jars, and in pound vacuum packed bags. Its shelf life is excellent when kept cold, and is OK at room temperature for months. I bought a pound of Red Star instant dry yeast at a wholesale club for only a couple of dollars and, after transferring it to a glass storage jar, it lasted for years in my freezer with no apparent loss of activity. The difference betwixt this kind and active dry yeast is in the drying process, with instant dry yeast not suffering from being coated with debris. This kind of yeast can be added dry to the other ingredients and mixed without any previous rehydration. Make sure that you use this kind if you do not plan to proof it. You can tell it apart from active dry yeast because active dry yeast looks like little spheres (more of less) and instant dry yeast looks more like tiny rods.
Bread yeast is fairly standard, only a couple of strains being used. They were developed especially for breadmaking, optimized for maximum carbon dioxide output, mild flavor, and optimum flavor development of the other bread ingredients. You can make beer and wine with it, but the products are usually inferior. For beer and wine, you need yeast strains specifically developed for them. Red Star has a large line of beer and wine yeasts, as do other suppliers. However, there are dozens of beer and wine yeast strains. For example, the yeast used for, say, a lager like Budweiser and the yeast used for Guinness Stout are radically different (to the point that some biologists even say that they are different species, as described already mentioned). Likewise, yeast to make Champagne is quite different than that used to produce port.
The differences in the types of yeast for winemaking have to do with the amount of alcohol desired in the final product (from around 7% in light, fruity Germans to up to 18% for some Champagnes), the flavors desired in the final product, and the kinds of grapes (or other fruit) used. Most wine yeasts will tolerate up to around 12% alcohol content before their growth is inhibited, but some Champagne strains have been developed that are good to 18%, they they become inhibited.
These alcohol tolerant yeasts are also of interest in production of distilled spirits, because it is more economical to start out with the highest concentration of alcohol possible in the mash before it is distilled, so that energy costs are reduced. This is also important in industrial alcohol production, and strains have been developed that combine high alcohol tolerance with very fast metabolism.
The fast metabolism of fuel alcohol yeasts seems to have a deleterious effect on the quality of the flavor of alcohol distilled from these mashes, but flavor is not a factor in fuel production, but time is. On the other hand, in producing beverage alcohol, time is not as much as a factor, expect for some very high volume beverage, like popular beers.
There are several other species of yeasts that should be mentioned. Candida albicans, along with several other species of Candida are human pathogens in some cases. Most people are familiar with "yeast infections", a common gynecological disorder that is often treated with OTC remedies. Normally, Candida infections are superficial and easily treated, but this is not always the case.
Back in the day when there were no good antifungal agents, little children would die from Candida infections of the oral mucosa, a condition called thrush. Children still get thrush, but it is not common any more and easily treated. In the old days, their mouths and throats could become so inflamed that they could not eat or drink, and they would die of dehydration and/or malnutrition. Although now an uncomfortable condition for children, it is rarely life threatening any more.
More serious Candida infections occur in people who are immunocompromised, and they can be fatal. Treatment involves aggressive treatment with powerful antifungal agents, and some of them are rather toxic. The problem is that Candida is part of the normal flora of the skin and intestinal track, so it is not possible to avoid the infectious agent. Lots of people who take antibiotics develop a temporary overgrowth of Candida in the intestines, with resulting pain and other effects, until normal intestinal flora balance is restored after the antibiotic regimen is finished. The antibiotics wipe out the "good" bacteria in the gut, allowing Candida to flourish.
This has just been a short survey of yeasts, but I hope that you have come out with the idea that yeasts are much more important than we normally think. They do much more than make our bread rise.
Well, you have done it again! You have wasted many more einsteins of perfectly good photons reading this bubbly piece. And even though Mitch McConnell stops believing his own rhetoric about "job killing taxes" and the like when he reads me say it, I always learn much more than I could possibly hope to teach by writing this series. Thus, keep those comments, questions, corrections, and other feedback coming. Remember, no science or technology issue is off topic here. I shall stick around tonight as long as comments warrant, and shall return tomorrow after Keith's show for Review Time. It looks like the producers have settled on the comedian Bill Nye as their science adviser. Go figure! Bill Nye???!!!
Doc, aka Dr. David W. Smith