Yeast in Brewing

Like many domesticated plants and animals, yeast has found it convenient to hitch a ride with human beings, who care for it and feed it the sugar and other nutrients it loves. Brewing, baking, winemaking, distillation, and even fuel ethanol all depend on yeast. The diversity of strains attests to its great age and versatility. It is believed that the ale yeasts we brew with today are direct descendants of yeast first brought into the domestic sphere about ten thousand years ago. Brew by brew, jugs of slurry have been handed off from brewer to brewer, treasured like the ember of a fire too precious to be allowed to go cold.

Sometime around 1500, a new hybrid yeast appeared that was happy in much colder conditions than ale yeast. This yeast, Saccharomyces pastorianus, developed along with a new type of cold-fermented beer, lager. It is now known that this yeast came from hybridization of an ale yeast with a cold-tolerant yeast called Saccharomyces eubayanus, which was recently found living in Patagonia. How this happened is a bit of a mystery, as the timing of lager and New World voyages don’t quite match.

Lager yeast stayed in Bavaria until the mid-nineteenth century, when the lager explosion took the world by storm. Viewed as a more modern type of beer, lager was subject to an enormous amount of research to improve its production. Lager yeast was purified, reduced to a single-cell culture by Emil Christian Hansen at Carlsberg in Denmark, who also isolated the Tuborg strain a little later. Today’s lager yeasts descend from these original purified strains, and as a result, there is very little genetic variation among lager strains. While there are some differences, they tend to be minor, perhaps more related to behavior in the brewery than any obvious effect on beer flavor.

Ale yeast remains vastly more complex. Some strains, like those used for altbier and Kölsch, are neutral and clean, and will tolerate some cold conditioning. Strains from Britain show a wide range of personality: spicy, fruity, woody, malty, and more. You will probably not be surprised to learn that it is Belgium that has the greatest variety of yeast. The beer there was never nationalized; separate languages and other regional differences have preserved many different beer styles, although much has been lost as well. Even in Belgium, one reads accounts from the early twentieth century that the beers were more complex before they went to single-cell cultures.

Belgian yeast strains are so distinctive that they will turn any kind of wort into a Belgian beer. Their flavors range from fruity and gently aromatic to a big complex esteriness to spice-tinged strains to those like saison that are dryly phenolic, and for which the common descriptor is “peppery.” And that’s just the brewer’s yeast. Several Belgian beer types use wild yeast and even bacteria like Pediococcus to ferment beer. A broad range of microorganisms in your beer is an almost inevitable consequence of fermenting in wood, but as breweries have modernized, specific yeasts like various Brettanomyces species have been cultured and added to the stainless tanks, and this is becoming popular elsewhere, too.

Wild critters are not limited to Belgium. Scratch the surface of lager-mad Germany and you will find survivors like Berliner weisse, made briskly sour by a Lactobacillus fermentation. Right in the very heart of Reinheitsgebot country, the popular Bavarian specialty, hefeweizen, uses a unique yeast, Torulaspora delbrueckii, to add a complex banana/bubblegum/clove aroma that is key to the style.

Yeast Biology

Yeast is a fungus, which is the kingdom intermediate between animals and plants. While we think of mushrooms and other fungi more like vegetables, fungal biology is actually more similar to animals than to plants. And although yeast is single-celled, it has a nucleus, which is a giant step above bacteria in evolutionary terms.

Though yeast might look like a simple little thing under the microscope, it is magnificently complex. Yeast is a complete living thing, capable of eating, ridding itself of toxins, reacting to its environment, and, finally reproducing. Yeast even has the ability to communicate with its neighbors, sending out messenger chemicals like heat shock proteins that warn of troublesome living conditions. Some yeasts can even kill other microbes.

Yeast Cell Biology

CYTOPLASM
The jelly-like liquid that fills the cell, and in which everything else is suspended.It is not inert. Plenty of cellular processes take place within it.

NUCLEUS
The brain of the cell, which directs activities and contains the genetic material needed for reproduction.

NUCLEOLUS
A structure contained within the nucleus, with specialized functions to create ribosomes, cellular structures where proteins are synthesized, and to produce ribonucleic acid (RNA).

ENDOPLASMIC RETICULUM (ER)
An extensive network of wobbly sheetlike structures throughout the cell but especially clustered near the nucleus. One type of ER temporarily shelters ribosomes, the cell’s protein manufacturers. Another type of ER has a complex function including lipid synthesis and metabolism of certain substances, and a variant of this regulates calcium levels within the cell. Both are connected to the nucleus by tubular structures.

BUD SCAR
The mark left after a daughter cell buds off of the mother.

MITOCHONDRION
A specialized organelle where energy production takes place. In yeast, sugars are broken down and converted to adenosine tri-phosphate (ATP), which can power various processes in the cell.

LIPID STORAGE DROPLETS
Lipids (fats) form an energy storage mechanism for cells.

GOLGI APPARATUS
Sort of a packaging department for cellular proteins that will be secreted from the cell. It is formed by a stack of cisternae, blobby disks with unique capabilities. The proteins arrive and depart in vesicles, and are processed by moving through the stack of disks.

VESICLES
Encapsulated droplets containing substances that need to be transported without being diluted in the cell’s cytoplasm.

VACUOLE
Essentially a reservoir, both for chemicals the cell may need to keep handy for certain functions and molecules require the aid of specialized proteins that act as gatekeepers.

CELL WALL
This is composed of a lipid membrane supported by a cytoskeleton made from fibers of a glucan carbohydrate. While some small molecules can pass through without assistance, larger molecules require the aid of specialized proteins that act as gatekeepers.

LYSOSOME
These organelles digest worn-out cellular parts and food particles and attack bacteria and viruses. They disgorge their contents into the vacuole.

PEROXISOMES
These are cellular organelles where fatty acids are degraded a process that is inhibited under typical fermentation conditions making them an interesting workhouse for production of fatty-acid-derived molecules.

Brief History of Yeast

For centuries, humans have mass-produced food and alcoholic beverages using fermentative yeasts, of which wine and beer are the best-known products derived from this process.  Its history can be  back over 5,000 years to the Egyptians who used yeast to make their bread, believing it to be a miracle. Recent genomic evidence suggests that the canonical beer and bread yeast, S. cerevisiae, originated in China before moving west via the route which would become the Silk Road.

For a long time, wild yeasts present in the air spontaneously fermented brewing vats, hence yeast’s nickname of “gift of god”. Each region would brew its own type of beer as yeast in the air varies, depending on its location! 

In 1516 the beer purity law was introduced by the Bavarians, better known as the Reinheitsgebot. The law made it illegal to brew beer containing anything other than water, barley malt, and hops. Yeast was not included in the ingredients at that time, because they did not know it existed.

The invention of the microscope by the Dutch lens manufacturer, Janssen, allowed the study of micro-organisms such as yeast. About 150 years later in 1680 Anton van Leeuwenhoek was first to observe, through a microscope, that yeast was composed of small, interconnected elements. Interestingly, he did not realize that it was alive. At that time, the most commonly accepted theory of fermentation was that it was a spontaneous process—a chemical reaction promoted by contact with the air—and the yeast was a chemical by-product.

People began to domesticate beer yeasts in the late sixteenth or early seventeenth century, when beer-making in Europe moved from homes to pubs and monasteries.

Louis Pasteur

In 1857, Louis Pasteur analyzed and understood the fermentation process. He claimed that yeasts were responsible for fermentation and demonstrated that the yeast cell could live with or without oxygen and was a key element in bread flavor and aroma. Louis Pasteur discovered that certain organisms, including yeasts, were able to live in the absence of air. He called them anaerobic organisms.In 1866, his work on wine saw the appearance of pasteurization, a technique consisting in heating a liquid before cooling it suddenly, with the aim of killing germs. The first scientific research on brewer’s yeast was conducted by Pasteur in 1876 (“Les études sur la Bière”) and allowed the selection and storage of the most appropriate brewing strains as well as the development of beer pasteurization.

Emil Christian Hansen

Hansen started working at Carlsberg as a researcher on organisms in beer. In 1883 Emil announced his system of pure yeast cultures. He revealed that bad beer was not only a result of bacterial infection, as French biologist, Louis Pasteur had assumed, but contamination by wild yeast. Until then brewers re-used beer from previous fermentations. Isolating yeast meant they could use “fresh” clean yeast every time, significantly improving the brewing process and taste consistency. He then worked to isolate a single cell of good yeast and propagated it into a pure culture. The new “Carlsberg bottom yeast n.1” was used for the1883. Its scientific name was Saccharomyces carlsbergensis or Saccharomyces uvarum (now S. pastorianus), but most brewers call it lager yeast.

Alfred Jörgensen & Axel Bergh

The first known patent that followed Pasteur’s insistence on sterility of both media and equipment was in 1891, by Alfred Jörgensen, director of his own lab by the same name, and Axel Bergh, directory of his own lab in Stockholm, Sweden and owner of several breweries. Their patent used a sterile aeration system, thus maintaining strain purity and enhancing growth. Sterile media and equipment are required to maintain pure cultures, and are now standard in modern yeast production. By the early 1900s, better aeration methods and the invention of centrifuges (replacing filters) increased production capability. This, in turn, allowed the expansion of the commercial baking industry, and by the 1920s commercial yeast as we know it was born. Today Jörgensen’s lab has been supplying breweries with pure culture yeast for almost 130 years.

Yeast Types

There are hundreds of different brewing strains in yeast banks around the world, and each strain produces a different flavor profile. Most major breweries generally have their own strain of yeast. These yeast strains have evolved with the style of beer being made, particularly if that brewery was a founder of a style, such as Anchor Steam.

The choice of yeast can push a wort in a particular direction: fruity, spicy, woody, peppery, hoppy, or neutral. Most of Europe’s still a huge variation, so knowing your Fuller’s from your Young’s from your Whitbread’s gives you an advantage when trying to choose from the many strains on the market. With the choices available these days, one can brew almost any beer that can be imagined.

ALE YEAST

Ale yeast (Saccharomyces cerevisiae) is called top-fermenting yeast because its vigorous fermentations produce lots of CO2 that pushes it to the top of the tank.  It likes warmer fermentation temperatures and tends to produce lots of fruity, spicy, or sweet esters that add lots of flavor to the beer. Ale yeast is crucial when brewing ales, stouts, porters,  Kölsch, Altbier, and wheat beers.

The ideal fermentation temperature for all strains is 68°F (20 °C), but they can tolerate temperatures between 50°F- 77°F (10°C-25°C), in consideration of the strain. Most strain are between 64.4°F-71.6°F (18°C-22°C).

LAGER YEAST

Lager Yeast (Saccharomyces uvarum, and S. pastorianusis) are the opposite of Ale yeast, and ferments on the bottom, as well as converts the sugars at a colder temperature than Ale yeast. This colder process is called lagering or lager style, and has a lower temperature range from 44.6°F-55°F (7°C-15°C).

Due to the lower degree in temperature the fermentation process does lake longer, yet it also reduces the off flavor productions. Lager beers have been considered to be “cleaner” or having a “crisp” flavor profile, such as Pilsners, Bocks, Marzen, Dortmunders, and American malt liquor, and other lagers styles.

WILD YEAST or BRETTANOMYCES

Brettanomyces, also referred to by brewers as Brett, is Greek for British Fungus, is a genus, and has several strains or species belonging to it. These species can provide beer with flavors and aromas that are unique enough for brewers to consider them as options for fermentation.

Brett has been mostly viewed as a spoilage yeast, except in Belgian lambic, Flanders red/brown beers, and a handful of styles of wine. More recently Brett has gained popularity in the United States (and subsequently the brewing industries of other countries) as a yeast that can contribute desirable and novel characteristics to beer and other alcoholic beverages.

YEAST-STRAIN FLAVOR AND PERFORMANCE CHARACTERISTICS

Most yeast can handle about 10 percent alcohol without any problem. High-alcohol beer types can go up to 12 percent or more as long as they’re pitched in sufficiently large quantities into well-oxygenated wort. Normal wine yeasts can easily reach 15 percent or slightly more, while specialized yeasts for distillation, sake, or tokay wine, can tolerate 20 percent or more. Brewers making super-strong brews usually start with alcohol-tolerant ale strains in wort at around 1.100 (24°P), adding extra sugar and a specialized yeast when the normal yeast conks out. Rousing (stirring), along with extra oxygen, is sometimes necessary to ferment very high-alcohol beers.

All the characteristics from esters to phenol and beyond. Lager yeasts have restrained aromas. Every ale strain has some amount of fruitiness, but only a few specialized ones produce noticeable phenol, giving spicy, sometimes peppery aromas. Each strain is different.

With yeast behavior, this is largely related to flocculence. A yeast that stays in suspension longer ferments more thoroughly. Beyond differences between strains, attenuation is greatly affected by pitching rate and wort aeration, as greater quantities of yeast are more effective at eating all the sugar in the wort. Alcohol tolerance of yeast obviously has an impact on attenuation of strong beers.

This is the tendency for yeast cells to clump together and drop out of the beer, a process that involves the binding of special proteins and other chemicals on cells’ surfaces. Some flocculence is useful, as it clears the beer, but with too much, the strain may be underattenuative, leaving the beer a bit sweet. So-called powdery or nonflocculent strains are highly attenuative, making a dry beer, but they may need to be fined with isinglass or gelatin in order to clear the beer.

Every yeast has a temperature range where it produces the most style-appropriate flavor. Below its working range, the yeast may go dormant; above it, estery and possibly phenolic flavors become too much. The high end is more a matter of personal preference and style norms, but the low end is a matter of yeast physiology, and the brewer has little choice over the matter.

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