What Is Yeast?
Yeasts are eucaryotic single celled fungi which reproduce by budding or fission. In brewing, Saccharomyces cerevisiae is the Genus and Species that is of the most significance. Ale and lager yeasts are currently both classified as Saccharomyces cerevisiae but historically have been considered separate species with lager yeasts in the Saccharomyces uvarum (carlsbergensis) classification. Yeasts are very small, typically 5 to 10 microns (1 micron = 10-4 centimeters) which is around 5 times the size of most bacteria.
Yeasts play an important role in human history. Without yeast, important staples like bread, beer and wine would not exist. As important as yeasts are in our common history, it was not until relatively recently (1841) that yeasts were recognized as the cause of fermentation. Since that time, yeast strains have been isolated and recognized for unique characteristics that help create specific styles of beer and add specific characteristics to wine, distilled spirits, and bread. There are literally hundreds (if not thousands) of strains of yeast used for fermentation to create alcoholic beverages in almost every society on earth.
Where Do Yeasts Come From?
Yeasts are everywhere! There are several hundred recognized species of yeasts, with many strains within each species classification. Basically, anywhere there is a carbohydrate (sugar) source, you will probably find yeasts that are adapted to consume it. Saccharomyces cerevisiae is a species that has the capability of utilizing a wide range of sugars such as, glucose, fructose, mannose, galactose, sucrose, maltose, maltotriose, and raffinose (lager yeasts can also metabolize melibiose). The ability to metabolize a wide range of sugars allows Saccharomyces cerevisiae adapt to many environments.
The first fermented beverages were purely accidental. A sugar solution of some kind (exactly what the first fermented beverage consisted of is a hotly contested issue) was left out and yeast dropped into it from the air or was on the surface of the fruit or grain used to make the solution. The solution fermented and someone drank it and liked the taste and most likely the effect. The cause of the transformation from juice to wine or grain to beer remained a mystery for thousands of years. However, humans were able to select certain traits in yeast without being aware of the manipulation. Cold northern regions selected yeast that could ferment in colder temperatures. The use of brewing paddles ensured that yeast was carried over from batch to batch. In winemaking, the lees (a mixture of yeast from the fermentation, grape skin particles, and precipitated proteins) were spread out on the vineyard, increasing the presence of high alcohol tolerant yeasts that had dominated the fermentation.
As civilization advanced, so did brewing and winemaking practices and their importance to society. Regions of the world developed specific styles and types of fermented beverages and the techniques required to ensure consistent flavor profiles. All of this refinement led to selection for and development of yeast strains with very specific characteristics and capabilities. By the time yeasts were “discovered” and identified as the engine that drives fermentation, the worlds known brewing and winemaking yeast and bacterial cultures were already faithfully carrying out their specialized duties without any recognition or real help from human kind.
The modern brewery and winery have excellent tools at their disposal for creating a predictable, consistent product. What makes this possible is the ability to access known pure cultures of yeast and to minimize the intrusion by wild yeast and bacteria that cause “off” aromas and flavors. There are some breweries and wineries that still use “natural inoculation” with great success but they are typically very small, specialized producers of very unique products. It is possible to genetically modify yeast and it is done frequently in the biotechnology sector, but it is not a common practice in the brewing and winemaking industry. As a result, most of the cultures in use today for brewing and winemaking have been in existence for hundreds of years.
What Does Yeast Do?
Yeasts are very efficient at taking wort, (a very complex medium consisting of sugars, amino acids, peptides, proteins, vitamins, nucleic acids, ions and many other compounds), and converting it to alcohol, CO2, and hopefully favorable flavor and aroma compounds. This process of converting wort to beer is called fermentation and is extremely complex with many variables that affect yeast performance. Each yeast strain produces different levels of flavor and aroma compounds as well as alcohol levels even if all conditions are identical. This variation between yeast strains gives the modern brewer a powerful tool for creating a particular beer style.
It is important to understand that yeasts are living organisms that will respond to their environment by producing more or less of certain compounds. The brewer controls the environment that the yeasts are put into and so it is very important for the brewer to know how yeast will respond to certain environmental factors. The most important factors that affect yeast performance that a brewer can control are:
- Wort gravity
- Pitch rate
- Yeast strain
- Nitrogen content
- Nutrient content
- Grain bill/Mash program (spectrum of fermentable sugars)
In typical wort fermentation, yeasts follow a predictable schedule of events: Lag Phase, Log Phase, and Stationary Phase.
The lag phase is the time during which the yeast become acclimatized to the wort and prepare to reproduce and consume massive amounts of sugar. As with all phases in the yeast life cycle, nothing is clear-cut. Not all of the yeast undergo the exact same process at the same time but generalizations are possible.
In this phase, oxygen is extremely important. Oxygen is used by yeast for synthesis of sterols and unsaturated fatty acids that are necessary growth factors. Without oxygen, these lipids can’t be biosynthesized and growth will be very limited. The sterols and fatty acids produced are also very important in the structure of the cell membrane and the ability of the yeast to respond to external and internal stresses. Adequate oxygen in the wort when the yeast is pitched is essential to a successful fermentation and long-term health of the culture.
During the lag phase, yeast also take in amino acids, peptides, other proteins, and various nutrients. Internal cellular energy reserves (glycogen) fuel the initial yeast activity until the membrane becomes permeable to the wort sugars.
The log phase is a time of exponential growth of the yeast culture. The preparation the yeast made during the lag phase allows rapid multiplication of cells and consumption of sugar. Yeasts reproduce asexually by budding; the adult cell forms a daughter cell that is an exact genetic copy itself. Nitrogen, amino acids, nutrients, and sugar are consumed while the cells are reproducing.
Many of the significant aromatic and flavor compounds are by-products of cell growth and are produced during the log phase. Many large breweries try to limit the amount of yeast growth by pitching larger quantities of yeast and therefore minimize ester synthesis. Keeping fermentations cold also limits ester production by limiting the rate of growth.
The stationary phase is the last stage where the yeast population reaches maximum density and the remaining sugars are consumed. Triggers that are not yet completely understood cause the yeast to begin increasing cellular energy reserves in the form of glycogen and trehalose. As the available sugar and nitrogen decreases, the yeasts begin to prepare for a period where there is a lack of food.
When the yeast have consumed all of the sugar, flocculation (the act of yeast aggregating in to “clumps” and settling out) begins. During this time, many brewers will allow the fermentation to “rest” (at fermentation temperature for ales and to rise in temperature for lagers). The rest allows time for the yeast to convert or reduce some less desirable compounds, especially diacetyl, to more acceptable or desirable compounds.
Yeasts are amazing organisms that perform many important tasks for humanity. The production of alcohol is a service that yeasts have performed for people for as long as recorded history. In our modern world, we have the capability to not only utilize a wide range of pure yeast strains, but to also control the environment in which the yeasts perform their valuable service. It is important for the modern brewer to not only know and understand the yeast strains available for use but to also understand how manipulating the environment in which the yeasts perform (wort, must, or dough) can drastically alter the finished product (beer, wine, or bread). The endless combinations of ingredients, yeast strains, equipment designs, and brewing techniques are what make brewing a fascinating and sometimes all consuming hobby.