Understanding Yeast Metabolism
Y
east are microscopic, unicellular fungi that are capable of converting various types of sugar into ethanol and other byproducts. Yeast take in sugars and anaerobically (without oxygen) metabolize them to produce energy, additional yeast cells, ethanol, carbon dioxide and other metabolic byproducts:
Sugar + Yeast → More Yeast + Ethanol + Carbon Dioxide + Metabolic By-products + Heat
The two species of yeast that are most widely used by brewers are Saccharomyces cerevisiae (ale yeast) and Saccharomyces pastorianus (lager yeast). These two species of yeast differ in several important ways, including their optimum fermentation temperatures (lager yeast can ferment at colder temperatures), ability to ferment different kinds of sugars (lager yeast can ferment maltotriose) and production of fermentation byproducts (lager yeast produces fewer esters and is generally described as producing “cleaner” beer).
Yeast metabolism
When fermenting wort, yeast have several distinct phases of metabolism, including cell wall synthesis/O2 uptake, sugar uptake, nitrogen uptake, fermentation, energy production, cell growth, acidification, production of byproducts (from the yeast’s perspective . . . CO2, ethanol, others). These stages are not discrete, in other words one blends into the other as the fermentation progresses and a cell may be doing more than one thing at a time. Different cells within the population of yeast will be in different phases. For example, when a fermentation slows, some cells are quicker to shut down and flocculate than others.
Fermentation is also sometimes described as proceeding through lag phase (where there is little or no indication of active fermentation at the macroscopic level), exponential growth phase (when the yeast are multiplying most rapidly and cell counts increase exponentially over time), stationary phase (when the cell count is relatively stable and the bulk of fermentation occurs) and death (or in beer production, flocculation and perhaps the cells are reused for another fermentation).
Some of the major metabolic pathways relevant to beer production are shown in this figure.
Cell wall synthesis /O2 uptake
In order for yeast to begin cell wall synthesis, the yeast cells must have access to cell-wall building blocks: sterols and oxygen. Sterols can be synthesized by the yeast, or they may be scavenged from the wort (if present). Additionally, the yeast cells must be permeable enough to allow nutrient and oxygen uptake to occur.
Synthesis of cell walls using sterols and oxygen requires the yeast to consume internal reserves by using internally-stored glycogen in order to supply the needed energy. (As the fermentation progresses, newly assimilated wort sugars also contribute.) Glycogen reserves are slowly depleted during the storage of yeast, so older yeast will have less stored glycogen. Low oxygen levels or low glycogen levels can lead to a sluggish fermentation.
Sugar uptake
After the yeast has built up a cell wall structure, the next step is sugar uptake. In an anaerobic environment, yeast generally take up and preferentially ferment sugars in the following order: glucose, fructose, sucrose, maltose (the most abundant) and maltotriose.
Different enzymes help process the different sugars. For example, the enzymes sucrose permease and sucrase transport sucrose inside the cell and split it into glucose and fructose. Different enzymes take in maltose and maltotriose and split them into their constituent glucose residues.
Nitrogen uptake
Nitrogen in the form of amino acids is next taken up by the yeast. Amino acids can be thought of as consisting of an NH3+ component that is attached to various carbon chain skeletons. The NH3+ component is used by the yeast cells for protein synthesis. Amino acids are also the precursors for the formation of fusel alcohols.
Under certain conditions, yeast may excrete essential nitrogen-containing compounds back into the wort. This can occur if a yeast cell experiences larger osmotic pressures than they can cope with due to high wort gravity. This phenomena is known as shock excretion.
Fermentation
From the perspective of the yeast, fermentation produces numerous byproducts (things not associated with making more yeast) including ethanol, acetaldehyde, fusel alcohols, esters and ketones.
Ethanol is a major excretion product from fermentation. Excretion of ethanol is a cell detoxification mechanism from pyruvate and acetaldehyde build up within the yeast cell. Ethanol is, of course, the two carbon alcohol that provides the “kick” to beer and is also toxic to many other microorganisms that could potentially spoil wort.
Acetaldehyde is an intermediate anaerobic fermentation product that is mostly converted to ethanol during the fermentation process. Acetaldehyde is the compound associated with a green apple flavor within finished beer. If yeast cells are present, acetaldehyde will continue to be converted to ethanol during aging.
Fusel alcohols — alcohols that contain more than two carbon atoms — are formed when the carbon skeletons from amino acids in the wort are taken up and biochemically reduced within the yeast cell to an alcohol of corresponding chain length. Different fusel alcohols have different flavors and aromas. Fusel alcohols can have flavors that are described as “hot,” solvent-like, rough, chemical, medicinal and rosy. These are not wanted in beer and can cause headaches for the drinker at moderate concentrations.
Esters are formed when an alcohol combines with an acetyl-CoA-produced fatty acid. Ethyl acetate, the most common beer ester, is formed from ethyl alcohol and acetyl-CoA. Ester compounds can have flavors described as fruity banana and are more prevalent in ales than lagers.
Ketones and vicinal diketones (VDKs) are formed by the oxidation of the amino acid synthesis intermediates valine and isoleucine. Ketone flavors are described as buttery or butterscotch (e.g. diacetyl), fruity, musty, honey (e.g. 2,3-pentadione) and rubber. These molecules are not desired in most beers, although some English-styles have a noticeable level of residual diacetyl. VDKs can be removed by yeast during the later stages of fermentation. This is the primary reason that brewers are advised not to separate their beer from their yeast to quickly following primary fermentation.
Factors in the fermentation process that impact yeast performance
Fermentation involves complex interactions of biological, chemical and physical factors. Factors such as wort temperature, wort gravity, yeast nutrient availability, dissolved oxygen content and yeast pitching rate all affect how the yeast will ferment the wort.
Conclusions
Yeast are the living organisms that do the biochemical work of making beer. The ability of yeast to ferment wort and produce beer is affected by myriad variables. In order to ensure that you have optimized the conditions for the best yeast performance you should use fresh, healthy yeast; ensure that the wort is fully aerated; pitch an appropriate amount of yeast (frequently, this involves making a yeast starter); fermentation within the optimal temperature range for quality beer production (varies with yeast strain) and don’t separate the beer from your yeast too soon.
