Fermentation is an incredibly complex process that can be mind boggling at times. Brewers like to think of yeast as a microscopic lifeform that transforms the sugars found in wort into alcohol, carbon dioxide gas, and metabolic byproducts that add flavor to the final product. However, in reality, yeast cells do not consume sugar. Yeast cells consume carbon, which they attempt to transform into energy. Alcohol and metabolic byproducts are the results of an inefficient metabolic pathway. The topic of this blog entry is how yeast cells transform compounds collectively known as carbohydrates into energy.
Brewers often hear the term “organic chemistry” used when describing fermentation. Organic chemistry is the study of carbon-based compounds. Sugar is carbon bound to water; hence, the term carbohydrate. All of the sugars found in wort are multiples of CH2O. The simplest sugars found in wort are known as monosaccharides. The monosaccharides commonly found in wort are glucose, fructose, mannose, and galactose. These sugars are also known as hexoses because they contain six carbon atoms. All of the hexoses share the shame chemical formula, which is C6H12O6. How the hexoses differ is in their linear form.
The four hexoses commonly found in wort belong to two different types of simple sugar. Galactose, glucose, and mannose are aldoses. Fructose is a ketose. An aldose is a sugar that contains one aldehyde group per molecule. A ketose is a sugar that contains one ketone group per molecule. Aldoses differ from ketoses in the location of something known as a carbonyl group. A carbonyl group is a carbon atom that is double bound to an oxygen atom. The carbonyl group appears at the end of the carbon chain in aldoses whereas it appears in the middle of the carbon chain in ketoses. Ketoses where the carbonyl group appears at the end of the molecule can isomerize into aldoses. The carbonyl group in d-fructose appears at the end of the molecule; therefore, it can isomerize into an aldose.
As we move up the scale in complexity from the monosaccharides, we discover a group of sugars known as disaccharides. A disaccharide consists of two monosaccharides bound by what is known as a glycosidic bond. The most abundant disaccharide found in wort is maltose. Maltose consists of two glucose molecules bound by a glycosidic bond. Sucrose is also a disaccharide found in wort, but to a lesser extent. Sucrose consists of a glucose molecule bound to a fructose molecule by a glycosidic bond. Another disaccharide that Saccharomyces pastorianus (S. pastorianus or simply lager yeast) can reduce to monosaccharides, but Saccharomyces cerevisiae (S. cerevisiae or simply ale yeast) usually cannot is melibiose. Melibiose consists of a glucose molecule bound to a galactose molecule by a glycosidic bond.
What is a glycosidic bond? A glycosidic bound is a type of covalent bond. In the case of glycosidic bonds, the bond occurs when atoms in two different sugar molecules share what are known as valence electrons. A glycosidic bond is formed via what is known as a condensation reaction. The outcome of a condensation reaction is another compound and an H2O molecule. For example, as mentioned above, maltose is a disaccharide that contains two glucose molecules bound by a glycosidic bond. Maltose is formed via the following condensation reaction:
C6H12O6 + C6H12O6 → C12H22O11 + H2O
By the way, like the monosaccharides, all disaccharides share the same chemical formula. The chemical formula for a disaccharide is C12H22O11.
The most complex sugars found in wort that affect fermentation belong to a family of carbohydrates known as trisaccharides. A trisaccharide consists of three monosaccharides bound by two glycosidic bonds. All trisaccharides share the chemical formula C18H32O16. The ability to reduce trisaccharides to simpler sugars is one of the attributes that affects how well changes in saccharification rest temperature affect final gravity. For example, maltotriose is the most frequently occurring trisaccharide found in wort. Maltotriose consists of three glucose molecules bound by two glycosidic bonds. One of the reasons why Lallemand Windsor leaves a high terminal gravity is because the yeast strain is maltotriose challenged. One way to offset this weakness is to rest one’s mash at a temperature of 65°C/149° or lower to produce an extract that contains a lower percentage of trisaccharides and dextrins. Many brewers refer to this type of wort as a more fermentable wort.
If yeast cells can only use monosaccharides directly, how do they reduce disaccharides and trisaccharides to monosaccharides? Yeast cells perform this feat via the inverse of a condensation reaction. The process is called hydrolysis. The roots of the word hydrolysis are from the Greek “hydros” for water and from the Latin “lysis” for break apart or deconstruct. Together, these words mean break apart via the insertion of water, and that is exactly what happens.
While breaking the glycosidic bond in a disaccharide is a one-step process, breaking the glycosidic bonds in a trisaccharide requires two steps. In the case of maltotriose, the first step involves breaking a maltotriose molecule into one maltose molecule and one glucose molecule.
C18H32O16 + H2O → C12H22O11 + C6H12O6
The maltose molecule is then split into two glucose molecules.
C12H22O11 + H2O → C6H12O6 + C6H12O6
Brewers who have delved into this area of fermentation have heard that S. pastorianus can use raffinose as a carbon source while S. cerevisiae can only partially metabolize raffinose. This limitation is due to the same limitation that prevents most S. cerevisiae strains from using melibiose as a carbon source. Raffinose consists of two glucose molecules and one galactose molecule. What happens when S. cerevisiae attempts to break the glycosidic bonds that hold raffinose together is that the raffinose molecule is split into one melibiose molecule and one glucose molecule. Unable to break the bond that holds melibiose together, this disaccharide is left undigested. Raffinose is lost during the malting of barley; therefore, it is absent from wort.
The rate at which hydrolytic reactions occur is shortened by the creation of enzymes. Enzymes are reaction catalysts. A reaction catalyst is a compound that increases the rate at which a reaction occurs. The enzyme responsible for catalyzing the hydrolysis of maltose into two glucose molecules is called maltase.
Enzymes are proteins, and proteins are encoded by cells via a process known as transcription. A cell’s DNA provides the blueprints for transcribing proteins. If one has ever wondered why different yeast strains yield different levels of attenuation given everything else equal, herein lies the reason. A yeast cell’s DNA controls the enzymes that can be encoded as well as the level at which the enzymes can be encoded.
What happens after the higher-level saccharides are broken down into monosaccharides? Well, the yeast cell goes about performing something known as catabolization. Catabolism is a metabolic process where the yeast cells attempt to turn carbon-based compounds into energy.
The primary catabolic process that occurs in yeast cells is called glycolysis. Once again, we see a word that ends in “lysis;” therefore, we know that this process involves the breaking apart or deconstruction of a compound. In the case of glycolysis, the compound is glucose. Glucose is the primary monosaccharide found in wort. It is also a building block for the most common disaccharides and trisaccharides found in wort. The goal of glycolysis is to turn glucose into a compound known as adenosine triphosphate (ATP). ATP is the fuel source for a cell. The transformation of glucose into ATP in the less efficient anaerobic metabolic pathway results the production of ethanol, higher alcohols, diketones, and organic acids. We can look at these metabolic byproducts as the yeast equivalent of incomplete combustion, as all of these compounds contain carbon.