Carbohydrates For Fermentation

Carbohydrates and their chemistry can be an extremely complicated subject. In this appendix, we're going to give you just a taste of that complexity, and then simplify back down the few simple points that apply directly to fermentation and distillation. Knowing about the complexity and the vast numbers of different kinds of sugars can help appreciate how valuable the traditional fermentable sources truly are.

Carbohydrates are materials made up exclusively of carbon, hydrogen and oxygen, in a ratio that is very close to one atom of oxygen and two of hydrogen for every carbon atom. The general carbohydrate formula is C(H2O)n. Chains of three or more carbon molecules are called sugars, and are given names that end in the letters -ose. Sugars with six carbons (the most common size) are called hexoses, and five-carbon sugars are pentoses.

Many different sugars have exactly the same chemical formula - they are isomers of one another, differing by the arrangement of the atoms on the chain. Here are the chemical structures, in chain form, of several different hexoses, all with the same formula, and the same molecular weight - 180.16. Of these, only glucose and fructose are readily fermentable.

AldoHexoses

AldoHexoses

To make things more complicated, each of these hexoses can form rings due to the interaction of oxygen molecules, and they can form several different kinds of rings. Here is an example of how this happens in glucose

Fig. A2-2 Glucyclation

The rings form and fall apart constantly, so all the different forms exist in a state of dynamic equilibrium. Here are diagrams of the various forms of glucose and fructose.

Fig. A2-2 Glucyclation

And to get one step more complicated, most of the ring forms can exist in different structural versions.

Fig. A2-3 Fructomers and glucomers

Here are the two forms of (alpha)-D-glucose.

Fig. A2-4 Glucboat and Gluchair)

Since enzymes interact with molecules on the basis of their shape, any particular enzyme usually only interacts with one of the many forms a sugar molecule can assume.

Fortunately, all we really need to remember is that fructose and glucose are the fermentable simple sugars, and that yeast can readily move these molecules into the cells for processing into ethanol and carbon dioxide.

Sugar molecules can join together into chains in a variety of forms to form polysaccharides. Groups of two sugar molecules are called disaccharides, three are trisaccharides, and so forth. Simple polysaccharides with 10 to 50 sugar molecules are collectively called dextrins. Polysaccharide molecules can be straight or branched, and can become very complex.

The most common disaccharide is sucrose (common table sugar), which is composed of one glucose and one fructose molecule. In the process of joining together the two hexoses, one molecule of water is given up, so the molecular weight of sucrose is 342.3 - 5.2% lower than the combined molecular weight of fructose and glucose. Another common disaccharide is maltose, formed from two glucose molecules, and a common trisaccharide is maltotriose is a glucose trisaccharide. Here are pictures of these three fermentable sugars.

Distillation Maltotriose
Fig. A2-5

Sucrose cannot be transported across cell walls, but some types of yeast and bacteria release enzymes into their environment that hydrolyze sucrose, adding back the missing water molecule, and producing one molecule of glucose and one of fructose. These are then taken into the cell and processed.

Many kinds of polysaccharides cannot be enzymatically broken down, and thus cannot be fermented. The dextrins produced in a beer-making wort are not fermentable, and give the beer its body. A few special types of carbohydrates are very widely made by plants. Two of the most familiar are starch and cellulose, both of which are made up of chains of glucose molecules. Starch and cellulose differ in how they are linked together. Starch is built in exactly the same way as maltose and amylose, with every glucose molecule joined to the next on their "lower" side (this is called an "alpha" linkage). In cellulose, the molecules are joined together from the lower side of one to the upper side of the next, called a "beta" linkage. Although the diagram shows all the sugar molecules "right side up", in the beta linkage, every other one is actually "upside down". Here are diagrams of starch and cellulose.

CHîOH CH2OH CH2OH

CHîOH CH2OH CH2OH

H OH H OH H OH

Cellulose

The alpha linkage of starch makes it fairly easy for enzymes to break down into glucose, which is why plants use starch as a "storage material" for food energy. When they need stored energy, they release enzymes called amylases which break down the starch. We use these same enzymes to prepare starchy material for fermentation.

Cellulose's beta linkage, on the other hand is much harder to break down, and it is used by plants as a primary structural material. There are a few organisms that make enzymes capable of hydrolyzing cellulose, but the process is very slow and expensive compared to starch.

Pectin is another complex polysaccharide that should be of interest to beverage makers. Pectin is mainly composed of methyl esters of galactose, joined by a beta linkage. It is a primary constituent of plant cell walls, and is especially abundant in fruit. The methyl esters are the "COOCH3" groups in the picture below. This represents (O=C-O-CH3). The non-methylated sugars have "COOH" (O=C-OH) in the same location. These two forms are mixed together in no particular order, and in amounts that

COOH COOCHs COOCHa

COOH COOCHs COOCHa

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