From Cereal Corn to Alcohol

Maltose Molecule
(pictures with friendly permission of Dr. Bernd Meynhardt/Universitaet Kiel)

Whisky by definition is a beverage distilled from grain. How do you produce alcohol from grains? By fermentation, of course. But you have to carry out a long series of chemical and biological reactions until you get alcohol.

Sugar

Grains consist primarily of starch. Other constituents include proteins, fats and trace elements. Starch is the basic material required for alcohol production. For a simpler explanation of the chemical context, I would like to begin not with starch, but with sugar. Sugars and starch both belong to the carbohydrate group. If you understand the basic characteristics of sugar, then you can easily appreciate the properties of starch.

Historically, our northern ancestors knew only of bee honey and fruits as a sugar supply. That changed with the development of contact to other parts of the world, when cane sugar from southern latitudes arrived. Only recently was sugar beet discovered as a sugar source. In the middle of the 19th century sugar plants were developed for northern latitudes. Today we produce sugar from sugar beet and starch sugar plants and additionally import substantial quantities of cane sugar into the European community (supplier e.g.: South Africa, Mauritius).

Everyone knows of some sugar forms from food advertisements. In addition to the sugars noted in the following chart, there are many other sugars, whose chemical structures are similar.

Description	Chemical Description	Formula
Cane Sugar	Saccharose	C₁₂H₂₂O₁₁ (= 2*C₆H₁₂O₆- H₂O)
Dextrose (Grapes)	Glucose	C₆H₁₂O₆
Fruit Sugar	Fructose	C₆H₁₂O₆
Milk Sugar	Lactose	C₁₂H₂₂O₁₁
Barley Sugar	Maltose	C₁₂H₂₂O₁₁

Common to all these sugars is the basic chemical formula (C₆H₁₂O₆). Thus a simple sugar molecule consists thus of six carbon, 12 hydrogen, and six oxygen atoms.

The important sugars Glucose and Fructose each have six carbon atoms (C₆H₁₂O₆). Note that there are sugars with five or seven carbon atoms (example: Ribose) where a H-C-OH group is missing or is additionally inserted into the chain.

A fruit sugar (Fructose) has a slightly different structure in the chain. The second H-C-OH group has been replaced by a Carbonyl group (C=O); but the aldehyde function at the end of the chain is missing.

What makes a sugar sweet? Sweetness is due to the OH groups, which react with the receptors on our tongue. But the quantity of the OH groups is not decisive; rather it is the relative positions of these OH groups in three-dimensional space. Only certain orientations can fit onto the sweet receptors on the tongue. There are non-sugar substances whose OH groups are arranged similarly in three-dimensional space (artificial sweeteners, Glycol...), and therefore also taste sweet.

The C¹ and C⁵ atoms of the sugar molecule may be connected by the doubly bound oxygen atom of the upper aldehyde group to form a ring of five carbon and one oxygen atom. No atoms are lost during this addition and all atoms are built into the newly formed ring. The sixth carbon atom points laterally away from the ring. The ring is structured in space in the form of an easy chair. This ring structure is energetically more favorable than the straight chain. Statistically, a mixture of 99% rings and 1% chains are found in glucose solutions.

The important sugars for alcohol production (maltose, glucose) typically form these rings. However cane sugar consists not only of rings of six (carbon atoms) but also of rings of 5.

Alternately, the OH group at the first c-atom (formerly group of aldehydes in the chain) can be exchanged with the H-atom at the same C-atom. This results in another spatial configuration, called a beta arrangement.

Starch

Starch is constructed from Glucose by connecting several of these rings to form long chains, by the splitting off of water molecules. The water is eliminated between C¹ and C⁴ atoms.

This compound is the barley sugar. You may write this compound in the following way:

This describes an alpha Glucose ring (Glc) connected by its first C-atom to a second ring via its fourth atom. Starch consists of repeated linkages of such sugars according to the generalized formula:

The formula above describes chemically pure starch, which is given the name Amylose. Pure Amylose is spatially bent. Natural starch is not structured quite so regularly. In addition to chains, branched forms appear.

If you chain beta glucose molecules by the same method you get cellulose, as found in the wood of whisky casks. Cellulose molecules are long chains which can connect themselves to each other and bind via hydrogen bonds. Therefore cellulose is fibrous and more stable than starch flour.

The enzyme Amylase, found in barley, can split grain starch at the O bonds. By contrast, it cannot detect the beta-forms of the sugar and thus does not react with cellulose. Goats are able to split cellulose by micro organisms in their intestine into sugars.

The enzyme attacks the chains, dividing them into two sugars (Dimere, Maltose) from the ends of the strand. If the entire chain is split, only two and three-chain constituents (Trimeres) remain. The Trimeres are not cut further by the enzyme.

Alcoholic Fermentation

The last step of cereal corn to alcohol reaction pathway is alcoholic fermentation.

Alcoholic fermentation is not performed by enzymes, but rather by yeasts. Yeasts are not bacteria but fungi. Yeasts are found in nature everywhere, particularly in the autumn when fruits ripen and their spores spread in large numbers in the air. These carry out a simple chemical reaction in sugar/water solutions according to the following formula:

The yeast fungi splits a Glucose molecule and produces two ethanol molecules (alcohol) and two carbon dioxide molecules per ring and energy in the form of heat. Additionally, fruity flavour materials (esters) develop, which provide a large taste variety to the whisky. The yeast fungi cannot act on pure starch, which remains untouched by the funghi.

In Scotland usually two different dry-yeasts (baker and brewery yeast) are used (see e.g.: Glen Moray or Dallas Dhu). The first yeast provides a fast start to the fermentation. At the same time the wash is acidified. The second yeast operates better in the sour environment and achieves its max. performance later. It ensures that the wash will have a high alcohol content by the end of the fermentation process. In America they especially favour the development of fruity components (esters) by the yeasts. Each Bourbon has therefore its own yeasts. These have been isolated from wild yeasts and patented by the companies. All yeasts are reproduced in their own private facilities (see e.g.: Four Roses and its laboratory) in large quantities. They are added to the wash in liquid form.

The carbon dioxide rises in the fermenting, bubbling solution and escapes to the air. The ethanol produced enriches itself in the solution. The fungus lives from the energy produced by the chemical reaction. This process persists for a long time, either until all the sugar is used up (typical for whisky) or until the alcohol concentration has increased so much that the yeast fungus is killed by its own products (typical for wine).

Vinegar - Acetic Acid - Vinegar Bacteria

Each distiller, brewer or winegrower has to confront himself with the existence of vinegar-producing bacteria. Vinegar bacteria are found in free nature in exactly the same environments as the yeast fungi. They nourish themselves on alcohol and produce acetic acid. If a fermentation container (wash back, fermenters) is stricken by vinegar bacteria, then the whole content is lost for the distiller. For this reason the wash backs in Scotland are cleaned with chemical substances and in the USA the fermenters are even sterilized with high temperature. The fundamental chemical reaction, which the vinegar bacteria execute, is as shown below:

Since the reaction needs oxygen, one can usually avoid an infection with vinegar bacteria by strict exclusion of air.

In addition to the vinegar bacteria, we described here, special bacteria exist, which produce initially alcohol and then acetic acid. There are also more highly developed fungi which compete with these vinegar bacteria and can work along the whole chain from the starch to the acetic acid.

Nature still performs miracles. But we can easily enjoy them without solving them.