From the Grain to Alcohol

By definition whisky is a distilled beverage made from grain. How does grain become alcohol? - By fermentation, of course. - But for that a long series of chemical and biological reactions must take place.

The 3D image of a Maltose molecule
Maltose molecule
(Pictures by courtesy of Dr. Bernd Meynhardt/University of Kiel)

1. Sugar

A grain consists mostly of starch. Further components are proteins, fats and trace elements. Starch is the basic material for alcohol production. For a simpler explanation of the chemical context, I'd like to start with sugar rather than with starch. Sugar and starch are both carbohydrates. Once you have understood the basic properties of sugar you can also understand starch easily.

Historically Europeans only knew honey and fruits as sugar sources. This changed abruptly with the exploration of the world, when sugar cane arrived from the southern hemisphere. Only recently has the sugar beet been discovered as a sugar source. Sugar beet plants have only been cultivated since the middle of the 19th century in Europe. Today we produce sugar in beet and starch sugar plants and in addition we import considerable amounts of cane sugar into the European Union (e.g. from South Africa or Mauritius). Some sugar types are well-known from food ads. Beside the sugars listed below there are many more types of sugar whose chemical structures are similar.

NameChemical nameChemical formula
Cane sugarSucroseC12H22O11
(=2 x C6H12O6-H2O)
Grape sugarGlucoseC6H12O6
Fruit sugarFructoseC6H12O6
Milk sugarLactoseC12H22O11
Malt sugarMaltoseC12H22O11

All these sugars have the basic chemical formula (C6H12O6) in common. A simple sugar molecule consists of 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. The following structure shows a stretched d-glucose molecule.

d-glucose molecule in 2D
d-glucose molecule

The important sugars glucose and fructose both have six carbon atoms (C6H12O6). But there are also sugars with five or seven carbon atoms (e.g. ribose). They lack an H-C-OH group in the chain or have an additional one.

Fructose has a slightly different chain structure. The second H-C-OH group is replaced by a keto group (C=O), while the aldehyde function at the end of the chain is missing.

d-fructose molecule
d-fructose molecule

What makes a sugar sweet? It's the OH groups, which react with the receptors on the tongue. However, the amount of OH groups is irrelevant. The decisive factor is the spatial position of these OH groups, since they can only make contact with the receptors on the tongue in a certain position. There are substances whose OH groups are spatially similarly arranged (artificial sweeteners, glycol,...), a property that makes them also taste sweet.

A ring of 6 carbon atoms
carbon ring in the glocose

Via the double-bound oxygen atom from the upper aldehyde group, the C1 and C5 atoms of the sugar can now connect to a ring consisting of 5 carbon atoms and one oxygen atom. No atoms are eliminated; they're all included in the new ring structure. The sixth carbon atom protrudes laterally from the ring. The ring has a chair-like structure in space, which is energetically more favourable than a straight chain. Statistically, a mixture of 99% rings and 1% chains is found in glucose solutions.

This ring formation is typical for the sugars that are important for alcohol production (maltose, glucose). Cane sugar, however, consists not only of rings of 6 carbon atoms but can also form rings of 5 atoms.

a glucose molecule

The OH group at the first C atom (formerly the aldehyde group in the chain) can now be substituted with the H atom at the same C atom. This results in a different spatial arrangement called beta-arrangement.

a beta glucose molecule

2. Starch

Starch is built from glucose by connecting several of these rings to a chain while eliminating a water molecule. The water is cut off between the C1 and the C4 atoms.

A starch molecule
Connection of two α-glucose molecules, forming maltose

This compound is maltose. In short you can write this for the compound:

a malt sugar molecule in a simple drawing
Malt sugar (maltose)

The chemical notation is as follows:

This means that the C1 atom of an α-glucose ring is connected to the C4 atom of a second α-glucose ring. Starch is formed by the repeated connection of such sugars according to the generalised formula:

The formula above describes chemically pure starch called amylose. Pure amylose is spatially bent. Natural starch is not structured quite so regularly. Beside chains also branched forms appear.

When you connect β-glucose using the same method, you get cellulose, which can be found in the wood of whisky casks. Cellulose molecules are long chains that adhere to each other and connect via hydrogen bonds. Therefore cellulose is more fibrous and stable than starch.

The enzyme amylase from the barley can now split the starch of the grain at the bonds. However, it can't split cellulose chains, since it can't recognise the β forms. Goats, in contrast, are able to split cellulose into sugar with the help of microorganisms in their intestines.

The enzyme attacks the chains, splits them and cuts off double sugars (dimers, maltose) from the end of the chain. When the whole chain is split only double and triple chains (trimers) remain. The enzyme also can't further split the trimers.

Note that the enzyme amylase only occurs in barley malt. However, it can also split starches in other types of grain, not only barley. That's why the mash for bourbon and grain whiskies usually contains 10% barley malt.

Large grain distilleries, starch sugar factories and also the American whiskey distilleries make use of another property of starch. Starch can also be split at high temperatures through acid hydrolysis. The bourbon distilleries use cookers where the corn is cooked in an acidic environment (sour mash) under slight overpressure (1.14 bar = 2 psi overpressure) at 105°C / 221°F for 25 minutes, which speeds up the splitting of starch.

The last step from the grain to alcohol is fermentation. Alcoholic fermentation is carried out by the yeast. Yeasts are fungi, not bacteria. They can be found everywhere in nature. Especially in autumn, when the fruits are ripe, their spores spread through the air in large numbers. They carry out a simple chemical reaction in sugar/water solutions according to the following formula:

3. Alcoholic Fermentation

The yeast fungus splits a glucose molecule, producing two ethanol molecules (alcohol) and two carbon dioxide molecules per ring, as well as heat energy. In addition, the reaction and its following byreactions produces aromatic substances (esters) that give whisky its great variety of taste. Yeasts can only split glucose, not starch. 

In Scotland usually two different dry yeasts are used: baker's and brewer's yeast. (see Glen Moray or Dallas Dhu for example). Baker's yeast makes the fermentation process start quickly. At the same time the wash is acidified. Brewer's yeast works better in an acidic environment and achieves its maximum performance later. It makes sure that the wash has a high alcohol content. American whiskey producers attach great importance to the fruity esters of special yeasts. That's why each bourbon has its own yeast(s). They have been isolated from wild yeasts and patented by the companies. All the yeasts are bred in in-house facilities (see Four Roses and their lab for example) and added to the mash in liquid form.

The carbon dioxide rises from the fermenting, bubbling solution and spreads in the air, while the ethanol accumulates in the solution. The yeast fungus lives off the energy that is released. This process either lasts until all sugar is used up (typical for whisky) or until the alcohol content has risen so far that the yeasts kill themselves off through their own products (typical for wine with residual sweetness).

Vinegar - Acetic Acid - Vinegar Bacteria

Each distiller, brewer or winemaker must deal with vinegar bacteria. Like yeasts, vinegar bacteria occur in nature. They feed on alcohol and produce acetic acid. Once a wash back (or fermenter) is infested with vinegar bacteria, the contents can't be used anymore. That's why Scottish wash backs are cleaned with chemical substances, while fermenters in the USA are even sterilised at high temperatures. The basic chemical reaction carried out by vinegar bacteria looks as follows:

Transformation of ethanol into acetic acid

Since the reaction requires oxygen, an infection with vinegar bacteria can usually be avoided by strict exclusion of air.

Beside the vinegar bacteria described here, there are special bacteria that produce their own alcohol before they produce acetic acid. But there are also more highly developed fungi that compete with the vinegar bacteria and can also react with every product occurring during the process, from starch to acetic acid.

Nature still has a lot of wonders in stock for us. Until they’re all investigated, we should simply enjoy the products of nature.

Slàinte mhath.

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