The Chemistry in Fermentation
Chemisches Hintergrundwissen
By definition, whisky is a distilled beverage made from grain. But how does grain become alcohol? Through fermentation, of course. Before this happens, a long chain of chemical and biological reactions must take place. We explain these in detail here.
Sugar
The cereal grain consists primarily of starch. The other components are proteins, fats and trace elements. Starch is the starting material for alcohol production. For a simpler explanation of the chemical relationships, however, I would like to start not with starch, but with sugar. Sugar and starch both belong to the group of carbohydrates. Once you have understood the basic properties of sugar, you can easily draw the conclusion about starch.
Historically, our local ancestors only knew bee honey and fruits as sources of sugar. This changed abruptly with the discovery of the world, when cane sugar came to us from southern latitudes. It was not so long ago that the sugar beet was developed as a source of sugar in our country. From the middle of the last century, more and more beet sugar factories were built in our latitudes. Today, we produce sugar in beet and starch sugar factories and also import considerable quantities of cane sugar into the European Community (suppliers e.g. South Africa and Mauritius).
Everyone knows some forms of sugar from food advertising. In addition to the sugars listed below, there are many other sugars whose chemical structure is similar.
| Name | Chemical Name | Formula |
|---|---|---|
| Cane sugar | Sucrose | C12H22O11 (=2*C6H12O6-H2O) |
| Dextrose | Glucose | C6H12O6 |
| Fructose | Fructose | C6H12O6 |
| Milk sugar | Lactose | C12H22O11 |
| Malt sugar | Maltose | C12H22O11 |
All these sugars have their basic chemical formula C6H12O6 in common. A simple sugar molecule thus consists of 6 carbon, 12 hydrogen and 6 oxygen atoms. The following structure shows a stretched D-glucose molecule.
All these sugars have their basic chemical formula C6H12O6 in common. A simple sugar molecule thus consists of 6 carbon, 12 hydrogen and 6 oxygen atoms. The following structure shows a stretched D-glucose molecule.
Now what makes a sugar sweet? It is the OH groups that react with the receptors on our tongue. But it is not the quantity of OH groups that is decisive, it is rather the position of these OH groups in space, which can only make contact with our receptors on the tongue in a certain position. There are substances whose OH groups are arranged similarly in space (sweetener, glycol, etc.), and therefore also taste sweet.
The C1 and C5 atoms of sugar can combine (add) via the double-bonded oxygen atom of the upper aldehyde group to form a ring of five carbon and one oxygen atom. In this process, no atom is split off, but all atoms are incorporated into the new ring structure. The sixth carbon atom protrudes from the side of the ring. However, the ring is not flat in space, but rather bent in an armchair shape. This ring structure is energetically more favourable than the free chain. Statistically, there is a mixture of 99% rings and 1% chains in a glucose solution.
Ring formation is typical for the sugars important for alcohol production (maltose and glucose). However, cane sugar does not only consist of 6-membered rings, but can also form 5-membered rings.
The OH group on the first C atom (formerly aldehyde group in the chain) can now be interchanged with the H atom on the same C atom. This results in a different spatial arrangement. In this case, one speaks of a beta arrangement.
Starch
From glucose, we now arrive at starch by linking several of these rings to form chains by splitting off a water molecule. The water is split off between the C1 and C4 atoms. This compound is the malt sugar (see picture on the right).
This compound is also written chemically as follows. This means that one alpha-glucose ring (Glc) connects to a second alpha-glucose ring with its C4 atom via the C1 atom.
This formula describes the chemically pure starch called amylose. Pure amylose is spatially coiled. In nature, starch is not quite as regularly structured. There are not only chains but also branches.
If you chain beta-glucose using the same method, you get cellulose, as found in the wood of whisky barrels. The cellulose molecules are long chains that can lie next to each other and bind via hydrogen bonds. This is why cellulose is fibrous and more stable than starch flour.
The enzyme amylase from barley can now break down the starch of the cereal grain at the O-bonds. However, it cannot do anything with cellulose. It cannot recognise the beta-forms. Goats, on the other hand, are able to split cellulose into sugars with the help of microorganisms in their intestines.
The enzyme attacks the chains, splits them and always separates two sugars (dimers and maltose) from the ends of the strand. Once the entire chain has been split, only chains of 2 and 3 (trimers) remain. The enzyme cannot break down the trimers any further either.
SPECIAL FEATURE: The enzyme amylase is found exclusively in barley malt. However, it can break down not only the starch of barley but also starches from other types of grain into sugar. For this reason, the mash for bourbon and grain whisky usually contains a 10% share of barley malt.
Large grain distilleries, starch sugar factories and also the American whiskey distilleries take advantage of a special feature of starch. Starch can be split acid-hydrolytically with the addition of heat. In the cookers of the bourbon distilleries, corn is cooked in an acid environment (sour mash) at a slight overpressure at 105 °C for 25 min (1.14 bar = 2 psi overpressure), thus accelerating the splitting of the starch. This can replace the use of barley.
Alcoholic Fermentation
The last step from grain to alcohol is alcoholic fermentation. What is alcoholic fermentation? Let's just play dumb ... (loosely based on Heinz Rühmann - Die Feuerzangenbowle)
The yeast fungus splits a glycose molecule and produces two ethanol molecules(alcohol) and two carbon dioxide molecules per ring and energy in the form of heat. In addition, fruity aroma substances (esters) are produced, which give the whisky its great variety of flavours. The yeast fungus cannot harm the pure starch. It is spared from the fungi.
In Scotland, two different dry yeasts (baker's yeast and brewer's yeast) are usually used. The first of the two ensures that fermentation starts quickly. At the same time, the wash is acidified. The second yeast works better in an acidic environment and only reaches its maximum performance later. It ensures that a high alcohol content is produced in the wash.
In the USA, special importance is attached to the fruity esters of special yeasts in whiskey production. Every bourbon therefore has its own yeast(s). They were isolated from wild yeasts and patented by the companies. All the yeasts used are produced in large quantities in their own propagation facilities and are also added in liquid form.
The carbon dioxide rises in the fermenting, bubbling solution and disperses into the air. The ethanol produced accumulates in the solution. The yeast fungus lives on the energy that is released. This process continues until either all the sugar is consumed (typical for whisky) or until the alcohol concentration has risen to such an extent that the yeast fungus kills itself through its own products (typical for wine).
Vinegar - Acetic Acid - Acetic Bacteria
Every distiller, brewer or vintner has to deal with vinegar bacteria by necessity. Vinegar bacteria occur in the wild just like yeast fungi. They feed on alcohol and produce acetic acid. Once a fermentation container (wash back, fermenter) is infested with acetic bacteria, the entire filling can no longer be used. For this reason, wash backs in Scotland are cleaned with chemical substances and in the USA fermenters are even sterilised at high temperature. The basic chemical reaction that the vinegar bacteria carry out looks like this:
Since the reaction requires oxygen, it is usually possible to avoid infection with acetic bacteria under strict air exclusion. Beyond the acetic bacteria described here, there are special bacteria that first produce alcohol themselves before they produce acetic acid. However, there are also more highly developed fungi that compete with these acetic bacteria and can also react through from starch to acetic acid. Nature still has many wonders in store for us. Until they are explored, we should simply enjoy the products of nature.
