Our Fellow, Dr. Heinz Weinberger from Whisky-Connaisseur, delves into the true heartbeat of Whisky production - fermentation...
Yeasts are micro-organisms consisting of only one cell. They belong to the class of ascomycetes, which represent the largest group of fungi. Yeast cells are about the size of red blood cells, however, they are so small that hundreds of millions of them fit into a teaspoon. One of the simplest forms of ascomycetes is the beer or baker's yeast Saccharomyces cerevisiae. It is not only the main figure in the brewing process but also plays a central role in other biotechnological industries such as baking bread or producing spirits, fine chemicals, enzymes and bioethanol. The predominant type of yeast used worldwide in the production of spirits is based on a small number of stable commercial S. cerevisiae strains, derived primarily from a bakery/brewery yeast heritage. While some Whisky distilleries – for example those in the USA – grow their own yeast cultures from laboratory stocks, the majority of distilleries rely on the supply of commercially available strains from yeast producers. Of the theoretically large selection of potential yeast strains that can be used to make spirits, relatively few S. cerevisiae strains of Scotch Whisky are actually commercially available. These include, but are not limited to Distiller's M and MX yeast (from Kerry Bio-Science), Pinnacle yeast (from AB Mauri) and DistillMax yeast (from Lallemand). These yeasts are grown on a large scale as pure cultures using molasses as a nutrient together with others. Distillers yeasts are available in various formats: either in dry form (active dry yeast), cake form (pressed yeast) or as liquid slurry (cream yeast)
The biological purpose of yeast is to divide, and for this it needs energy. This energy can be gained by both respiration and fermentation, as yeast is the only living microorganism that can switch from respiration to fermentation. It is thus able to utilize sugar both in the presence of oxygen (aerobic conditions) and under its exclusion (anaerobic conditions). In the presence of oxygen dissolved in the wort, the yeast "respirates" the sugars in the wort (mainly glucose, maltose and maltotriose) completely into carbon dioxide (CO2) and water. It gains the energy it needs to build new cells and thus to grow. Under these aerobic conditions, a total of 38 molecules of adenosine triphosphate (ATP), an important energy carrier for the cells, are produced from one sugar molecule of glucose.
Due to the formation of CO2 and the increasing lack of oxygen in the wort during fermentation, the environment becomes increasingly threatening for the yeast cells. In this hostile situation, the yeast switches its metabolism to the fermentation programme. Fermentation helps the yeast to bridge life-threatening times. Under this oxygen deficiency, however, glucose is metabolised only incompletely to alcohol, or more precisely to ethanol, and CO2. The energy gain is lower because only 1-4 molecules of ATP are formed. However, this is sufficient for the yeast to survive. As fermentation progresses, however, the yeast finds itself in an increasingly emergency environment. It is confronted with an increasing alcohol concentration, a lack of nutrients and an ever higher proportion of organic acid in the wort. Thus, the yeast cells no longer divide with increasing ethanol content and rising temperature, stop their metabolism completely and finally die. This causes their cell walls to dissolve and the aroma-rich ingredients are thus released into the resulting wash. At this point, about 40 to 48 hours of fermentation have passed.
Now the third phase of fermentation starts caused by various bacteria. Since, in contrast to beer production, Whisky is not produced under sterile conditions, microorganisms – especially lactic acid bacteria (Lactobacillus) and wild yeasts – can enter the wort. These bacteria are no longer in competition with the previously superior number of yeast cells and can thus make their contribution to the aromas. As a result of the release of their fermentation products lactic acid and acetic acid, the pH value of the resulting wash decreases. If this fermentation phase lasts too long, however, there is a risk that the wash will become too acidic and can then no longer be used for Whisky production. Lactic acid and acetic acid enter into chemical reactions with the alcohols formed by the yeast, producing aromatic chemical compounds, so-called esters. For example, lactic acid reacts with the main alcohol ethanol to form the ester ethyl lactate, which has a pleasantly mild odour and is described as fruity, creamy and reminiscent of coconut.
It was shown that the main part of the compounds is formed under the influence of yeast, which ultimately influences the aroma of the later Whisky. These include congeners responsible for the flowery, fruity, grassy, soapy, oily, sulphurous, waxy odour and taste aromas. The aromatic and flavour-active compounds obtained from yeast can be roughly listed as ethanol, CO2, carbonyl compounds (such as aldehydes and ketones), medium and higher alcohols (fusel oils), esters (above all ethyl acetate), diketones, fatty acids and organic acids, glycerol and derivatives as well as sulphurous compounds. Ethanol and carbon dioxide are the main products produced during fermentation. The most important higher alcohols are propanol, amyl alcohol, isoamyl alcohol and phenethyl alcohol. During fermentation, acetaldehyde is the carbonyl compound with the highest concentration. This can be further reduced to ethanol or oxidised to acetic acid. Diketones, such as diacetyl and pentane-2,3-dione, both give a distinctive aroma and a characteristic buttery, honey-like or toffee-like flavour. The formation of congeners is strongly influenced by fermentation temperatures: at higher temperatures less esters and more higher alcohols are formed. Sulphur compounds are formed from sulphur-containing amino acids in the proteins and by the reduction of sulphate salts (from water) in the wort and mash. If there is a high content of pungent hydrogen sulphide and organic sulphur-containing compounds, parts of these may remain in the wash and, if not carefully distilled, impair the quality of the new make spirit.
There are now efforts in Scotland to develop yeast strains that are particularly suitable for worts with a higher sugar content, shorter fermentation times and better utilisation of higher sugars. It remains to be hoped that the aroma profile of the yeasts will also be taken into account and that different strains will be investigated in order to improve the taste profiles. Some distilleries are already working on yeast as a "flavour tool" and related trials have already been started.
Fermentation is the heartbeat of the Whisky, because this is where the alcohol is produced...
-Dr Heinz Weinberger
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