I talked about the role of yeast strains in part one of this series, so I shall restrict myself here to the fermenting vessels or wash backs and the process of fermentation. Washbacks nowadays are either stainless steel or the more traditional larchwood or pitch pine. It used to be the case that the main difference between the two was the bacteria found in the wooden variety. However, modern cleaning methods allow both types to be properly sterilised so that the most obvious benefit of stainless steel has been lost. But wooden washbacks, as well as giving a pleasingly traditional look to the tun room, also insulate the fermenting wort from cold draughts – useful in winter in some locations.Most malt distillery washbacks are relatively small, so the heat generated by fermentation dissipates naturally. Seasonal fluctuations in ambient temperature can be accommodated simply by adjusting the wort cooler to give a higher or lower starting (or ‘setting’) temperature. As distilling yeasts have been improved, becoming more vigorous in recent years, the efficiency of fermentation has benefitted from generally lower setting temperatures. I am confident that these moderate adjustments to setting temperatures have little or no effect on spirit character. There is however one very important aspect of fermentation which has a major effect on spirit character, and this is the length of time allowed for the fermentation. The yeast will have converted almost all of the fermentable wort sugars to alcohol in about 44 hours, and in busy distillery there is a natural temptation to distill it straightaway. In contrast, if demand for whisky is slack, there will be operational benefits in leaving at least some of the washbacks, say over a weekend, to be distilled 100-120 hours later. These two extremes of short and long fermentations will produce notably different spirits if distilled and collected separately. In the short versions, residual bacterial action will be minimal, wash acidity will be low and nutty or spicy characters will dominate in the spirit. In the long fermentations, yeast cells will be fully dead and substantially autolysed which together with bacterial action will produce many new flavour compounds and a much higher acidity (lower pH) in the final wash. The character of whiskies from these long fermentations will be more complex, depending on the design and operation of the subsequent distillation plant – particularly on the opportunity for the acidic wash to dissolve copper. But suffice to say here that a distiller seeking consistency of character must look to the length of his fermentation and manage this factor through times of high and low distillery activity.Distillation
The final stage before maturation has now been reached and we can leave behind the biochemistry of malting, mashing and fermentation, moving into the physical chemistry of distillation. The process is no less complex and the science is far from fully understood, but the effects of pot still distillation on spirit character are very pronounced. No two distilleries are the same and the main variations can include the size and shape of stills; the method and rate of heating; the cut-off points for spirit collection; the method and rate of cooling and condensing, and the frequency of rests.These topics warrant a textbook of their own but any whisky enthusiast will readily appreciate that the volatile flavour components or congeners emerging from the top of say, short squat stills which are heated (or ‘driven’) hard, will be more robust and less delicate than those from tall, gently heated stills which allow for lots of recondensing (or ‘reflux’) action. Similarly the choice of cut-off points between light, ethereal foreshots, the central, very special spirit and the heavier, less desirable feints are obviously very significant to final character.The chemical reactions between spirit and copper surfaces are absolutely vital – an average distilling plant will lose several kilograms of copper each week in this way. Copper plays an essential cleansing role in removing heavy sulphurous flavour compounds, and relatively more copper will be dissolved by the high acidity washes from the long fermentations I described earlier. Similarly, the high surface area of modern ‘shell and tube’ copper condensers will provide more dissolved copper than traditional worms – the latter therefore generally producing whiskies that are heavier in character. A very cold condensing temperature will yield less dissolved copper, whilst over-zealous still cleaning might expose too much fresh copper surface. Conversely, these internal copper surfaces can become exhausted or blinded by continuous use but will be regenerated by frequent rests – preferably opening the man-doors to allow air into the still. There is no doubt that if copper had not been the metal of natural choice available to our forefathers for still-making, then we would not have a Scotch whisky industry today.With all these potential variables in the still-house, it is hardly surprising that unique and individual spirits can emerge. It is also apparent why each generation of distillers must jealously guard its process and be extremely cautious about any change to distillery plant or pattern of working.Up to this point I have tried to describe those factors which may affect the character of new make spirit – maturation and casks will follow in part three of this series, in the next issue of Whisky Magazine. In summary, whilst there are many variables which have subtle but important flavour effects, in my opinion the major factors during the distillery processes are the peatiness of malt from kilning; the length of fermentation time; the design of the distillation equipment (stills, intermediate vessels, worms and condensers), and still-house operation (heating, cut-off points, air rests and spirit temperature).I hope from the foregoing that it is not difficult to envisage why, from the 750 or so Scotch distilleries historically recorded, some have evolved and survived to produce the great Scotch malt whiskies of today. This evolution, which I like to term our Industrial Darwinism, is a unique heritage and cannot be overvalued in its contribution to the worldwide success of Scotch whisky.