The Production Process – Fermentation
Fermentation is the process that transforms the sugary liquid, known as wort, into wash, which is similar to a strong beer with an alcohol content between seven and ten percent.
Before the wort enters the fermentation vessel, or known as washback in Scotland, it has to be cooled down to approximately 20 degrees Celsius. Then yeast is added to the wort in the washback, where it breaks down the sugars in the wort, converting them to alcohol and carbon dioxide. This also generates whisky’s unique flavor compounds.
In fermentation, yeast multiplies as it consumes the sugars and amino acids in the wort. Initially, fine bubbles are produced; in time, as the process accelerates, white bubbles will cover the entire surface of the washback. This is the peak of fermentation and indicates that the yeast is producing ever more ethyl alcohol and carbon dioxide. The temperature rises until reaching approximately 35 degrees Celsius, at which point the yeast stops multiplying. Having completely consumed the amino acids and sugars in the wort, the yeast now begins to starve. To this point the process has taken roughly 24 hours. Once it begins to starve, the yeast will perish over a period of 15-16 hours, during which time lactic acid bacteria will increase. From here, the liquid requires an additional 10-20 hours to mature as the lactic acid bacteria feeds on the remaining sugars that the yeast could not consume, as well as on the amino and fatty acids emitted from the dead yeast. This process generates both lactic acid and unique flavors.
In a little over 60 hours, the fermentation process is complete. The resulting fermented liquid is called wash and has an alcohol content of approximately seven to ten percent. In addition to controlling the starvation and destruction of the yeast, it is also important to properly gauge the maturation period of the wash. If the maturation period is too short, the resulting liquor will be crude. By allowing the wash time to mature, lactic acid fermentation will progress and produce a clean liquor.
The wash consists mostly of water, ethyl alcohol, esters and fatty acids. Additionally, it contains several hundred recognized flavor compounds that are important in whisky. The unique balance of these compounds produces unique flavors. That balance is affected by various factors, such as the type of yeast and conditions of fermentation to best suit individual yeasts.
There are two types washback: wooden and stainless steel, where different vessels produce different character and flavors of wash, which ultimately has an influence on the style of the whisky.
Wooden washbacks, which traditionally are made of either Douglas Fir or Oregon Pine wood, tend to produce a liquor with richer flavors compared to stainless steel ones. On the other hand, stainless steels are advantageous because they make it easy to regulate the fermentation temperature, and to keep the washback clean. Regardless of material used, malt distillery washbacks have a capacity of approximately 30,000L, whereas the grain distillery ones could be up to 10 times larger!
在麥芽汁進入發酵容器 (蘇格蘭被稱為發酵桶) 之前，必須冷卻至約攝氏20度。然後將酵母加入麥芽汁中，將麥芽汁中的糖份轉化為酒精 (即乙醇) 和二氧化碳，同時衍生威士忌的獨特風味。