Vodka production: Fermentation
Whether vodka is to be distilled from grain, potatoes, sugar beet, grapes or even rice, the first step is to produce alcohol using yeast. The process of culturing yeast under conditions to produce alcohol is called fermentation.
When certain species of yeast, most notably Saccharomyces cerevisiae, metabolize sugar they produce ethanol and other alcohols, carbon dioxide and heat. If sugar is present in the raw material being used, as is the case with molasses or fruit, then this can be directly fermented as the yeast has immediate foodstuff. So the fermentation of grapes, molasses and other ingredients containing sugar is straightforward. Indeed, grapes tend to have yeast cells on their skins so merely crushing grapes and leaving them is often enough to start a natural fermentation.
If grain (barley, wheat, corn/maize, rye) is being used the grain will first have to be heated to form a mash. Amylase and diastase enzymes are added to break down the long chains of starch molecules into simple sugars which are perfect for fermentation - mainly glucose and a small amount of maltose.
Potatoes must first be cleaned and their skins removed by scrubbing as the peel is mainly fibre and cannot be fermented. The peeled potatoes are then chopped and moved to a mash vessel where they are cooked by steam to gelatinise the starch. The term 'mash' when discussing potatoes would more usually suggest the mashing of potatoes i.e. macerating the potatoes into small pieces, rather than the conversion of starch to fermentable sugars by heating. Both uses of the term 'mash' are true of this particular mashing process, in which potatoes are continually pumped around, thus reducing the size of the lumps to make a runny mashed potato, so producing a larger surface area for the enzymes to work on.
The runny potato is cooled to 60°C (140°F), an ideal temperature for the action of the enzyme which is added to the gelatinised potato to help the conversion of starch to sugars. The mixture is then cooled to around 30°C (86°F), at which point the yeast can be added.
Distillers of old would rely on natural airborne yeasts for their fermentation but today's distillers usually add commercially prepared distiller's or brewer's yeasts, or in some cases their own cultured strains of yeast.
Small propagation or starter tanks, rich in sugars may be used to start the yeast before it's added to the main fermentation tank. This allows the numbers of yeast cells to multiply rapidly, making the yeast stronger and better able to process the mash. This practice is particularly necessary when fermenting molasses.
Ethanol is toxic to yeast, thus limiting the ethanol concentration obtainable by fermentation alone. The most ethanol-tolerant strains of yeast can only survive up to approximately 15% ethanol alcohol by volume.
Saccharomyces cerevisiae yeast functions in temperatures from around 64-33°C (91°F). As the yeast metabolizes the sugar to produce ethanol it also produces heat so the temperature has to be monitored and in some cases controlled by pumping cold water through pipes or surrounding jacket. If the temperature is allowed to rise much above 35°C (95°F) then the yeast will die and the fermentation will stop.
Saccharomyces yeast not only produces ethanol, but it also produces higher alcohols at temperatures above 23.8˚C (74.8°F). These alcohols are called fusel alcohols or fusel oils and tend to have a spicy or peppery flavour. A slow, well-controlled fermentation will produce a better quality wash with lower levels of methanol and fusel.
Depending on the base ingredient, the fermentation process, which can last anything from one day to two weeks, produces a beer-like liquid called 'wash' at 7-15% alc./vol..