Fusel oil is the common name for by-products as well as higher alcohols formed in the fermentation process. The principal ingredient of fusel oil is amylalcohol which comprises 65-80% of fusel oil. It comprises all forms of isobutylcarbinol and d-amylalcohol. It also contains 15-25% of isobutyl- and approximately 4-7% of n-propyl alcohol. Amyl-, butyl- and propyl-alcohols comprise the principal components of fusel oils, but there are other substances although none of these is present in significant quantities. They appear in such small amounts that one needs only consider the principal components.
The make up of fusel oil depends principally on the ingredients of the fermentation and the fermentation temperature, and fusel oil is the aroma of the mash. For example, in brandy and other fruit-base spirits (for example, slivovitz, calvados, etc) the fusel oil content is 0.6% or more. This is the principal aroma of the drink and after storage and maturing most of the fusel oil constituents have taken the form of esters.
In basic raw spirit distilled from mash based on sugar the fusel content is usually between 0.4-0.7% of the 95% alcohol. In an experiment with 200 grammes of sugar in 2 litres of water and using 40 grammes of baker's yeast the fusel content of the raw spirit was 0.40%. As this mash was no more than 6% one must reckon with a little more fusel in practice (despite the overdose of yeast in the experiment).
The addition of ammonium salts to the mash reduces the formation of fusel oil, i.e. yeast nutrient salt (ammomium phosphate). In total, fusel oil is soluble in water up to 33%. When the raw spirits is diluted down to 40-50% some of the fusel oil goes out of solution and takes on an oily consistency. (The process is facilitated if kept cool). These are the insoluble fusel oils, principally amylalcohols. The separated fusel oil floats up to the surface due to its lower specific weight, where it can be removed by various methods. If the temperature is a maximum of 15 °C and circumstances good one can separate 0.3% of the fusel oil (1.5 cl = 15 ml of 5 litres of raw spirits) calculated on concentrated raw spirits. This is over one third of the fusel oil present. Some home distillers usually fills the diluted raw spirits into a bottle right up to the stopper. When the surface has become oily one discards the first millimetre and the rough separation of the fusel oil has been done. Then the results are purified using activated carbon.
Purification using activated carbon
manufacturing process. It is principally down to purification for the best results. The purification method I describe gives 70% better results than those usually used. It is the only method that gives an absolutely pure spirit. If a good activated carbon is used of a small grain size then 2 liters (0,5-1 kg) is sufficient for 4-5 litres of spirit (diluted to under 50%). The same purification method is used around the world. The only difference is that commercial spirits manufacturers filters the spirit from below, percolating upwards for 2.5 meters, with a constant flow rate of the spirit. This is in order to be able to precisely control the filtering at a speed of 0.2 - 0.5 metres per hour.
Activated carbon can be compared to small sponges full of holes. The absorption capacity of the activated carbon is measured in the area of these holes per gramme (expressed in m2/gramme). The grain size of the activated carbon determines how fast it absorbs impurities. Effective activated carbon should have a grain size of a maximum of approx. 1 mm. Larger grains work too ineffectively and are unable to use the surface area inside the grain. Powdered carbon can not be used for the best purification method as the powder consolidates and blocks the process. The impurities are absorbed by the channels in the activated carbon, including fusel oils and the flavour of yeast. To take best advantage of the channels one filters slowly through a high layer of activated carbon (1.5-2.0 metres).
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