Fermentation
Once the sugar wort has been pumped from the mashtun to the washback fermentation can be initiated. Washbacks can be made from a number of materials but in the main stainless steel and various species of wood are predominantly used in the whisky industry.
Stainless steel vessels usually have domed tops completely sealed with the exception of a man door, a vent for CO2 extraction and possibly a switcher (rotating blade for knocking back froth during fermentation). They usually have domed bottoms to aide emptying and can be fitted with side entry mixers for keeping the yeast in suspension. Wooden vessels are usually fitted with lids also made of wood which provide an adequate seal for fermentation. They can also be fitted with switchers and most now have CO2 extraction fitted. The wooden washbacks are usually set at an angle to aide emptying.
Antifoam which alters the surface tension of the liquid in the wash back preventing foam forming can also be used when required.
Yeast, Saccharomyces Cerevisiae, used for fermentation in the distilling industry can be purchased in a number of ways, pressed, slurry and dry. Here at Springbank we use pressed yeast, which is manufactured in the UK specifically for the distilling industry. Yeast is a living organism and must be stored in a clean refrigerated environment until used.
The essential properties of a good distilling yeast are as follows.
Good flavour production and complete and rapid fermentation of wort sugars .
Lack of flocculence and minimal frothing coupled with the ability to grow well even above 30ºC and able to withstand a wash alcohol level of up to 10%abv.
The concentration of yeast added to the washback will depend on the character we want to produce in the washback. As long as an adequate amount of yeast is added per tonne of malt mashed complete fermentation will occur.
At the beginning of fermentation the yeast culture uses the sugars in the wort to multiply, this is known as aerobic fermentation. Here at Springabnk the original gravity will vary between 1038 and 1048 degrees depending on which mash it is during the week. Once the reservoir of oxygen in the washback has been depleted the yeast begins to convert the sugars into various alcohol's, carbon dioxide and energy in the form of heat, alcohol's are a byproduct of what is called anaerobic fermentation. During fermentation the gravity of the wash will decrease due to the conversion of sugars to alcohol's.
Complete fermentation will take a minimum period of forty eight hours but fermentation time can be as short as forty hours or as long as one hundred and thirty hours. Many flavour compounds are produced during fermentation and distillers “ tailor “ their fermentation's to their specific requirements.
During fermentation the temperature of the worts, now known as wash, will rise from 16 - 20ºC up to 34ºC. Temperatures above 35ºC will destroy the living yeast cell, so should be avoided.
Once fermentation is complete the final gravity of the wash as measured against water should be around 0. 998 but can be as low as 0.995. The alcohol content of the wash will depend on the original gravity of the wort. Some distilleries like Springbank will produce wash with an alcohol content of up to 5% by volume where other distilleries may produce wash with an alcohol content of between 8 and 10% by volume.
As always feel free to drop us an e-mail should you have a question. Stuart Robertson, Distillery Manager.
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