Chapter 11 Production of Scotch and Irish whiskies: their history and ...

Production of Scotch and Irish whiskies: their history and evolution 151a massive evolution of heat. While many of thelarger grain whisky distilleries have fermentersfitted with cooling coils, these are absent, or iffitted are relatively inefficient in most malt whiskydistilleries where temperature can rise by the endof the fermentation to as high as 35-37 o C. Thedistiller is concerned about the temperature riseduring fermentation since this can cause thefermentation to stop or become ‘stuck’.Temperature rise can be controlled by using alower starting temperature or, because glycolysisof sugar is a heat-producing process, by using alower initial concentration. Strains ofSaccharomyces cerevisiae are well suited for maltwhisky distillery fermentations since they canferment efficiently over a wide temperaturerange. Fermentation is usually continued for atleast 36 hrs and frequently longer, at which timethe ethanol content of the wash is 7-11% (v/v).In larger distilleries, particularly those in the US,the carbon dioxide evolved is collected, liquefiedand sold. Smaller distilleries, particularly the maltwhisky distilleries in Scotland, usually do not havethis facility.It should be noted that mashes in malt whiskydistilleries are not boiled, so any enzyme activitymanifested at the temperature of the mash andany microorganisms that can survive at thattemperature will continue to be active during thefermentation. The continued activity of limitdextrinases in unboiled distillery mashesincreases the concentration of sugars availablefor fermentation by the yeast. Hopkins andWiener (1955) calculated that with amylasesalone the yeast cannot metabolize the equivalentof the final 12-16% of the starch.Another important consequence of usingnon-sterile conditions in distillery fermentationsis the activity of bacteria that pass through in themash, which are encouraged to some extent bythe relatively high temperatures to which thefermentations can rise. In addition to lactic acidbacteria, the flora can include other Grampositiveas well as Gram-negative strains. Theconcentration of the flora depends on a numberof factors including the extent to which the lacticacid bacteria grew during yeast propagation, theextent of the flora on the cereal raw materialsand on the standard of hygiene in the distillery.There is no doubt, however, that the controlledactivity of this bacterial flora, and particularly ofthe lactic acid bacteria, is accompanied byFigure 7. Time course removal of fermentable sugars from a Scotch grain whisky mash (Pyke, 1965).