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

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150 T.P. LyonsFigure 6. Changes in specific gravity, optical rotation, pH value and acidity during fermentation of a mash in theproduction of Scotch malt whisky (Dolan, 1976).Scotland and Ireland, particularly the pot whiskydistilleries, rarely have their own yeastpropagation equipment. They rely on speciallypropagatedpressed yeast for use infermentations. In Scotch malt whisky distilleries,this pressed yeast is augmented, usually on anequal weight basis, with surplus brewery yeasttypically supplied in compressed form frombreweries that may be as far as 500 km from thedistillery.The requirements for a strain of Saccharomycescerevisiae yeast used in distillery practicehave been described by Harrison and Graham(1970). Apart from the obvious need to select astrain that maintains very high viability in thepressed state (containing 25% dry weight), othervery important properties of these strains areability to tolerate concentrations of ethanol onthe order of 12-15% (v/v) and the capacity tometabolize oligosaccharides such as maltotrioseand maltotetraose in order to maximize theconversion of starch into ethanol and carbondioxide.Whisky worts usually have a specific gravityin the range 1.050-1.080, a pH value of around5.0, a total acid content of 0.1% and an opticalrotation of +30 o . After inoculation, the yeastcontent is 5-20 million cells/ml. The bacterialcount varies with the cleanliness of the plant andthe extent to which the raw materials wereendowed with a microbial flora. Scotch grainwhisky fermentations create little if any foambecause of their large content of suspendedsolids. However, in most Scotch malt whiskyfermentations only a small proportion of thesuspended solids in the mash is retained in thefermentation vessel. These fermentations tendto foam and the distillers have resorted to theuse of various types of antifoams.Changes over the time course of a typicalfermentation in a Scotch malt whisky distilleryare depicted in Figure 6. Fermentation proceedsvigorously for the first 30 hrs, during which timethe specific gravity falls to 1.000 or below andthe optical rotation to around zero. The sugarsin the wort are utilized in a particular sequencewith glucose and fructose being fermented first,followed by maltose and then maltotriose. Theremoval of sugars during fermentation of aScotch grain whisky mash is shown in Figure 7(Pyke, 1965). Over the first 30 hrs the pH value,after declining to around 4.2, rises to about 4.5.During the first 30 hrs the specific gravity dropsat a rate of about 0.5 o per hour accompanied by
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).
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