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

Production of Scotch and Irish whiskies: their history and evolution 153the mash. Table 3 shows changes in theconcentrations of Gram-negative and Grampositivebacteria and (separately) of lactobacilliduring fermentation of a minimally-infected mashand of a heavily-infected mash. The time courseof fermentation of an unacceptably-infected maltdistillery mash (Figure 8) shows, in comparisonwith similar data for fermentation of anacceptable mash (Figure 6), a greater rise in theacid content of the mash after about 35 hrs anda lower optical rotation of the mash after about40 hrs. In the fermentations there is often adifference of up to 4 hrs from the time a rise inthe acid content is detected to the point whenthe pH value of the fermentation begins to fall.Dolan (1976) attributes this to the bufferingcapacity of the mash. The data in Table 4 showthe effect of different levels of infection after 30hrs fermentation of a malt distillery mash on spirityield and the associated financial losses to thedistiller. Dolan (1976) recommends upper limitsof 1,500 bacteria, 50 Gram-positive and 10 lacticacid-producing bacteria per million yeast cells inthe mash at the start of fermentation.Much less has been published on the effectof retaining solid material in the fermenting mash.However, marine microbiologists have longknown that the presence of solid particles in aliquid medium can affect bacterial growth,probably because of the concentration ofnutrients at the solid-liquid interface (Heukelekianand Heller, 1940; Zobell, 1943). Moreover,Cromwell and Guymon (1963) found that formationof higher alcohols during fermentation ofgrape juice is stimulated by the presence of grapeskins or inert solids. Beech (1972) made similarobservations on cider fermentations. Merritt(1967), in the only detailed report on the role ofsolids in whisky distillery fermentations, states thata dry solid concentration of 50 mg/100 ml mighttypically be expected, although much will clearlydepend on the design of the mash tuns used inindividual distilleries. Merritt went on to reportthat a concentration of dry solids as low as 5mg/100 ml causes an increase in yeast growth,and that solids also enhance the rate ofproduction of ethanol and glycerol. There wasalso an effect on production of higher alcoholsby the yeast (Table 5). With the possibleexception of n-propanol, production of all of themajor higher alcohols was increased in thepresence of solids, the effect being particularlynoticeable with isobutanol and 2-methylbutanol.Figure 8. Changes in specific gravity, optical rotation, pH value and acidity during fermentation of a Scotch maltwhisky mash containing an unacceptably high concentration of bacterial infection (Dolan, 1976).