From "The Alcohol Textbook", the chapter entitled Production of Scotch and Irish whiskies: their history and evolution. Scotch does not rely on lactic souring though as you'll see it's not as absent as people might guess.
Next, info on a mash which relies on lactic souring - Rye whiskey.Whisky worts usually have a specific gravity in the range 1.050-1.080, a pH value of around 5.0, a total acid content of 0.1% and an optical rotation of +30 degrees. After inoculation, the yeast content is 5-20 million cells/ml. The bacterial count varies with the cleanliness of the plant and the extent to which the raw materials were endowed with microbial flora. Scotch grain whisky fermentations create little if any foam because of their large content of suspended solids. However, in most Scotch malt whisky fermentations only a small proportion of the suspended solids in the mash is retained in the fermentation vessel. These fermentations tend to foam and the distillers have resorted to the use of various types of antifoams.
Changes over the time course of a typical fermentation in a Scotch malt whisky distillery are depicted in Figure 6. Fermentation proceeds vigorously for the first 30 hrs, during which time the specific gravity falls to 1.000 or below and the optical rotation to around zero. The sugars in the wort are utilized in a particular sequence with glucose and fructose being fermented first, followed by maltose and then maltotriose. The removal of sugars during fermentation of a Scotch grain whisky mash is show in Figure 7 (Pyke, 1965). Over the first 30 hrs the pH value, after declining to around 4.2, rises to around 4.5. During the first 30 hrs the speicifc gravity drops at a rate of about 0.5 per hour accompanied by a massive evolution of heat. While many of the larger grain whisky distilleries have fermenters fitted with cooling coils, these coils are absent, or if fitted are relatively inefficient in most malt whisky distilleries where temperature can rise by the end of fermentation to as high as 35-37C. The distiller is concerned about the temperature rise during fermentation since this can cause the fermentation to stop or become 'stuck'. Temperature rise can be controlled by using a lower starting temperature or, because glycolysis of sugar is a heat-producing process, by using a lower initial concentration. Strains of Saccharomyces cerevisiae are well suited for malt whisky distillery fermentations since they can ferment efficiently over a wide temperature range. Fermentation is usually continued for at least 36 hrs and frequently longer, at which time the ethanol content of the wash is 7-11% (ABV). In larger distilleries, particularly those in the U.S., the carbon dioxide evolved is collected, liquefied and sold. Smaller distilleries, particularly the malt whisky distilleries in Scotland, usually do not have this facility.
It should be noted that mashes in malt whisky distilleries are not boiled, so any enzyme activity manifested at the temperature of the mash and any microorganisms that can survive at that temperature will continue to be active during the fermentation. The continued activity of limit dextrinases in unboiled distillery mashes increases the concentration of sugars available for fermentation by the yeast. Hopkins and Wiener (1955) calculated that with amylases alone the yeast cannot metabolize the equivalent of the final 12-16% of the starch.
Another important consequence of using non-sterile conditions in distillery fermentations is the activity of bacteria that pass through in the mash, which are encouraged to some extent by the relatively high temperatures to which the fermentations can rise. In addition to lactic acid bacteria, the flora can include other Gram-positive as well as Gram-negative strains. The concentration of the flora depends on a number of factors including the extent to which the lactic acid bacteria grew during yeast propagation, the extent of the flora on the cereal raw materials and on the standard of hygiene in the distillery. There is no doubt, however, that the controlled activity of this bacterial flora, and particularly of the lactic acid bacteria, is accompanied by excretion of compounds that contribute to the organoleptic qualities of the final whisky (Geddes and Riffkin, 1989).
During the first 30 hrs or so of malt whisky fermentation there is vigorous fermentation and the majority of the aerobic bacteria die. This, however, provides ideal conditions for grown of anaerobic or microaerophilic bacteria, principally lactic acid bacteria (mainly strains of Lactobacillus brevis, L. fermenti and Streptococcus lactis) with the result that the concentration of lactic acid in the fermented mash can be as high as 30 mg/L (MacKenzie and Kenny, 1965.) A wide range of lactobacillus species have been identified in Scotch whisky fermentations including L. fermentum, L. brevis, L. delbrueckii, L. plantarum, L. casei and a bacterium resembling L. collinoides, in addition to Leuconostoc spp., Streptococcus lactis and Pediococcus cerevisiae (Bryan-Jones, 1976). More recently Barbour (1983) isolated many species that did not conform to recognized species of lactic acid bacteria, a point emphasized by Walker et al. (1990) who used DNA hybridization techniques to classify distillery bacteria. Growth of lactic acid bacteria is probably enhanced by yeast excretion of nitrogenous nutrients at the end of a vigorous fermentation. Kulka (1953) demonstrated the ideal nature of yeast autolysate for growth of lactobacilli. Bacterial activity in the fermenting wort also leads to removal of some acids. Actively growing yeast secrete citric and malic acids, but MacKenzie and Kenny (1965) attribute the lower concentrations of these acids in malt distillery worts (as compared to brewery worts) to their partial removal by bacteria.
Occasionally, the extent of the bacterial flora in the fermenting wort can become too large. This causes problems due to sugar utilization by the bacteria that lead to an overall decrease in spirit yield. In addition, the bacteria may produce organoleptically-undesirable compounds and also release hydrogen ions causing the pH value of the wort to fall too low, thereby providing suboptimal conditions for action of certain enzymes. Examples of undesirable compounds that may be excreted by bacteria are hydrogen sulfide and other sulfur-containing compounds (Anderson et al., 1972). Lactobacilli can also metabolize glycerol (excreted by the yeast during fermentation) to produce B-hydroxypropionaldehyde, which subsequently breaks down on distillation to give acrolein (Harrison and Graham, 1970). Acrolein imparts a pungent, burnt and often peppery odor to the whisky (Lyons, 1974). In a later paper, Dolan (1976) concentrated on the problems arising in malt whisky distilleries when there is an unacceptably high concentration of bacteria in the mash. Table 3 shows changes in the concentrations of Gram-negative and Gram-positive bacteria and (separately) of lactobacilli during fermentation of a minimally-infected mash and of a heavily-infected mash. The time course of fermentation of an unacceptably-infected malt distillery mash (Figure 8 ) shows, in comparison with similar data for fermentation of an acceptable mash (Figure 6), a greater rise in the acid content of the mash after about 35 hrs and a lower optical rotation of the mash after about 40 hrs. In the fermentations there is often a difference of up to 4 hrs from the time a rise in the acid content is detected to the point when the pH value of the fermentation begins to fall. Dolan (1976) attributes this to the buffering capacity of the mash. The data in Table 4 show the effect of different levels of infection after 30 hrs fermentation of a malt distillery mash on spirit yield and the associated financial losses to the distiller. Dolan (1976) recommends upper limits of 1,500 bacteria, 50 Gram-positive and 10 lactic acid-producing bacteria per million yeast cells in the mash at the start of fermentation.
Much less has been published on the effect of retaining solid material in the fermenting mash. However, marine microbiologists have long known that the presence of solid particles in a liquid medium can affect bacterial growth, probably because of the concentration of nutrients at the solid-liquid interface (Heukelekian and Heller, 1940; Zobell, 1943). Moreover, Cromwell and Guymon (1963) found that formation of higher alcohols during fermentation of grape juice is stimulated by the presence of grape skins or inert solids. Beech (1972) made similar observations on cider fermentations. Merritt (1967), in the only detailed report on the role of solids in whisky distillery fermentations, states that a dry solid concentration of 50mg/100 ml might typically be expected, although much will clearly depend on the design of the mash tuns used in individual distilleries. Merritt went on to report that a concentration of dry solids as low as 5 mg/100 ml causes an increase in yeast growth, and that solids also enhance the rate of production of ethanol and glycerol. There was also an effect on production of higher alcohols by the yeast (Table 5). With tth possible exception of n-propanol, production of all of the major higher alcohols was increased in the presence of solids, the effect being particularly noticeable with isobutanol and 2-methylbutanol.
The effect of low insoluble solids content is a factor relevant to congener levels in malt whisky fermentations. In grain whisky production, where 'all-grains-in' fermentations are generally used, the degree of rectification during distillation is the principle determinant of higher alcohol levels in the spirit.