HIGH-GRAVITY BREWING 27
pared with the 20°P MS wort, with a corresponding decrease in the concentration of glucose plus fructose. The three worts were fermented in the International Centre for Brewing and Distilling (ICBD) 2 hL pilot brewery (Fig. 2-22) with a lager yeast strain at 13°C, and the concentrations of ethyl acetate and isoamyl acetate were determined throughout the fermentation (Figs. 2-23 and 2-24, respectively). The profiles were similar for both esters. The concentrations of both esters in the 20°P MS fermented wort were twice those observed in the 12°P MS fermented wort. However, the ester concentration in the 20°P VHMS wort was approximately 25% reduced compared with the 20°P MS wort (68). This observation confirms the findings employing synthetic media with single sugars, that maltose fermentations produce less ethyl acetate and isoamyl acetate than glucose fermentations. In addition, similar to synthetic media fermentations, the wort with elevated concentrations of maltose produced higher yeast viabilities than the wort containing lower levels of maltose (Tables 2-7 Fig. 2-21. Wort sugar profiles. VHM = very high maltose. (Re and 2-8). produced, by permission, from Younis and Stewart, 1998) (67)
Fig. 2-22. The pilot brewing plant at Heriot-Watt University, Edinburgh, Scotland. (Courtesy International Centre for Brewing and Distilling, Heriot-Watt University)
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Chapter 2
Fig. 2-23. Ethyl acetate concentration in worts of differing gravities and sugar compositions. MS = maltose syrup, and VHMS = very high MS. (Re足pro足duced, by permission, from Younis and Stewart, 1998) (67)
Fig. 2-24. Isoamyl acetate concentration (mg/L) in worts of differing gravities and sugar compositions. MS = maltose syrup, and VHMS = very high MS. (Re足pro足duced, by permission, from Younis and Stewart, 1998) (67)
HIGH-GRAVITY BREWING 29
Influence of HG Wort (Wash) on the Production of Grain Whisky Scotch whisky has been defined in United Kingdom law since 1909. The current definition is contained in the Scotch Whisky Regulations 2009, which replaced the Scotch Whisky Act 1988. The regulations define Scotch whisky as whisky produced in a distillery in Scotland from water and malted barley to which only whole grains of other cereals (for example, corn and wheat) may be added. The 2009 regulations prohibit the production in Scotland of whisky other than Scotch whisky. Previous legislation had only governed the way that Scotch whisky was produced. However, Fig. 2-25. A typical pot still, employed for the batch distillation the Scotch Whisky Regulations 2009 set additional of malt whisky. (Courtesy F. O. Robson) rules for the labeling, packaging, and advertising of Scotch whisky. Also, all single-malt Scotch whisky has to be bottled in Scotland. The 2009 regulations and the accompanying European Union legislation both specify a minimum sales alcoholic strength of 40% by volume (27). There are two distinct types of whisky: malt and grain, each of which has quite different characteristics: • Malt whisky has a pronounced bouquet and taste and is made exclusively from malted barley and yeast by the pot-still method (Fig. 2-25). This method is a batch process that does not enable continuous production. Consequently, the whisky is made in separate batches (both fermentation and distillation), each of which is similar although not identical. The average annual capacity of a malt distillery is approximately 2.5 million liters of pure alcohol, although may be considerably higher. In 2013, there were 98 functioning malt distilleries in Scotland. The average concentration of unfermented wort is 12–14°P, and the average yield in 2012 was approximately 412 liters of alcohol per metric ton of malted barley (27). • Grain whisky is made from a mixture of malted barley, maize (corn), wheat, yeast, and water in the average proportion of approximately 16% (w/v) barley malt and 84% (w/v) maize, wheat, or a combination of both, although this proportion varies from one distillery to another. Unlike malt whisky, the grain product is produced by a continuous distilling process (modeled on the Coffey still) that lends itself to large-scale production (Fig. 2-26). In addition, some plants employ a continuous fermentation process, whereas others are batch fermented (details later in this chapter and in chapter 4). Grain whisky has less well-defined characteristics than malt whisky, which makes it suitable for blending purposes. Unlike malt whisky, grain whisky varies little
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Chapter 2
in taste from one distillery to another. Consequently, the industry regards it as a commodity product, and it is traded from one distilling company to another at a set price. In 2013, there were seven operating grain distilleries in Scotland. The average concentration of unfermented wort is 18–20°P (0 G), and the average alcohol yield of such a facility in 2008 was 385 liters of alcohol per metric ton of cereal employed (27). There are essentially three types of Scotch whisky (Table 2-9). Only strains of Saccharomyces cerevisiae (ale type) are employed for the fermentation of whisky worts. Fermentation is conducted at higher temperatures (28–32°C) than brewing in different geometry fermenters (also called washbacks). Unlike in brewing, the yeast is only used once; it is not reused. At the end of fermentation, the fermented wort plus yeast goes directly into the batch (malt whisky) (Fig. 2-25) or continuous (grain whisky) still (Fig. 2-26). As already discussed, grain whisky is produced by both batch and continuous fermentation processes. A schematic of a continuous fermentation process is shown in Figure 2-27. Yeast is purchased (in cream, cake, or dried form) from a yeast supplier. This yeast is usually grown on a molasses medium. To acclimatize (liven) the yeast to a cereal-based fermentation
Fig. 2-26. Coffey still, employed for the continuous distillation of grain whisky. (Courtesy F. O. Robson)
HIGH-GRAVITY BREWING 31
environment, the yeast is incubated in grain (wheat) wort in a trub vessel for 24 hr. The acclimatized yeast is incubated with grain wort (also termed wash) in a continuous fermenter (also termed a wash back) in flow-through mode at 30–32°C for approximately 36 hr. When a steady state has been established, the rate of wort addition is in balance with the rate at which the fermented wort leaves the fermenter to be held in a holding tank prior to distillation in a continuous still. In 2005, a grain distillery in Scotland employing Fig. 2-27. Schematic of a fermentation process for grain whisky continuous fermentation was successfully fermentproduction. (Reproduced from Stewart, 2010) (55) ing 21°P grain wort yielding 11% (v/v) alcohol in the fully fermented wort. This situation prevailed early in 2006, but late in 2006 problems began to be encountered with a decreased yield of alcohol to 9.6% (v/v) because of incomplete utilization of wort maltose and particularly maltotriose (Fig. 2-28). This decrease equated to a reduced alcohol yield from 385 to 370 L of alcohol per metric ton of grain. In addition, because of the wort maltose and maltotriose, the resulting distillers dried grain had a sticky consistency and was not acceptable to use as an animal feed. In an attempt to overcome this problem, the original gravity of the wort was reduced to 19°P (Table 2-10) (55). This
Fig. 2-28. Fermentation trends for a 21°P grain wort (July 2005–September 2008). ABV = alcohol by volume. (Reproduced from Stewart, 2010) (55)
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Chapter 2
reduction resulted in complete fermentation of the wort with no residual maltose and maltotriose and improved the consistency of the distillers dried grain. However, the distillery’s overall alcohol yield was reduced below productivity levels. The reasons for the deterioration in yeast efficiency regarding maltose and maltotriose uptake in 2006 are still unclear. It appears that the 21°P wort exerts stress effects on the pitching yeast, with inhibitory effects on maltose and particularly maltotriose uptake. Stress effects on sugar uptake, especially maltose and maltotriose, have been described previously (61) and will be further discussed in a subsequent chapter. The exact reasons for this inhibition are not completely clear, but that maltose and maltotriose require energy (active transport) to be taken into the cell cannot be ignored (9). It is worthy of note that in 2013 this distillery began using maize (corn) instead of wheat. Problems with incomplete fermentation have been largely eliminated; reasons for this fermentation improvement are unclear (personal communication).
HG Brewing, Yeast Centrifugation, and Flow Cytometry The use of centrifuges has become an established way to increase brewery throughput, because they improve beer production clarification times. Centrifuges can have several different roles within a production brewery (28): • Cropping of nonflocculent yeast at the end of primary fermentation • Reducing the yeast quantity from green beer before the start of secondary fermentation • Beer recovery from cropped yeast • Separation of hot break (trub) after wort boiling • Removal of cold break (trub) at the end of maturation Yeast quality is influenced by the way that yeast is cropped, and centrifuges play an important role in this regard. Yeast quality affects beer stability and instability, which concerns a number of complex reactions involving proteins, carbohydrates, polyphenols, metal ions, thiols, and carbonyls. There are many diverse types of beer
instability that involve a number of different microorganisms, chemical species, and reactions. An understanding of these reactions has progressed since the 1980s, but a complete comprehension of beer instability reaction systems is not available. Passage of yeast through a centrifuge imposes mechanical and hydrodynamic shear stresses (18). Previous reports (28,59,64) have shown that these stresses can cause a reduction in cell viability, a decrease in flocculation, cell wall damage, increased extracellular proteinase A (PrA) levels, hazier beers, and reduced beer foam stability. Despite evidence of cell damage, little was reported before 2008 (19) about the effect of repitching yeast that has been cropped with a centrifuge on yeast and beer quality, especially in a HG wort environment. During the course of these studies (20), flow cytometry was employed to assess a number of yeast cellular parameters before and after centrifugation cycles at high g-force with a 5–6 hL/hr disk stack centrifuge (Westfalia Separator) (Fig. 2-29) in tandem with the ICBD 2 hL pilot brewery (Fig. 2-22). Increasing g-force exposes yeast cells to detrimental effects of hydrodynamic forces. SEM analysis (Fig. 2-30) provided visual evidence of yeast damage and the release of cell wall components as a result of disc stack centrifugation at high g-force.
HIGH-GRAVITY BREWING 33
Fig. 2-29. Westfalia Separator disc stack centrifuge. (Repro duced, by permission, from Chlup et al, 2008) (19)
Fig. 2-30. Environmental scanning electron microscopy of ale yeast strain damage: A, cells before passage through a disc centrifuge, and B, cells following passage through a disc centrifuge operating at a high g-force. (Reproduced, by permission, from Chlup et al, 2008) (19)