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Adams et al. (1992) Adams et al. (1992)

flocculation. Whether this reflects some predilection of brewing yeast or, perhaps more likely, the visual nature of such changes is not clear.

Over the years, workers at Guinness have reported changes in flocculation and utilisation of maltotriose, characteristics 'which tend to change spontaneously' (Donnelly & Hurley, 1996). Early observations originate from two doyens of brewing yeast physiology, V.E. Chester (1963) and R.B Gilliland (1971a) working, respectively, in London and in Dublin. Both demonstrated that flocculation was prone to instability. Notably, cultures derived from a single cell became less flocculent on sub-culturing (Chester, 1963). Gilliland (1971a) reported practical observations of an ale yeast switching from flocculent (Class II) to non-flocculent (Class I) (see Section Opposite but less common was the process observation that a lager strain became more flocculent. Gilliland (1971a) also noted 'the second serious mutation was the loss of ability to ferment maltotriose and this has been found on a number of occasions, both in top fermentation breweries and in lager breweries'.

This theme was extended by Hammond and Wenn (1985), who reported sluggish fermentation problems that 'disappeared' after seven to ten fermentations together with flocculation changing from flocculent Class II to non-flocculent Class I (Gilliland, 1971a). Intriguingly, the 'new' problem yeast grew well in laboratory cultures on an oxidative carbon source (glycerol) but exhibited much weaker growth on the wort trisaccharide, maltotriose. Conversely the established 'normal' yeast showed strong growth on maltotriose and weak growth on glycerol. The case for these observations being linked to genetic instability coupled with selective advantage was reinforced by laboratory observations where maltotriose utilisation could occasionally be induced in the problem yeast by repeated subculturing on maltotriose. In conclusion, Hammond and Wenn (1985) argued that these events are caused by some subtle mitochondrial malfunction, which evades detailed genetic analysis of mitochondrial DNA.

A similar but seemingly less frequent observation on the switching of flocculation in production yeast strains has been made in Bass Brewers. Here, a major lager yeast strain becomes dramatically more flocculent which results in cropping difficulties. Typically, the pumped yeast solids increase from 40^15% to 60-75%. As is shown in Fig. 4.20, the timing of the switch is not consistent but, once made, is stable. Two

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