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Class IV

Present gravity

Fig. 4.6 The fermentation performance of yeasts from the four Gilliland flocculation classes (redrawn from Gilliland, 1951).

fermentation systems have their place, the two-litre 'tall tube' fermenter - based on the dimensions of a cylindroconical fermenter - has found particular application in studies of fermentation performance (Walkey & Kirsop, 1969; Gilliland, 1971a; Thorne, 1975; Bryant & Cowan, 1979). Although a fruitful approach, tall tube fermentations are regarded by many as 'user hostile'. The Carlsberg Research Centre has overcome such concerns via the development of automated systems such as the 'multiferm' (Sigsgaard, 1996) and Octo-Duo-Ferm (Skands, 1997) which, respectively, consist of 60 and 16 two-litre tall tube fermenters. Other developments have focused on data handling (Bryant & Cowan, 1979) and product analysis (Gilliland, 1971; Thorne, 1975).

There is consensus (Burns, 1941; Walkey & Kirsop, 1969; Gilliland, 1971; Thorne, 1975; Bryant & Cowan, 1979) that the fermentation performance of brewing yeast can be described broadly by five criteria. These include (i) the formation of a yeast head, (ii) deposit formation (flocculation), (iii) rate of fermentation, (iv) extent of fermentation and (v) clarification after fining. Although clearly adding value, analysis of beer flavour through tasting panels or 'analytically' (Gilliland, 1971; Thorne, 1975) offers little diagnostic benefit.

Unfortunately, 'fermentation performance' lacks sufficient sensitivity to differentiate between closely related brewing yeasts. Indeed, in a major survey, Walkey and Kirsop (1969) used the above 'five criteria' to characterise 153 brewing strains in tall tube fermentations from the National Collection of Yeast Cultures. This approach was extended by Bryant and Cowan (1979) to include a further 82 strains from the NCYC but with sophisticated data treatment of all 235 strains by principal coordinates analysis.

Although essentially similar conclusions can be drawn from both studies, Bryant and Cowan (1979) identified five distinct groups dependent on head or deposit formation (see Table 4.13). Group 1 consisted of low head (score of 1) and high deposit (score of 5) formers. Conversely group 5 consisted of head forming, low deposit strains. Groups 2-4 catered for strains falling between the extremes. Further subdivision was effected by 'A' (high clarification) or 'B' (higher extent of attenuation) subgroups.

Table 4.13 Classification of 235 brewing strains by fermentation performance (from Bryant & Cowan, 1979).

Group

Number of

Head

Deposit

Degree of

Rate of

Clarification

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