Question 7 - Laura Cooley

Compare and contrast the fermentation process of top and bottom fermented beer

In 1883, Emil Hansen used serial dilutions to separate yeast cells based on morphology and show that top and bottom fermenting strains produce unique fermentations (Rank et al., 1988).

In ale beers, the strain Saccharomyces cerevisiae is used and is known as a ‘top’ fermenting yeast. On the other end of the beer scale, lager fermentation requires the use of the yeast strain Saccharomyces pastorianus, (formerly known as Saccharomyces carlsbergenisis), which is a ‘bottom’ fermenting yeast strain. Although both strains belong to the same species, they are distinguishable by their biochemical and physiological properties (Leskošek and Stojanović, 2002).

 The names ‘top’ and ‘bottom’ fermenters do not refer to the area of the vessel where fermentation takes place it is, in fact, referring to the type of yeast used and how the yeast grows during the fermentation process. A ‘top’ fermenting yeast produces low density clumps as it grows in the wort. These clumps trap carbon dioxide and rise to the surface of the fermentation vessel. In contrast to this, ‘bottom’ fermenting yeasts flocculate which then settles on the bottom of the fermenter vessel (Hutkins, 2006).

‘Top’ and ‘bottom’ fermenters were originally categorised due to their flocculation behaviour. The behaviour of each type is so distinct that the two main classes of beer, ale and lager, are based on the yeast types (Lodolo, et al., 2008). The temperatures at which these yeasts ferment are different. Top fermenting yeasts, used in ale production, ferment at fairly high temperatures of 18˚C to 27˚C. While, in contrast, lager production uses yeast strains that are capable of fermentation at temperatures below 15˚C. In ale production, fermentation is followed by a short aging period or even no aging period, however lager production, after fermentation is subjected to a long aging period, which can last up to a few weeks, also known as ‘lagering’. Within these categories there can be a range of differences including alcohol content, colour, flavour etc (Kodama, et al., 2006).

Top fermentation is the oldest method of beer production, and until the middle of the 19^th century was the only method used. Beers produced from top fermenting yeast differs from bottom fermenting yeast in their ingredients and by their aroma, which is primarily induced by the strain used, S. cerevisiae (Michael Eblinger, 2009).

Top and bottom fermenters have many differences. For example, top fermenting yeasts are chains of budded cells, only ferment one third of raffinose, have a greater yield crop after fermentation and have a high enzyme content while in contrast, bottom fermenting yeasts are single cells or pairs of cells, ferment raffinose completely, produce a lower yield crop after fermentation and have a low enzyme content. (Scannell, 2012).

References

Hutkins, Robert, W. (2006). Microbiology and Technology of Fermented Foods. Oxford: Blackwell Publishing. p320.

Kodama, Y., Kielland-Brandt, Morten C.,Hansen, J.. (2006). Lager Brewing Yeasts. In: Per Sunnerhagen, Jure Piskur Comparative Genomics Using Fungi as Models. Berlin: Springer Berlin. p145 - 164.

Leskošek, I.,Stojanović, M. . (2002). A possible application of ale brewery strains of Saccharomyces cerevisiae in lager beer production. World Journal Of Microbiology And Biotechnology. 9 (1), p70-72.

Lodolo,Elizabeth J., Kock, Johan, L.F., Axcell, C, Barry., Brook, Martin.. (2008). The yeast Saccharomyces cerevisiae– the main character in beer brewing. FEMS Yeast Research. 8 (7), p1018–1036.

Michael Eblinger, Hans (2009). Handbook of Brewing. Germany: Wiley-VCH. p222

Rank GH, Casey G & Xiao W (1988) Gene transfer in industrial Saccharomyces yeast. Food Biotech 2: 1–41.

Scannell, Amalia (2012) Lecture material, file 5 – Yeast.