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.

Question 8-What esters in beer produce undesirable flavours and where in the process can their production be controlled? Erin Fleming

Erin Fleming-11209224

Assignment 1-Task 2

Question 8

Due to their high volatility and low thresholds, esters contribute significantly to the unique flavours and aromas associated with specific varieties of beer. Over 100 different esters have been identified in beer (Eblinger 2009) but most brewing esters are formed by esterification of ethanol with fatty acids and acetyl coenzyme A (Yoshioka et al. 1981).  This reaction results in the formation of ethyl acetate, an ethyl ester which is the most common ester found in beer. Although these esters make up the largest percent of all esters in beer, those which are formed by the acetates of the higher alcohols, also called “banana esters” or “acetate esters” also play a role in the flavour and aromas (Briggs et al 2004).  At high levels and in certain varieties, acetate esters may be regarded as off-flavours.  Therefore, their formation must be controlled.

Controlling the formation of acetate esters in the brewing process can be done in a number of ways.  Manipulating the yeast strain, the wort specific gravity and sugar profile, wort oxygen and lipid content, genetic modification of the yeast, as well as the fermentation temperature will impact the level of acetate esters synthesized. Fewer acetate esters results in a beer with fewer off-flavours.

One of the most important factors affecting ester production is the yeast strain selection (Verstrepen et al. 2003).  The strain of yeast used will affect both the average ester production as well as the relative proportion of each individual ester produced (Verstrepen et al. 2003).  Equally, wild yeast strains such as Hansenula and Pichia produce large quantities of ethyl acetate by aerobic fermentation (Briggs et al. 2004). Therefore, if one wishes to minimize the acetate esters produced, the yeast population should be regulated to avoid wild strains.

Another important influence on esterification is the specific gravity and sugar profile of the wort.  High-gravity operations, which are very commonly used today, result in an unbalanced flavour profile due to the overproduction of acetate esters (Verstrepen et al. 2003).  This overproduction leads to beers which are “over-fruity” or “solvent-like.” The relative amounts of various sugars contained in the wort also affects the formation of acetate esters.  In general, worts which contain higher levels of glucose and fructose produce more esters than those containing more maltose (Younis et al. 2000).  Therefore, by adding a supplemental maltose syrup, one could reduce the acetate ester concentrations.

The formation of volatile esters is diminished with an increase in wort oxygen and lipid content.  Therefore, wort aeration or oxygenation is recommended as a possible means by which to decrease unwanted esters (Verstrepen et al. 2003).  Regarding lipids, unsaturated fatty acids in the wort also decrease the synthesis of unwanted esters.  This is most likely due to the repression of the ATF gene transcription (Meilgaard 2001).  Manipulation of the wort lipid content can be achieved through changes in the filtration process (Verstrepen et al. 2003).  

Generally, a decrease in temperature during fermentation also decreases ester synthesis (Eblinger 2009).  Likewise, increased fermentation temperatures ranging from 10-25°C results in an increase in ester production (Verstrepen et al. 2003).  The exact increases are strain-dependent, but by simply decreasing the temperature at which fermentation occurs, one can minimize esterification.

Acetate ester formation in brewer’s yeast can also be controlled with the genetic modification of specific genes.  According to Verstrepen et al., it is possible to “create new yeast strains with desirable ester production characteristics.”  Additionally, the manipulation of the ATF gene specifically is one of the most important control points affecting ester synthesis (Verstrepen et al. 2003).

References:

Briggs, D; Boulton, C; Brookes, P; Stevens, R (2004). Brewing Science and Practice. New York: Woodhead Publishing. 683-687.

Eblinger, H (2009). Handbook of Brewing Processes, Technology, Markets. Online: Wiley-VCH Verlag GmbH & Co. 134.

Meilgaard, M (2001). Effects on Flavour of Innovations in Brewery Equipment and Processing: a Review. The Journal of the Institute of Brewing and Distilling.107, 271-286.

Verstrepen, K; Derdelinckx, G; Dufour, J; Winderickx, J; Thevelein, J; Pretorius, I; Delvaux, F. (2003). Flavor-Active Esters: Adding Fruitiness to Beer. Journal of Bioscience and Bioengineering. 96 (2).

Yoshioka, K. Hashimoto, N. (1981). Ester Formation by Alcohol Acetyltransferase from Brewers' Yeast. Journal of Agricultural Biology and Chemistry.45

Younis, 0; Stewart, G (2000). The Effect of Wort Maltose Content on Volatile Production and Fermentation Performance in Brewing Yeast. Brewing Yeast Fermentation Performance.1(2).