Many microbreweries will have at some time experimented with bottle conditioned beer as either to show commitment to traditional products or to add a high value product to a standard portfolio of filtered beers. Many of us will also have experienced difficulties, if not in hygiene then in consistency.
In theory bottle conditioned beer should be an extension of primary fermentation only in a smaller package than bulk tank or cask. Racking directly into bottle should mean that the same yeast remains active and simply completes its work in a different environment.
However, as with many apparently simple processes the truth can be frustratingly different. Let’s just list some of the common complaints of bottle conditioned beer seen by brewers and, sadly, by customers:
- Sour – clear acidic taste often accompanied by other unconventional flavours including staling.
- Stale – oxidised and cardboard character, may also be stale.
- Phenolic – medicinal character, may be accompanied by over carbonation and gushing.
- Gushing – over-carbonation, beer foams or spurts from the bottle.
- Cloudy – yeast rises in the bottle and into the glass, often seen with over- carbonation.
- Flat – no or limited carbonation, beer lacks life and sparkle, also shows little head retention.
- Flat – no head produced or retained, may be evident on well carbonated beers as well as on flat beer.
- Over-sweet and cloying palate – dominantly sweet flavour.
Quite an impressive and intimidating list but also probably familiar to many of us. The strange thing is that bottle conditioning is no new science but one with long achievement in much of the traditional brewing industry, notably overseas in Belgium but also historically in the UK. Since its use to the major brewing industry has remained historic little information has been reported for some time. However, a Belgian, study conducted on a range of breweries using bottle conditioning and on laboratory experiments has now cast some useful light on the factors involved (J Am Brew Chem, 2006, v64 pp206-213).
Leaving aside the importance of good hygiene and counter pressure filling to avoiding faults 1, 2 and 3 this study focused on investigating whether the sugar used in secondary fermentation affected the subsequent bottle conditioning. Taking filtered bright beer with limited residual fermentable material bottles were primed with glucose, sucrose or maltose and left to condition at 22-24oC for 2-3 weeks but samples monitored daily. Fresh yeast was added at a level of one million cells per ml.
A clear result was obtained between bottles primed with glucose or sucrose and those primed with maltose. The former fermented fully while the latter achieved less than 20% of its potential leaving bottles flat, over-sweet and also with less yeast sedimentation.
This result was also obtained using a range of commercial priming syrups ranging from those based on simple glucose to others containing mainly maltose and maltotriose. In all cases maltose and maltotriose fermented poorly.
Now the implications of this are important when we look at the bottle conditioning process. Use glucose or sucrose to prime and condition will be achieved giving the beer good carbonation. Use maltose, or perhaps partly fermented wort, and you’ll get flat, cloudy beer with a sweet flavour from unfermented sugars.
Further investigations attempted to identify the reasons behind this finding. These were based on an assessment of the different ways in which yeast use these sugars. In essence glucose and fructose are taken into the yeast cell by simple diffusion while sucrose is mostly digested into glucose and fructose before uptake. Maltose by contrast needs an active uptake which requires energy and is only activated once glucose has been used. Of course, normal fermentation uses maltose adequately although it has to be said that cells adapted to fermenting high levels of glucose will inhibit the uptake of maltose and stuck fermentations may result.
In the study two factors which affect maltose uptake were targeted as possible causes – alcohol and carbon dioxide concentration being the most likely as these are particularly prevalent in bottle conditioned beers.
The possibility of ethanol affecting uptake was investigated by comparing secondary fermentation in a basic maltose wort and the same wort with 7% alcohol added. Although the ethanol did reduce fermentation compared to the control poor fermentation was observed in both. Experiments looking at lower concentrations of ethanol on maltose uptake showed more limited effect suggesting ethanol isn’t the cause of poor secondary fermentation.
Carbon dioxide, in contrast, had a highly significant effect. Bottles of wort were primed with maltose and sucrose and compared for secondary fermentation after artificial carbonation and, as a control, leaving others uncapped but layered with oil to maintain anaerobic conditions.
Full fermentation was obtained in the carbonated bottles primed with sucrose but limited fermentation in bottles primed with maltose. Maltose was, however, fermented in bottles with oil. As a further confirmation of this effect carbonated bottles primed with maltose were opened after 3 weeks and started immediate fermentation.
What the actual cause of this effect are is unclear but pressure on yeast cell structure and enzyme activity are suggested. Further studies may clarify this but the lesson for small scale bottle conditioning is clear. Use simple glucose or sucrose for priming rather than wort rather than bottle with residual sugars as these will be maltose and maltotriose and are more likely to remain unfermented when carbon dioxide builds up.
Additional consequences of incomplete maltose fermentation were also apparent in this study. Firstly, clarity was reduced, possibly because residual maltose would inhibit flocculation and so cause poor adhesion of yeast to the glass. Secondly, yeast viability declined more in maltose primed bottles than in sucrose primed bottles. Poor viability will not only decrease fermentation but also release undesirable flavours and could possibly produce poor head retention. All features we wish to avoid.
Extensive though this study is it doesn’t completely explain bottle conditioning. Analysis of commercial bottle conditioned beers did show considerable variation suggesting that other factors are important, possibly yeast strain but perhaps processing. In addition, there are certainly examples of adequately conditioned UK bottle conditioned beers produced directly from primary fermentation. Whether these have the best balance of carbonation, residual sugar and yeast viability is, perhaps, worth assessing.
Written by Dr Keith Thomas
Brewlab Director