Recent developments in high gravity brewing.

Stewart, G.G., Bothwick, R., Bryce, J., Cooper, D., Cunningham, S., Hart, C. and Rees, E.

Abstract
The well known economic advantages of high gravity brewing are accompanied by a number of problems, including the reduced extract production efficiency during lautering when high gravity worts are obtained directly from the mash (instead of by adding a liquid sugar adjunct to a wort of normal gravity), reduced bittering efficiency (i.e. less of the alpha acid content of the added hops is afterwards found as iso alpha acid in the boiled wort), poorer foam stability (apparently because high gravity worts contain no more foam positive polypeptides per unit of volume than low gravity worts produced from the same materials, so that the concentration of these polypeptides in the finished beer is reduced by dilution) and the fact that the fermentation of high gravity worts imposes more stress on the yeast due to increased osmotic pressure, higher ethanol concentrations in the later stages of fermentation and differences in the nutrient balance in the wort (although there is considerable variation among brewers' yeast strains in the extent to which these stresses affect their performance). Possible solutions include the replacement of lauter tuns by thin bed mash filters (which are more efficient at extracting the fermentable material and, because they use less sparging liquor, produce a higher gravity wort from a mash of the same composition than lauter tuns), the addition of isomerized hop extract to the beer to bring its bitterness up to specification (which also improves foam stability) and the replacement of some of the barley malt in the grist by wheat malt to increase foam stability. Investigations into the effects on yeast of fermenting high gravity worts have revealed problems such as loss of viability, a reduction in the number of fermentations that can be carried out by pitching with yeast cropped from the previous fermentation in the series, and impaired glucose uptake when the yeast used for a high gravity fermentation has been acid washed. It has also been found that the performance of at least some yeast strains in high gravity fermentations can be somewhat improved by increasing the magnesium ion content of the wort.
Keywords : beer brewers' yeast composition efficiency fermentation high gravity brewing performance production quality survey wort