Very high gravity brewing : an understanding of the maximum gravity in the Brewhouse

While brewers are routinely operating with high gravity wort at 17,5-18°P, with significant cost savings, the use of very high gravity (VHG) worts – 20-24°P – has not yet been adopted by the brewing industry for the production of standard beers, being associated with decrease in brewhouse material efficiency, reduced kettle hop utilisation and fermentation rates, elevated ester formation, extended lag phase duration, foam instability and increased concentrations of residual sugars in beer. Consequently, VHG wort fermentation has extensively been studied in the last 30 years, using mixes of produced wort and high maltose syrup as an input. In the brewhouse, mashing in and filtration equipment technology allow very concentrated mashes – 1,8:1 water/equivalent malt ratio – that theoretically optimise wort gravity. In between, there is a missing link. Investigating the limits of very high gravity in the brewhouse opens a series of perspectives to build it. Mass balance simulation, integrating the axial dispersion model to calculate sparging extract losses, tests the theoretical highest gravity for a newly designed process that maximises first wort and final wort concentration. This new iterative process recycles second wort to replace, completely or partly, water during mashing in and sparging, reaching theoretically 44°P first wort and 30°P final wort after 6 iterations. It was tested at pilot scale, achieving a proof of concept with a 35,3°P first wort and a 24 to 25°P final wort, without syrup addition, after one iteration. Fermentations of these very high gravity worts, with adapted conditions of aeration and 300 ppb zinc complement, showed a lower level of ethanol production, 10,4%ABV for a 24,4°P wort, but a higher rate related to fermentable sugars : worts coming from the very high gravity process have a different sugar composition, with 10% less fermentable sugars, especially maltose, demonstrating a deficiency in β-amylase and α-amylase activity. Fermentation was 0 to 4 days longer than standard high gravity fermentations, showing no FAN, lipid or zinc deficiency. Higher ester/higher alcohol ratio is confirmed. Exogeneous endo-1,4-xylanase and cellulase brought no fundamental change to fermentation results or brewhouse parameters. Viscosity is therefore the key parameter of very high gravity brewing. Its impact on enzymatic kinetics, saccharification, more specifically on filtration and sparging time and efficiency are demonstrated to be the limitations to a possible business case for the wort recycling process. Perspectives are existing. Good level of FAN and low β-glucans allow a different temperature brewing diagram to decrease viscosity and improve amylase kinetics. Laboratory scale trials showed that a combination of saccharification time, calcium and enzyme addition could increase fermentable sugars to a standard level. And many exogeneous enzymes could still be tested to decrease viscosity in the brewhouse. Wort recycling process could also be tested with lower first wort gravities. Many passionate investigations are still to come… Keywords: VHG, brewhouse, sparging, model, wort recycling, viscosity, sugars.