O-9
Modeling the effects of fermentation process variables on beer flavor
volatiles. The use of high temperatures, high pitching rates and high gravities have all
been suggested as ways of intensifying the fermentation process. Such techniques
do indeed increase the overall rate of fermentation and lead to increases in
productivity and better asset utilisation. However there are a number of
disadvantages, specifically altered beer flavour (especially higher alcohols and
esters) and detrimental effects on yeast viability and vitality. Investigations
at BRI have sought to understand how process variables affect beer flavour
profiles with the aim of developing intensified processes which produce beers
with normal flavour characteristics. In order to assess the interactive effects
of process variables on beer flavour production, a group of statistically
designed fermentation trials were performed using a range of process conditions.
The conditions varied were temperature (12-28 degrees Celsius), pitching rate
(0-30 million cells/ml), wort gravity (11-20 degrees Plato) and initial
dissolved oxygen level (0-100% air saturation). The fermentations were carried
out in 1 litre stirred batch fermentation vessels using standard lager wort and
Saccharomyces cerevisiae NCYC 1324. The kinetics of flavour volatile
production were monitored to enable maximum specific production rates to be
calculated. Fermentations were followed until specific gravity reached 2.5
degrees Plato. Statistical analysis of the data obtained suggests that the
process variables have significant effects on the average yields (mass volatile
produced per degree Plato consumed) of various esters and higher alcohols during
batch fermentation. All process variables affected the fermentation time (time
taken to reach to 2.5 degrees Plato). Empirical correlations have been derived
for predicting the average yields of those flavour volatiles where the data were
statistically significant. An empirical correlation was also derived for
predicting fermentation time. As expected, the predictions for total
fermentation time indicated that higher temperatures and pitching rates yielded
faster fermentations. Wort gravity and initial dissolved oxygen levels also
affected the fermentation time. Temperature and pitching rate had strong
interactive effects on the yields of higher alcohols and of some esters.
Additionally, the predicted average yields for some flavour volatiles were
observed to change when wort gravity or initial dissolved oxygen levels were
altered. Comparison of the predicted average flavour volatile yield "maps" with
the predictive maps for process time indicating operating zones where process
intensification occurs, without changes to flavour volatile concentration.
After obtaining a Ph.D. in Chemical Microbiology at Cambridge University,
John spent 14 years with Guinness as their senior microbiologist. He joined
Brewing Research International in 1985 as Head of Fermentation. Between 1995 and
1999 John was successively in charge of Quality Services, Information and
Communications and finally Membership Services. In 1999 he was appointed
Operations Director with overall responsibility for the Pilot Plants and
Laboratories at BRI. John is a Fellow of the Institute and Guild of Brewing and
of the Royal Society of Chemistry and is a former chairman of the EBC
Microbiology Group.
JOHN R.M. HAMMOND and Alistair Brown. Brewing Research International, Nutfield,
Surrey, UK.