De novo Formation of Sesquiterpene Oxidation Products during Wort Boiling and Impact of the Kettle Hopping Regime on Sensory Characteristics of Pilot-Scale Lager Beers

Abstract

Many brewers aim at a balanced ‘kettle hop’ aroma in their lager beers and therefore add aroma hops to the 
boiling kettle. Whereas the application of ‘late’ hop additions to acquire an intense ‘kettle hop’ aroma with a 
‘floral/citrusy’ bouquet is scientifically quite understood, brewers have also been adding rather expensive 
(European/noble) aroma hops at the onset of boiling in an empirical way to impart ‘noble kettle hop’ aroma, 
typically described by delicate ‘spicy/herbal’ notes, to their beers. Although many researchers suggested 
generation of hop oil-derived terpene oxidation products during wort boiling and associated oxygenated 
sesquiterpenoids with these refined ‘spicy/herbal’ notes, actual de novo formation of such compounds during 
wort boiling has up to date not been proven unambiguously in real brewing practice and consequently, there 
remain many questions with regard to this subject. This study tackles this problem by investigation of 4 conventionally hopped lagers, thereby varying the time point of hop addition (pellets cv. Saaz). HS-SPME-GC-MS 
analysis of samples taken along the wort boiling process of an ‘early’ hopped beer revealed de novo formation 
of oxygenated sesquiterpenoids. The impact of the hopping regime on the hop-derived flavour of the beers 
was demonstrated via sensory analysis by our taste panel. The ‘early’ hopped beer clearly expressed ‘spicy/
herbal’ aroma. These notes were also clearly detected in the beer hopped with a combination of ‘early’ and 
‘late’ hopping, and, moreover, this beer expressed ‘floral/citrusy’ notes and was scored highest for both ‘kettle 
hop’ flavour and general appreciation. Our observations suggest that expression of ‘noble kettle hop’ aroma 
characteristics in lager beer might not simply be dependent on the absolute level of (flavour-active) oxygenated sesquiterpenoids present, but also on the ratio of volatiles imparting ‘floral’ aroma and ‘spicy’ aroma.