Abstract
Diacetyl reduction during lager maturation is essential for producing clean flavour because excessive diacetyl creates buttery notes and lowers sensory acceptability. This study investigates the biochemical drivers of diacetyl reduction during lager maturation. Controlled lager fermentations were matured under defined temperature, yeast contact time, nutrient condition, and dissolved oxygen levels. Alpha-acetolactate, diacetyl concentration, acetoin formation, yeast viability, amino acid uptake, pH, ethanol concentration, attenuation, and sensory flavour quality were monitored. The results show that active yeast contact was the main driver of diacetyl reduction during early maturation, as viable cells converted diacetyl into less flavour-active compounds. Moderate maturation temperature accelerated reduction without increasing unwanted ester or higher alcohol formation. Low yeast vitality slowed diacetyl cleanup and extended maturation time. The study demonstrates that lager maturation efficiency depends on maintaining yeast metabolic activity after primary fermentation. Proper temperature control and yeast health management support faster diacetyl reduction and cleaner lager flavour.