Effects of branched-chain fatty acids derived from aged hops on the aroma of beer brewed with flavour hop varieties

Hops stored at room temperature for an extended period are commonly referred to as 'aged hops'. Degradation of alpha and beta acids occurs during ageing, so the ageing process is accompanied by a significant increase in the concentration of branched-chain fatty acids (BCFAs): isobutyric acid (IBA), isovaleric acid (IVA), and 2-methylbutyric acid (2MBA). These three compounds, having rancid, cheesy, and/or sweaty flavours, are regarded as "off-flavours". It was previously reported that BCFAs contained at their threshold levels could enhance the flavour intensity of monoterpene alcohols (linalool, geraniol, and beta-citronellol; LGC) and change their aroma profiles. In this study, the behaviours of BCFAs during a 12-months ageing process were compared using five Japanese hop varieties. All BCFAs in the hops increased in all varieties during the total ageing period and a drastic increase was especially observed from 9 to 12 months. The content of linalool gradually decreased but plateaued at relatively high levels. In thiol-containing hops, 3-sulfanyl-4-methylpentan-1-ol (3S4MP) decreased drastically from 6 to 12 months, whereas 4-methyl-4-sulfanylpentan-2-one (4MSP) gradually decreased but was plateaued at relatively high levels. Next, to investigate the sensory effect of aged hops on aroma and taste profile of beer, model beers spiked with several combinations of monoterpene alcohols (LGC), 4MSP, and each BCFA were prepared. As a result, BCFAs commonly enhanced the characteristics of fruity, fullness, and maturity in the model beer spiking LGC. BCFAs also more enhanced the characteristics of tropical in coexistence with 4MSP. The sensory effect of BCFAs was expected to enhance hop-derived flavour of beer brewed with both geraniol-rich hops and 4MSP-rich hops. However, the sensory tests also showed an increase in rancid characteristics due to the presence of BCFAs. Therefore, test beers were brewed using fresh and aged 4MSP-rich hops. As a result, although the negative characteristics of rancid gradually increased depending on the dosage of aged hops, the positive characteristics of fruity, tropical, fullness, and maturity increased by mixing only 0.1 g/L of aged hops.