Aroma profile and bitter acid characterization of hop cones (Humulus lupulus L.) of five healthy and infected Polish cultivars

The characteristic hoppy aroma is directly correlated with the chemical composition of its volatile profile and essential oil. The aim of this work was to analyse the essential oils and volatile fractions of five popular Polish hop cultivars (Sybilla, Lubelski, Pulawski, Magnat and Marynka), healthy and infected by viruses and viroids by SPME and GC–MS. Hop fraction of hard resins and alpha- and beta acids were determined according to the official analytical method (modified Wöllmer method EBC 7.7 and HPLC method EBC 7.5, respectively). Hop Mosaic Virus (HpMV), Apple Mosaic Virus (ApMV), Hop Latent Viroid (HLVd) are commonly found in plants of all hop cultivars commercially produced in Poland. Monoterpenes hydrocarbons showed higher percentages in the aroma profile of infected cones (82.5%–94.3%) than in the healthy ones (81.1%–91.4%) and increased from the healthy to the infected plants, except for 'Sybilla' that had an opposite trend (from 90.1% in healthy to 77.9% in infected hop). Hydrogenated sesquiterpenes were the second main class of aroma constituents with major amount in healthy cones with the exception for 'Sybilla'. Myrcene was always the most abundant chemical constituent in all the infected cultivars, except for 'Sybilla' (from 86.3% in infected to 74.6% in healthy plants), followed by α-humulene in four of the five cultivars except for 'Marynka', where (E)-β-farnesene showed higher amount. The EO yields related to the five healthy cultivars showed variations between 0.2% in 'Lubelski' up to 1.1% in 'Marynka', very similar to those observed in the five infected cultivars. Monoterpenes predominated over sesquiterpenes in healthy and infected 'Sybilla' and 'Marynka' EOs, while in healthy and infected 'Lubelski' and 'Magnat' prevailed an opposite trend. 'Magnat' is the only one cultivar that showed different amount of the two main classes of constituents between the healthy and infected plants (sesquiterpenes 64.6% in healthy cones and 50.5% of monoterpenes in the infected ones). Myrcene was the main constituent in all the infected EO samples (29.9%–67%), while α-humulene predominated only in three healthy cultivars ('Lubelski', 'Pulawski', and 'Magnat' with 34.2, 36.8, 31.0%, respectively). The content of bitter substances (α and β-acids as well as hard and soft resins) varied in healthy and infected hop plants and this behaviour was strongly dependent on the selected cultivars but differences between healthy and infected plants were not statistically significant. In conclusion virus/viroid infection seems not to interfere in a substantial way with the normal hop secondary metabolite biosynthesis, but only in the biomass yields.