Investigation of Stereoisomer Distribution and Thermochemical Conversion of Eight Terpene Alcohols Derived from Different Varieties of Chinese Hops (Humulus lupulus L.)

Terpene alcohols play an important role in hop aroma, and their stereostructure is closely correlated with aroma characteristics. The stereoisomer distributions among R-(−)- and S-(+)-linalool, nerol and geraniol, S-(−)- and R-(+)-β-citronellol, (2Z,6E)- and (2E,6E)-farnesol in five varieties of Chinese hops were investigated using Headspace-Solid Phase Micro-Extraction Enantioselective-Gas Chromatography-Mass Spectrometry (HS-SPME-EsGC-MS). Different hop varieties showed differences in the stereoisomeric distribution of the terpene alcohols. The R-form of linalool, geraniol, and the R-form of β-citronellol existed in all hop samples; while the S-form of linalool was not detected in any of the varieties tested. A boiling model, using four different timepoints, that mimics whirlpool conditions in the brewhouse was established and the stereoisomeric changes that can occur to different terpene alcohols as a result of boiling were investigated. The two forms of linalool, geraniol and R-(+)-β-citronellol continuously increased across all boiling timepoints. Nerol initially decreased in concentration with early boiling timepoints but as boiling continued, it started to increase in concentration. Based on intramolecular rearrangement, geraniol can convert into linalool, β-citronellol, and a small fraction to nerol. Likewise, the conversions of nerol to geraniol and linalool, as well as farnesene into farnesol were observed. Specifically, (2Z,6E)-farnesol was detected only in SA-1 and Lemon hops, and its content fluctuated slightly, which suggested that (2Z,6E)-farnesol was a relatively stable configuration. Cluster analysis (CA) and principal component analysis (PCA) further showed the result on a two-dimensional space. The CA results showed that aroma varieties of hops had essential distinctions in the flavor matrix compared with other varieties. PCA results indicated that geraniol, R-(+)-β-citronellol, (2Z,6E)-, and (2E,6E)-farnesol could be used to discriminate hop varieties; while nerol and two forms of linalool are important components for reflecting the influence of boiling time.