It is well-known that various beers contain many flavor compounds derived from barley malts, hops, yeast fermentation, and other raw materials. Among these flavor compounds, terpenoids are mainly derived from hops. Linalool, one of the monoterpene alcohols, has been found in various beers and been regarded as an important factor for a hop-derived beer flavor. We focus on contributions of other monoterpene alcohols (geraniol, beta-citronellol, nerol, and alpha-terpineol) to hopped beer flavor. Several researchers have reported that monoterpene alcohols are biotransformed by yeast and that geraniol is mainly transformed to beta-citronellol during the first 2-4 days in model fermentation. In this study, we investigated the biotransformation of monoterpene alcohols during fermentation of hopped beer by using various hop cultivars. As a result, geraniol drastically decreased during the first 3 days. beta-Citronellol was almost absent in wort and gently increased during the total fermentation period. The concentrations of geraniol and beta-citronellol in finished beer increased, depending on the initial concentration of geraniol in the wort. The continuous increase of beta-citronellol did not correspond to the fast decrease of geraniol. This increase of beta-citronellol might be partly explained by an occurrence of glycosidically bound flavor precursor and a glucoside hydrolase activity secreted from lager yeast. In addition, we examined flavor characteristics of monoterpene alcohols and found that there was an additive effect among linalool, geraniol, and beta-citronellol and that only 5 microg/L of geraniol and beta-citronellol were enough for this effect. Therefore, it is suggested that not only linalool but also geraniol and beta-citronellol might contribute to hopped beer flavor at lower levels, at which OAVs of these compounds become below 1.0.
J. Inst. Brew. 116(3), 251-260, 2010The behaviour of hop-derived monoterpene alcohols during fermentation by lager yeast was previously investigated in this laboratory. It was suggested that the concentration of geraniol and β-citronellol in the finished beer increased depending on the initial concentration of geraniol in the wort. In addition, an additive effect among linalool, geraniol and β-citronellol was found and 5 ug/L of geraniol and β-citronellol was enough for this effect. In this paper, conditions regarding the enrichment of the initial concentration of geraniol in the wort were investigated. From the screening of various hop cultivars, Citra hop was selected as a geraniol-rich cultivar. In addition, it was observed that coriander seed, which can be used in beer production as a flavourant, contained not only linalool but also geraniol at high levels. The use of Citra hop or coriander seed was effective for enriching the concentration of geraniol and β-citronellol in the finished beers. In the Citra beer and in the coriander beer, the content of linalool was excess in comparison with the content of geraniol and β-citronellol. Therefore, the synergy of geraniol and β-citronellol, under coexistence of excess linalool, was examined. It was found that the flavour impression of excess linalool became more fruity and citrus by coexistence with geraniol and β-citronellol and that the coexistence of all three monoterpene alcohols was effective for this synergy. The flavour characteristics of the Citra and coriander beer and the importance of geraniol metabolism for a citrus flavour in beer are discussed.
A viroid-like RNA was detected in low molecular weight RNAs extracted from virus-free hops by two-dimensional polyacrylamide gel electrophoresis (PAGE). It migrated considerably faster than hop stunt viroid in 5% PAGE containing 8M urea under a denaturing condition. A cDNA fragment was amplified from the low molecular weight RNAs containing the viroid-like RNA by reverse transcription and polymerase chain reaction (RT-PCR) using a pair of hop latent viroid (HLVd) specific primers. The amplified cDNA product of expected size was detected by agarose gel electrophoresis. The full-length cDNA of the viroid-like RNA in RT-PCR products was cloned and sequenced. The established nucleotide sequence of the inserted cDNA was completely identical to that previously reported for HLVd. The total nucleic acids were extracted from hops cultivated in Japan, and the presence of HLVd were examined by dot blot hybridization and RT-PCR. In a hop sample, HLVd was not detected by dot blot hybridization, but detected by RT-PCR. The result indicates that RT-PCR is potentially more sensitive than dot blot hybridization using a radioactive probe. HLVd was detected in all mother cultivars tested. However, the two virus-free clones obtained by meristem culture showed negative reaction in both dot blot hybridization and RT-PCR detection methods. It is also shown that HLVd probably can be eliminated by meristem culture as indicated in several plant viruses and viroids.
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