Stability of Active Constituents of Hops (Humulus lupulus) Strobiles and their Ethanolic Extracts during Storage

Abstract

Aims: The purpose of this study was to evaluate the stability of three major active constituents (humulones, lupulones and xanthohumol) in dried hops (Humulus lupulus) strobiles (whole and ground) as well as their ethanolic extracts during storage. Methodology: A comparative study of humulones, lupulones and xanthohumol levels of H. lupulus strobiles during storage was carried out. Dried whole strobiles and cryogenically ground dried strobiles stored at -15ºC  as well as ethanol extracts of the strobiles prepared using different ethanol concentrations (10%, 30%, 50%, 70%, and 95%) and stored at room temperature, were analyzed by HPLC to quantify each constituent. These hops samples were analyzed immediately after preparation, and then one year and two years later to determine the concentrations of the constituents. Results: HPLC analysis indicated that the amount of all three constituents in the ground strobiles and in the ethanol extracts decreased gradually during the storage period. The 10% and 30% ethanol extracts had very low amounts of constituents initially and were practically devoid of constituents at the end of two years. The 50% ethanol extract contained considerable amounts of humulones and xanthohumol, and low levels of lupulones initially, but lost substantial amounts over time. The 70% and 95% ethanol extracts showed higher levels of all three constituents, while the 95% H. lupulus ethanol extract contained the highest constituent levels throughout the experimental period. The ethanol content of the extract had a direct correlation to the constituent levels; the higher the ethanol level, the higher the initial and subsequent constituent levels. Conclusion: Both dried hops and ethanol extracts lose active components over storage time. When preparing extracts, at least 70% ethanol is necessary to extract the highest levels of three bioactive constituents and to retain them over a two-year period. Ethanol concentration is a critical factor to be considered in hops extraction process.