Nearing maturity, female hop plants develop inflorescences called hop cones. Inside of a mature hop cone is the lupulin gland where glandular trichomes are present. Within the lupulin gland, the glandular trichomes secrete alpha acids (α), and beta acids (β). Hop bitter acids are extremely sensitive to photolysis and oxidation reactions causing the degradation of α-acids and β-acids which negatively affect their use in beer, and the beer brewing industry accounts for 98% of world use of hops (Fandino et al., 2015). Determining hop storage index (HSI) is a way to measure the amount of α and β-acids lost during postharvest handling. HSI is a ratio determined by measuring the spectrophotometric UV absorption of hop extract at 275nm for oxidative compounds, and 325nm for bitter acids. Bitter acids and any oxidative decreases were determined in eight cultivars of hops (‘Cascade’, ‘Chinook’, ‘Comet’, ‘Mount Rainier’, ‘Newport’, ‘Tahoma’, ‘Willamette’, ‘Zeus’) produced at the Cimarron Valley Research Station in Perkins, Oklahoma. Mature hop cones were hand harvested at 80% moisture and dried at ambient temperature to 8-10% moisture using a centrifugal fan. Hops were stored no longer than six months frozen under nitrogen in vacuum sealed bags until analysis. Hop bitter acids were extracted using toluene and UV absorbance was measured at 355, 325, and 275nm using a spectrophotometer. Based on the HSI, hops were assigned a quality score of good (0.4). Seven of the eight cultivars of hops were of good quality (‘Cascade’, 0.20; ‘Chinook’, 0.19; ‘Comet’, 0.17; ‘Mount Rainier’, 0.12; ‘Newport’, 0.23; ‘Tahoma’, 0.18; ‘Willamette’, 0.22). ‘Zeus’ hops were of questionable quality (0.31) but contained a slightly higher moisture content at storage (11%) than the other cultivars (7-10%), which could have negatively affected hop storage quality. Understanding sources of oxidative stresses to hop bitter acids during postharvest handling is valuable information for determining and maintaining hop quality.
