Chemometric studies of hops degradation at different storage forms using UV-Vis, NIRS and UPLC analyses

Abstract

Hops (Humulus lupulus L.) are one of the vital raw materials of brewing and their form of storage directly influences the final organoleptic sensations of beers. When hops are incorrectly stored, the degradation of important bitter compounds occurs fast. In the present work, it was used the ultraviolet/visible and near infrared regions, maximized by Ultra-Performance Liquid Chromatographic (UPLC) analyses, to identify the best way to store hops, by varying the (i) storage temperature, (ii) contact with atmospheric air, and (iii) storage time. For that, three different varieties of commercial hops were stored for six months (Hersbrucker, Magnum and Zeus). The chemometric results obtained with the Ultraviolet/ Visible (UV-Vis) and Near Infrared Spectroscopy (NIRS) data demonstrated the hop degradation kinetics under different storage conditions, while the chromatographic results provided the quantification of this degradation. Together, the results indicated that hops stored at low temperatures (≤ -10 °C) under a vacuum plastic bag presented the lower α - acids degradation rates over the months of the study.

The Stability of Hop (Humulus lupulus L.) Resins during Long-Period Storage

“…Overall, the shape did not have a statistically significant effect on alpha-acid loss. The greatest amount of alpha-acid was retained under anaerobic conditions in the cold room, in accordance with previous studies [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Mikyška and Krofta [ 13 ] found no difference in alpha-acid content after one year under anaerobic conditions at 2 °C, which is not consistent with our results, where pellets lost between 8.5 and 22.2% (after one year).…”

“…Losses under other conditions after six months and one year were similar to those of the present study. Verissimo [ 17 ] showed that presence/absence of atmospheric air was not statistically important at very low temperatures (below −10 °C), whereas in our study, the presence of oxygen was an important factor at low temperatures (4 °C). This finding indicates that the presence of oxygen is no longer important only at temperatures below the freezing point.…”

“…Unlike the findings reported by Tembo, Canbaş et al found that beta-acids were not particularly stable, as the losses ranged from 3.2 to 6.3% at 3 °C, and from 10.7 to 22.7% at room temperature. In contrast to these studies, chemometric studies of hop degradation under different storage conditions [ 17 ] revealed that the presence of oxygen did not play a significant role in alpha-acid deterioration at low temperatures (≤−10 °C), whereas storage at aerobic conditions and at 27 °C led to complete deterioration of co-, n-, ad-humulone after six months of storage. Táborský et al [ 18 ] investigated pellets of eight different varieties after storage at 4 °C under anaerobic conditions and found once again a variety-dependent loss of alpha- and beta-acids and a decrease in the alpha/beta-acid ratio in all varieties by the end of a nine-month storage experiment.…”