Impact of pre-freezing and microwaves on drying behavior and terpenes in hops (Humulus lupulus)

Abstract

Hop buds (Humulus lupulus) are paramount to beer bittering, flavoring, and microbiological stability. To optimize post-harvest processing, fresh and pre-frozen hops were subjected to freeze-drying, hot air and microwave-assisted hot air drying. Pre-freezing occurred at − 80°C, prior to drying at 35°C, 50°C, and 65°C, with different microwave power (0 W, 100 W and 200 W, where 0 W represented conventional hot air drying). Results show that hops drying kinetics can be described using the predictive Page and Logarithmic mathematical models. Obtained R2, SSE, and RMSE values ranged between 0.999 and 0.982, 0.035–0.001, and 0.058–0.004, respectively. Irrespective of the drying condition, pre-freezing reduced drying time by 0.17–85.9 %. Pre-freezing hop buds increases the effective moisture diffusion coefficient, and it increases with higher drying temperature and microwave power, ranging between 5.91 × 10−10 m2 s−1 and 2.43 × 10−7 m2 s−1. SEM analyses indicate that pre-freezing causes structural damage to lupulin glands. The average concentration of myrcene, limonene, caryophyllene, and humulene for fresh hops were 15.08 mg g−1, 0.27 mg g−1, 3.09 mg g−1, and 6.52 mg g−1 respectively. For the dried samples under the various conditions, the concentration ranged from 12.20 mg g−1 to 0.53 mg g−1 (myrcene), 0.26 mg g−1 to 0.12 mg g−1 (limonene), 1.49 mg g−1 to 0.31 mg g−1 (caryophyllene), and 2.69 mg g−1 to 0.52 mg g−1 (humulene). Results affirm that pre-freezing plant material prior to drying can shorten postharvest processing times, and this method can potentially be applied to other industrial crops. This study highlights the importance of controlled postharvest processing to ensure industrial crop quality.

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