Evaluation of Hop Drying Efficiency Based on Moisture Content Measurement with Different Variable-temperature Drying Modes

In order to solve the problems of long drying time and low drying sufficiency, the drying experiment was examined to verify the drying efficiency at different variable-temperature modes using the hop variety ‘Tuopu No.1’ obtained from Gansu Province A modified oven was taken as the baking media with a single-layer curing barn. The moisture content of the hop cones and the RH values of the hop layers at different drying temperatures were measured. Two optimal variable-temperature drying modes were selected, that was, in the drying process of hops, the temperature was set from 30 to 50 ℃(30~50 ℃) and from 30 to 60 ℃ and then down to 40 ℃(30~60~40 ℃). Baking with these two drying modes, the moisture content of hops could be reduced by 54.48% and 49.82% within 7 h, respectively. The drying efficiency was furtherly verified in the hop bakery of the workshop and the drying curves were analyzed. The linear models could give a good fit to the drying curves of the drying mode of 30~50 ℃(R²>0.96), while Weibull distribution models(β<1) fitted well to the drying mode of 30~60~40 ℃. It was proved that the two drying models belong to constant rate drying and falling rate drying, respectively. It was further verified that the drying efficiency was higher under the drying mode of 30~50 ℃. Hops drying at 30~60~40 ℃ had higher essential oil content, which was about 43% higher than that at 30~50 ℃. The above results indicated that the hops should be dried with a low temperature(30 ℃) in early stage and followed a high temperature(50 ℃) in late stage; In order to obtain more essential oil content in the final hops, the drying temperature could be reduced to some extent(40 ℃).