Parameters of the drying medium and dried hops in belt dryer

Abstract

Rybka A., Heřmánek P., Honzík I., Krofta K. (2017): Parameters of the drying medium and dried hops in belt dryer. Res. Agr. Eng., 63 (Special Issue): S24-S32.An important factor in hop growing is the process of drying. For this purpose belt dryers with follow-up conditioning are the most widespread but they are not ideal. In this respect, an analysis of the drying process was carried out in the belt dryer of Agrospol Velká Bystřice Co., Ltd. for the 'Saaz' hop variety. Drying parameters were monitored by means of fixed sensors, continuously sensing data loggers and samples taken for laboratory analysis (hop moisture, alpha and beta bitter acids, Hop Storage Index (HIS). The process of drying showed that hops are practically dry (10 ± 2.0% of moisture) already at the end of the second belt or possibly at the beginning of the third belt. It was also proved that hops are over-dried (moisture of 4 to 8%) and then they are adjusted by conditioning to the final moisture content of 8-10%. Excessive drying leads to cone disintegration, which makes any manipulation with hops for purposes of further processing difficult and results in greater losses of lupulin. The current state in the field of technology of hop drying and conditioning is not ideal. This refers to both operating costs and qualitative features of the final product. In belt dryers prevailing so far, following stationary picking lines, their drying process is directly linked to the continuous process of harvest. In belt dryers hop cones are dried at a drying air temperature of 55-60 o C for 6-8 h coming from original 75-85% of moisture to final 8-10% prior to conditioning. Bracts themselves, however, have moisture content of only 5-6%, while strigs may have a moisture content of up to 30%. Percentage of strig weight to the total cone weight (6-12%) has a great effect on subsequent moisture permeability during the conditioning of hop cones (Kořen et al. 2008). Controlled conditioning requires roughly up to a 1/3 of the total energy requirements needed for hop growing (Doe, Menary 1979). Stability of alpha bitter acids, being the key hop substance from the point of view of brewing technology, is sufficient at drying temperatures reaching up to 60°C. However, for some heat-labile substances the drying temperatures between 50 and 60°C in the final stage of drying are too high. They lead to irreversible transformations and losses. Such substances are for instance hop essential oils that are contained in the amount of 0.5-3.5%, depending on hop variety (Hofmann et al. 2013;Kumhála et al. 2013). The pilot studies showed a decrease of 15 to 25% of the overall content of essential oils present in the hops prior to drying at the current drying conditions (Kieninger, Forster 1973;Kirchmeier at al. 2005). Besides this amount, the sensory profile changes too, due to a loss of the more volatile components. With special aroma hop varieties, so called "flavour hops", whose 25Vol. 63, 2017, Special Issue: S24-S32 Res. Agr. Eng.doi: 10.17221/35/2017-RAE content and ...

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