Optimization of radio frequency heating protocols based on free radical control to improve the storage stability of highland barley

Abstract

Highland barley (HB) is prone to deterioration due to the lipid rancidity induced by lipase (LA) activity. Therefore radio frequency (RF) heating was applied to inactivate the LA of HB. The objectives of this study were to reveal the critical reason for triggering lipid deterioration during RF heating and to establish effective RF treatment protocols by controlling this trigger. After RF stabilization treatment, HB was packaged by three methods: Ziplock bags (ZB), vacuum (Va) and vacuum combined with an oxygen absorber (Va-OA). The changes in the free fatty acid value (FFAV), peroxide value (POV), free radical intensity, hexanal content and free fatty acid (FFA) composition in RF-treated HB samples were also compared. The optimal operating conditions of RF heating to inactivate LA and control free radical intensity were target temperature of 95 °C with holding for 0 min, electrode gap of 120 mm and water supplementation of 2%. Under these conditions, the free radical intensity of RF-stabilized HB decreased to the lowest level of 0.1367, and the inactivation rate of LA reached 44.19%. After 30 d of storage at 37 °C, the hexanal content showed the lowest level of 2.103 μg/g, and LA activity did not increase significantly during the whole storage period. During the 6-month storage under ambient temperature, the FFAV, POV and free radical intensity of RF-stabilized HB packaged by ZB, Va and Va-OA were all significantly lower than those of untreated HB, while their FFA composition changed more slowly. The lowest hexanal content was observed in Va-OA packaged RF-treated samples highlighting the importance of free radical control during RF heating and the subsequent control of oxygen accessibility during storage. In summary, this study suggested the great potential of RF heating technology for stabilizing HB efficiently.