Optimization of Combined Microwave–Hot Air Roasting of Malt Based on Energy Consumption and Neo-Formed Contaminants Content

ABSTRACT: To produce specialty malt, malts were roasted by combined microwave–hot air at various specific microwave powers (SP = 2.5 to 3 W/g), microwave heating times (t_mw = 3.3 to 3.5 min), oven temperatures (T_oven = 180 to 220 °C), and oven heating times (t_oven = 60 to 150 min). The response variables, color, energy consumption by microwave (E_mw) and oven (E_oven), total energy consumption (E_tot), quantity of neo-formed contaminants (NFCs), which include hydroxymethylfurfural, furfural, furan, and acrylamide were determined. Response surface methodology (RSM) was performed to analyze and predict the optimum conditions for the specialty malt. Production using combined microwave–hot air roasting process based on minimum energy consumption and level of NFCs. At 95% confident level, SP, T_oven, and t_oven were the most influencing effects with regard to E_tot, whereas t_mw did not affect E_tot. T_oven and t_oven significantly affected malt color. Only T_oven significantly influenced the NFCs content. The optimum parameters were: SP = 2.68 W/g for 3.44 min, T_oven = 206 °C for 136 min for coffee malt, SP = 2.5 W/g for 3.48 min, T_oven = 214 °C for 136 min for chocolate malt, and SP = 2.5 W/g for 3.48 min, T_oven = 211 °C for 150 min for black malt. Comparing with conventional process, combined microwave–hot air reduced E_tot by approximately 40%, 26%, and 26% for coffee, chocolate, and black malts, respectively, and reduced HMF, furfural, furan, and acrylamide contents by 40%, 18%, 23%, and 95%, respectively, for black malt. Practical Application: An important goal for research institutions and the brewery industry is to produce colored malt by combining microwave and hot air roasting, while saving energy, getting desirable color, and avoiding the formation of carcinogenic and toxic neo-formed contaminants (NFCs). Therefore, one objective of this study was to compare energy consumption and content of NFCs during roasting of malt by hot air-only and combined microwave–hot air processes as well as to determine the effect of specific power, microwave processing time, oven temperature, and oven processing time during combined microwave–hot air roasting. Another objective was to predict the optimum conditions for the production of coffee, chocolate, and black malts.