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 degrees 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 degrees C for 136 min for coffee malt, SP = 2.5 W/g for 3.48 min, T(oven) = 214 degrees C for 136 min for chocolate malt, and SP = 2.5 W/g for 3.48 min, T(oven) = 211 degrees 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.