Analysis of drying kinetic of brewer´s spent grains: effect of the temperature on the physical properties and the content of bioactive compounds

Background: Brewer´s spent grain (BSG) is a biomass by-product generated in large volumes during industrial beer production. BSG has become a growing environmental problem, as most breweries discard it inappropriately, negatively impacting the environment. Alternatives for the exploitation of this by-product have consisted of elaborating food supplements for farm animals, obtaining biofuels, developing adsorbents, and obtaining substances for the food industry. However, the high moisture content in BSG (approximately 70%), poses a significant challenge in exploring various reuse alternatives. Therefore, the implementation of a pre-drying process becomes essential. Objective: This study aimed to analyze the BSG drying kinetics at different temperatures and the effect of the drying temperature on the physical properties and the content of bioactive compounds. Methods: BSG samples were dried at different temperatures (50, 60, 70, 80, 90, and 105°C) and analyzed for their moisture ratio, water activity, total polyphenol content (TPC), and DPPH (1,1-diphenyl-2-picrylhydrazil) radical scavenging activity. Also, four kinetics models were fitted to the drying data. Results: It was determined that the effective diffusivity was between 5.23x10-10 (m2/s) and 2.49x10-09 (m2/s), and the value of the activation energy was 28.05 kJ/mol. In addition, it was found that the content of phenolic compounds (1.27±0.120 mg gallic acid equivalents /g) and the DPPH radical scavenging activity (0.21±0.015 mg gallic acid equivalents /g) were not significantly affected by the variation in the drying temperatures studied. Conclusions: From an operational point of view, the most suitable temperature for the drying process of BSG was 105°C since it would allow to reach shorter drying times, and the TPC was not affected markedly by the range of temperature studied.