Thin-layer drying of spent grains in superheated steam

An empirical equation with regression-determined coefficients was developed to describe thin-layer drying of brewers' spent grain (BSG) and distillers' spent grain (DSG) in superheated steam (SS). A small moisture gain on the sample surface due to steam condensation at the beginning of the drying caused the initial moisture ratio of the sample to increase to a value between 1.00 and 1.20 for BSG drying or between 1.00 and 1.30 for DSG drying. The moisture gain increased linearly with decreasing the temperature and/or velocity of steam. Unlike in hot-air drying, not only steam temperature but also steam velocity influenced the drying rate of BSG and DSG in SS. At 145 °C, an increase in steam velocity from 0.3 to 1.1 m/s cut the drying time by half for both spent grains. Increasing the SS drying temperature from 110 to 180 °C decreased the starch content in samples dried to the equilibrium moisture at the SS velocity of 0.66 m/s by 14.5% and 11.5% for BSG and DSG, respectively. Drying to the equilibrium in 145 °C at a SS velocity of 0.66 m/s caused the starch content in the samples to decrease by 22% for BSG and 23% for DSG compared with that in the non-dried material. The SS drying parameters had no effect on β-glucan, pentosan, and protein contents in the dried samples.