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 degreesC, 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 degreesC 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 degreesC 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 beta-glucan, pentosan, and protein contents in the dried samples. (C) 2004 Elsevier Ltd. All ri-hts reserved.