Sorption isotherms for amaranth grains

Abstract

Compiling of EMC-ERH data for amaranth grains (Amaranthus cruentus L.) in the range of water activity from 0.029 to 0.979 and temperature from 25 to 90°C was performed. Included data sets comprised experimental values of EMC-ERH that summarize 78 identified points for desorption, 53 for adsorption and 16 not discerned points that were considered for mean sorption. Five isotherm equations for grains included in the ASAE Standards (Modified Henderson, Modified Chung-Pfost, Modified Halsey, Modified Oswin and GAB) were evaluated for their ability to fit sorption data from the literature (M c vs. aw for adsorption, desorption and mean sorption). The goodness of fit for each isotherm was quantified through the correlation coefficient (R2), the sum of squares (RSS), the standard error of the estimate (Sy), the mean relative deviation (MRD) and the plots of residuals. The three-parameter GAB isotherm was the best and gave a good correlation (R2>0.9817, RSS<0.0293, MRD<0.1380, S y<0.0141, and random residuals-plots) for the general data-fit in the range of aw from 0.1 to 0.9, of interest in seed storage and processing. The Modified Halsey equation was rejected because it gave poor statistic parameters of agreement and patterned residual plots. For desorption, the Modified Chung-Pfost model gave the lowest mean relative deviation; the Modified Henderson equation was the second best in describing the EMC-ERH data, followed by the Modified Oswin and GAB models. For adsorption, the GAB equation presented the lesser MRD, followed by the Modified Chung-Pfost, Henderson and Oswin models. When mean sorption data were analyzed, the Modified Chung-Pfost equation was the best. However, when the GAB isotherm was adjusted at each temperature, a higher quality of agreement was obtained compared with the other isotherms, demonstrating the adequacy of GAB model to describe the experimental data of EMC-ERH for amaranth.