Sorption Isotherm Characteristics of Distillers Dried Grains with Solubles (DDGS)

Abstract

Distillers Dried Grains with Solubles (DDGS) is widely recognized as a highly nutritious animal feed ingredient. With the exponential growth of the fuel ethanol industry in the past several years, significant quantities of distillers grains are now being produced. To effectively utilize these feeds in the domestic market, however, these coproduct streams are increasingly being transported greater distances, and must be stored in various structures until final use. Unfortunately, DDGS flow is often problematic, as it frequently becomes restricted by caking and bridging during storage and transport. This issue may arise from a number of factors, including storage moisture, temperature, relative humidity, particle size, time, or temperature variations. The objective of this study was to develop sorption isotherms for DDGS with varying soluble levels, in order to provide facility designers and operators with appropriate storage and transport information. Equilibrium moisture contents (EMC) of DDGS with four different soluble levels (10, 15, 20 and 25% db) were determined using the static gravimetric method at 10, 20, 30 and 40oC over four equilibrium relative humidity (ERH) conditions, ranging from 0.560 to 0.910. The sorption capacity of DDGS increased with increasing temperature and soluble level, and followed a type III isotherm, which is commonly observed in high sugar foods. The observed EMC values for 10, 15, 20 and 25% (db) solubles ranged from 8.61% to 47.07% (db), 11.58% to 83.49% (db), 13.72% to 90.70% (db), and 15.03% to 132.01% (db), respectively. Nine models were applied to fit the isotherm data. The Modified Henderson and modified Chung-Pfost models did not fit the data, however, as the regression coefficients did not converge. The Modified Halsey and modified Exponential models, on the other hand, were found to perform well for the isotherm data. As there was no common model to predict the sorption isotherms of DDGS with various soluble levels, a new EMC model was developed. This new model, termed the GMR (Ganesan-Muthu-Rosentrater) model, incorporated soluble level as one of the effects, along with temperature and moisture content. The GMR model (R2=0.9395 F=977.55), followed by a new modified Exponential 2 (NME2) model (R2=0.9368 F=934.43), produced the best fits for DDGS with varying soluble levels.