Impact of alkaline hydrogen peroxide pretreatment on sorghum wet-milling and quality of sorghum starch

Sorghum is an important food security crop and a potential alternative source for industrial starch production. Sorghum starch is, however, characterized by dull off-colors, attributed to the high levels of polyphenol compounds in the grain. Under alkaline conditions, hydrogen peroxide produces perhydroxyl anions, which react with chromophore groups in colored molecules. This work evaluated the impact of alkaline hydrogen peroxide (AHP)-pretreatment on sorghum wet-milling and the quality of sorghum starch. AHP-pretreatment of sorghum was investigated to (1) determine the optimum H2O2 concentration and pH at which acceptable starch could be obtained; (2) evaluate effect of pretreatment on the recovery of wet-milling products and the quality of the wet-milled starch fraction; and (3) evaluate the hydration kinetics of the pretreated grains during soaking in water. The first study varied H2O2 concentration and pH based on a central composite design experiment to determine its effect on sorghum starch color and recovery. The change in starch color was correlated with the properties of the wet-milled starch. A response surface model determined an acceptable degree of lightness (L-value) of 90 for sorghum starch at a pH of 11.84 and 20% (w/w) H2O2 concentration. Varying pH during AHP-pretreatment had a significant effect (p < 0.05) on the starch color. A response surface model for starch recovery did not adequately fit the experimental data (Lack-of-Fit = 0.460). Changes in starch color did not influence the starch recovery and most of the starch properties analyzed, however, it was significantly correlated with the starch pH (r = 0.62), swelling power (r = -0.57) and final pasting viscosity (r = 0.62) at p > 0.01, as well as oil absorption capacity (OAC) (r = 0.48) at p > 0.05. The second study evaluated the effect of AHP-pretreatment and further steeping in water for 24 hours at 55oC on the recovery yields of wet-milling products (coarse fraction, fine fraction, starch fraction, and total solids recovery) and the properties of the wet-milled starch obtained from four sorghum cultivars. The presence or absence of tannin in the sorghum cultivars did not significantly influence (p < 0.05) the recovery yields of wet-milled fractions. AHP-pretreated grains produced significantly higher (p < 0.05) levels of starch fraction yields and total starch recoveries. AHP-pretreatment also produced brighter starches particularly from the tannin-containing cultivars, whereas, steeping in water alone reduced the brightness of starches from the tannin-free sorghum cultivars. Most starch functional properties were not significantly affected (p > 0.05) by AHP-pretreatment or steeping in water. In the third study, the hydration kinetics of AHP-pretreated sorghum was evaluated by fitting Peleg's sorption model to experimental water absorption data with R2 values for the fitted model ranging between 0.976 and 0.996. The capacity constant (K2) was independent of the soaking temperature and pretreatment, with an average value of 0.02%-1. AHP-pretreatment resulted in a significantly higher (p < 0.05) initial rate of hydration (1.86 g/min to 5.10 g/min) compared to the control (1.56 g/min to 3.92 g/min) at 60 °C. The saturation moisture content for AHP-pretreated grains (56.67%, d.b. to 82.37%, d.b.) and the control (52.02%, d.b. to 76.49%) were consistent for each grain cultivar. The entire body of work demonstrates the effectiveness of AHP-pretreatment to enhance the color of wet-milled sorghum starch without significantly changing its functionality.