Effect of steeping treatment on pasting and thermal properties of sorghum starches

Abstract

Chemical treatments in wet milling could improve the physico-chemical properties of starch isolated from high-tannin sorghums. Sorghums Chirimaugute (medium-tannin), DC-75 (high-tannin), and SV2 (tannin-free) were steeped in water, dilute HCl (0.9%, v/v), formaldehyde (0.05%, v/v), and NaOH (0.3%, w/v) solutions before wet milling and starch separation. Pasting, textural, and thermal properties of starch were determined. Steeping in NaOH resulted in starches with higher peak viscosity (PV), cool paste viscosity (CPV), and setback than when water, HCl, and formaldehyde were used. The time to PV (Ptime) and PV temperature (Ptemp) were markedly reduced by treatment with NaOH. NaOH could have caused a degree of pregelatinization. HCl treatment gave starches with higher Ptemp and Ptime, presumably due to delayed granule swelling. Gel hardness was largely determined by the starch amylose content. The low hardness of DC-75 starch gels was slightly improved in NaOH-treated grains. Gelatinization temperatures of sorghum starches were generally low, regardless of steeping treatment. Starch from NaOH-treated grain generally had slightly higher gelatinization temperatures than when water, HCl, or HCHO was used. Chemical treatments during steeping of sorghum grains greatly affected starch properties. Dilute alkali steeping during wet milling could be used to improve properties of starch isolated from tannin-containing sorghums.