Metal chelation of corn protein products citric acid derivatives generatedvia reactive extrusion

Citric acid (CA) was thermochemically reacted with corn coproducts from the ethanol industry, i.e. corn gluten meal (CGM), produced from corn wet mill ing, and distillers' dried grains (DDG), produced from corn dry milling, to generate value-added, acid-insoluble reaction products with enhanced metal -binding properties. Heated CA produced an anhydride that reacted with the nucleophilic hydroxyl groups of starch and fiber as well as the hydroxyl, s ulfhydryl, and amino groups of protein to produce ester or acyl derivatives . Short-term reactive extrusion of 1:I w/w ratios of each corn product with CA by two passages at 20 rpm of each blend through a Brabender Plasti-Cord er PL 2000, 1.9 cm single screw, 30:1 (L:D) fitted with a 1:I screw and no die, through zone temperatures of 140, 200, and 196 degrees C yielded react ion products that possessed similar degrees of carboxylation as a lengthy, 24 h oven-baking procedure at 120 degrees C previously reported. Metal ion binding of the extruded derivatives increased with increasing pH from 3.0-5 .0 where the solubility of the metal ions was unaffected by pH. At a pH of 4.5, where the charged ligand acts as an electron pair donor and at a metal ion concentration of 200 mg/l, the equilibrium binding efficiency for CGM/ CA derivative under this set of conditions was: 0.54, 0.33, 0.85, 1.00, 0.5 9, 0.28, 0.30, 0.72, and 0.45 mmol/g for Cd2+, Co2+, CU2+, Fe2+, Pb2+, Mn2, Ni2+, Agi, and Zn2+, respectively; whereas, for the DDG/CA derivative the metal binding was: 0.78, 0.66, 1.04, 1.08, 0.64, 0.51, 0.60, 1.02, and 0.7 3 mmol/g, respectively. The ability of citrate derivative to chelate metals varied with each derivative. Based on a comparison of metal binding data t o heated CGM and DDG controls with no CA, the pendant carboxyl groups contr ibuted by the CA moiety gave significantly higher metal ion binding. Compar ison of metal binding ability for both the CGM/CA and DDG/CA derivatives wi th a sulfonic acid, styrene type analytical grade cation exchange resin, AG 50W-X8, showed that CGM/CA possessed 13-44% of the capacity to bind metals, whereas DDG/CA possessed 23-47%. The corn-based derivatives produced in th is study were biodepradable and could be produced at a considerably lower c ost compared with petroleum-based resins. Published by Elsevier Science B.V .