Thermochemically‐modified soybean and corn protein products with enhanced metal‐binding properties

Abstract

Citric acid (CA), also known as 2-hydroxy- 1,2,3 propane tricarboxylic acid, was thermo-chemically reacted with food quality soy protein isolate (SPI), distillers' dried grains (DDG), produced from corn dry milling, and corn gluten meal (CGM), produced from corn wet milling, to generate acid-stable products with enhanced metal-binding properties. CA dehydrates at high temperature to form an anhydride that can interact with nucleophilic functional groups of protein or carbohydrate to generate ester or acyl derivatives. Effects of temperature, CA concentration, pH and reaction time were evaluated to show that SPI, DDG, and CGM, when heated in the range 1 10-120 °C with CA at 1 : 1 w/w ratio, under endogenous acidic conditions for 24 h. yielded reaction products with reaction efficiencies >60% which possessed 4.13, 4.19 and 4.26 mmol COOH/g, respectively. FTIR data of original heated protein compared with their respective CA products demonstrated additional absorbances indicative of ester and carboxyl linkages. The SPI/CA, DDG/CA, and CGM/CA products effectively bound 1.18, 1.07, and 0.98 mmol Cu 2+ /g, respectively, when analysed by ion plasma spectrometry. Solid state 27 Al NMR supported the metal binding characteristics of CA reaction products and demonstrated that Al 3+ was bound ionically to carboxyl groups present on the reaction product. Amino acid composition studies showed diminished amounts of amino acids with nucleophilic reactive groups in all three CA reaction products. The CA reaction products are highly resistant to acid hydrolysis with 6 N HCI for 4 h at 145°C. The biobased products generated in this study possess cation-exchange capability and potential biodegradability that may have an outlet for industrial wastewater treatment.