Optimization processes and functionality of citric acid esterified glutinous rice starch synthesized via infrared radiation

Abstract: Glutinous (waxy) rice is one of the most popular varieties for food production in East and Southeast Asia. Currently, citric acid treatment is widely used to synthesize the glutinous esterified rice starch. Native starch can normally be modified to enhance the physicochemical and mechanical properties for better industrial applications. Esterification of starch is then one of the most important chemical modifications to incorporate ester functional groups. Taking glutinous rice starch as the research object, this study aims to optimize the preparation process under Infrared Radiation (IR) for a high production efficiency of citrate starch using single factor test and Response Surface Method (RSM). Citric acid was set as the esterification agent. A degree of substitution was also selected as the indicator. The physicochemical properties and structure of starch were determined before and after modification to reveal the IR effectiveness in starch esterification. In specific steps, 50.0 g product was first uniformly distributed in a stainless-steel sample holder, and then irradiated using a piece of ceramic infrared equipment. The distance between the infrared lamp system and the sample holder was set to 25 cm. The process parameters were adjusted separately, including the intensity of infrared radiation, time, pH, and mass ratio. The dried citrate starch was washed with the distilled water three times, and then cleaned with absolute ethanol once to remove the unreacted citric acid, subsequently dried at 50 °C to a moisture level of 8 % in an oven, finally ground and passed through a 200-mesh sieve. The results showed that the optimum processing conditions were achieved as follows: 2 474 W/m2 of infrared radiation intensity, 2.96 of pH value, and 0.50 of mass ratio. There was no significant difference (P > 0.05) in the dry starch using the Infrared Radiation Method (IRM, 0.156) and Dry Heating Method (DHM, 0.158). The data demonstrated that a seven-minute Infrared radiation/citric acid Method (ICM) could obtain dry starch properties in a five-hour Dry Citric acid Method (DCM) treatment. Since IR high penetration and energy transfer efficiently supplied sufficient energy for the reaction system, more molecules were energized to accelerate the hydroxyl and carboxyl groups of citric acid that formed covalent ester bonds with hydroxyl groups in starch. Correspondingly, the energy field at high temperature was an inevitable condition for the esterification reaction. No significant differences were observed in the physicochemical properties and structure of citrate starch, compared with the conventional process (P > 0.05), both indicating better thermal stability, shear, and digestion resistance than those of Native Starch (NS). The content of Resistant Starch (RS) in ICM- and DCM-treated starch increased by 50.15% and 50.35%, respectively, compared with NS. The reason was probably that the derivative groups and cross-linking structure formed the high steric hindrance that resisted enzyme attack. Fourier-Transform Infrared (FT-IR) pattern presented a new peak at 1 749 cm?1, indicating that the esterification occurred between the citric acid and starch with the difference of 0.05. It inferred that the IRM and DHM treatments achieved basically equivalent degree of substitution. No alterations were detected in the X-ray pattern of citrate starch, compared with the native glutinous rice starch, indicating that the modification could not alter the crystal type of starch. IRM- and DHM-treated starch exhibited the reduced peak and relative crystallinity in a more diffuse diffraction curve, indicating that the cross-linking of citric acid destroyed the crystalline structure to generate the crystalline defects under limited molecular mobility. Consequently, the IR technology can be expected to serve as a potential starch modification for the highly efficient preparation in the citrate starch with better performance in modern food production.