红外辐射制备柠檬酸糯米淀粉酯工艺优化及功能特性

为提高柠檬酸淀粉酯制备效率,拓展淀粉在工业领域的应用价值,以糯米淀粉为研究对象、柠檬酸为酯化剂,红外辐射为核心制备技术,取代度为衡量指标,用单因素和响应面法优化柠檬酸淀粉酯的制备工艺条件,并对改性前后淀粉的理化性质、微观结构测定分析。最佳工艺参数为:红外辐射照度2 474 W/m2、红外辐射时间7.1 min、pH值为2.96、柠檬酸与淀粉质量比0.50。在优化条件下,取代度的验证试验结果为0.156,与响应面拟合模型预测值(0.158)吻合良好,红外辐射处理7.1min可达传统干热制备法5h的制备取代水平且两者理化性质和分子结构不存在显著差异性(P0.05),两者抗性淀粉含量分别提高50.15%和50.35%,表现较强的热稳定性、抗剪切和抗消化特性;傅立叶红外光谱图显示在1 749 cm~(-1)位置出现酯类C=O伸缩振动特征吸收峰,证实酯化反应的发生。表明应用红外辐射技术辅助高效制备良好性能的柠檬酸淀粉酯是有效的。

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 1749 cm?1, indicating that the esterification occurred between the citric acid and starch with the difference of 0.005. 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.