超声辅助制备抗冻融大豆分离蛋白工艺优化

为了提高大豆蛋白冻融稳定性,研究了超声波辅助下大豆分离蛋白(soybean protein isolate,SPI)与葡聚糖(dextran,D)发生美拉德反应改性蛋白的方法,以乳化稳定性(emulsion stability index,ESI)和乳析指数(creaming index,CI)为响应值,建立了优化工艺的Box-Behnken模型。验证试验表明,模型具有重现性和可靠性,在SPI质量浓度40 mg/m L、超声温度80℃、比功率5 W/m L条件下,与未改性SPI相比,改性后SPI乳化稳定性提高了43.80%,经1、2、3次冻融循环后乳析指数分别降低了57.76%、75.33%、96.20%。接枝物制备的乳液经冻融循环后粒径维持在50~55μm。红外光谱分析接枝物在1 010 cm-1处的C-N共价键振动增强,说明SPI和葡聚糖是以共价键的方式结合。扫描电镜结果表明,改性后蛋白颗粒更加疏松,分子间聚集程度降低。研究结果为冷冻食品专用大豆分离蛋白的产业化生产提供了理论和技术指导。

Processing optimization for improving freeze-thaw stability of soybean protein isolate by ultrasonic assisted glycosylation

Soybean protein isolate (SPI) has been widely used in food industry because of its ability to improve texture which is contributing to the nutritional and health benefits of protein-based foods. It can be used as emulsifiers in food emulsions due to the surface-active properties of their constitutive proteins, the storage globulins 7S and 11S. Oil-in-water (o/w) emulsions are thermodynamically unstable systems and sensitive to environmental changes, for example cooling and freezing. Freezing and frozen-food storage can maintain microbiological and chemical stability, while extending the shelf life of food products. There are many potential applications for o/w emulsions that can be frozen and then thawed prior to use, such as refrigerated and frozen food. Nevertheless, most of o/w emulsions are highly unstable when they are frozen and will have rapid breakdown after thawing. When an o/w emulsion is stored at low temperature, a variety of physicochemical processes can occur including ice formation, fat crystallization, freeze-concentration, interfacial phase transitions and biopolymer conformational changes. These phase transitions may lead to creaming, oiling off, coalescence and flocculation of the emulsions, which limit its utilization in frozen food. In order to improve the freeze-thaw stability of SPI, the SPI-D (dextran) grafts were prepared by ultrasonic-assisted glycosylation. Based on the early study of single-factor experiment, we found that the emulsion stability index (ESI) before freeze-thaw and the creaming index (CI) after freeze-thaw had high correlation. So the ESI and CI were set as the response values, the concentration of SPI, ultrasonic temperature and ultrasonic power were as factors, the Box-Behnken model of optimizaiton process was established to improve the emulsifying properties and freeze-thaw stability of SPI. The results showed that under the condition of the concentration of 40 mg/mL SPI, 80 ℃ ultrasonic temperature, 5 W/mL power density, the freeze-thaw stability of glycosylation products attained the optimal level. Compared with the control SPI, the CI of the emulsions prepared by ultrasonic SPI-D grafts decreased by 57.76%, 75.33% and 75.33% respectively after freeze-thaw cycles of 1, 2 and 3 times, the ESI increased by 43.80%, and the particle size of the emulsions maintained at the range of 50-55μm. Comparative study of freeze-thaw stability of emulsions prepared by the mixture of SPI and D, the wet heating SPI-D grafts, the ultrasonic SPI-D grafts and the native SPI was also performed. The results indicated that the CI and ESI of SPI and D mixture had no significant difference compared with the native SPI. The addition of dextran with heating had a little effect on the freeze-thaw stability of SPI, but far more than that of ultrasonic SPI-D grafts. Infrared spectrum analysis showed that the grafts in 3 500-2 990 cm-1 had the wide stretching vibration, the absorption peak migrated from 3 280 to 3 270 cm-1, and the free hydroxyl had the vibration absorption. SPI had the absorption peak in 1 060 cm-1, which migrated to 1 010 cm-1 for the grafts, and the grafts’ absorption of vibration was better than SPI, the carbon-nitrogen (C-N) covalent bond vibration was strengthened, the degree of graft attained 32.43%, indicting that SPI and dextran were based on a combination of covalent bond. From the fluorescent spectra we could see that the maximum absorption wavelength of SPI was red-shifted from 344 to 345 nm, the fluorescence intensity increased to 1 014; scanning electron microscopy showed that the particle state of glycosylation protein became more loose, the intermolecular aggregation degree decreased significantly, and thus the freeze-thaw stability improved significantly. The result provides us a new method for the preparation of SPI with high freeze-thaw stability, which is to modify SPI through ultrasonic-assisted glycosylation. This method provides a theoretical and technical guidance for the industrialization production of SPI which is more suitable for frozen foods.