该文通过测定不同样品浓度、离子强度、温度对大豆分离蛋白(soybean protein isolate,SPI)、酪蛋白酸钠(sodium caseinate,SC)、蔗糖酯(sugar esters,SE)制备乳液的平均粒径、乳析率、微观结构的影响,评估了这3种食品级乳化剂的乳化性能。结果表明,SPI为20 g/L、SC为12 g/L、SE为2.0 g/L时均可以制备粒径细小且稳定的乳液,平均粒径d4,3分别为(0.724±0.238)、(0.451±0.003)、(0.541±0.003) μm。在乳化过程中,当乳化剂不足以覆盖油/水界面时,不同乳化剂制备的乳液发生的不稳定机制也不同:SC、SE乳液仅发生了聚结,而SPI乳液可能由于不溶性蛋白颗粒存在导致发生桥联絮凝;向SPI、SC、SE乳液中添加NaCl后,通过激光共聚焦显微镜(confocal laser scanning microscopy,CLSM)可以观察到絮凝体的存在,但是激光粒度分析仪无法测定出SC、SE乳液絮凝体的大小,表明NaCl对液滴的静电屏蔽作用使SC、SE乳液产生比较弱的絮凝,这种絮凝体容易受外界作用力而分散开来;SPI乳液中NaCl浓度为0.15 mol/L时,90 ℃水浴加热会导致SPI构象变化并且球蛋白在油水界面缓慢展开,使更多的疏水基团暴露发生疏水相互作用,导致絮凝加剧,表明SPI在环境压力下对抗聚结的稳定性好。
The effects of emulsifier concentration, ionic strength, and heating on the emulsions formed by soy protein isolate (SPI) sodium caseinate (SC), and sucrose ester (SE) were investigated to evaluate their emulsifying properties. Results showed that fine and stable emulsions could be prepared at 20 g/L for SPI, 12 g/L for SC, and 2.0 g/L for SE, with the average diameter of (0.724±0.238), (0.451±0.003), and (0.541±0.003) μm for d4,3, respectively. When the emulsifier concentration was not enough to saturate the oil/water interface created during emulsification, the instability mechanisms of emulsions formed by different emulsifiers were different. Emulsions formed by SC and SE only showed droplet coalescence, however, emulsions formed by SPI showed bridging flocculation of droplets probably caused by the insoluble protein particles existed in SPI dispersion. The addition of NaCl into the emulsions formed by SPI, SC, and SE could cause emulsion creaming and droplet flocculation as observed using confocal laser scanning microscopy (CLSM). However, the sizes of droplet flocs could not be measured using Mastersizer 3000, because they were broken up by the sample preparation procedures required for Mastersizer 3000, suggesting that the droplet flocculation was caused by the electrostatic shielding effect of NaCl were rather delicate. The heat of the 90 ℃ at NaCl concentration of 0.15 mol/L resulted in a conformational change of the SPI and a slow unfolding of the globulin at the oil-water interface, exposing more hydrophobic groups to hydrophobic interactions and leading to increased flocculation, indicating that the SPI is stable against coalescence under environmental stresses.
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