Dickinson Eric, Ettelaie Rammile, Murray Brent S, Du Zhiping
Food Colloids Group, Procter Department of Food Science, University of Leeds, Leeds, LS2 9JT, United Kingdom.
J Colloid Interface Sci. 2002 Aug 1;252(1):202-13. doi: 10.1006/jcis.2002.8405.
The rate of shrinkage of air bubbles of initial radii, r, from 50 to 150 microm injected beneath a planar air-water interface has been measured. Bubbles were stabilized by 0.05 wt% protein in approximately 0.1 mol dm(-3) ionic strength buffer at pH 7.0 and at room temperature. Four proteins were studied: commercial whey protein isolate (WPI), sodium caseinate, gelatin, and pure beta-lactoglobulin. Bubbles in all systems showed shrinkage due to diffusion of gas from the bubbles, which accelerated as the bubbles got smaller. Within approximately 1 h all bubbles had disappeared, having shrunk to below approximately 1 microm, so that in no cases was there evidence of stabilization via a surface rheological mechanism. The rates of shrinkage with the different proteins were not significantly different except in the case of gelatin, which at any given bubble size appeared to give a slightly higher rate, probably because the surface tension is higher for this system. A new theoretical analysis of the dissolution kinetics for the case of a bubble close to a planar interface has been developed. For caseinate and WPI a simple model incorporating a constant surface tension and a constant bubble-interface separation appears to account for the kinetics. Interestingly, the model predicts a linear dependence of r(n) versus time when n is closest to 3, in contrast to n = 2 expected from previous work. For gelatin and pure beta-lactoglobulin, the introduction of modest dilatational elasticities of approximately 2.3 and 7 mN m(-1), respectively, gives good agreement between theory and experiment. This is particularly the case for beta-lactoglobulin, where there is a noticeable slowing, but not cessation, of the shrinkage as the bubbles get smaller. In the light of these findings the practical significance of surface rheology with respect to stability to disproportionation is discussed. Finally, we present experimental evidence that a bubble stabilized by beta-lactoglobulin shrinks to a nonspherical protein particle consisting of the completely collapsed protein film.
已测量了初始半径(r)从50至150微米的气泡在平面空气 - 水界面下方的收缩速率。气泡在pH 7.0、室温下，于离子强度约为0.1 mol dm⁻³的缓冲液中，通过0.05 wt%的蛋白质得以稳定。研究了四种蛋白质：商业乳清蛋白分离物（WPI）、酪蛋白酸钠、明胶和纯β - 乳球蛋白。所有体系中的气泡均因气体从气泡中扩散而收缩，随着气泡变小收缩加速。在约1小时内所有气泡均消失，收缩至约1微米以下，因此在任何情况下均未发现通过表面流变学机制实现稳定的证据。除明胶外，不同蛋白质的收缩速率无显著差异，在任何给定气泡尺寸下，明胶的收缩速率似乎略高，可能是因为该体系的表面张力更高。已针对靠近平面界面的气泡情况开展了溶解动力学的新理论分析。对于酪蛋白酸盐和WPI，一个包含恒定表面张力和恒定气泡 - 界面间距的简单模型似乎可以解释动力学。有趣的是，该模型预测当(n)最接近3时(r^n)与时间呈线性关系，这与先前工作预期的(n = 2)相反。对于明胶和纯β - 乳球蛋白，分别引入约2.3和7 mN m⁻¹的适度膨胀弹性后，理论与实验结果吻合良好。对于β - 乳球蛋白尤其如此，随着气泡变小，收缩明显减缓但并未停止。鉴于这些发现，讨论了表面流变学对歧化稳定性的实际意义。最后，我们给出实验证据，表明由β - 乳球蛋白稳定的气泡收缩为一个由完全塌陷的蛋白质膜组成的非球形蛋白质颗粒。
