Hindle S, Povey MJ, Smith K
Procter Department of Food Science, The University of Leeds, Leeds, LS2 9JT, United Kingdom
J Colloid Interface Sci. 2000 Dec 15;232(2):370-380. doi: 10.1006/jcis.2000.7174.
Droplet collision is shown to be important in the propagation of nucleation through supercooled oil-in-water emulsions by the use of a novel membrane technique. On the other hand, nucleation mediated by Tween 20 micelles is shown to be of relatively much less importance in both n-hexadecane and cocoa butter oil-in-water emulsions. The droplet collision phenomenon probably occurs via a surfactant bridge between the colliding droplets. When this process is taken into account we show that the Turnbull model for crystal nucleation kinetics explains very well nucleation in cocoa butter oil-in-water emulsions by seed crystals. On the basis of this model we characterized the seed crystals in cocoa butter via isothermal crystallization studies at 14.2, 15.0, 15.5, and 15.8 degrees C. We suggest that there are few seed crystals whose size exceeds 0.28 µm at 80 degrees C. In our cocoa butter samples there were between 10(16) and 10(17) seed crystals m(-3) whose average size we inferred to be less than 0.09 µm. A value of 0.133 mJ m(-2) is obtained for the Gibbs free energy of the nucleating surface in our West African cocoa butter samples. There is evidence that the alpha-polymorph of POS comprises the nucleating layer in the seed crystal. There is no evidence that surfactant influences the primary nucleation of oil crystals. However, the surfactant has a big effect on the kinetics of the secondary nucleation process, mediated by droplet collision. Copyright 2000 Academic Press.
通过一种新型膜技术表明，液滴碰撞在过冷的水包油乳液中晶核形成的传播过程中很重要。另一方面，在正十六烷和可可脂水包油乳液中，由吐温20胶束介导的成核作用相对来说重要性要小得多。液滴碰撞现象可能是通过碰撞液滴之间的表面活性剂桥发生的。当考虑到这个过程时，我们表明特恩布尔晶体成核动力学模型很好地解释了可可脂水包油乳液中晶种的成核情况。基于这个模型，我们通过在14.2、15.0、15.5和15.8摄氏度下的等温结晶研究对可可脂中的晶种进行了表征。我们认为在80摄氏度时，尺寸超过0.28微米的晶种很少。在我们的可可脂样品中，每立方米有10¹⁶到10¹⁷个晶种，我们推断其平均尺寸小于0.09微米。在我们的西非可可脂样品中，成核表面的吉布斯自由能值为0.133毫焦每平方米。有证据表明，POS的α-多晶型物构成了晶种中的成核层。没有证据表明表面活性剂会影响油晶体的初级成核。然而，表面活性剂对由液滴碰撞介导的二次成核过程的动力学有很大影响。版权所有2000年学术出版社。
