Nayebzadeh Kooshan, Chen Jianshe, Dickinson Eric, Moschakis Thomas
Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, UK.
Langmuir. 2006 Oct 10;22(21):8873-80. doi: 10.1021/la060419o.
This work investigates surface properties of a protein particle gel and effects of polysaccharide on the surface microstructure of such a protein gel. Whey protein isolate (WPI) was used as the primary gelling agent, and a polysaccharide (xanthan) was investigated for its surface smoothing effects. The surface properties of heat-set WPI gels with and without the presence of xanthan (0, 0.05, and 0.25%) were characterized using a surface friction technique. The surface friction force of a gel against a stainless steel substrate was found to be highly dependent on the sliding speed for all three gel samples, and the addition of xanthan caused a general reduction of surface friction. The gel containing no xanthan has the largest surface friction and behaved in the most load-dependent manner, whereas the gel containing 0.25% xanthan has the lowest surface friction and showed the least load dependency. It was inferred that the WPI gel containing no xanthan has the roughest surface among the three samples and the presence of xanthan leads to a smoother surface with probably a thinner layer of surface water. Surface features derived from surface friction tests were confirmed by surface microstructure observation from confocal laser scanning microscopy (CLSM) and environmental electron scanning microscopy (ESEM). Surface profiles from CLSM images were used to quantify the surface roughness of these gels. The mean square root surface roughness R(q) was calculated to be 3.8 +/- 0.2, 3.0 +/- 0.2, and 1.5 +/- 0.2 microm for gels containing 0, 0.05, and 0.25% xanthan, respectively. The dual excitation images of protein and xanthan from CLSM observation and images from ESEM observation indicate a xanthan-rich layer at the surfaces of the xanthan-containing gel samples. We speculate that the creation of the outer surface of a particle gel is based on a different particle aggregation mechanism from that leading to network formation in the bulk.
本研究考察了蛋白质颗粒凝胶的表面性质以及多糖对该蛋白质凝胶表面微观结构的影响。以乳清分离蛋白（WPI）作为主要胶凝剂，并研究了一种多糖（黄原胶）的表面平滑作用。采用表面摩擦技术对添加和未添加黄原胶（0%、0.05%和0.25%）的热致凝胶WPI的表面性质进行了表征。发现所有三种凝胶样品在不锈钢基底上的表面摩擦力都高度依赖于滑动速度，添加黄原胶会使表面摩擦力普遍降低。不含黄原胶的凝胶表面摩擦力最大，且对负载的依赖性最强，而含有0.25%黄原胶的凝胶表面摩擦力最低，对负载的依赖性最小。据推断，在这三个样品中，不含黄原胶的WPI凝胶表面最粗糙，而黄原胶的存在使表面更光滑，且表面水层可能更薄。通过共聚焦激光扫描显微镜（CLSM）和环境电子扫描显微镜（ESEM）对表面微观结构的观察，证实了由表面摩擦试验得到的表面特征。利用CLSM图像的表面轮廓来量化这些凝胶的表面粗糙度。含0%、0.05%和0.25%黄原胶的凝胶的均方根表面粗糙度R(q)分别计算为3.8±0.2、3.0±0.2和1.5±0.2μm。CLSM观察得到的蛋白质和黄原胶的双激发图像以及ESEM观察得到的图像表明，含黄原胶的凝胶样品表面存在富含黄原胶的层。我们推测，颗粒凝胶外表面的形成基于一种与导致本体网络形成不同的颗粒聚集机制。
