Food Science Department, University of Guelph, Guelph, ON, Canada.
Food Science Department, University of Guelph, Guelph, ON, Canada.
Food Res Int. 2020 Mar;129:108846. doi: 10.1016/j.foodres.2019.108846. Epub 2019 Dec 3.
Cows' milk is a commodity used worldwide to make many dairy products. We have used the ultra small angle X-ray scattering (USAXS) technique to reveal the fat globule and casein micelle structures of some dairy products. USAXS covers the q-range 5 × 10 Å < q < 10 Å, thereby allowing the study of micron-scale structures present in those dairy products. We measured the USAXS intensity, Iq, as a function of the scattering vector magnitude, q, for samples of skim milk, non-homogenized whole milk, homogenized whole milk, half and half and heavy cream, at two temperatures, 7 °C and 45 °C. The data collected from the scattering experiments were fitted using the Unified fit model run under the IRENA software from the Advanced Photon Source, Argonne National Laboratory (Illinois, USA). The fittings were carried out when the data were plotted as log[I(q)] vs log[q]. We observed a combination of linear regions (LRs) and knees. Two parameters of interest were obtained from the fittings, a radius of gyration, R, and a Porod exponent, P. Unified fit allowed us to fit up to four structural levels. One of the knees was centered at q ≈ 8 × 10 Å for all samples measured at 7 °C, but vanished at 45 °C. Two LRs were identified as being either due to casein micelles (CMs) or to fat globules (FGs). The porod exponent obtained from these LRs allowed us to describe the surface morphology of CMs and FGs. Two of the R values gave a rough estimate of the FGs and CMs sizes. FGs were identified for samples of homogenized whole milk, half and half and heavy cream in the q-region 2 × 10 < q < 8 × 10 Å. We found that, in the absence of chymosin, or changes in pH, CaCl concentration or temperature changes, skim milk and non-homogenized whole milk displayed a Porod exponent that indicated a behavior characteristic of aggregation. Using computer simulations, we found that, seemingly, bovine CMs spontaneously formed approximately 1-dimensional aggregates possibly analogous to swollen randomly branched polymers.
牛奶是一种在全球范围内被广泛用于生产各种乳制品的商品。我们利用超小角 X 射线散射（USAXS）技术来揭示一些乳制品中的脂肪球和酪蛋白胶束结构。USAXS 的 q 范围为 5×10 Å<q<10 Å，因此可以研究这些乳制品中存在的微米级结构。我们测量了脱脂乳、非均质全脂乳、均质全脂乳、半脱脂乳和高脂奶油在 7°C 和 45°C 下的 USAXS 强度 Iq 与散射矢量大小 q 的关系。从散射实验中收集的数据使用在 IRENA 软件下运行的统一拟合模型进行拟合，IRENA 软件来自美国阿贡国家实验室（伊利诺伊州）的先进光子源。数据以 log[I(q)]对 log[q]作图进行拟合。我们观察到线性区域 (LR) 和拐点的组合。从拟合中得到了两个感兴趣的参数，一个是转动半径，R，另一个是 Porod 指数，P。统一拟合允许我们拟合多达四个结构层次。在所有在 7°C 下测量的样品中，有一个拐点出现在 q ≈ 8×10 Å 处，但在 45°C 下消失了。两个 LR 被确定为要么是由于酪蛋白胶束 (CMs)，要么是由于脂肪球 (FGs)。从这些 LR 获得的 Porod 指数允许我们描述 CMs 和 FGs 的表面形态。两个 R 值给出了 FGs 和 CMs 尺寸的大致估计。在 q 区域 2×10<q<8×10 Å 中，我们发现均质全脂乳、半脱脂乳和高脂奶油的样品中存在 FGs。我们发现，在没有皱胃酶、pH 值变化、CaCl 浓度变化或温度变化的情况下，脱脂乳和非均质全脂乳显示出表明聚集行为特征的 Porod 指数。通过计算机模拟，我们发现，似乎牛 CMs 自发地形成了大约 1 维的聚集体，可能类似于溶胀的随机支化聚合物。
