Using membrane filtration it is possible to selectively concentrate proteins and, in the case of microfiltration, concentrate casein micelles. During filtration, water is often added and this practice, called diafiltration, causes further release of permeable components and maintains filtration efficiency. Filtration causes changes in composition of the protein as well as the soluble phase, including soluble calcium, which is a critical factor controlling the gelation properties of the casein micelles in milk. It was hypothesized that concentrates obtained using membrane filtration with or without diafiltration would have different gelation behavior. To test this hypothesis, two concentrates of similar casein micelle volume fraction were prepared, using spiral wound polymeric microfiltration membranes with a 800 kDa molecular weight cutoff, with or without diafiltration. The concentrates showed a gelation behavior comparable to that of skim milk, with a similar gelation time and with a higher firmness, due to the higher number of protein linkages in the network. In contrast, the hydrolysis of κ-casein by chymosin and casein aggregation were inhibited in diafiltered casein micelle suspensions. When the concentrates were recombined with the original skim milk to a final concentration of 5% protein, which re-established a similar soluble phase composition, differences in gelation behavior were no longer observed: both treatments showed similar gelation time and gel firmness. These results confirmed that membrane filtration can result in concentrates with different functionality, and that ionic environmental conditions are critical to the aggregation behavior of casein micelles. This is of particular significance in industrial settings where these fractions are used as a way to standardize proteins in cheese making.

中文翻译：

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渗滤影响通过过滤获得的浓缩酪蛋白胶束悬浮液的凝胶化特性

使用膜过滤可以选择性地浓缩蛋白质，在微滤的情况下，可以浓缩酪蛋白胶束。在过滤过程中，通常会添加水，这种称为渗滤的做法会导致可渗透成分的进一步释放并保持过滤效率。过滤会导致蛋白质以及可溶相（包括可溶性钙）的组成发生变化，这是控制牛奶中酪蛋白胶束凝胶特性的关键因素。据推测，使用有或没有渗滤的膜过滤获得的浓缩物将具有不同的凝胶行为。为了验证这一假设，使用截留分子量为 800 kDa 的螺旋缠绕聚合物微滤膜制备了两种类似酪蛋白胶束体积分数的浓缩物，有或没有渗滤。由于网络中蛋白质键的数量较多，浓缩物显示出与脱脂奶相当的凝胶化行为，具有相似的凝胶化时间和更高的硬度。相反，凝乳酶对κ-酪蛋白的水解和酪蛋白聚集在渗滤的酪蛋白胶束悬浮液中受到抑制。当浓缩物与原始脱脂奶重新混合至最终浓度为 5% 的蛋白质时，重新建立了相似的可溶相组成，不再观察到凝胶行为的差异：两种处理都显示出相似的凝胶时间和凝胶硬度。这些结果证实膜过滤可以产生具有不同功能的浓缩物，并且离子环境条件对酪蛋白胶束的聚集行为至关重要。