Department of Human Health and Nutritional Sciences, University of Guelph, Guelph, ON, Canada N1G 2W1.
Research and Development Department, Gay Lea Foods Co-operative Ltd., Hamilton, ON, Canada L8H 3R7.
J Dairy Sci. 2021 Jun;104(6):6609-6619. doi: 10.3168/jds.2020-19355. Epub 2021 Mar 25.
Milk protein concentrates (MPC) are typically dried high-protein powders with functional and nutritional properties that can be tailored through modification of processing conditions, including temperature, pH, filtration, and drying. However, the effects of processing conditions on the structure-function properties of liquid MPC (fluid ultrafiltered milk), specifically, are understudied. In this report, the pH of liquid MPC [13% protein (70% protein DM basis), pH 6.7] was adjusted to 6.5 or 6.9, and samples at pH 6.5, 6.7, and 6.9 were subjected to heat treatment at either 85°C for 5 min or 125°C for 15 s. Sodium dodecyl sulfate PAGE was used to determine the distribution of caseins and denatured whey proteins in the soluble and micellar phases, and HPLC was used to quantify native whey proteins as a measure of denaturation, based on the processing conditions. Both heat treatments resulted in substantial whey protein denaturation at each pH, with β-lactoglobulin denatured more extensively than α-lactalbumin. Changes in liquid MPC physicochemical properties were monitored at d 1, 5, and 8 during storage at 4°C. Viscosity increased after heat treatment and also over time, regardless of pH and heating conditions, suggesting the role of whey protein denaturation and aggregation, and their interactions with casein micelles. The MPC samples processed at pH 6.9 had a significantly higher viscosity than those heated at pH 6.5 or 6.7, for both temperature and time conditions; and samples processed at 85°C for 5 min had higher viscosity than those heated at 125°C for 15 s. Particle size analysis indicated the presence of larger particles after 5 and 8 d of MPC storage after heating at pH 6.9. Acid-induced gelation of the liquid MPC led to significantly higher gel firmness after processing at 85°C for 5 min, compared with 125°C for 15 s. Also, gels made from MPC adjusted to pH 6.5 had higher storage moduli, with both time and temperature combinations, demonstrating the role of pH-dependent association of denatured whey proteins with casein micelles in gel network formation. These findings enable a better understanding of the processing factors contributing to structural and functional properties of liquid MPC and can be helpful in tailoring milk protein ingredient functionality for a variety of food products.
牛奶蛋白浓缩物（MPC）通常是高蛋白干燥粉末，具有功能性和营养性，可以通过改变加工条件（包括温度、pH 值、过滤和干燥）来调整。然而，加工条件对液体 MPC（超滤牛奶）结构-功能特性的影响还研究得不够充分。在本报告中，我们将液体 MPC[13%蛋白质（70%蛋白质 DM 基础），pH 值 6.7]的 pH 值调节至 6.5 或 6.9，并将 pH 值为 6.5、6.7 和 6.9 的样品分别在 85°C 下加热 5 分钟或在 125°C 下加热 15 秒。十二烷基硫酸钠 PAGE 用于确定可溶相和胶束相中酪蛋白和变性乳清蛋白的分布，HPLC 用于定量天然乳清蛋白作为变性的衡量标准，这是基于加工条件。在每种 pH 值下，两种热处理都会导致乳清蛋白大量变性，其中β-乳球蛋白的变性程度比α-乳白蛋白更严重。在 4°C 下储存的第 1、5 和 8 天，监测液体 MPC 物理化学性质的变化。无论 pH 值和加热条件如何，热处理后粘度都会增加，并且随着时间的推移也会增加，这表明乳清蛋白变性和聚集的作用及其与酪蛋白胶束的相互作用。在 pH 值为 6.9 下加工的 MPC 样品比在 pH 值为 6.5 或 6.7 下加热的样品具有更高的粘度，无论温度和时间条件如何；在 85°C 下加热 5 分钟的样品比在 125°C 下加热 15 秒的样品具有更高的粘度。颗粒大小分析表明，在 pH 值为 6.9 的 MPC 加热后储存 5 天和 8 天后，存在更大的颗粒。酸诱导的液体 MPC 凝胶化导致在 85°C 下加热 5 分钟后凝胶硬度显著增加，而在 125°C 下加热 15 秒后凝胶硬度显著增加。此外，用 MPC 制成的凝胶，其 pH 值调节至 6.5，具有更高的储能模量，无论时间和温度组合如何，这表明 pH 值依赖性的变性乳清蛋白与酪蛋白胶束的缔合在凝胶网络形成中起作用。这些发现有助于更好地理解导致液体 MPC 结构和功能特性的加工因素，并有助于为各种食品产品定制牛奶蛋白成分的功能。
