Department of Food Science, The Pennsylvania State University, University Park 16802.
Department of Food Science, The Pennsylvania State University, University Park 16802.
J Dairy Sci. 2022 Oct;105(10):7926-7939. doi: 10.3168/jds.2021-21299. Epub 2022 Aug 12.
The present work aimed to improve acid and rennet milk gelation properties with mild thermal and pH changes to skim milk, with emphasis on heating temperatures below the denaturation temperature of whey proteins. We hypothesized the heat-induced, pH-dependent micellar changes, namely the shifts in casein and calcium equilibria between the micellar (or colloidal) and serum phases, result in firmer acid and rennet milk gels and reduced gelation time. Homogenized, pasteurized skim milk was adjusted to pH values in the range of 6.4 to 7.3, heated at temperatures in the range of 50 to 80°C, cooled to refrigeration temperature, and restored to native pH (pH 6.7). Then, acid and rennet gels were made by the addition of glucono-δ-lactone and chymosin, respectively. We monitored the storage modulus (G', Pa) during gel formation with small-amplitude oscillatory shear and the gelation time and maximum G' (G'max, Pa) of acid and rennet gels, were measured at 3 and 2 h, respectively. When skim milk was heated at 50°C for 15 min, there was a 58 and 163% increase in the G'max of acid and rennet gels, respectively, as the pH at heating was raised from pH 6.7 to 7.3. Increases in gel strength were greater for skim milk heated at 60°C for 15 min. There was a positive correlation between G'max of acid gels and the heat-induced casein protein exchanges between the micellar and serum phases on heating milk at pH in the range from 6.4 to 7.3 (r = 0.78). We also found positive correlations between the variation in G'max of rennet gels with the heat-induced, pH-dependent migration of casein (r = 0.83) and calcium (r = 0.80) from the micelle into the serum phase, as determined by PAGE and atomic emission spectroscopy. Under these mild heating temperatures (50 and 60°C), rennet coagulation time was significantly reduced from 45 ± 5 to 27 ± 3 min when the pH at heating was raised from pH 6.7 to 7.3. The ability to enhance milk gelation properties with a scalable pretreatment allows for the expression of novel functionality of casein.
本研究旨在通过温和的热和 pH 变化来改善脱脂乳的酸凝和凝乳酶凝乳特性，重点关注低于乳清蛋白变性温度的加热温度。我们假设，热诱导的、依赖于 pH 的胶束变化，即酪蛋白和钙在胶束（或胶体）和血清相之间的平衡转移，导致更坚固的酸凝和凝乳酶凝乳，并减少凝胶化时间。将均质、巴氏杀菌的脱脂乳调整至 pH6.4 至 7.3 的范围内，在 50 至 80°C 的温度下加热，冷却至冷藏温度，并恢复至天然 pH（pH6.7）。然后，通过添加葡萄糖酸-δ-内酯和凝乳酶分别制备酸凝和凝乳酶凝乳。我们通过小振幅振荡剪切监测凝胶形成过程中的储能模量（G'，Pa），并分别在 3 和 2 小时测量酸凝和凝乳酶凝乳的凝胶化时间和最大储能模量（G'max，Pa）。当脱脂乳在 50°C 下加热 15 分钟时，随着加热时 pH 值从 pH6.7 升高到 7.3，酸凝和凝乳酶凝乳的 G'max 分别增加了 58%和 163%。当脱脂乳在 60°C 下加热 15 分钟时，凝胶强度的增加更大。在 pH6.4 至 7.3 的范围内，加热乳时，酸凝胶的 G'max 与胶束和血清相之间的热诱导酪蛋白蛋白交换呈正相关（r = 0.78）。我们还发现，凝乳酶凝乳的 G'max 变化与胶束中的酪蛋白（r = 0.83）和钙（r = 0.80）在 pH 依赖性迁移之间存在正相关关系加热乳至 pH7.3 时，通过 PAGE 和原子发射光谱法从胶束进入血清相。在这些温和的加热温度（50°C 和 60°C）下，当加热时的 pH 值从 pH6.7 升高到 7.3 时，凝乳酶凝固时间从 45 ± 5 分钟显著缩短至 27 ± 3 分钟。通过可扩展的预处理来增强乳凝胶特性的能力允许表达新型酪蛋白功能。
