首页> 外文期刊>Journal of dairy research >pH-Dependent behaviour of soluble protein aggregates formed during heat-treatment of milk at pH 6.5 or 7.2
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pH-Dependent behaviour of soluble protein aggregates formed during heat-treatment of milk at pH 6.5 or 7.2

机译:pH值在6.5或7.2的牛奶热处理过程中形成的可溶性蛋白质聚集体的pH依赖行为

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The pH-dependent behaviour of soluble protein aggregates produced by the pre-heating of reconstituted skim milk at 90℃ for 10 min was studied, in order to understand the role of these aggregates in acid gelation of heated milk. The following milk samples were prepared: (1) control (unheated reconstituted milk, pH 6.5); (2) milk heat-treated at pH 6.5 (mHtd6.5) and (3) milk heat-treated at pH 7.2 (mHtd7.2). They were centrifuged and the supernatants (SPNT 1) pH-adjusted to yield a series of pH values ranging from 6.5 or 7.2 to 4.6 using HCl at 20℃ or GDL at 20 and 38℃. pH-Adjusted SPNTs 1 were re-centrifuged. The resulting supernatants (SPNTs 2) were analysed by OD (at 600 and 280 nm) and SDS-PAGE in order to characterise proteins still soluble as a function of pH. Particle size in SPNTs 1 was analysed by Steric Exclusion Chromatography. The OD600 nm revealed that during acidification soluble casein in both control and heat-treated samples exhibits variations in its optical properties or size as previously shown with micellar casein. In heat-treated samples, soluble casein and heat-induced covalent soluble aggregates precipitate at the same pH value. A progressive acidification of the soluble phase did not separate them. Increasing the temperature of acidification from 20 to 38℃ resulted in an increase in the precipitation pH of the proteins. However choice of acidifier did not have a significant effect on OD profiles. The soluble covalent aggregates from mHtd7.2 were smaller, more numerous, and had a higher content of κ-casein than mHtd6.5. Both types of aggregates began to precipitate at the same pH value but precipitation occurred over a narrower pH-range for soluble aggregates prepared from mHtd7.2. This may explain the higher gelation pH of mHtd7.2 compared with mHtd6.5.
机译:研究了将重组脱脂乳在90℃下预热10分钟产生的可溶性蛋白质聚集体的pH依赖性行为,以了解这些聚集体在加热牛奶的酸化凝胶中的作用。制备了以下牛奶样品:(1)对照品(未加热的再生牛奶,pH 6.5); (2)在pH 6.5(mHtd6.5)下热处理的牛奶和(3)在pH 7.2(mHtd7.2)下热处理的牛奶。将它们离心并使用20℃的HCl或20和38℃的GDL调节pH的上清液(SPNT 1)的一系列pH值,范围从6.5或7.2至4.6。将pH调节的SPNT 1重新离心。通过OD(在600和280 nm处)和SDS-PAGE分析所得的上清液(SPNTs 2),以表征仍可溶的蛋白质,其pH值随功能变化。通过立体排阻色谱法分析SPNT 1中的粒径。 OD600 nm表明,在酸化过程中,对照和热处理样品中的可溶性酪蛋白均表现出光学特性或大小的变化,如先前胶束酪蛋白所示。在热处理的样品中,可溶性酪蛋白和热诱导的共价可溶性聚集体在相同的pH值下沉淀。可溶性相的逐步酸化没有将它们分离。将酸化温度从20升高到38℃会导致蛋白质沉淀pH值的增加。但是,选择酸化剂对OD分布没有显着影响。与mHtd6.5相比,来自mHtd7.2的可溶性共价聚集体更小,数量更多,并且κ-酪蛋白含量更高。两种类型的聚集体开始在相同的pH值下沉淀,但是对于mHtd7.2制备的可溶性聚集体,沉淀发生在较窄的pH范围内。这可能解释了mHtd7.2的胶凝pH值高于mHtd6.5。

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