Abstract
Biopolymer casein was isolated from cow’s milk by acid coagulation, which was initiated by acetic acid and sodium acetate in the first, and hydrochloric acid in the second process. The casein isolated by acid coagulation, i.e. by first process, and commercial casein were separated on α-, β-, (α+κ)- and κ casein by urea fractionation. The aim of this study was to compare various properties of commercial casein fractions with casein fractions isolated from cow’s milk. The structure of casein and casein fraction samples were monitored by Fourier transform infrared spectroscopy (FTIR), and the obtained vibrational bands showed structural differences between isolated and commercial casein (presence of various amino acids), as well as their fractions. Differential scanning calorimetry (DSC) was used to determine glass transition temperature. The results showed that the glass transitions of the isolated and commercial casein were below room temperature (Tg = 2–30 °C) due to the destruction of the samples structure that provides molecules mobility and leads to a phase transition. Thermal degradation obtained by thermogravimetric analysis (TGA) of all samples occurred in multiple steps. From the results, it is evident that 5 mass % of the each sample degraded at significantly different temperatures (T95), and it can be concluded that isolated casein and its fractions showed better heat stability than commercial casein and its fraction.
Highlights
Casein has been manufactured during most of the 20th century because of the simplicity of its isolation and its usefulness in the manufacturing, pharmaceutical, cosmetic and food industries
It could be a glass transition temperature because a and b fractions do not have a wide interval of molecular weight distribution, i.e. values of molecular masses are very close to each other (Mm a casein = 22 068 to 723 g mol–1; Mm β casein = 939 to 24 089 g mol–1), and the Tg can be precisely determined[1]
Separation of casein samples onto fractions resulted in the destruction of its structure, which was confirmed by Fourier transform infrared spectroscopy (FTIR) analysis
Summary
Casein has been manufactured during most of the 20th century because of the simplicity of its isolation and its usefulness in the manufacturing, pharmaceutical, cosmetic and food industries. Milk contains several protein types, among which casein accounts for about 80 %, and it is organized as micelles[2,3] with diameter ranging from 50 to 500 nm. It includes four protein fractions a1-, a2-, b- and k casein (molar ratio ∼4:1:4:1, respectively) and they are different in their molecular weights[1,4,5,6]. The casein under study (99 % purity, moisture content
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