Multiscale Characterization of Casein Micelles Under NaCl Range Conditions

Abstract

Micellar casein (MC) dispersions were studied at a constant protein concentration of 5 wt % in high NaCl environment. The micellar edifices were characterized as to their morphology, size, and content of proteins in the supernatant after ultracentrifugation. Additionally, changes in secondary structures of the protein upon salt increase were followed by Fourier Transform Infrared Spectroscopy (FTIR). For the first time, the estimations of secondary structural elements (irregular, ß-sheet, α-helix and turn) from Amide III assignments were correlated with results from Amide I. Casein micelles dispersions in water were characterized by Transmission Electron Microscopy (TEM) by a spherical shape and a size between 100 and 200 nm. A salt increase resulted to a destabilization of the micelle and the formation of mini-micelles more or less aggregated. The size of the new edifice was almost similar to the native micelle. These TEM observations were confirmed by a constant casein micelle hydrodynamic diameter determined by Dynamic Light Scattering (DLS) and ranging between 150 and 180 nm. Upon salt increase, FTIR revealed an increase in irregular structures and a concurrent decrease in ß-sheet structures. Secondary structural elements percentages were almost similar from Amide I and Amide III. The use of these multiscale techniques led to a better understanding of the micellar edifice under high salt environment. Around 3% NaCl addition, a good correlation was observed between destabilization of the micellar edifice, modifications of the caseins secondary structure and repartition of caseins between supernatant and pellet after ultracentrifugation.

Influence of succinylation on the conformation of yak casein micelles

“…The spectroscopy of FTIR were obtained with a VERTEX 70V Bruker spectrometer (Bruker Corporation, Karlsruhe, Germany) equipped with an ATR cell (Total Attenuated Reflection mode) and a MCT (Mercury Cadmium Telluride) detector cooled with liquid N 2 as described by Hussain, Gaiani, Aberkane, Ghanbaja, and Scher (2011).…”

“…Peak assignment of Amide I region was performed using the results of former studies for caseins (Hussain et al, 2011). During the iteration process the peaks positions were allowed to shift which was less than 2 cm À1 .…”

“…Bovine caseins have never been crystallized; therefore, there was little information on their structures (Hussain et al, 2011). Nevertheless, tentative assignments of secondary structural components in bovine caseins have been performed (Curley, Kumosinski, Unruh, & Farrell, 1998).…”

“…The results revealed that there were 36.42 ± 1.84% ß-sheet, 35.33 ± 2.04% turn, 18.37 ± 2.53% irregular and 9.88 ± 1.95% a-helix. Hussain et al (2011) reported that 5% cow casein micelles in water (w/w) contain 12% a-helix, 45% ß-sheet, 26% turn, and 16% irregular structure for FTIR amide I. The results were probably attributed to differences in the amino acid sequence from different bovine breeds (Bai et al, 2010).…”

Hot brining of pasta filata cheese: effect of sodium and calcium chloride on composition, yield, and hardness

“…Nevertheless, it has been reported that adding sodium chloride in this step increases the yield (Locci et al 2012). Sodium chloride is reported to change the conformation of casein proteins (Hussain et al 2011) and modify the solubility of the different casein fractions (Post et al 2012). Since cheese is a soft gel of concentrated casein micelles with embedded fat globules and a continuous serum phase, the swelling of this concentrated protein network increases in the presence of sodium chloride (de Kruif et al 2015;Guo et al 1997).…”