Analysis of bovine milk caseins on organic monolithic columns: An integrated capillary liquid chromatography–high resolution mass spectrometry approach for the study of time-dependent casein degradation

Abstract

Casein proteins constitute approximately 80% of the proteins present in bovine milk and account for many of its nutritional and technological properties. The analysis of the casein fraction in commercially available pasteurized milk and the study of its time-dependent degradation is of considerable interest in the agro-food industry. Here we present new analytical methods for the study of caseins in fresh and expired bovine milk, based on the use of lab-made capillary organic monolithic columns. An integrated capillary high performance liquid chromatography and high-resolution mass spectrometry (Cap-LC–HRMS) approach was developed, exploiting the excellent resolution, permeability and biocompatibility of organic monoliths, which is easily adaptable to the analysis of intact proteins. The resolution obtained on the lab-made Protein-Cap-RP-Lauryl-γ-Monolithic column (270mm×0.250mm length×internal diameter, L×I.D.) in the analysis of commercial standard caseins (αS-CN, β-CN and κ-CN) through Cap-HPLC–UV was compared to the one observe using two packed capillary C4 columns, the ACE C4 (3μm, 150mm×0.300mm, L×I.D.) and the Jupiter C4 column (5μm, 150mm×0.300mm, L×I.D.). Thanks to the higher resolution observed, the monolithic capillary column was chosen for the successive degradation studies of casein fractions extracted from bovine milk 1–4 weeks after expiry date. The comparison of the UV chromatographic profiles of skim, semi-skim and whole milk showed a major stability of whole milk towards time-dependent degradation of caseins, which was further sustained by high-resolution analysis on a 50-cm long monolithic column using a 120-min time gradient. Contemporarily, the exact monoisotopic and average molecular masses of intact αS-CN and β-CN protein standards were obtained through high resolution mass spectrometry and used for casein identification in Cap-LC–HRMS analysis. Finally, the proteolytic degradation of β-CN in skim milk and the contemporary formation of low-molecular-weight proteose–peptones (PP) with exact monoisotopic Mr between 9444.0989Da and 14098.9861Da was confirmed through the deconvolution of high resolution mass spectra and literature data.