Characterisation of triacylglycerols from bovine milk fat fractions with MALDI-TOF-MS fragmentation

In this study, we used MALDI-TOF-MS to profile and characterise the triacylglycerol (TAG) species of anhydrous bovine milk fat (AMF) and its low melting (olein) and high melting (stearin) fractions obtained by dry fractionation. Silver-ion solid phase extraction (Ag+-SPE) cartridges were utilised to separate the TAGs according to saturation degree. Then, several TAG species were selected and fragmented via post-source decay (PSD) fragmentation. MALDI-TOF-MS TAG profiles and fragmentation patterns were compared to the TAG and fatty acid (FA) compositions obtained by gas chromatography-flame ionization detector (GC-FID). We found that, olein was rich in medium length chain TAG species like CN38:0 and CN40:1, whereas stearin was rich in saturated long chain TAG species from CN42:0 to CN52:0. Separation of the TAGs based on saturation degree allowed successful selection of the TAG parent-ion for fragmentation by eliminating the interferences of TAG species that have the same carbon number but vary in number of double bonds. The TAG fragmentation patterns indicated significant differences between AMF, olein and stearin as a result of the dry fractionation process. Compared to AMF, olein yielded in higher fragments of short-chain saturated and middle-chain unsaturated FAs. Whereas, stearin yielded in saturated and monounsaturated long chain FA fragments. Fragmentation of unsaturated long chain TAGs showed that the TAGs in olein contained more C18:1 and C18:2 than that of AMF and stearin. Stearin on the other hand, contained higher amount of TAG species containing C16:0. These results were in line with the FA compositions obtained from GC-FID and suggest that Ag+-SPE cartridges coupled with MALDI-TOF-MS offer an informative and practical approach to characterise fats and oils with complex TAG composition.