Analysis of isomeric diacylglycerolic classes to evaluate the quality of olive oil in relation to storage conditions

Abstract  The % content and the % fatty acid (FA) composition of the three isomeric classes of diacylglycerol (DAG) fraction (sn-1,2-, sn-2,3-, sn-1,3-DAG) were valued in four groups of extra virgin olive oils, stored in different conditions of temperature and time, with the objective to study the influence of the cited parameters on olive oil DAG classes. The separation and % quantitation of all isomeric DAG classes were carried out by normal phase-high performance liquid chromatography (NP-HPLC) on total DAG fraction previously isolated by TLC and subjected to derivatization step with (S)-(+)-(1-naphtyl)-ethylisocyanate; moreover the % FA composition of each DAG class was determined by HRGC analysis. The results showed notable differences on the % content of each DAG class as well as the ratios between the classes. The samples analysed as soon as they were produced showed the highest % contents for sn-1,2-DAG and the lowest for sn-1,3- and sn-2,3-DAG. The samples stored at 30 °C generally showed the highest % contents of sn-1,3-DAG, because of the isomerization processes of sn-1,2-DAG to the more stable sn-1,3-DAG. The results of structural analysis of the sn-1,2-DAG class besides confirmed that isomerization processes occurred and that the storage temperature had a predominant role; this research emphasizes the importance of the valuation of % contents together with the structural analysis of the isomeric DAG classes as useful analytical parameters to evaluate the storage conditions and the conservation status of olive oils.