Kinetics of diglyceride formation and isomerization in virgin olive oils by employing 31P NMR spectroscopy.: Formulation of a quantitative measure to assess olive oil storage history

Diacylglycerol isomers and free acidity were determined for five extra virgin olive oils of different initial acidities by employing a facile P-31 NMR methodology as a function of storage time and storage conditions. The kinetic treatment of the hydrolysis of triacylglycerols (TGs) and the isomerization of 1,2-diacylglycerols (1,2-DGs) to 1,3-diacylglycerols (1,3-DGs) during storage of 18 months at ambient temperature in the dark and light and at 5 degreesC in the dark showed that the isomerization is strongly dependent on the rate of the TGs hydrolysis, the initial free acidity (Ho) of the virgin olive oil samples, and storage conditions. Although the time-evolution of the diacylglycerols (DGs) depends on the TGs hydrolysis, the ratio D of the concentration of 1,2-DGs to the total amount of DGs was found to be independent of this factor. From the kinetic expression of the ratio D, a quantitative measure was formulated that allows the estimation of the storage time or age of virgin olive oils. Application of this quantitative measure to several olive oil samples of known and unknown storage history resulted in a very good agreement with respect to the actual storage time for up to 10-12 months of storage. For a longer storage period, where the isomerization of DGs is close to its equilibrium state, the calculated age index is only indicative.