Determining the oil and water content of single olives using magnetic resonance imaging (MRI) spectroscopy

Abstract

The nuclear magnetic resonance (NMR) spectra of olives are dominated by oil and water. Chemical shift imaging (CSI), a specialized version of magnetic resonance imaging (MRI), was examined as a noninvasive technique to determine both oil and water content. To make the method quantitative, internal oil and water standards were used. CSI was used to spatially separate the oil and water signals from the olive from known masses of oil and water. Single intact Olea europaea 'Mission' olives were examined to test the method. In principle, the ratio of the integrals of the oil and water signals at the olive position to the integrals at the phantom (oil and water controls) positions in CSI images will give the oil and water contents of the olive. We found that both oil and water in an olive have short spin lattice relaxation times (T2 of 28 and 13 ms, respectively). We also found that the spin echo version of CSI was required, since the free induction decay method lost significant signal due to sample heterogeneity. Moreover, short T2 requires a fit of a series of CSI images to determine oil and water content. Intensities versus echo times were fitted to an exponential, and extrapolation to zero echo time gave the absolute masses of water and oil in the olive. The same olives were subsequently analyzed by classical Soxhlet extraction. The Soxhlet values correlated poorly with the NMR values, and this may have been due to the difficulty of accurately extracting single olives. For the NMR data, shimming, phasing and choice of CSI integration limits as possible sources of error were also considered.