Postharvest ripening study of sweet lime (Citrus limettioides) in situ by volume-localized NMR spectroscopy

Spatially resolved NMR-especially volume-localized spectroscopy (VLS)-is useful in various fields including clinical diagnosis, process monitoring, etc. VLS carries high significance because of its ability to identify molecular species and hence track molecular events. This paper reports the application of VLS at 200 MHz to study the postharvest ripening of sweet lime (Citrus limettioides) in situ, including a comparative study of normal and acetylene-mediated ripening. Localization to a cubic voxel of 64μL was achieved with point-resolved spectroscopy (PRESS). Glucose, sucrose, fructose, and citric acid are found to be among the main constituents in the fruit. In the natural process, the sugar to acid ratio increases with ripening. Ethanol generation is seen to occur at a faster rate in acetylene-mediated ripening. Whereas NMR imaging experiments including parametric imaging (e.g., T ₁ or T₂ maps) may be employed for "macro" monitoring of processes such as these, this work demonstrates that the molecular imprint of the process may be tracked noninvasively by VLS.