Raman a banana: mapping banana peel composition and ripening process by Raman spectroscopy

Banana peel was analyzed by Raman microspectroscopy and its chemical components were identified by comparison of the acquired spectra with reference substances, including plant pigments, pectin, and flavonoids. Compounds of pharmaceutical and food-industry relevance - chrysin, lutein, and pectin - were detected. Raman mapping revealed microscopic inclusions of calcium oxalate in the epidermis, the size of which increased during the ripening process. The progress of ripening was associated with changing relative intensities of the Raman signals of chlorophyll, carotenoids, and further banana peel constituents: over the whole ripening period the intensity of the chlorophyll band at 1328 cm^-1 decreased 2.7-fold while that of the carotenoid band at 1527 cm^-1 increased 1.7-fold. A normalized difference index derived from the two bands increased monotonically with storage time and provides a self-referencing measure of ripening independent of the absolute instrument response. Standardized colorimetric analysis of the same fruits confirmed the ripeness classification by independent instrumental means. The results shed light on important aspects of the banana ripening process and demonstrate the usefulness of Raman spectroscopy for qualitative and semi-quantitative, non-contact and non-destructive analysis of banana peel composition. They further provide a reference for Raman spectroscopic evaluation of growth parameters as an extension of hyperspectral or multimodal imaging, and support the consideration of banana peel as a renewable, eco-friendly source of valuable compounds.