Insights into the FTIR Spectral Fingerprint of Saffron (Crocus sativus L.) Stigmas After Gentle Drying Treatments

Abstract

Drying of the delicate, red stigmas of the Crocus sativus L. flower is necessary to produce saffron, the most expensive spice in the world. So far, laborious and sample destructive methods were applied to get vital insight into this process following key physicochemical changes. Vibrational spectroscopy tools that allow molecular fingerprinting of plant tissues via multivariate data analysis are still not exploited. This study aimed at gaining new insights into the Fourier-Transform Infrared (FTIR) spectra of saffron on different gentle drying treatments in vacuum or by short-time heating with varying sample loading, energy input, duration etc. Diagnostic spectral bands that were exposed using Principal Component Analysis were assigned to C=O stretching in vinyl or cyclic esters, amides or other inter-molecular interactions of importance for functionality. Above all, the peak at 1160 cm−1 (typical of C-O-C glycosidic bridges) proved a distinguishing feature of short-time heated vs vacuum-dried saffron. Other critical quality attributes of the dried stigmas (physical structure, color, chemical composition), assessed with Scanning Electron Microscopy (SEM), colorimetry, UV-Vis spectrometry and High-Performance Liquid Chromatography (HPLC), indicated both positive and negative effects per drying method. Our work highlights the novelty to combine non-destructive FTIR spectroscopy with conventional techniques for a more insightful evaluation of desired or undesired changes after saffron dehydration. Moreover, the spectral fingerprinting approach offers a cost-effective, eco-friendly solution for rapid, non-invasive control of the raw material that is of high interest for food and nutraceutical applications.