Résumé
Calyces of Hibiscus sabdariffa are traditionally used to prepare a red beverage rich in anthocyanins (delphinidinand cyanidin-3-O-sambubioside) by decoction in water. These molecules are subjected to degradation during storage leading to a undesirable color intensity decay and change from red to brown in few months. The chemical origins of this quality loss have been already identified as scission and mainly condensation reactions leading to uncolored or brown polymers of anthocyanins (Sinela et al., 2016). Scission and condensed products are known to be instable and/or of various molecular weight. Their concentration is therefore very difficult to get unless numerous time- and money-consuming analysis. An alternative of getting mecanistic and kinetic insigth into anthocyanin degradation during processing or storage is the UV-VIS spectra analysis with chemometric tools. Indeed, UV-VIS spectra are easy to get, but may be complex to interpret as they result from the contribution of all molecules that absorb in this spectral region, which is the case of most of the polyphenols. The objective of this study is to use spectral deconvolution by mean of Multivariate Curve Resolution- Alternating Least Squares (MCR-ALS) on spectral evolution of Hibiscus sabdariffa extract obtained by soaking calyx powder in water at a ratio 1/9 (g/g) during one month of storage at 37°C. In this work, we propose a twosteps approach: a first deconvolution on two fractions of the extract, low molecular and high molecular weigth fractions obtained by size-exclusion chromatography and a MCR done on the spectrum of the whole extract. Absorbance from 250 to 700 nm (each 0.5 nm) was recorded on 10 samples at different storage times. MCRALS was carried out on the 10 spectra using a toolbox developped by Jaumot et al. (2005) on Matlab (The MathWorks, Inc., Natick, MA, USA) with a non negativity constraint algorithm. To improve the mathematical resolution and the chemical validity, the data set was augmented with pure spectra obtained with standards in solution. The results showed that the deconvolution made on the fractions obtained with size-exclusion chromatography was useful to extract qualitative information in the form of spectra of the low and high molecular weigth compounds. Three spectra were clearly identified as anthocyanin (with a spectral contribution equivalent to their relative concentration), chlorogenic acid, scission products (gallic and protocatechuic acids) and two spectra were attributed to condensed molecules (because of their absorbance in the brown region) and other colorless anthocyanin forms in equilibium (because of their peak in the UV region). The five spectra were used to improve the MCR-ALS applied to the whole extract. For this second step, the relative concentration of each specy was obtained. The anthocyanin evolution could be modeled by a first-order kinetic with a rate value consistent with litterature. Chlorogenic, gallic and protocatechiuc acids were found stable as expected. A significant evolution was evidenced for condensed molecules whose relative concentration increased by a 1.6-fold. To conclude, MCR results on Hibiscus sabdariffa extracts, fractionnated and not, were interesting since they could successfully represent the evolution trend of the different species in solution during storage: the anthocyanin but also their degradation products. (Résumé d'auteur)