Structural elucidation of pectin extracted from cocoa pod husk (Theobroma Cacao L.): Evaluation of the degree of esterification using FT-IR and 1H NMR

Nowadays, the increasing demand for cocoa derivatives is generating a serious disposal problem in close proximity to cultivated lands and farms owing to the huge quantity of cocoa pod husk (CPH) waste. Based on this context, this paper deals with the utilization of CPH as raw material for pectin extraction using citric acid and its structural characterization by NMR and FT-IR. Regarding the FT-IR technique, pectin from CPH was compared with commercial pectin by the ATR-IR method, while the second-derivative FT-IR method was conducted to explore masked bands in the normal CPH pectin spectrum. In addition, a deconvolution process was carried out for estimating the degree of esterification through the bands at 1725 cm(-1) and 1610 cm(-1). On the other hand, structural elucidation of CPH pectin was performed by 1D and 2D NMR to validate the presence of rhamnose groups attached to galacturonic acid groups suggesting a RG-I structure. Besides, the degree of esterification was evaluated using H-1 NMR and compared with the FT-IR method, which showed similar values around 35-37% and depicted a low degree of esterification in CPH pectin. Additionally, the antioxidant activity of CPH pectin was explored reaching a value of 11.79 +/- 1.10%, a bit higher compared with the average value for polygalacturonic acid (10.31 +/- 3.29) and other pectin-based materials. Finally, an amorphous structure was observed by XRD, and also, the presence of cadmium on pectin surface was discarded by EDX analysis, whereas the morphology was explored by FESEM observing a sheet-like structure.