Pomegranate peel fermented by Saccharomyces cerevisiae: a study on changes in tannin content and polysaccharide structure

The pomegranate fruit is highly appreciated for its nutritional properties and numerous uses in the food field. After processing pomegranate fruits to produce juice, significant quantities of waste are generated and the peel is the main byproduct (approximately 50% of the total weight of the fruit). Peel contains several micronutrients, hydrolysable tannins and appreciable amount of polysaccharides, mainly pectin [1]. As regards the fermentation of the pomegranate fruit, the most widely described processes are those involving the juice through the use of lactic acid bacteria, namely Saccharomyces c. to obtain a sort of wine [2]. Among the procedures proposed in the literature, fermentation processes using only the peel of the pomegranate fruit have only been poorly explored and aimed at ethanol production [3]. Since there is a demand for simple and sustainable methods that can favor the obtaining of valuable products from pomegranate peel, the aim of the study was to evaluate the changes induced by fermentation with a common yeast such as Saccharomices cerevisiae on the tannins and polysaccharide of the peel (mesocarp and endocarp). Two varieties (Wonderful and G1) were fermented applying 3 processes: i) fermentation in closed tubes (1N method); ii) fermentation in closed tubes after a blanching of few minutes of the peel at 100°C (2PbN method); iii) fermentation in open-air, with tubes closed only with a cotton cap (3Na method). The experiments were done in triplicate with and without the presence of yeast, to have for each method of fermentation a blank sample (B) characterized by the absence of the yeast (L). All the fermentations were carried out at 25°C adding water to reach a ratio of 1/10 w/v and the same amount of yeast (15 mg/g of dried peel); the samples were collected from 0 to 96 hrs of fermentation. The polysaccharides were recovered after decoction of the whole fermented samples and successive precipitation with ethanol. HPLC-DAD profiles of tannins in blank samples and yeast samples were very similar or overlapping, within each fermentation method. From a quantitative point of view, the % yields in dry aqueous extracts from fermented samples, calculated on the dried skins, were decidedly higher for the white samples compared to the fermented ones. Interestingly, the concentration of total tannins in the dry extracts obtained from fermentation with Saccharomices c. after 48 and 96 hrs was found to be approximately 50% higher than in the corresponding blank samples. As confirmed by DLS measurements and NMR-DOSY experiments, fermentation reduced the molecular weight of native peel pectin. Since polysaccharides from unfermented peels have previously shown prebiotic activity in vitro [4], an increase in their fermentability is expected as a consequence of the reduced molecular weight obtained after the fermentation processes.