Effects of fermentation with Saccharomyces cerevisiae on pomegranate peel; a study of phenolic compounds and polysaccharides

The fruit of Punica granatum L. (Punicaceae) is widely used in popular medicine in many countries. Fruit juice and peel extracts of pomegranate have shown numerous health benefits, e.g. antimicrobial, antioxidant and antidiabetic effects [1]. Pomegranate peel is approximately 38-50% of fresh fruit weight and is the main by-product of juice production. Fermented foods play a significant role in the diets of many cultures and improve nutritional and nutraceutical properties, e.g. facilitating the absorption of nutritional and bioactive components [2]. Fermentation technology has been widely applied for the treatment of fruits and vegetables, but few studies on the fermentation of pomegranate peels are available so far [3]. Aim of the study was to evaluate the chemical changes determined by the Saccharomyces cerevisiae on the pomegranate peel from Wonderful and G1 varieties. Samples of dried peel were wet with water (1g/10 mL), and fermented as such in open and closed bottles under agitation in Thermo-Shaker. Some peel samples were boiled for 2 minutes before fermentation to reduce the activity of endogenous microorganisms. Commercial yeast (25 mg/g DW) was used to inoculate the samples, blank samples were yeast free. All samples were fermented at 25°C for 48 and 96 hours, then centrifuged twice to collect the supernatant which was successively treated and analyzed. In particular, the tannins were determined by HPLC-DAD and the polysaccharides by DLS and 1H-NMR. The yields in dry extract were close to 50% of the weight of the dried peels. The results showed a large amount of ethanol produced by yeast fermentation, while alcohol was absent in the blank samples. The tannin content was similar in all samples and varied between 442 mg/g and 534 mg/g of dry extract. Polysaccharides (precipitated after ethanol addition) ranged from 7.4% to 10% for yeast-fermented and blank samples, respectively. The analyzes by DLS allowed to evaluate the molecular dimensions of the polysaccharides which were similar for blank samples and yeast added peels, indicating a partial hydrolysis of the polysaccharides even without the use of yeast. The predominant polymer (90-99% of the total polysaccharide pool) showed a hydrodynamic volume close to 8.2 *105 kDa, with a molecular size ranging from 282-414 nm. It should be emphasized that after a 48-hour fermentation with "autochthonous" microorganisms and with the addition of yeasts, the size of the pectic polysaccharides was reduced of 50-70% compared to the native polysaccharides recovered after the decoction of the peel [4]. These preliminary results suggest that the aqueous extracts of fermented pomegranate peel can be suitable substrates to evaluate the prebiotic properties, already observed in vitro for the native polysaccharides of the fruit [5]. Pomegranate peel fermentation can be proposed as a natural and low-cost process to obtain new functional ingredients from this by-product potentially able to improve human microbiota health by providing both prebiotic compounds and a high amount of hydrolysable tannin.