Preparation of a Punica granatum L. peel extract rich in punicalagin and evaluation of its antifungal activity

Pomegranate (Punica granatum L.) is one of the richest fruits in polyphenols, compounds with valuable antioxidant and antimicrobial properties. In recent decades, there has been an ever-increasing demand for its fruits, primarily for juice production, accompanied by a growing generation of agri-food waste, estimated at 9 tons for each ton of pomegranate juice produced.

In this study, the peels of the pomegranates discarded for colour and shape, inappropriate for sale, granted by an Italian farm, were extracted with 95% ethanol. Subsequently, the enrichment of the phenolic compounds in the extract was achieved through solid phase extraction (SPE) by sequential elution with water, water/acetonitrile 1:1 v/v and acetonitrile as mobile phase. The phenol-rich extract was subjected to HPLC-PDA, revealing the presence of high amount of punicalagin (36.1 ± 3.2 %), along with ellagic acid and ellagic acid derivatives.

To evaluate the possible application of the punicalagin-rich extract within formulations for environmental or human/veterinary use, it was tested on six phytopathogenic fungi (from the culture collection of the Fungal Biodiversity Laboratory of Sapienza), reported also as pathogenic for humans or animals, assessing fungistatic and fungicidal activity through the microdilution broth method. MIC and MFC values for the extract and fluconazole, as drug reference, were determined. For four species (FBL number: 4, 57, 59, 659) MIC 90 was observed to be in the range of 500-1000 μg/mL, while the respective MFC was 2000 μg/mL; however, only three (FBL 4, 57, 659) showed MIC 100 at 1000 or 2000 μg/mL. Interestingly, FBL 4 was more sensitive to pomegranate extract than to fluconazole (MIC90 > 64 μg/mL).

In conclusion, in this study, the preparation of a pomegranate peel extract rich in compounds with valuable biological activities paves the way for an innovative valorisation of pomegranate waste products, representing an efficient example of circular economy.