Hazelnut shells represent a waste material (about 42% of the total biomass) deriving from hazelnut harvest. These are mainly used as a heating source; however, they represent an interesting source of polyphenols useful in health field. The impact on phenolic profile and concentrations of hazelnut shell extracts obtained by three extraction methods (maceration, ultrasonic bath, and high-power ultrasonic), as well as temperature, extraction time, and preventive maceration, was studied. The prepared extracts were characterized in terms of chemical composition, antioxidant and antimicrobial activities. Eighteen different phenolic compounds were identified and quantified by chemical analysis and gallic acid was the most abundant in all the extracts analyzed. Other relevant compounds were chlorogenic acid, protocatechuic acid and catechin. Preventive maceration had a positive effect on the extraction of different types of compounds regardless of the method performed. Application of the high-power ultrasonic method had different effects, either positive or negative, depending on the type of compound and extraction time. All the prepared extracts showed antioxidant activity especially those prepared by maceration, and many of them were able to inhibit the growth of both B. subtilis and B. cereus.

Corylus avellana L. is one of the most cultivated species in the world. Mainly utilized with the purpose of obtaining food material, hazel trees cannot guarantee constant kernels productions given the threats related to pathogens and to adverse conditions, especially in a globalisation and global changes scenarios. This matter led us to consider the opportunity of using hazel tree in all its parts and for several purposes, due to its multifunctional characteristics. As a pioneer species, it is a precious plant useful for forest restoration purposes and for forest successions/wildlife facilitation. Its roots enter into symbiosis with truffles making this species exploitable for hazelnuts and truffles production. The precious elements contained in what is considered "waste" deriving from hazel crops (i.e., leaves, skins, shells, husks and pruning material), could be reused and valorised in the perspective of a circular economy that is opposed to a linear one. In particular, a list of several phenolic compounds detected in hazelnut shells has been reported in literature to prevent and delay many human diseases due to their antioxidant properties and to free radical scavenging activities, with implications potentially useful even in the fight against COVID-19. All this makes hazel crop by-products interesting to be valorised as a chemical compound source for human health, even more than a biomass fuel or for bio-char applications. The multiple possible uses of the hazel tree would lead to alternative productions than the only nut productions, avoiding significant economic losses, would decrease the cost of disposal of crops residues and would increase the sustainability of agro-ecosystems by reducing, among other things, the production of wastes and of greenhouse gases deriving from the usual burning of residues which often happens directly in fields.