Hazelnut skin as an industrial coproduct: Novel source of polyphenols and dietary fibre for functional food applications and health benefits.

Global hazelnut production generates significant quantities of skin, an underutilized coproduct currently restricted to low-value applications despite its rich bioactives, dietary polyphenols, and dietary fibres. Valorising this residue aligns with circular economy principles by transforming agro-industrial waste into high-value resources. Hazelnut skin is identified as an exceptional source of dietary fibre (∼70%), vitamin E, and dietary polyphenols, exhibiting antioxidant activity significantly superior to hazelnut kernel. Its unique fibre composition, particularly specific pectin and xyloglucan structures, has been proposed, primarily based on in vitro and animal studies, to contribute to gut health. These proposed mechanisms include the stimulation of certain microbial genera (e.g., Prevotella and Lactobacillus) and increased short-chain fatty acid production; however, these structure-function relationships remain hypothetical and require validation in human studies. Furthermore, hazelnut skin serves as a novel source of functional ingredient in a range of food matrices, including bakery, dairy, beverage, and meat products, among others. Overall, the evidence suggests that hazelnut skin represents a promising and sustainable functional ingredient, the valorisation of which may support gut health-related benefits while contributing to the development of circular and resource-efficient food systems. This review critically examines the current knowledge on the chemical composition, functional properties, technological applications, and health-related benefits of hazelnut skin, with particular emphasis on modulation of colonic microbiota, providing a foundation for future research exploring its novel applications in functional food development.