Crack the Shell by Unlocking the Polyphenol Power of Hazelnut Waste with Ultrasound

Hazelnut (Corylus avellana L.) shells, typically discarded as agro-industrial by-products, represent a potentially valuable source of bioactive polyphenolic compounds with significant antioxidant properties. This study aimed to evaluate and compare the polyphenol composition and antioxidant capacity of the kernels and shells of two hazelnut varieties, ‘Rimski’ and ‘Istarski duguljasti’. High-intensity ultrasound-assisted extraction (UAE) was applied to enhance the recovery of bioactive compounds under optimized conditions (80% ethanol, high amplitude, and 25 min treatment). The extracts were analyzed for total polyphenols, total flavonoids, total non-flavonoids, and individual phenolic compounds. Hazelnut shells exhibited significantly higher levels of total polyphenols, flavonoids, and antioxidant capacity compared to kernels. The dominant individual polyphenolic compounds identified in the shell were kaempferol, gallic acid, naringin, rutin trihydrate, quercetin-3-glucoside, chlorogenic acid, quercetin, ferulic acid, rosmarinic acid, and vanillic acid. Application of UAE notably improved extraction efficiency and overall yield compared to conventional extraction methods. The findings underscore hazelnut shells as a nutritionally and functionally valuable by-product and confirm UAE as a green, efficient extraction technique. These results provide a strong basis for developing high-value-added products for the cosmetic, pharmaceutical, and food industries, thereby supporting circular bioeconomy and sustainable chemistry principles.