Impact of altitudinal variation on nutrient dynamics, and bioactive composition of hazelnut (Corylus avellana L.) kernel

This study aims to investigate the effects of altitudinal variation on the biochemical composition, antioxidant capacity, fatty acid profile, and volatile compounds of three Turkish hazelnut (Corylus avellana L.) cultivars—Tombul, Palaz, and Çakıldak. Kernels were harvested from three altitudes (300 m, 400 m, 590 m) in Ordu Province and analyzed for total phenolic content, antioxidant activity (DPPH and FRAP), protein, moisture, ash, sugars (sucrose, glucose, fructose), organic acids (oxalic, citric, succinic, tartaric), total lipid content, saturated and unsaturated fatty acids, and volatile aroma compounds. Since altitude is known to influence plant metabolism, understanding its impact on hazelnut quality is essential for breeding, production planning, and functional product development. Comprehensive analyses revealed significant genotype × altitude interactions. Total phenolic content ranged from 43.93 to 231.03 mg GAE/100 g, with the highest value recorded in Çakıldak at 590 m. DPPH scavenging activity peaked in Çakıldak (84.50%), while FRAP values were highest in Palaz and Tombul at 400 m. Protein content varied between 17.60% and 22.87%, and moisture and ash levels were influenced by both cultivar and elevation. Sucrose and glucose contents increased with altitude, reaching 5.73 g/100 g and 0.83 g/100 g, respectively, in Çakıldak at 590 m. Organic acids also showed altitude dependence, with oxalic (33.47 mg/100 g), citric (430.85 mg/100 g), and succinic acid (402.26 mg/100 g) being most abundant in Çakıldak at 590 m. Fatty acid profiling revealed oleic acid (69.04–81.15%) as the dominant MUFA and linoleic acid (6.45–17.55%) as the primary PUFA. PUFA content increased with altitude, especially in Çakıldak, while Palaz maintained a stable MUFA profile across altitudes. Volatile profiling showed cultivar-specific aroma signatures. However, altitude subtly modifies VOC intensities without changing cultivar identity. Tombul was aldehyde-rich with fresh green notes (e.g., hexanal), Palaz was abundant in alcohols and esters contributing fruity-sweet aromas (e.g., linalool, menthyl acetate), while Çakıldak was enriched in ketones and terpenes with roasted and resinous notes (e.g., limonene, pulegone). Multivariate PCA and heatmap clustering confirmed distinct grouping patterns based on altitude and cultivar. The study shows that altitude may play a decisive role in shaping the nutritional, functional, and aromatic properties of hazelnuts. Çakıldak demonstrated superior antioxidant potential, PUFA enrichment, and sugar/organic acid accumulation at high altitude, highlighting its value for health-oriented applications. Palaz emerged as a stable oil-quality source with consistent oleic acid content, while Tombul’s aldehyde-rich profile offers distinct flavor potential. These findings provide a holistic perspective on how altitude and genotype shape hazelnut quality, offering valuable guidance for breeding programs, ecological zoning, and the development of functional and gastronomic hazelnut products.