Investigation of the mechanisms and strategies for reducing shell cracks of hazelnut (Corylus avellana L.) in hot-air drying

During postharvest drying hazelnuts experience shell-cracks, which could accelerate quality deterioration and microbial contamination of nuts during storage. This study investigated shell-cracking mechanisms and developed strategies to reduce cracking during hot-air drying. Physicochemical properties of shells or kernels, moisture desorption isotherm, structure of inshells, and morphological property of inner fiber attached to shells were correlated with cracking ratios for four hazelnut cultivars (Barcelona, Jefferson, Yamhill, and Wepster). A systematic experimental approach combining Taguchi design and gradient drying was utilized to reduce shell cracks while retaining drying efficiency. Among the four cultivars, Jefferson with hygroscopic and lower density shell, larger air-gap between shell and kernel, and thicker and strongly bound shell fibers had the highest cracking ratio. During hot-air drying, relative humidity (RH) was the most contributing factor inducing shell cracks of Jefferson than temperature and air velocity. Using gradient drying through increasing RH from 50 to 60% or reducing temperature from 38 to 32 °C when inshells reached ∼16 g/100 g moisture content (MC) reduced cracking ratio < 30% and drying time <15 h for Jefferson. This study generated new knowledge of hazelnut shell cracking mechanisms and provided a practical drying strategy for reducing shell cracks.