Moisture dependence on mechanical properties of pine nuts from Pinus pinea L.

Abstract Mechanical properties of pine nut shell are very important in the cracking process. Before industrial cracking shell moisture is adjusted by water soaking in order to avoid the breaking up of the nuts. In this work the moisture dependence of the mechanical properties of pine nut shells were studied in order to improve the industrial cracking process. Relationship between nut size and moisture content was studied and no significant differences were found for shell dimensions at four different moisture levels, 9.79%, 16.71%, 20.98% and 25.05% (dry basis). Compression tests were carried out on pine nuts at nine different moisture contents between 1.38% and 25.48% (db) and their main mechanical parameters were evaluated. Nut shell has a characteristic behavior under compression loading. Rupture force, rupture deformation, elasticity and rupture energy of shell were found to be moisture dependent. Values of rupture force decreased linearly from 0.24 to 0.49 kN, rupture deformation and rupture energy increased initially with moisture until stabilization at asymptotic values of about 1.8 mm and 0.22 J, respectively. Elasticity index of the shell decreased linearly with moisture from 1 to 0.4 kN/mm.