Physical and mechanical properties of morula (Sclerocarya birrea subsp. Caffra) nuts.

Abstract

Morula nuts were characterized for several physical and mechanical properties relevant to food process engineering. Mean moisture content of the nuts which had been naturally dried under the sun was 7.65% (w.b.). Mean nut length, width and thickness were, respectively, 25.64, 21.99 and 18.03 mm, while the derived geometric mean and equivalent diameters were 21.62 and 21.72 mm, respectively. Mean surface and projected areas were 1 475.74 and 445.02 mm2, respectively, while mean nut volume was 5 371.90 mm3. Mean apparent density of the nuts was 811.30 kg m-3 while bulk density and bulk porosity were determined as 476.43 kg m-3 and 41.28%, respectively. Morula nuts are generally spherical, with mean sphericity of 84.40%, and aspect and flakiness ratios of 0.86, and 0.83, respectively. Mean angle of repose against a plywood surface was 19.13o while coefficient of friction against a galvanized iron surface was 0.24. Peak force upon mechanical compression in a texture analyser was 2 738.33 N, while energy expended during compression was 3 284.49 Nmm, with mean hardness being 774.02 N mm-1 and tensile strength recorded as 6.60 MPa. It may be concluded from these results that morula is a tough, moderately sized spherical nut whose density is lower than that of water, having high bulk porosity and low frictional properties.