Physical Characteristics Of Macadamia Nuts During Drying

As a kind of high economic value nut, macadamia nut is rich in fat and protein. Recent years the production of macadamia nut has increased significantly in Yunnan Province, which requires specialized processing unit urgently. However, there is no enough information in the scientific literature with regards to the processing of macadamia nut in China except a few preliminary researches, since the industrial system of macadamia starts later and structure of macadamia nut is complex. The existing processing equipments are so limited that they could not meet demand of macadamia nuts'modern production. The physical characteristics during drying process are the basis of most processing, which would significantly influence the later processing of products. Therefore, this work studied the physical and mechanical characteristics of macadamia nut during drying process and found out the variation between shrinkage properties and stress distribution with moisture content, which provided theoretical basis for the processing of macadamia nut.The basic physical parameters of macadamia nut, like moisture content, dimension (nut and kernel), shell thickness, density (nut, shell and kernel) and kernel ratio were determined first. Then the shrinkage curve, linear strain and linear moisture expansion coefficient of shell at different moisture contents and positions as well as the shrinkage equation of shell during drying process were obtained. The modulus of elasticitv and broken load of macadamia nut at different factors and levels also were studied. The regression equations between the three factors (the loading position, moisture content and loading rate) and modulus of elasticity as well as break load were established through the ANOYA. Finally the shrinkage equauon and modulus of elastic obtained were applied to th model of stress balance unit to find the distribution of shell during drying process.The rcsults(indicat that the drying curves, which show different variation trends for nut, shell and kernel, are exponential curves. Horizontal diameter is generally greater than the width and hilum diameter for nut and kernel. With decrease of moisture content, the three axis dimensions of nut and kernel decrease. The shell thickness is nonuniform with thick top and thin middle. The densities of nut and kernel are increased firstly and then decrease during drying process. The kernel ratio decreased from0.259to0.320with decrease of moisture content. The shrinkage of shell, which show the width direction<hilum direction<horizontal direction, is different from each other at three directions. Furthermore, the linear strain and linear moisture expansion coefficient of shell both nonlinear increase as the moisture content decrease. And the shrinkage of shell gradually increase from inside to outside, which presente curvilinear growth with decreased of moisture content. The modulus of elasticity of macadamia is significantly affected by the loading position and moisture content. The break load is significantly affected by the loading position and the moisture content. The loading speed has little effect on both break load and modulus of elasticity of macadamia nut. The regression equation can be described as: F=-4.3865M5+116.89M4-1145.1M3+5056.5M2-9653M+6946.3, and that for the modulus of elasticity is:E=-0.108M5+2.4323M4-18.315M3+49.995M2-17.587M+10.136. The variation of tangential stress and radial stress is similar to the linear strain, which were significantly affected by the moisture content. The external drying stress is greater than the internal. There is the maximum stress at the moisture content around10.9%. The maximum value of radial stress is0.51MPa while that for tangential is0.3MPa.The conclusions could be described as follow:(1) Macadamia could be simplified as a sphere. The dimensions of width direction could be selected as grading standards when graded by size.(2) Macadamia nut should be guided to horizontal with loading speed of30～40mm/min before cracking. The moisture content of nuts is about1.09% then broken load and modulus of elasticity are little.(3) The nut, shell and kernel can be separated by density after broken.(4) The crack of macadamia appears first on the hilum line during drying process, which probably resulted froi the fact that radial stress is larger than the tangemial stress and the outer stress is larger than that of the inner.