Rheological model for swida wilsoniana fruits in process of cold pressing based on stress-relaxation tests

Swida wilsoniana is one of the important woody oil plants supported by the State Council, mainly distributed in Hunan, Guizhou and Jiangxi Province, of which the whole fruits contain about 30% oils with high nutritional value. Mechanical pressing and solvent extraction are 2 major ways for fruit oil, and the lack of equipment for Swida wilsoniana fruits with the characteristics of hard core make it imperative to carry out this work. In order to study the rheological property of Swida wilsoniana fruits in cold pressing process, the stress relaxation tests under different loading strain levels were carried out by using the self-made straight cylinder pressing device. Based on the basic model of rock rheology, the relationship between the stress and strain was simulated, and its rheological model was established. The results showed that the relationship of stress and strain in the loading process of Swida wilsoniana fruits was divided into 2 parts which were defined as linear period and nonlinear period, and the critical yield strain was 0.4, meaning that the relationship between stress and strain was linear when the strain level was lower than 0.4 and was nonlinear as strain level was higher than 0.4. The amount of stress relaxation rose with the increase of pressing strain level, especially as the strain level was higher than 0.5, due to the oil bursting out of oilseeds, and the higher strain levels, the greater amount of oil, and the greater amount of stress relaxation. Segmented simulation was used to analyze and determine the parameters of the model. Firstly, the Maxwell model generalized by elastic element and damper mode was used to simulate the linear relaxation process. Then linear mode and the Kawakita's superposition model jointly simulated the process of nonlinear relaxation. Finally the rheological model was established and the parameters were obtained. Error analysis showed the application range and simulation effect. The relative error was large when the strain was 0.5 due to the rupture occurring in the period of loading, and the average relative errors were less than 7%when the strains were larger than 0.55, which could provide basic data of stress and strain parameters for squeezing process and special equipment. However, pressing pressure was only one of the pressing factors which would affect the pressing effect for Swida wilsoniana fruits. Other factors such as form of oilseeds, composition of main contents, pressing speed and temperature were also important factors influencing squeezing effect. Morphological change of Swida wilsoniana fruits was just a reflection for pressing, which had some limitations. The main evaluation index for pressing effect was the oil yield rate or residual oil of cake. So establishing the relationship between stress, strain and residual oil was an important direction of research. Therefore, a comprehensive analysis of the impact of various factors should be considered to establish an appropriate system for oilseed processing conditions, and to realize a cold and effective pressing technique for oil Swida wilsoniana fruits.