Effect of temperature and storage time on mechanical properties and color of sweet and bitter olives

Storage conditions of product after harvest and during storage are important factors that affect the quality parameters of the products such as color and mechanical properties. The observance of effective issues on the fruit quality, such as weather conditions, harvest methods, timing of harvesting, temperature of storage and time of storage in maintaining of olive quality and reduce waste, will be very effective. Performance of mechanical testing and mechanical and colorimetric properties of products has an important role in study of qualitative and quantitative properties during storage. Moreover, the study of color and mechanical properties of products, such as olive is necessary, for use in machine design for processing and recovery processing lines in order to reduce waste.
In this study, two types of olives, bitter and sweetened with 1.5% NaOH were harvested and tested to study the effect of storage time on their color and mechanical properties. Olive fruits were stored in two temperatures (environmental conditions with 25 and refrigerator with 4) for 3 months. During this period, experiments were carried out at the intervals of 10 days. Universal testing machine and color meter devices were used for determining the mechanical properties and the color of olives, respectively. By performing a factorial experiment under completely randomized design, the effects of independent factors including storage time, storage temperature and type of olive on the measured properties (shear modulus, shear force, shear energy, modulus of elasticity, penetration force, yield strain, L*, a*, b*, h and c*) were studied.
The results of analysis of variance for shear modulus showed that the main effect of olive type, temperature, time of storage and the dual effects of these parameters were significant at the level of one percent. Analysis of variance for shear force showed that the effect of type, temperature, time of storage and interaction effects of type× temperature and type× time of storage were significant at the level of one percent. The results of data analysis for shear energy showed that the independent parameters, including the type, temperature, time of storage and the effects of dual and triple of their were significant at the level of one percent. According to the graphical results, shear modulus and shear energy of any type of olive on the thirtieth day with the fast steep increased. Water surface evaporation, changes in the texture of olive and a sharp drop in humidity are reasons for the sudden increase of shear modulus and shear energy on the thirtieth day. With increasing time of storage, shear strength and other mechanical properties for bitter and sweet olive increased. This result conformed to the results of Lavassani et al and Nanos et al. Analysis of variance of mechanical properties for olive fruit in penetration test showed that the effect of type and interaction effects of type and time of storage on modulus of elasticity were not significant. Analysis of variance of yield strain showed that the only effect of type was significant at the level of one percent. The results of mean comparison with Duncan test showed that during storage, penetration force and modulus of elasticity for bitter and sweet olive increased. This result correlated with the results of De Castro et al. The penetration force, yield strain and modulus of elasticity of sweet olives during storage were higher in comparison to bitter olives. The penetration force and modulus of elasticity of bitter and sweet samples stored at 4 were higher in comparison to the same sample in temperature of 25. This result correlated with the results of Nanos et al. Analysis of variance of colorimetric properties showed that the interaction effects of type and time of storage on L* were not significant. The effects of type and time of storage on a* and all effects on b* and c* were significant at 1% probability level. The results of mean comparison showed that the brightness and yellow of bitter and sweet samples increased with increasing time of storage compared to the first, tenth and twentieth days. The increase in L* and b* color characteristics correlated with the results of Piga et al. Color characteristics L*, b* and c* of bitter samples in each of the temperatures and days of storage were higher in comparison to sweet samples. In addition, the values of L*, b* and c* of bitter and sweet samples stored at 25 were higher in comparison to the same samples in temperature of 4.
The results of mechanical and colorimetric tests showed that by increasing the storage time, stiffness, shear strength and other mechanical parameters increased in both types of olives, and the L* and b* values of the samples were higher in comparison to the first, tenth and twentieth days. Bitter and sweet samples stored at 4 temperature displayed higher stiffness, modulus of elasticity, shear modulus, shear force and shear energy values and lower L*, b* and c* in comparison with the same samples stored at 25 temperature.