Evaluation of some mechanical properties of two varieties persimmon in order to improve post-harvest systems at different ripening stages

Introduction: Fruit and vegetables are an important component of the human diet and consumers usually expect such products to have premium quality. The texture is a major quality attribute that influences consumer acceptance, shelf life, resistance, and transportability. Also, the quality of fruits and vegetables can be determined by their external and internal characteristics. Recognition of agricultural product characteristics may help to design new machines, industrial processes to reduce the damages (Tavakkoli hashtjin, 2003).
The first step in the codification of qualitative standards for the agricultural product is the recognition of different properties and different modifications of these products caused by various factors (Mohsenin 1986). Persimmon (Diospyros) is a member of Ebenaceae family and originated from China and Japan. D. Lotus, D.Virginiana and D.Kaki are three important persimmon cultivars in Iran. According to the FAO statistics, the Iran persimmon harvested area was about 1692 ha in 2017. In this year Iran produced about 24326 tons of persimmon (with an average yield of 14.3 ton/ha) which ranked 11^th in the world. Although many researchers conducted some investigations on the mechanical properties of agricultural products, but studies on persimmon mechanical properties have been very limited. Hezbavi et al. (2008) studied the physical and mechanical responses of Japanese cultivar of persimmon (D.Kaki) and reported that there was a significant difference in all mechanical properties between soft and stiff persimmon, except fruit deformation. Altuntas et al (2013) determined the physical, mechanical and chemical properties of medlar during physiological maturity and ripening period. The physical properties such as geometric mean diameter, sphericity, bulk and true densities, porosity, projected area and color characteristics were measured during physiological maturity and ripening period of medlar. Mechanical properties such as rupture force, deformation and rupture energy and chemical properties (total soluble solid content, titratable acidity and pH) of medlar fruit were determined.  The results of Altuntas et al. (2010) have shown that the correlation coefficients between the physical parameters of persimmon fruits were significant. The coefficient of static friction was greater on plywood as compared to the chipboard and galvanized metal surfaces. They reported that the required force for punching persimmons along the Y-axis was higher than along the X-axis. Review of literature showed that the effect of harvesting time, cultivar and loading speed of D.Kaki and D.virginiana persimmon cultivars on some mechanical properties and coefficient of restitution of persimmon have not been studied. Therefore, in this research, some mechanical properties of two persimmon cultivars (D.Kaki and D.virginiana) at three harvests time (immature, semi-mature and mature) and coefficient of restitution were studied.
Material and methods: In this study, some tests were conducted to determine these mechanical properties of two persimmon varieties   D. Virginiana and D. Kaki at the three harvest times with three different loading speeds of 50, 100 and 200 mm/min using fruit texture analyzer and to obtain resilience coefficient used an invented device equipped with sonic sensor, so that it can be used as a criteria for bruising damage. The persimmon fruits (D.Kaki and D.virginiana) at three harvest times were provided from gardens of East Azarbaijan province. Then, the samples were transferred to the biophysical and mechanical lab of university of Tabriz. The moisture content of fruits was determined by the standard method (ASAE 1998). The acoustic test was used to determine the resilience coefficient that is a criterion for determining persimmon bruising damage. For this purpose, persimmons were dropped from three heights 10, 20 and 30 cm on the plate equipped with an acoustic sensor located underneath that plate. The amplitude-time curve was obtained using Praat software for each drop test. According to this curve, rebound time (the time required for the first and second peaks of curve) was determined.
Results and discussion: According to the results, the main effects of variety, harvesting time and speed of loading and also interaction of variety*harvesting time were significant at the probability level of 1% and the other interactions were not significant. It means, in general, as expected, the mean values of puncture force ​​for the two varieties, at three harvesting times and in three different loading speeds, had a significant effect at the probability level of 1%. The reason for the significance of interaction of variety*harvesting time is the behavior of puncture force at different times of harvesting persimmons. The results showed that the required mean value of puncture force in D. Virginiana variety was greater than D. Kaki variety and the average force required to punch the persimmon fruit, with Magness Taylor probe for the first harvesting time (immature stage) is nearly doubled compared with the third harvesting (mature stage). It shows that if persimmon has been marketed in the immature stage or in semi-mature stage, mechanical damage can be decreased to one-half value. By increasing the loading speed, the average force required to punch the persimmon increased. The average energy required to punch of persimmon fruit, using Magness Taylor probe for the first harvesting time (immature stage) is nearly doubled in comparison with the second harvesting  time (semi-mature stage) and nearly tripled in comparison with the third harvesting time (mature stage). By increasing the loading speeds, the average energy required to punch the persimmon fruit  increased and also the same results were obtained for three harvesting stages. It can be concluded that, for example, during sorting operation, whatever Persimmon move at a slower speed, minimum energy can cause  mechanical damage. When the product getting ripe the mean value of puncture energy decreased and by increasing loading speeds the mean value of puncture force increased. The difference between mean values of resilience coefficients of fruits released from different heights at the three different harvest times for both two varieties was significant at the probability level of 1%.