Effects of local compression on the mechanical damage of tomato with different maturity

Tomato is the most consumed vegetable in the world. The annual yield of tomato products has been over 50 million tons in China in recent years. Mechanical damage easily occurs on tomato during automatic picking, which affects the quality and edibility of tomato. Many tomatoes with little damage were discarded during harvest, which resulted in serious losses in the picking-selling system. Therefore, it is necessary to establish an evaluation model of tomato mechanical damage, while the effect of pressing pressure on mechanical damage of tomato should be explored to reduce the losses during harvest. Therefore, this study aims to explore the effect of local compression on mechanical damage of tomatoes at three maturity stages (Breakers, Turning and Pink). Three points of the tomato top were pressed by a hardness tester, where all tomatoes were placed under ambient temperature and pressure for 30 days. The damage phenomenon was observed, and the number was recorded on the first day. An electronic scanning microscope was then used to characterize the microstructure change of pressed points on the tomato. Four levels of mechanical damage were studied, including decay, rind wrinkle, indentation occurred, and no indentation. The evaluation indicators were also set as the day that the damage phenomenon was observed. A compression test was performed to clarify the influence of local pressure on the mechanical damage of each level. The area of pressure distribution was obtained at each damage level of tomato with each maturity, thereby establishing the local mechanical damage and classification model of tomato. Finally, a grabbing test was carried out in an articulated three-finger electric manipulator. The number of tomatoes was counted, whose damage level was consistent with that in the local mechanical damage assessment and classification model in the test. The results showed that there was a very high coincidence degree between the test data and the damage assessment and classification model, not less than 95 %. Experimental results indicated that: 1) The mechanical damage of tomato increased, as the compression pressure and maturity raised. 2) The days of observable damage decreased with the increase of maturity. There was little correlation between the days when tomatoes decayed with the compression pressure. The days when tomato rind wrinkled decreased with the increase of compression pressure. 3) A median level was used to represent the compression pressure. In the first level mechanical damage, the represented compression pressure from low to high was 366, 355, and 337 kPa, according to maturity, about 30% higher than that of the second level mechanical damage in a sequence of 265, 245, and 225 kPa. In the third level mechanical damage, the represented compression pressure was ranked in descending order of 165, 115, and 90 kPa according to maturity. Tomato decay occurred in the range of 3-7 days, about 50% ahead compared with the wrinkled days of the tomato rind in the range of 7-17 days. Consequently, the findings can provide a sound reference for the design of a multi-finger harvesting manipulator in modern tomato production.