The effect of dropping impact on bruising pomegranate fruit

The pomegranate journey from orchard to supermarket is very complex and pomegranates are subjected to the variety of static and dynamic loads that could result in this damage and bruise occurring. Bruise area and bruise volume are the most important parameters to evaluate fruit damage occurred in harvest and postharvest stages. The bruising is defined as damage to fruit flesh usually with no abrasion of the peel. The two different types of dynamic loading which can physically cause fruit bruising are impact and vibration. The impact and vibration loadings may occur during picking or sorting as the pomegranates are dropped into storage bins and during transportation. The focus of this work was on the impact loading as this appeared to be the most prevalent. In view of the limitations of conventional testing methods (ASTM D3332 Standard Test Methods for Mechanical Shock Fragility of Products), the method and procedure for determining dropping bruise boundary of fruit were also established by adapting free-fall dropping tests.
After the ‘Malas-e-Saveh’ pomegranates had been selected, they were numbered, and the weight and dimension of each sample were measured and recorded. Firmness in cheek region of each fruit was also measured. Fruit firmness was determined by measuring the maximum force during perforating the sample to a depth of 10 mm at a velocity of 100 mm min-1 with an 8 mm diameter cylindrical penetrometer mounted onto a STM-5 Universal Testing Machine (SANTAM, Design CO. LTD., England). Free-fall dropping tests with a series of drop heights (6, 7, 10, 15, 30 and 60 cm) were conducted on fresh ‘Malas-e-Saveh’ pomegranates. Three samples were used for each dropping height, and each sample was subjected to impact on two different positions. Before the test was started, it was necessary to control the sample's drop position. The cheek of sample was placed on the fruit holder. An aluminum plate mounted on upper part of the piezoelectric force sensor was the dropping impact surface of the device. After dropping impact, the sample was caught by hand to prevent a second impact due to sample rebound. After impact, the samples were stored at room temperature for 48h, during which time bruise tissues and arils turned brown. The bruise area and bruise volume of each sample were calculated according to equations (1 and 2).
Dropping impact acceleration versus time curves for the typical samples at ten drop heights are shown in figure 5. Drop height notably affected the impact acceleration. The peak force increased while contact times decreased with increasing drop height, which resulted in an increase of peak acceleration. Figure 6 shows the dropping impact velocity change during contact by theoretical calculation. The results showed that the velocities at the beginning of contact and the rebound velocities of the samples increased with increasing the drop height. Critical drop height of pomegranate in certain bruise area was determined and linear relationship between drop height and bruise volume for ‘Malas-e-Saveh’ pomegranates were obtained. It is clear that there were obvious differences between dropping bruise boundaries of pomegranates and the conventional damage boundary of products (as shown in figure 9). For the conventional damage boundary, the vertical line, critical velocity (Vc), represents the velocity change below which no damage occurs, regardless of the peak pulse acceleration. The horizontal line, critical acceleration (AC), represents the acceleration at which the product will be damaged if velocity exceeds VC. At the same time, for a conventional product, there is only one damage boundary at one shock condition. However, for fruit, a change in drop height (velocity) will lead to a change in bruise ratio. A series of bruise boundaries can be determined for different bruise ratios. Moreover, even if the velocity approaches zero, the fruit can still be bruised if its acceleration exceeds a certain value. These relationships provide an effective basis to predict and control drop bruising, which may be achieved through the design of reasonable cushioning packaging for fruit.
This research applied the concept of dropping bruise for pomegranate fruits. Because of the limitations in using conventional testing methods to test product of a viscoelastic nature, such as fruit, free fall dropping tests were adapted to determine dropping bruise fragility and bruise boundary for ‘Malas-e-Saveh’ pomegranates at different drop heights. For viscoelastic products such as fruit, even if the dropping impact velocity approached zero, the fruit could be bruised as long as the impact acceleration exceeded a certain value (critical acceleration). A series of bruise boundaries can be established for different levels of bruise ratios, i.e., a contour of constant bruise ratio can be drawn on the velocity acceleration plane.