Study and modeling of apricot bruise susceptibility as affected by impact and fruit properties

Introduction: Fruits confort several main procedures including harvesting, sorting, packaging, storage, and transportation. During these processes, fruits could encounter either static or dynamic loadings that potentially cause mechanical damage (Polat et al., 2012). Bruising is the most common type of post-harvest mechanical injury in fruits. Around 30–40% of the agricultural products become decayed due to mechanical damage. Mechanical injuries, such as bruises are difficult to limit. However, proper understanding the mechanism of bruises occurrence allows to reduce this undesirable phenomenon (Komarnicki et al.; 2017). Susceptibility measurement of the bruising can provide useful information toimprove postharvest handling and storage operations (Zhu et al., 2016). The bruise susceptibility of fruits and vegetables is themeasurment of response to external loading. Bruise prediction models can provide useful information about the influence of fruit properties on bruise susceptibility, leading to recommendations for fruit handling (Van Zeebroeck et al., 2007b). Bruise prediction models connect the impact characteristics (drop height, contact force and impact energy) to bruise damage, taking into account some fruit properties (radius of curvature, stiffness, temperature, color, etc) that determine bruise sensitivity. Apricot is one of the most vulnerable fruits to mechanical injuries. Therefore, controlling and reducing the mechanical injuries can improve the quality of apricot and increase its export. For this reason, the aim of present study was to evaluate the apricot bruising susceptibility and developing bruise prediction models for apricot fruit.
Material and methods: In the present study, two Iranian apricot varieties "Shahroudi" and "Ordubad" were used for experimental tests. All the fruits were harvested at their traditional maturity in June 2018. In order to avoid and minimize any pre-bruising, all the apricots were carefully hand-picked. Any extremely small or large fruits were excluded from the tests. The physical and mechanical properties of apricots and main characteristics were measured. A curvature meter was used to measure the radius of curvature locally at the point of impact. The acoustic impulse technique was used to measure the fruit firmness non-destructively. Constant height multiple impact (CHMI) method was used to measure apricot critical energy level. In this study bruise susceptibility of the studied cultivars was evaluated using a pendulum impact device to measure required parameters including the impact force, impact velocity, and impact energy. All apricots were placed in the special chamber and a spherical metal impactor was used to impact the apricots. Absorbed energy criteria have been used to quantify bruise damage. In this study, three different impact levels applied in the experimental tests. To obtain the absorbed energy the difference between impact energy and rebound energy was calculated and recorded for each impact test. Different regression models (linear, multiple-linear, and logarithmic) were developed to predict bruising in apricot fruit and were evaluated by proper criteria (R2, R_adj^2, and RMSE). In these models, impact characteristics (impact energy or peak contact force) along with fruit characteristics (acoustic stiffness and radius of curvature) were considered as independent variables and the absorbed energy as the dependent variable. Two separate regression models were developed. In the first model (model 1) independent variables were peak contact force (PF), the apricot radius of curvature at the location of impact (R), and apricot acoustic stiffness (S). In the second model (model 2) PF was replaced by the impact energy (Ei) and the other two variables were the same as the first model. Data analyses were performed using SPSS (version 25.0) and JMP Pro (version 13) software packages.
Results and discussion: Experimental data analysis showed both radii of curvature and acoustic stiffness had a negative effect on apricot absorbed energy in Shahroudi and Ordubad cultivars. In Shahroudi cultivar, the average radius of curvature was 20% less than the Ordubad cultivar. Also, the average acoustic stiffness in the Ordubad cultivar was 19% higher than acoustic stiffness in the Shahroudi cultivar. Therefore, the average bruising ratio in the Shahroudi cultivar was 5% higher than the Ordubad cultivar. In both cultivars, apricots with a lower radius of curvature at the contact area had high absorbed energy than those with a higher radius of curvature. By increasing the peak impact force from 16.5 to 30 N, the average absorbed energy in the Ordubad cultivar was increased by 71%. While this amount was 78% in Shahroudi. Also, the mean difference between the absorbed energy between the initial (19 mm) and final (27 mm) range of the curvature radius in Shahroudi and Ordubad cultivars was 18 and 29%, respectively. Also, the mean energy absorption difference between the initial and final acoustic stiffness ranges in Shahroudi and Ordubad cultivars was 27% and 32%, respectively. In other words, with increasing acoustic stiffness, the amount of absorbed energy in Shahroudi and Ordubad cultivars was decreased 27 and 32%, respectively. The effect of the impact energy parameter on the absorbed energy was similar to the peak contact force. Results of prediction models indicated that all of the studied independent parameters, including peak contact force, impact energy, the radius of curvature, and acoustic stiffness, and some of their interactions were significant on the absorbed energy at the 5% probability level. The results showed that increasing the two parameters of impact energy and peak contact force increases the energy absorbed in apricots.
Conclusion: This study was conducted to develop statistic models among different kinds of regressions, to estimate the apricot fruit bruising susceptibility by absorbed energy. Absorbed energy parameter is a good estimation property to quantify bruise damage in apricot. There were significant main effects and significant interactions between fruit properties (radius of curvature and acoustic stiffness) and the impact properties (peak contact force or impact energy). In both apricot cultivars, damage to the fruit decreased with the increase of acoustic stiffness. So, softer apricots developed a higher amount of absorbed energy. Also, smaller radii of curvature led to more bruise damage in both cultivars. It was concluded that Shahroudis cultivar was more susceptible to bruising rather than Ordubad cultivar. The results of this study in predictive regression models showed that a multiple-linear model was selected as the best model for fitting experimental data.