Determination of apple bruise resistance based on the surface pressure and contact area measurements under impact loads

- There is a clear relationship between the contact surface and the bruise volume.
- The resistance threshold can be determined by ratio of pressure to contact surface.
- Bruise volume as well as contact surface ensure precise bruise resistance evaluation.
- Relationship of bruise volume and contact surface is similar to each surface type.

The bruises as well as other affects in fruit quality cause lower selling prices and generate loss for fruit growers. For proper identification of damages in biological material, the impact load, contact pressure as well as force affecting on apple skin should be examined. In this paper, the authors present an experimental method in assessing of the bruise resistance as well as the bruise threshold for the 'Gala' apple cultivar consisting a relationship between the impact loads during free drop against four rigid surfaces. The authors measured the contact surface between tested fruit and fixed material to determine the bruise resistance. The Tekscan® measuring system was applied to determine the contact surface and the surface pressures at the moment of collision during impact test. Damaged tissue was photographed and subjected to the computer image analysis in order to determine the bruise volume. In this study, the bruise resistance index (BRI) as relationship between determined bruise volume and surface pressure at varying drop heights was presented. Due to the difficulty and time-consuming process of the bruise volume evaluation, the authors decided to replace commonly used method with functional relation of the contact surface. It allowed for assessing the alternative BRIc indicator which was based on the relationship between the surface pressure and the contact surface. Both the indicators as well as verifying linear regression analysis showed, that proposed BRIc power model with high precision describes power curves of the BRI indicator. From conducted analysis results that the BRIc indicator allows for the determination of the bruise resistance for the 'Gala' apple cultivar and precisely describes the BRI indicator. Based on the BRIc curves and changes in the bruise area-drop height relationship, a graphical method in assessment of the bruise resistance and the bruise threshold was proposed. The presented method can be used as effective tool in mechanical damage assessing.