Impact behaviour of freeze-dried and fresh pomelo (Citrus maxima) peel: influence of the hydration state

Abstract

Pomelos (Citrus maxima) are known for their thick peel which-inter alia-serves as energy dissipator when fruits impact on the ground after being shed. It protects the fruit from splitting open and thus enables the contained seeds to stay germinable and to potentially be dispersed by animal vectors. The main part of the peel consists of a parenchymatous tissue that can be interpreted from a materials point of view as open pored foam whose struts are pressurized and filled with liquid. In order to investigate the influence of the water content on the energy dissipation capacity, drop weight tests were conducted with fresh and with freeze-dried peel samples. Based on the coefficient of restitution it was found that freeze-drying markedly reduces the relative energy dissipation capacity of the peel. Measuring the transmitted force during impact furthermore indicated a transition from a uniform collapse of the foam-like tissue to a progressive collapse due to water extraction. Representing the peel by a Maxwell model illustrates that freeze-drying not only drastically reduces the damping function of the dashpots but also stiffens the springs of the model.

Keywords: Citrus maxima; Maxwell model; cellular material; coefficient of restitution; energy dissipation.

Figure 1.

Schematic drawing of the drop weight test rig. The drop height of the impactor is adjustable in order to achieve different impact velocities. The whole test rig was mounted on a massive granite pedestal (ca 95 kg).

Figure 2.

Representative velocity versus time curve of the impactor, covering the three phases of a drop weight test: (1) free fall of the impactor, (2) impact phase and (3) rebound of the impactor. The intersection of the linear regression lines, calculated from the velocity versus time data points of the impact phase and the rebound phase was defined as the rebound velocity (v₂). (The unfilled circles were not used for calculating the regression lines).

Figure 3.

Freeze-dried pomelo peel sample. Top view showing the epidermis where some freeze-cracks are visible (a) and lateral view with the epidermis at the bottom (b). Scale bars, 1 cm.

Figure 4.

Coefficient of restitution for fresh and for freeze-dried pomelo peel samples versus drop height of the impactor (the lines show the regression of the locally weighted scatter plot smoothing, the light coloured area represents the 68% confidence interval).

Figure 5.

Fraction of the kinetic energy dissipated by the samples during impact against drop height of the impactor (the lines show the regression of the locally weighted scatter plot smoothing, the light coloured area represents the 68% confidence interval).

Figure 6.

Force transmitted from the impactor through a pomelo peel sample to the force sensor versus time. The impactor was dropped from 0.125 m (a) and 1.25 m (b). The shaded areas under the curves represent the impulse.

Figure 7.

For validation of the force signal via momentum comparison, the momentum as calculated from the momentum balance (I_mb) and the momentum as calculated as the integral of the force versus time curve (I_i) are compared. Ideally, the ratio of both should be exactly 1. Significant differences (Wilcoxon test) from unity are marked with asterisks (*p<0.05, ^**p<0.01).

Figure 8.

Impact duration versus sample height. Each box represents a different impactor drop height. Each circle represents an individual drop test.

Figure 9.

Measured peak force versus sample height. Each box represents a different impactor drop height. Each circle stands for an individual drop test.

Figure 10.

Maximum strain up to which the pomelo peel samples were compacted during impact. In order to compare the different values (difference between measuring method (red asterisks) and difference between sample hydration state (black asterisks)), the Wilcoxon signed-rank test was performed (paired in case of comparison of the different methods). (p-values: *p<0.05, ^**p<0.01, ^***p<0.001).