Bruise Damage of Pomegranate during Long-term Cold Storage: Susceptibility to Bruising and Changes in Textural Properties of Fruit

ABSTRACT

This study evaluated the bruise susceptibility of pomegranate fruit and textural properties of impact bruised fruit during long-term storage. Pomegranate fruit were stored at 5ºC and 85% relative humidity for 90 d. Impact tests were performed by dropping fruit from three drop heights (20, 40, and 60 cm) onto a flat hard surface to obtain different impact energy levels. The energy absorbed at impact, bruise volume (BV, mm3), bruise area (BA, mm2) and bruise susceptibility (BS, mm3/mJ) of pomegranate fruit were determined at 30 d interval for each impact dropped fruit. Textural properties for whole bruised and control (non-bruised) cold-stored fruit were carried out at 14 d intervals to determine the mechanical strength based on compression, cutting and puncture resistance tests. Based on impact tests, the increase in BV and BA of pomegranate fruit with increased drop heights and duration of cold storage were found significant (p < .05) during the first 2 months of storage and then decreased in the last month of storage time. Pearson correlations indicated a strong linear relationship between the increase in both BV and BA with an increase in impact energy throughout the 90 d storage period. Fruit puncture resistance and whole fruit cutting force, cutting energy and power as well as fruit compression strength properties; firmness, and related textural properties increased with higher impact bruising and prolonged storage. The results obtained in this study demonstrate that long-term storage, indeed, influenced the susceptibility of pomegranate fruit to bruise, and altered fruit texture.

KEYWORDS:

• Postharvest losses
• Textural properties
• Bruise volume

Introduction

Production of pomegranate (Punica granatum L.) fruit has become a widely grown commercial fruit to many parts of the globe. Large commercial orchards of pomegranate trees are presently grown in many geographical regions, from the Mediterranean basin in the northern hemisphere to the southern hemisphere (Fawole et al., 2013; Holland and Bar-Ya’akov, 2009; Opara et al., 2009). Consumption of pomegranate fruit has remarkably increased due in part to its unique sensory and nutritional properties coupled with medicinal benefits that are attributed to the fruit’s high content of health-promoting phytonutrients and high antioxidant capacity (Ayhan and Esturk, 2009; Opara et al., 2009).

Mechanical damage occurring between harvesting and consumption has become one of the major contributing factors for the decreasing market value and quality loss of many fruits, including pomegranates (Opara and Pathare, 2014; Shafie et al., 2015). Damage on fruit (i.e. bruises, cuts or abrasions) is caused mainly by mechanical impacts predominantly occurring during harvesting, handling, and transportation (Opara and Pathare, 2014; Tabatabaekoloor, 2013). Bruising is the most prevalent form of mechanical damage in many fruits which is caused largely by excessive impact and/or compression forces concentrated on small area of the fruit surface against a rigid body or fruit against fruit mainly involved during mechanical handling (Kitthawee et al., 2011; Stopa et al., 2018; Stropek and Gołacki, 2013). Bruising is defined as the damage to the fruit caused by subcutaneous tissue failure without which may not necessarily involve rupture of the skin of fresh produce, usually indicated by the discoloration of the injured tissue (Blahovec and Paprštein, 2005; Opara and Pathare, 2014; Shafie et al., 2015).

Studies have been carried out to assess the factors influencing bruise damage susceptibility of fruit during postharvest storage. Findings from previous research have shown that storage duration influences the impact or compression response of fruit (Kitthawee et al., 2011; Yurtlu and Erdoúan, 2005). An increase in bruise sensitivity with increasing storage time has been ascribed to the changes in fruit’s physical characteristics during postharvest storage (Vursavus and Ozguven, 2003; Yurtlu and Erdoúan, 2005). In addition, factors such as temperature and humidity have a considerable effect on changes in overall quality, mechanical properties and bruise susceptibility of fruit (Albaloushi et al., 2012; Ekrami-Rad et al., 2011; Yurtlu and Erdoúan, 2005). Yurtlu and Erdoúan (2005) showed that increasing duration of cold storage of apples and pears decreased the bioyield point force, elastic moduli, and fruit firmness and led to a subsequent increase in impact bruising (bruise area and bruise susceptibility) for both fruits. Vursavus and Ozguven (2003) evaluated the effect of duration on bruised susceptibility of peaches using pendulum impactor at the preselected amount of impact energy. Reported results revealed that while the mechanical properties such as modulus of elasticity, bio-rupture force, and rupture stress decreased over storage, there was a non-linear relationship between the increase in bruise susceptibility and storage duration at each impact.

There is a dearth of published data on bruise damage susceptibility of pomegranate fruit, which might be partly explained by the complex structure of the fruit (Ekrami-Rad et al., 2011). Developing an understanding of bruise damage susceptibility of fruit during storage and handling is a complementary approach in developing improved harvesting and handling practices and could also influence the choice of storage methods as well as transportation systems (Albaloushi et al., 2012; Ekrami-Rad et al., 2011; Shafie et al., 2015). Furthermore, analysis of the changes in mechanical properties of fruit following the impact bruising could also help in the provision of useful recommendations to reduce further losses (Albaloushi et al., 2012). This highlights the need for an in-depth understanding of bruise damage susceptibility of pomegranate fruit during postharvest handling. Additionally, it is still unknown to what extent bruising could affect the textural or mechanical properties of pomegranate fruit during long-term storage. This study investigated the effect of storage duration on bruise susceptibility of pomegranate and analyzed the textural profile of bruise-damaged fruit during a 3-month storage period.

Materials and Methods

Fruit Preparation and Storage

Pomegranate cv. Wonderful fruit were handpicked at commercial maturity from Blydeverwacht commercial orchard located in Wellington (latitude 33° 38’ S, 19° 00’ E, altitude 252 m) in the Western Cape Province, South Africa. Fruit were packed in well-cushioned boxes to avoid on-transit bruising and transported in an air-ventilated vehicle to the Postharvest Technology and Research Laboratory, Stellenbosch University. Upon arrival to the laboratory, fruit for the experiment were carefully sorted to ensure the use of sound fruit free of any physical defects such as cracking, sunburn and husk scald.

Impact Test and Bruise Measurement

In the first experiment, we studied the bruise susceptibility of pomegranate ‘Wonderful’ fruit as affected by storage condition and duration. Sorted fruit were kept in cold storage (5ºC, 85 ± 5% relative humidity) for 90 d to simulate long-term storage. Sampling for impact test and bruise measurement was done at 14 d intervals for the duration of 12 weeks. Prior to testing, fruit were weighed individually using a Mettler weighing balance (± 0.01 g), and the mass of individual pomegranate fruit varied from 250 to 300 g. Impact bruising of whole fruit was performed by a drop test technique using the method reported by Montero et al. (2009). Forty-five individual pomegranate fruit were bruised by subjecting to different levels of drop impacts by letting the fruit fall from 20, 40 or 60 cm drop heights (15 fruit per drop height) onto a rigid flat surface. In this particular experiment, each pomegranate fruit was dropped twice from the same drop height onto two opposite sides of the fruit while ensuring that the two impacts are allocated at equidistant points on the cheek position of the pomegranate fruit. Six fruit were dropped per drop height to make 12 replications per drop height (i.e. 6 biological replicates; 6 fruit x 2 drop impacts per fruit for each drop height). In each test, the fruit was caught by hand after the first rebound to avoid multiple impacts onto fruit. After each impact, the bruised region of each fruit was marked using a marker in order to facilitate the detection of bruise damage during measurement. To allow bruise manifestation on damaged tissue after impact tests, fruit were incubated at ambient condition (19–22°C, 60 ± 5% relative humidity) for 48 h. Impact energy (E, mJ) resulting from each drop impact on fruit was calculated from Equation (1)(1) $\mathrm{E}\mathrm{i}=\mathrm{m}\mathrm{g}\mathrm{h}$(1) ;

(1) $\mathrm{E}\mathrm{i}=\mathrm{m}\mathrm{g}\mathrm{h}$(1)

where m is the mass of each individual pomegranate fruit, g is the gravitational constant (9.81 m/s²), and h is the drop height.

Bruise measurement was performed by slicing the fruit through the center of the marked impact region. Bruise damage of the fruit was identified by the presence of visibly bruise damaged tissue on the marked region which was clearly distinguishable from other non-bruised parts of the same fruit. Measurement of bruise dimensions comprised; $w_1$ and $w_2$ for major and minor width, respectively, and $d$ for bruise depth, all performed using a digital caliper (Mitutoyo, ± 0.02 mm). Results of bruise damage size were presented as bruise area (BA, mm²), bruise volume (BV, mm³) and bruise susceptibility (BS, mm³/mJ) expressed as the ratio of bruise volume to the energy absorbed during impact using Equations 2(2) $\mathrm{B}\mathrm{A}=\left(\frac{\pi }{4}\right)\ast {\mathrm{W}}_1{\mathrm{W}}_2$(2) –4(4) $\mathrm{B}\mathrm{S}=\frac{\mathrm{B}\mathrm{V}}{\mathrm{E}\mathrm{i}}$(4) (Opara and Pathare, 2014).

(2) $\mathrm{B}\mathrm{A}=\left(\frac{\pi }{4}\right)\ast {\mathrm{W}}_1{\mathrm{W}}_2$(2)

(3) $\mathrm{B}\mathrm{V}=\left(\frac{\pi }{8}\right)\ast {\mathrm{w}}^2\mathrm{d}$(3)

(4) $\mathrm{B}\mathrm{S}=\frac{\mathrm{B}\mathrm{V}}{\mathrm{E}\mathrm{i}}$(4)

Textural Profile

In the second experiment, changes in some mechanical properties of bruised pomegranate fruit during cold storage were investigated. Prior to storage, a batch of sound pomegranate fruit were selected and pre-conditioned at 22 ± 5ºC and 60 ± 5% relative humidity for 24 h. After stabilization at this temperature, 90 fruit were subject to different drop impact heights by letting the fruit fall from 20, 40 or 60 cm drop heights (30 fruit per drop height) using similar procedures as described in section 2.2. For the batch of fruits used for textural profile experiments, each fruit was dropped only once from the same drop height onto a cheek position of the fruit. Dropping of fruit was controlled to ensure that the impact is allocated on the midst point of the cheek position of the fruit. After drop impact treatments, fruit were stored at a cold temperature (5ºC, 85 ± 5% relative humidity) for 90 d. Sampling for puncture resistance, cutting and compression tests of fruit was conducted at 14 d interval throughout the storage duration.

Puncture Resistance Test

Measurement of fruit puncture resistance was conducted using the fruit texture analyzer (GÜSS-FTA, model GS, South Africa), with a 5 mm cylindrical probe programmed to puncture 15 mm into the fruit at the speed of 10 mm/s. Ten randomly selected pomegranate fruit from each drop impact heights (20, 40, 60 cm) and 10 non-bruised (control) fruit were punctured on three different positions of the fruit placed on a steel test platform with the stem calyx axis parallel to the platform. An average puncture resistance for each fruit was determined. Peak force required to puncture the fruit surface was taken as puncture resistance and the values were presented as mean ± S.E of 8 replications.

Cutting Test

Fruit cutting test was performed using texture profile analyzer XT Plus (Stable MicroSystem, Godalming, UK) with a blade set knife (a stainless steel cutting probe with sharpened edges). Ten randomly selected pomegranate fruit from each drop impact level (20, 40, 60 cm heights), each test was carried by cutting the pomegranate whole fruit into halves while positioned with its stem calyx axis parallel to the platform. The texture profile analyzer was set at 1 mm/s pretest speed, 1 mm/s test speed, 10 mm/s posttest speed, 1000 N cutting force and 20 mm cutting distance. The later was set to prevent the cutter from touching the plate during the downward movement of the probe. Data obtained from the textural profile analyzer were processed using Exponent v.4 software (Exponent v.4, Stable MicroSystem Ltd). Fruit cutting force, energy, and power were all evaluated from the force–deformation curve generated. Cutting energy was calculated by measuring the area under the force–deformation curve, whereas power required for cutting the whole pomegranate fruit into halves was calculated using equation 5(5) ${\mathrm{P}}_{\mathrm{c}}=\frac{{\mathrm{E}}_{\mathrm{c}}{\mathrm{R}}_{\mathrm{c}}}{60000\mathrm{x}}$(5) .

(5) ${\mathrm{P}}_{\mathrm{c}}=\frac{{\mathrm{E}}_{\mathrm{c}}{\mathrm{R}}_{\mathrm{c}}}{60000\mathrm{x}}$(5)

where${\mathrm{P}}_{\mathrm{c}}$is the cutting power (Watt), ${\mathrm{E}}_{\mathrm{c}}$is the cutting energy (N.mm), and ${\mathrm{R}}_{\mathrm{c}}$and x are the rate of fruit cutting (probe test speed, mm/min) and cutting displacement (mm), respectively. Results (average values of 8 replication fruit) for cutting force, energy and power were expressed as mean ± S.E.

Compression Test

Texture profile analyzer (TA-XT Plus, Stable Micro Systems, England, UK) was used to perform the fruit compression test. Prior to testing, the texture profile analyzer was calibrated with a 10 Kg load cell. Machine operating conditions were set at; pretest speed 1.5 mm/s, probe test speed 1 mm/s, posttest speed 10.0 mm/s, compression force 1000 N and deformation distance 20 mm. Using a 75 mm diameter compression plate, fruit were compressed on the cheek position with the stem/calyx axis parallel to the flat steel platform where each compression test was done per fruit. Ten fruit from each bruise drop impact height (20, 40, 60 cm) and eight non-bruised (control) fruit were randomly selected and compressed. For each compression test, a force–deformation curve was generated automatically and used to determine the compression properties of fruit using texture profile analyzer software (Exponent v.4, Stable MicroSystem Ltd.). From the force–deformation curve, the initial slope which gives an indication of the fruit tendency to deform elastically when a force is applied (elastic modulus, N/mm²), the maximum force (N) required to compress the fruit to a distance of 20 mm (firmness, N) and the energy required to compress the fruit (toughness, N mm) were obtained. Energy at bioyield, the point at the compression curve where the biological material is said to have failed in internal cellular structure (or permanent deformation) during compression at a given force was obtained by calculating the area under the force–deformation curve (Arendse et al., 2014). Furthermore, the power (W) to compress the whole pomegranate fruit was determined by using the equation below:

(6) ${\mathrm{C}}_{\mathrm{p}}=\frac{{\mathrm{E}}_{\mathrm{c}\mathrm{p}}{\mathrm{R}}_{\mathrm{c}\mathrm{p}}}{60000\mathrm{x}}$(6)

where ${\mathrm{C}}_{\mathrm{p}}$ is the compression power, ${\mathrm{E}}_{\mathrm{c}\mathrm{p}}$ is the compression energy (N mm), and ${\mathrm{R}}_{\mathrm{c}\mathrm{p}}$and x are the rate of fruit compression (probe test speed, mm/min) and compression displacement (mm), respectively. Results of eight replicates for compression force, compression energy and power were expressed as mean ± S.E.

Statistical Analysis

Statistical analysis was carried out using Statistica software (Statistical version 13, StatSoft Inc., Tulsa, OK, USA). Factorial analysis of variance (ANOVA) was used to evaluate the effects of storage duration and bruising on bruise damage and fruit mechanical properties, respectively. Duncan’s multiple range test (DMRT) was used to separate mean values of measured parameters based on their statistical differences.

Results and Discussion

Effect of Storage Duration on Pomegranate Fruit Bruising

The bruise size determined in each sampling day and impact test are shown in Figure 1. Bruise volume (BV, mm³) and bruise area (BA, mm²) of pomegranate fruit increased in the first 2 months in storage. The BV of medium (40 cm) and high (60 cm) impact dropped fruit increased significantly from 5072 to 5803 mm³ on day 0 to reach the respective peak values of 6867 and 8637 mm³ after 2 months of cold storage (Figure 1(a)). During the same period, BV for low (20 cm) impact dropped pomegranate fruit also increased albeit at a much lower magnitude (from 3712 mm³ day 0 to 4619 mm³ after 2 months). At the end of 3-month storage, fruit impacted at low, medium and high drop heights displayed 22.9%, 29.6%, and 16.2% respectively, lower BV, suggesting increased resistance to bruising during storage. The bruise area measured in pomegranate fruit subjected at low and medium drop impacts followed a similar trend to that of BV (Figure 1(b)). The consistent increase in BA for fruit dropped at medium and high drop impact was observed throughout the storage duration reaching 901 and 1010 mm² after 2 months from the respective initial values of 684 and 740 mm² on day 0.

Figure 1. Bruise size of ‘Wonderful’ pomegranate fruit; (a) bruise volume (b) bruise area, and (c) bruise susceptibility at different drop impact energy levels; low (20 cm), medium (40 cm) and high (60 cm) during 3-months cold (5ºC) storage.0 = before storage (day 0), Dh = drop height (cm) and S = storage duration (months)

Overall, an increase in bruise size (BV and BA) of pomegranate fruit with increasing storage duration could be attributed to the decrease in turgor pressure in the course of cold storage (Shafie et al., 2015). Fruit contain a large percentage of water that is responsible for turgidity in soft tissues (Singh et al., 2014), which tends to decline due to continuous moisture loss in the course of storage. Reduction in fruit turgor presumably leads to a consequential reduction in tissue sensitivity to bruise damage (Hussein et al., 2018; Singh et al., 2014). In the present study, the tendency of excessive loss in moisture is likely caused by impact bruising on pomegranate fruit at medium and high drop impacts. Lower sensitivity of pomegranate fruit to impact bruising observed in the end of 3-months storage could be the result of drying effect of the pomegranate fruit peel due to excessive moisture loss in fruit stored for more than 2 months leading to hardening of the fruit peel (Aktas et al., 2008; Polat et al., 2012).

The bruise susceptibility (i.e. ratio of the bruise volume to the energy absorbed during impact, mm³/mJ) of pomegranate fruit followed the trend of BV for all drop impact levels as shown by sharp rise in the first month of storage followed by a consistent decline until the end of storage (Figure 1(c)). The values of BS were expectedly higher in the order of 20 cm > 40 cm > 60 drop impacts for the first 2 months of storage. Accordingly, the relationship between bruise volume and the absorbed energy has been described as a simple linear function (Opara and Pathare, 2014; Van Zeebroeck et al., 2007). In the present study, the difference in impact energy due to differences in investigated drop impact heights (20, 40 and 60 cm) reduced the effect of BV for each calculates the ratio of BV to the impact energy-measured ratio for two parameters. Thus, in the present study, the BS may not be suitable as the parameter to compare the sensitivity of pomegranate fruit to impact bruising.

Bruise Size versus Impact Energy Relationship during Long-term Storage

Figure 2 represents the relationship between bruising and the impact energy absorbed at monthly interval. BV increased with impact energy throughout the storage duration, with high and positive correlation coefficients of 0.928, 0.971 and 0.934 after 1, 2 and 3 months of cold storage, respectively. The results also suggest that the linear change of BV of pomegranate fruit with absorbed impact energy increased after a month in cold storage and reached the peak after 2 months of storage. This tendency was also observed in the BA–Ei relationship as shown in Figure 2(b). Furthermore, Pearson correlations in Table 1 show that the plot of BS – Ei followed the trend like that of BV but in the opposing direction. The results revealed that higher values of BS were linearly associated with lower impact energy and vice versa. There was a general decline in linearity between the BS and impact energy with increasing duration of storage from strong lineal relation (r = −0.964) on day 0 to moderate (r = −0.744) at the end of 3-month storage.

Table 1. Pearson correlation coefficients and p-values of bruise size – impact energy relationship fitted by simple linear regression

Storage duration (months)	BV – Ei		BA – Ei		BS – Ei
	r	p-value	r	p-value	r	p-value
0	0.934	0.0001	0.926	0.0002	−0.964	< 0.0001
1	0.928	0.0002	0.862	0.0014	−0.944	< 0.0001
2	0.971	< 0.0001	0.963	< 0.0001	−0.882	0.0008
3	0.934	0.0001	0.923	0.0002	−0.744	0.0108

P-values of all variables are statistically significant at p < 0.05. r = Pearson correlation coefficient; BV = bruise volume; BA = bruise area; BS = bruise susceptibility; Ei = impact energy.

Figure 2. Scatter pots for bruising of pomegranate fruit cv. Wonderful versus impact energy relationship for different storage durations; 0 = before storage (day 0); 1 = month one; 2 = month 2 and 3 = month 3 of cold storage; (a) bruise volume, (b) bruise area and (c) bruise susceptibility

Overall, these results highlight that freshly harvested pomegranate fruit tend to bruise less at higher impact energy absorbed compared to stored fruit as evidenced by lower values of bruise size (BV and BA) before storage corresponding to higher impact energies across all studied drop impact levels (Figure 3). This could be explained by the high strength of the cell wall of freshly harvested fruit tissue that presumably supersedes the effect of high turgor pressure on the cell walls of fruit prior to cold storage (Mirdehghan et al., 2006; Singh et al., 2014). Furthermore, this finding suggests that in order to reduce fruit bruising incidences, most handing of pomegranate fruit, such as packing, sorting, and transportation, should be performed within a short time (<1 month) after harvest. Energy absorbed at impact decreased during storage presumably due to decreasing fruit mass resulting from moisture loss. However, the decline in impact energy in the course of storage had no effect on the increase in bruise size in the first 2 months of storage as described in the previous section.

Figure 3. Impact energy plotted against storage time at 5ºC for pomegranate fruit cv. Wonderful dropped at low (20 cm), medium (40 cm) and high (60 cm) drop heights ; 0 = before storage (day 0)

Puncture Resistance

The effect of impact bruising (p < .0001) and storage duration (p < .0001) on fruit puncture resistance was significant. There was generally higher puncture resistance in medium and high drop impact bruised fruit throughout the 12-week storage period (Figure 4). A significant difference in puncture resistance was observed between bruised and non-bruised (control) fruit from week 6 until the end of storage. During this period, an increase in puncture resistance was significantly higher in medium drop impact bruised fruit (152.9 N), and high drop impact bruised fruit (148.3 N) in comparison to that of control (136.3 N) or low impact bruised fruit (131.2 N). During the same period, the effect of storage on fruit resistance to puncture was also significant (p < .05), reaching the peak value of 160.9 and 158.7 N for medium and high impact bruised fruit, compared to 143.8 and 143.2 N for control or low drop impact bruised fruit, respectively (Figure 4). Changes in fruit resistance to puncture could be attributed to the hardening of pomegranate fruit peel resulting from moisture loss during storage. In agreement with the current study, it has been reported that moisture loss from the fruit led to a hardening of the fruit peel during long-term storage and increased the resistance of fruit to puncture (Arendse et al., 2014). This further suggests that the effect of bruising aided fruit moisture loss during storage, and hence, more hardening of fruit peel resulting in higher resistance to puncture compared to non-bruised control fruit.

Figure 4. Puncture resistance for pomegranate fruit cv. Wonderful subjected to low (20 cm), medium (40 cm) and high (60 cm) drop impact bruising and evaluated for 12 weeks in cold (5 C) storage. Non-bruised fruit were included as control; 0 = before storage (day 0)

Cutting Force, Energy, and Power

Fruit cutting characteristics, including force, energy, and power for bruised and control (non-bruised) pomegranate fruit during storage in cold (5ºC) condition, are shown in Figure 5(a) significant difference in the maximum cutting force between drop impact bruised and non-bruised pomegranate fruit was evident from week 6 until the end of cold storage (p < .05). During this period, fruit bruised at medium and high drop impact had 6–11% higher resistance to cutting than non-bruised or low drop impact bruised fruit that was maintained for the rest of storage duration. Similarly, storage time had a significant effect on the fruit cutting force (p < .0001). During the first 4 weeks of storage, average values of cutting force for medium and high impact bruised fruit were significantly lower than those observed in the subsequent weeks of storage.

Figure 5. Changes in maximum cutting force, cutting energy and power for whole pomegranate fruit cv. Wonderful subjected to low (20 cm), medium (40 cm) and high (60 cm) drop impact bruising and evaluated for 12 weeks in cold (5ºC) storage. Non-bruised fruit were included as control; 0 = before storage (day 0)

The maximum cutting energy for medium and high drop impact bruised pomegranate fruit was generally higher than those of non-bruised or low impact bruised fruit. Pomegranate fruit bruised at medium (40 cm) or high (60 cm) drop impact heights had up to 36–39% higher cutting energy than non-bruised fruit whereas no significant difference (p < .05) was observed between non-bruised and low impact bruised fruit until the end of 12-week storage time. The trend for cutting energy was also observed for cutting power (Figure 5(c)). At the end of 12-week storage, the cutting power was the highest in medium (0.173 W) and high drop impact bruised fruit (0.176 W), and the lowest was recorded for non-bruised (0.127 W) and low impact bruised fruit (0.141 W). Similarly, there was a general increase in the cutting energy and power for pomegranate fruit with an increase in storage duration (p < .0001). The average cutting energy increased from 1472.9 N mm for fresh fruit before storage (BS) to average values of 2674.9 N mm (for non-bruised or low impact) and 3508.6 N mm (for medium and high impact bruised fruit). Overall, these results highlight that both storage time and bruise damage had a significant effect on the mechanical strength of pomegranate fruit and peel. It has been revealed that storage conditions such as temperature, humidity, and storage time have a considerable effect on changes of both the quality and mechanical properties of fruit (Bentini et al., 2009; Ekrami-Rad et al., 2011).

The present study has confirmed that impact bruising increase the resistance to fruit cutting that subsequently affected the force, energy, and power required to cut the pomegranate fruit into two halves, a typical procedure of most pomegranate processing machines prior to removing the arils. Ekrami-Rad et al. (2011) found an increasing trend both in flavedo peak cutting force, force of cutting, whole fruit cutting energy and whole fruit cutting power with an increase in storage time. Nonetheless, the difference in cutting properties for pomegranate fruit between the previously reported data (Arendse et al., 2014; Ekrami-Rad et al., 2011) and the results of the present study highlight the influence of bruising on changes in mechanical properties of pomegranate fruit. For instance, Ekrami-Rad et al. (2011) reported 46% and 80% less force of cutting and cutting power, respectively, after 6 months of cold storage, in comparison to the results of the present study for medium and high drop impact bruised pomegranate fruit. Similarly, Arendse et al. (2014) reported 34% and 59% lower cutting force and cutting energy, respectively, for non-bruised ‘Wonderful’ pomegranate fruit stored at 5ºC for 3 months. Therefore, the results of the present study demonstrated that impact bruising of pomegranate fruit could lead to the increased cost of labor required for manual removal of arils for processing due to increased energy required to separate the arils from the fruit. Bruise damage could also lead to the downgrading of fruit not destined for processing due to deformation resulting from peel hardening and concomitant visual appearance of fruit. In worst-case scenario, fruit could be disposed of due to excessive hardening of peel.

Fruit Compression Profile

Compression Force, Energy, and Power

Results of the compression test for whole pomegranate fruit are presented in Tables 2 and 3. There was a significant (p < .05) difference in the force required to compress the fruit (firmness), the compression energy and power at different drop impacts (drop heights). After 6 weeks of cold (5ºC) storage, firmness of bruised fruit at medium (40 cm) and high (60 cm) drop heights were significantly higher than low (20 cm) or non-bruised fruit (p < .05). During this period, the differences in firmness values between fruit bruised (at medium and high drop impacts) and non-bruised fruit ranged between 30.29 N and 39.85 N. Furthermore, there were differences in values of firmness albeit not significant between medium and high drop impact bruised fruit, and between low drop impact bruised and non-bruised fruit. The trend of change in compression energy and power of pomegranate fruit with both drop impact levels and storage duration was similar to the results obtained in fruit firmness. After the 4-week storage period, there were higher values of compression energy and power in medium and high drop impact bruised fruit in comparison to low drop impact bruised (20 cm) and non-bruised fruit that progressed until the end of the storage trials. For the compression energy and power only high drop impact bruised fruit exhibited a significant increase after 4 weeks of storage. High values of firmness, energy, and power required to compress the fruit that were observed in bruised pomegranate fruit from the first 4-weeks storage period could be due to hardening of fruit peel caused by excessive moisture loss leading to increased fruit stiffness, which is characterized by toughening and increased mechanical strength (Ekrami-Rad et al., 2011).

Table 2. The compression strength properties for impact bruised pomegranate (cv. Wonderful) fruit during cold (5ºC) storage for 12 weeks

Storage duration (weeks)	Drop height (cm)	Firmness (N)	Compression energy (N mm)	Compression power (W)
0	Control	180.43 ± 6.10^ih	411.55 ± 11.67^ih	0.02 ± 0.00^ih
	20	176.30 ± 9.12^i	400.33 ± 5.76^i	0.02 ± 0.01^i
	40	170.88 ± 5.29^gi	395.16 ± 47.77^gi	0.02 ± 0.01^gi
	60	178.25 ± 0.50^ih	406.69 ± 5.01^ih	0.02 ± 0.00^ih
2	Control	184.34 ± 4.71^ih	470.56 ± 3.44^ih	0.02 ± 0.00^ih
	20	188.95 ± 6.21^i	458.63 ± 10.80^i	0.02 ± 0.00^i
	40	181.31 ± 7.61^gie	457.40 ± 8.62^gie	0.02 ± 0.00^gie
	60	201.09 ± 7.61^gif	460.04 ± 15.27^gif	0.02 ± 0.01^gif
4	Control	188.10 ± 6.09^gi	518.30 ± 46.63^gi	0.03 ± 0.01^gi
	20	189.06 ± 7.09^ih	488.64 ± 31.21^ih	0.02 ± 0.01^ih
	40	208.40 ± 6.18^gch	567.81 ± 30.70^gch	0.03 ± 0.00^gch
	60	218.91 ± 3.61^gic	558.84 ± 6.07^gic	0.03 ± 0.00^gic
6	Control	199.36 ± 13.67^gid	539.66 ± 5.70^gid	0.03 ± 0.01^gid
	20	210.56 ± 12.06^gic	535.14 ± 3.87^gic	0.03 ± 0.00^gic
	40	220.89 ± 10.13^a-f	657.32 ± 54.50^a-f	0.03 ± 0.00^a-f
	60	225.95 ± 8.98^a-e	606.70 ± 22.87^a-e	0.03 ± 0.01^a-e
8	Control	218.67 ± 3.50^gic	637.35 ± 5.59^gic	0.03 ± 0.00^gic
	20	214.74 ± 5.29^gid	615.53 ± 12.46^gid	0.03 ± 0.00^gid
	40	228.34 ± 27.64^a-d	673.59 ± 18.45^a-d	0.03 ± 0.01^a-d
	60	237.34 ± 13.02^ab	705.82 ± 27.71^ab	0.04 ± 0.00^ab
10	Control	215.36 ± 3.46^gch	611.13 ± 5.45^gch	0.03 ± 0.00^gch
	20	200.54 ± 5.64^gch	641.82 ± 56.46^gch	0.03 ± 0.00^gch
	40	245.65 ± 6.48^a-c	703.89 ± 9.92^a-c	0.04 ± 0.00^a-c
	60	240.31 ± 6.67^ab	727.78 ± 6.30 ^ab	0.04 ± 0.00^ab
12	Control	209.04 ± 3.07^gb	588.80 ± 49.72^gb	0.03 ± 0.01^gb
	20	205.20 ± 5.52^gb	568.90 ± 48.63^gb	0.03 ± 0.00^gb
	40	240.36 ± 5.47^ab	680.24 ± 21.00^ab	0.04 ± 0.00^ab
	60	231.13 ± 12.07^a	661.50 ± 6.04^a	0.04 ± 0.00^a
Significance level	Drop height (A)	<0.0001	0.003	0.003
	Storage duration (B)	<0.0001	<0.0001	<0.0001
	A × B	0.5632	0.8919	0.8919

Values are presented as mean ± standard error (SE). Means of each parameter followed by different letters across drop impact level and storage duration differs significantly (p < 0.05) according to Duncan’s multiple range test.

Table 3. Elastic modulus and energy at bioyield point of the force–deformation curves for compressed bruised pomegranate ‘Wonderful’ fruit during cold (5ºC) storage for 12 weeks

Storage duration (weeks)	Drop height (cm)	Elastic modulus (N/mm^2)	Bioyield energy (N mm)
0	Control	19.42 ± 1.01^ih	261.93 ± 9.65^ihe
	20	20.33 ± 1.11^i	226.22 ± 21.17^ih
	40	19.26 ± 1.64^gi	281.39 ± 9.36^gi
	60	18.12 ± 0.97^ih	272.51 ± 13.90^ih
2	Control	20.73 ± 1.90^ih	252.35 ± 61.14^ih
	20	22.24 ± 1.53^i	229.64 ± 63.95^i
	40	22.90 ± 2.14^gie	317.11 ± 17.31^gie
	60	23.79 ± 1.55^gif	300.03 ± 14.59^gif
4	Control	23.34 ± 1.50^gi	282.16 ± 3.89^gi
	20	24.12 ± 1.01^ih	270.40 ± 60.43^ih
	40	23.12 ± 2.06^gch	362.95 ± 83.28^gch
	60	25.68 ± 0.95^gic	350.95 ± 22.38^gic
6	Control	25.14 ± 1.90^gid	345.02 ± 10.38^gid
	20	28.85 ± 2.54^gic	347.32 ± 102.62^gic
	40	27.56 ± 1.64^a-f	422.17 ± 25.92^a-f
	60	27.92 ± 3.29^a-e	435.66 ± 9.53^a-e
8	Control	27.42 ± 1.80^gic	355.04 ± 45.45^gic
	20	29.14 ± 0.59^gid	345.27 ± 13.61^gid
	40	32.87 ± 2.31^a-d	448.52 ± 16.05^a-d
	60	30.43 ± 1.83^ab	496.90 ± 8.00^ab
10	Control	26.14 ± 1.71^gch	370.83 ± 9.10^gch
	20	27.83 ± 3.19^gch	368.26 ± 18.13^gch
	40	31.57 ± 1.53^a-c	475.19 ± 16.64^a-c
	60	31.14 ± 2.26^ab	505.29 ± 14.43^ab
12	Control	26.84 ± 1.65^gb	401.67 ± 53.63^gb
	20	25.92 ± 4.32^gb	405.87 ± 34.49^gb
	40	29.67 ± 2.83^ab	525.17 ± 13.60^ab
	60	30.14 ± 2.29^a	536.11 ± 28.39^a
Significance level	Drop height (A)	0.0684	<0.0001
	Storage duration (B)	<0.0001	<0.0001
	A × B	0.9559	0.9884

Values are presented as mean ± standard error (SE). Means of each parameter followed by different letters across drop impact level and storage duration differs significantly (p < 0.05) according to Duncan’s multiple range test.

Changes of fruit firmness, energy, and power required to compress the fruit were significant (p < .05) with respect to the duration of storage. At the end of 12-week storage, medium and high drop impact bruised fruit had respective 1.41 and 1.29 – fold increase in firmness. The maximum firmness was reached after 10 weeks of storage for fruit bruised at low (205.21 N), medium (245.65 N) and high drop impact (240.31 N), which is similar to the results observed for fruit compression energy and power (Table 3). Furthermore, there was a general trend of decline in firmness, compression energy and power across all bruised and non-bruised fruit during the last week of storage. The present study has revealed that bruise damage influenced loss of moisture from the pomegranate fruit peel during storage in cold (5ºC) condition that was associated with shriveling and an increase in the fruit stiffness. The results of an increase in pomegranate fruit firmness with storage time were similar to those obtained in pomegranate cv. Malas Saveh fruit (Ekrami-Rad et al., 2011) during the first 2-month storage.

Generally, the force required to compress the fruit is said to decrease with extended duration of postharvest storage, as reported in pomegranate fruit cv. Wonderful, during 5-month storage (Arendse et al., 2014) and cv. Mollar de Elche (Mirdehghan et al., 2006). Similarly, Ekrami-Rad et al. (2011) reported the decrease in fruit firmness during the last 4-month storage period (from 2 to 6 months) for ‘Malas Saveh’ pomegranates. The decrease in overall fruit stiffness during postharvest storage is attributed to the decrease in turgor pressure and loss of cell-wall integrity of pomegranate arils, resulting from the breakdown of pectin substances (Ekrami-Rad et al., 2011; Mirdehghan et al., 2006). Furthermore, a decrease in firmness of pomegranate fruit during long-term cold storage is also attributed to chilling injuries leading to increased loss of cell-wall integrity (Arendse et al., 2014; Shafie et al., 2015). The present data have been compared with previously reported physico-mechanical data (Arendse et al., 2014; Ekrami-Rad et al., 2011) on the tendency of decreasing firmness of fruit during postharvest storage and found the contrasting results. Possibly, the reason for this disparity could be due to the effect of impact bruising reported in the present finding, in comparison to previously reported data. Changes in compression textural properties observed in the present study were caused by fruit hardening due to increased moisture loss in bruise-damaged pomegranate fruit during storage. The implication of this could be more energy requirements for the processing of bruise-damaged pomegranate fruit that could subsequently lead to the increased cost of processing as opposed to non-bruised fruit.

Modulus of Elasticity and Bioyield Energy

Table 3 shows the modulus of elasticity and bioyield energy for bruised and non-bruised pomegranate fruit during the 12-week storage. Modulus of elasticity of fruit increased gradually and significantly (p < .0001) with prolonged storage, in particular, from 0 to 8 weeks. During this period, there were no significant differences (p = .0684) in modulus of elasticity between bruised and non-bruised fruit. Afterward, modulus of elasticity declined until the end of storage albeit not significant. The effect of pomegranate fruit bruising on the modulus of elasticity was evident during the last 4 weeks of storage duration. The highest and lowest values of elastic moduli were 32.86 N/mm and 26.13 N/mm observed on the eighth week of storage for medium drop impact bruised and non-bruised fruit, respectively. Bentini et al. (2009) stated that when the fruit is exposed under mechanical loading, it exhibits the visco-elastic behavior which depends on both the amount of force applied, the rate of loading and the duration of storage. Fresh tissue is characterized by elastic, hard and brittle behavior which could be affected by moisture loss and drying leading to loss of tissue elasticity and increased tissue deformability (Mayor et al., 2007). This could mean that the elastic behavior of fruit tends to decrease with the storage duration. In the light of the present study, the postharvest storage of bruised pomegranate fruit did not influence fruit elastic behavior. This could be explained by the fact that hardening effect of pomegranate fruit peel due to excessive moisture loss observed in bruised fruit resulted in toughening and an increase in mechanical strength of flavedo (the outer and colored portion of the peel) while maintaining the freshness of the inner content (arils) of the fruit that maintained the elasticity of the fruit. An increase in the elastic modulus of fruit during the first 8 weeks of storage corroborates with the previous work for ‘Malas Saveh’ pomegranate fruit by Ekrami-Rad et al. (2011) who reported 13% increase in modulus of elasticity with an increase in storage time from 0 to 2 months.

There was a significant increase in bioyield energy for bruised fruit, which was about twofold higher than the initial values (day 0) for medium and high drop impact bruised fruit. An increase in bioyield energy with increasing storage duration was also observed in low drop impact bruised and non-bruised fruit, although this change was not significant. Furthermore, pomegranate fruit bruised at medium and high drop impact generally had higher bioyield energy than non-bruised fruit. Furthermore, a significant difference between fruit bruised (at medium and high drop impact level) and non-bruised fruit was evident after 8 weeks of storage until the end of storage duration. From the force–deformation curve for the fruit compression test where bioyield points are determined, the bioyield point occurs where there is an increase in deformation with a decrease or no change in force. Polat et al. (2012) stated that the presence of bioyield point is an indication of initial cell rupture in the cellular structure of the material. Hence, this could mean that an increase in bioyield energy demonstrates decreasing deformability of fruit to compression test as the storage period progressed. The results in the present study conflict with other previous findings, where an increase in the duration of storage was found to increase the fruit deformation in nectarine (Polat et al., 2012) and ‘Wonderful’ pomegranate (Arendse et al., 2014). The decrease in fruit deformation during storage could be the result of excessive moisture loss in bruise-damaged pomegranate fruit, which was characterized by hardening of fruit peel and subsequent increase in resistance to fruit compression.

Conclusion

Changes in bruise damage susceptibility and mechanical properties of impact bruised ‘Wonderful’ pomegranate fruit were investigated during long-term cold storage. There was a general increase in susceptibility to bruising for pomegranate fruit within the first 2 months of storage and then decreased in the last month. These findings highlight the need to carry out most of the fruit handling operations such as sorting, packing and transportation as soon after harvest as possible to reduce bruising. The results of mechanical properties of pomegranate fruit such as puncture resistance and whole fruit cutting force, cutting energy and power as well as fruit compression properties showed that these parameters were influenced by both the level of drop impact bruising and duration of cold storage. Overall, the results of the present study suggest that there was a rapid increase in stiffness for bruised fruit peel with an increase in the duration of storage. This subsequently modified the mechanical strength of the fruit peel. The present work has also revealed that the studied mechanical properties and bruising susceptibility of pomegranate fruit are strongly related to the duration of storage. Hence, these results further demonstrate that the design of pomegranate fruit processing machines for separation of the arils from its rind and/or compression of whole fruit to obtain juice should take into consideration the effect of bruising and storage duration altogether. Overall, the results of textural attributes of pomegranate fruit induced by impact bruising reported in this study highlight some important aspects of fruit processing such as energy requirements for fruit cutting and compression. Processing of bruise-damaged pomegranates may require more energy that could lead to the increased cost of processing, especially during manual removal of arils or processing. On the other hand, excessive moisture loss, hardening of fruit peel and shrinkage, which are all associated with pomegranate fruit bruising potentially alter the morphological appearance of fruit hence impairing the fruit visual quality and consumer acceptability. Hence, the present results provided a contribution to the knowledge to better understand the extent of impact bruising on textural attributes of pomegranate fruit during long-term storage.

Acknowledgments

This work is based on the research supported wholly/in part by the National Research Foundation of South Africa (Grant Numbers: 64813). The opinions, findings and conclusions or recommendations expressed are those of the author(s) alone, and the NRF accepts no liability whatsoever in this regard.

Additional information

Funding

This work was supported by the National Research Foundation [64813].