Mechanical Damages Increase Respiratory Rates of Citrus Fruit

Abstract

The present work intended to evaluate the effects of impacts and compression forces on the respiration rates of citrus fruit. The fruits were compressed with an instrumented sphere and hydraulic jack and impacted by dropping at different heights onto a rigid surface. Five treatments were applied in a completely randomized design. Respiratory rates increased during the first 6 hr after the treatments, and increases depended on the compression intensity and drop height applied. Impacts produce a greater effect on respiration rates of all species evaluated. Oranges submitted to compressions and limes submitted to impacts presented the greatest increases of respiration.

Keywords:

• CO₂ production
• mechanical injury
• oranges (Citrus sinensis)
• tangerines (Citrus deliciosa)
• limes (Citrus latifolia Tanaka)
• tangors (Citrus reticulata x Citrus sinensis)

INTRODUCTION

High postharvest losses and poor fruit quality are important issues in citrus production (Ghafoor et al., 2010). The predominant postharvest problems in citrus are: green mold, caused by Penicillium digitatum (Pers.: Fr.) Sacc.; sour rot, caused by Galactomyces citri-aurantii E. E. Butler (anamorph Geotricum citri-aurantii (Ferraris) Butler); blue mold caused by P. italicum Wehmer and handling injuries, which comprise oleocellosis (Skaria et al., 2003). Reports on postharvest losses of fresh produce, including citrus, range from 5 to 40% of the total production (Ghafoor et al., 2010). In Brazil, there is a lack of studies that identify the total losses of citrus fruit caused by mechanical damages. Fischer and colleagues (2009) determined increases in oleocellosis in Valência oranges varying from 28.8% at arrival to the packinghouse to 53.4% at the packing tables.

From production to consumption, horticultural products are prone to the action of static and dynamic forces during the processes to which they are submitted (Couto et al., 2002). Static forces result in compression damages and dynamic forces in impacts. Probably the most common types of damages in citrus are mechanical injuries, such as cuts and abrasions. Citrus fruit are comparatively resistant to bruising, but they are very susceptible to decay after being scratched or scraped enough to cause small, often invisible, breaks in the peel (Miller et al., 2001).

The resistance to bruising sometimes is understood as the resistance to mechanical damage; however, mechanical damages do not only affect external fruit characteristics, they might also change internal quality parameters, such as flavor and nutraceutical compounds (Montero et al., 2009). Mechanical damages also influence respiration rates in fresh produce (Agar and Mitcham, 2000; Durigan et al., 2005; Mattiuz and Durigan, 2001; Henz et al., 2005). Respiration rates are related to shelf life lengthening of fresh fruits and vegetables with extensive literature on fresh-cut fruits (Soliva-Fortuny and Martín-Belloso, 2003; Lee et al., 2003; Marrero & Kader, 2006; Arruda et al., 2008), as well as on whole fresh fruits and vegetables (Bron et al., 2005; Bassetto et al., 2005; Villanueva et al., 2005; Trinchero et al., 2004).

As the main physiological process of harvested products, respiration is used to predict storage potential (Lau, 1985; Patterson, 1989), and any factor that might have an effect on respiratory rates is critical for postharvest shelf life. Therefore, the objective of the present work was to evaluate the effect of impacts and compressions on respiration rates of citrus fruit.

MATERIALS AND METHODS

Tangerines, limes, and oranges were harvested from private groves and transported by car on the same day to the postharvest laboratory located at the Departamento de Horticultura e Silvicultura in Porto Alegre, Rio Grande do Sul, the southernmost state in Brazil. The harvested fruits were submitted to different degrees of impact and compression forces.

Impact forces were applied by allowing the fruit to drop from 40, 60, 80, or 100 cm heights onto a rigid surface. Each fruit was dropped twice from the same height. Compression treatments were applied with a hydraulic jack fixed onto a metallic plate (Fig. 1). Fruit were set one by one between the compressing device in a series with an instrumented sphere. The instrumented sphere used in the present experiments is part of an analyses system consisting of an aluminum instrumented sphere made up with three metal rings equipped with strain gauges, and an external unit containing the data acquisition system. According to Müller et al. (2008) the instrumented sphere is calibrated to measure static forces. Thus, while the hydraulic jack is activated, the fruit and the sphere are compressed simultaneously and the applied forces are real time identified via software. Compression forces of 15 N, 30 N, 60 N, or 120 N were applied to limes, tangors, and tangerines, and 31N, 62.5 N, 125 N, or 250 N were applied to oranges.

FIGURE 1 Equipment for compression application, formed by a hydraulic jack, two acrylic plates, and an instrumented sphere connected to a computer (a), detail of a treatment application on an orange (b).

After the treatment application, the fruits were maintained at room temperature (20 ± 2°C) for 7 days in glass recipients. The recipients were sealed for 1 hr prior to headspace CO₂ concentrations with a gas analyzer (Climasul Rua Lourenço Pergher, RS, Brazil) equipped with a zirconia detector. Respiration rates were expressed as mL CO₂/kg fruit/hr.

Each treatment was replicated three times and each experimental unit consisted of six fruits in each species. Experiments were conducted in a completely randomized design. Data were submitted to regression analysis by SAS (SAS 9.1, SAS Institute, Cary, NC, USA).

RESULTS

Compression and impact mechanical damages affected the respiration rates of the citrus species (Figs. 2 and 3). Increases in CO₂ production were observed from 1 to 6 hr after mechanical damage application. The higher CO₂ production of the tangor Murcott and Montenegrina tangerines after the 24-hr evaluation period are mainly attributable to the development of rots like Penicillim digitatum.

FIGURE 2 CO₂ production of Murcott tangor (a, e, b), Valência orange (c, e, d), Tahiti limes (e, e, f) and Montenegrina tangerines (g, e, h) submitted to mechanical damage of impact (a, c, e, e, g) and compression (b, d, f, h) and evaluated along 7 days.

FIGURE 3 CO₂ production as a function of the treatment intensity. Mechanical damage of compression (a) and impact (b) in Murcott tangor, Valência orange, Tahiti limes, and Montenegrina tangerines.

CO₂ production rates are dependent on the treatment intensity (Fig. 3). The greater the intensities of the force and the drop heights, the greater the respiratory rates. Increases on CO₂ production of 66.2, 51.8, 53.2, and 25.7% were observed on Tahiti limes, Murcott tangors, Valencia oranges, and Montenegrina tangerines, respectively, when the highest drop height treatments were applied. The most severe compression forces resulted in increases of 41.8, 42.7, 44.9, and 5.6% on the same species. Among the studied species, greater increases on CO₂ production were observed in Valência oranges submitted to compression forces and Tahiti limes submitted to impacts. When comparing mechanical damage effects on CO₂ production, impacts were more severe than compression forces and the effects were more noticeable in all the evaluated species.

DISCUSSION

Cellular respiration is a metabolic process through which chemical energy is produced that is used for internal vital reactions and on the processes involving cellular synthesis and maintenance (Saquet and Streif, 2002). As the main physiological process taking place in fresh harvested products, respiration intensity might be used to predict the storage potential of produce. The greater the respiration of a fresh vegetable, the lower the storability (Lau, 1985; Patterson, 1989) and the lower its shelf life (Kader, 1985; Blanke, 1991; Wills et al., 1998).

Vegetative organs submitted to vibrations and mechanical damage in general increase their respiratory rates in comparison to not injured controls (Pisarczyk, 1982; Salveit and Locy, 1982; Mao et al., 1995). Yet, there are studies that have not determined significant differences in cellular respiration related to mechanical damage (Steffens et al., 2008). Nonetheless, there are citations in the literature of increases in respiratory rates due to mechanical damages in many horticultural crops, such as pears (Agar and Michan, 2000), guavas (Mattiuz and Durigan, 2001), arracacha (Henz et al., 2005), and Tahiti limes (Durigan et al., 2005). Durigan and coworkers (2005) noticed increases in respiratory rates 1.5 times higher for compression damages, 2.2 times higher for cuts, and 3 times higher for impacts in comparison to the not injured fruit. These results point to larger effects of impacts on cellular respiration of the limes, similar to the data of the present work.

Respiratory rates are indicative of tissue metabolic activity and might be practical to infer how fast senescence starts off (Blanke, 1991). Accordingly, the results obtained in the present study corroborate metabolism and senescence progression of citrus fruit after mechanical injury, which might support the rationale of qualitative losses detected in previous studies (Montero et al., 2009).

CONCLUSIONS

There is an increase on the respiratory rates of Valencia oranges, Tahiti limes, Murcott tangors, and Montenegrina tangerines submitted to compression and impact mechanical damages. The increases are influenced by treatment intensity and they occur in the first hours after the treatment application.

Dynamic forces, such as impacts, produce more evident effects on respiration rates of all evaluated citrus species. Valencia tangerines submitted to compression and Tahiti limes submitted to impacts showed the greatest increases on CO₂ production.

ACKNOWLEDGMENTS

The authors wish to thank Ecocitrus for the fruit given for these experiments and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior for the scholarship.