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INTRODUCTION
Figs are one of the most important commercial fruits around the world because of their nutritional compounds, taste, and traditional production. Figs are commercially grown in Brazil and 28,044 t of the fruit were produced in 2014, mainly in the southeastern and southern Brazilian regions (IBGE, 2016), for fresh fruit consumption or for industries. In Brazil, the fig for fresh consumption - called "mature fig" -is harvested at the maturation stage and the fig for processing - called "green fig" - is harvested before ripening.
However, certain concerns about fig tree (Ficus carica L.) plantations have prompted studies to find solutions, such as the need to address the perishable nature of the fresh fig, which rapidly decreases the quality of the fruit after harves, because it is subject to rapid physiological breakdown. This characteristic affects the efficiency of the commercialization and exportation of the fruit around the world (Gözlekçi et al., 2008; Paula et al., 2007; Sharma and Singh, 2013). The postharvest life of the fresh fig fruit is normally 7-10 d even when it is stored under low temperatures (Ozkaya et al., 2014).
Some studies have been conducted to investigate the effects of substances applied on figs to improve its postharvest conservation, such as calcium chloride, fungicides, sodium hypochlorite, and 1-methylcy-clopropene (1-MCP) (Gözlekçi et al., 2008; Irfan, 2013; Paula et al., 2007; Watkins, 2008). 1-MCP is an unsaturated cyclic olefin that acts as a competitive ethylene antagonist, i.e. it blocks ethylene receptors and can be used to control ethylene production, respiration rate, and softening and extends the shelf-life of a wide range of fruits (Freiman et al., 2012; Sozzi et al., 2005; Terra et al, 2014; Watkins, 2008). However, 1-MCP applications on fruits may not always have similar results on the postharvest quality since its effect has been shown to vary according to the climacteric fruit species, cultivar, maturation and ripening stages of the fruit, and 1-MCP application forms (Freiman et al., 2012; Sozzi et al., 2005; Watkins, 2006; Zhang et al., 2017).
There are few reports on the postharvest behavior of figs treated with 1-MCP. Gözlekçi et al. (2008) showed that 10 µg L-1 of 1-MCP slowed the softening of Bardakci figs and retained fruit firmness during a 15-day storage period in styrofoam trays wrapped with commercial cling film at 0°C and 9092% relative humidity (RH). D'Aquino et al. (2003) treated Bianca figs with 400 µg L-1 of 1-MCP at 20°C for 24 h and concluded that 1-MCP treatment of the figs and storage for 7 d at 15°C may improve their storability.
To the best of our knowledge, very few studies have reported on the postharvest management of fresh fig cultivars and even fewer have considered post-harvest behavior after 1-MCP-treatment. The aim of this work was to evaluate the effects of 1-MCP applications on the postharvest quality of Roxo-de-Valinhos figs.
MATERIALS AND METHODS
Study area
Figs were harvested from plants grown in a orchard located in Pinhais County, Paraná State, Brazil (25°26' S and 49°16' W; 947 m a.s.l.), with a Cfb Koeppen climate, average temperature of 24°C maximum and 11°C minimum, and annual rainfall average of 1,500 mm. After harvest, the figs were immediately transported to the Postharvest Laboratory for the experiment procedures.
Plant material
The fig trees were 5-year-old Roxo-de-Valinhos cultivars. The plants were cultivated in an organic system, subjected to annual drastic pruning, and spaced at 1x3 m. Annual fertilization was done using cattle manure at 60 L per plant, three times every 60 d starting from August. Diseases were controlled by spraying with lime sulfur after pruning and a Bordeaux mixture during the vegetative and production stages of the fig trees. To control weeds, mechanical weeding in the crop rows and mowing were used, integrated with black oat (Avena strigosa L.), planted in July once a year. The figs were hand-harvested on 12 April, 2014, collecting fruits that showed changing skin color, green to reddish, at stage 5 (Freiman et al., 2012).
Experiment description
Harvested figs were separated into lots at the Post-harvest Laboratory so that each lot had 10 figs, enabling storage for 0, 4, 8, 12, and 16 d, and they were placed in a paper box. Thereafter, the figs were ready to be treated with 1-MCP.
Postharvest 1-MCP treatments
1-MCP was used in the form of SmartFresh® powder (0.14% active ingredient). The figs were treated with the 1-MCP solution of the corresponding concentration. The 1-MCP doses were measured with a precision balance, and then they were dissolved in distillated water to obtain each deluded solution. The 1-MCP concentrations were 5, 10, and 20 µg L-1. The untreated control (0 µg L-1) only used distilled water.
The treatments were applied in plastic gallon containers (70 L), which already contained figs indoor. The 1-MCP solutions were injected into the containers using plastic syringes, which were immediately sealed in order to preserve the vapor of the solutions. Thus, the 1-MCP powder was introduced into a syringe, supplemented with water until a specific volume for the required concentration, and immediately applied to the containers. The duration of treatments was 24 hours.
Storage conditions
Following the 1-MCP treatment, the figs were placed in paper boxes capable of storing 8 fruits and stored at 4±1°C and 90-95% RH for 16 d. For each treatment, three replicates of each sample with 10 figs per replicate were used for the physical and chemical quality analyses; two figs were analyzed immediately post 1-MCP-treatment and the other 8 figs were stored. There were five storage periods (0, 4, 8, 12, and 16 d) in order to evaluate the postharvest quality conservation.
Physical and chemical qualities
For measuring the firmness, total soluble solids (TSS) concentration, total titratable acidity (TTA), and weight loss (Instituto Adolfo Lutz, 2008; Leonel and Tecchio, 2008), samples were periodically withdrawn from the refrigerator and two figs per replicate were used for the analyses every 4 d. The peeled-fruit firmness was determined using a manual penetrometer (PTR-100) with a 7.9-mm-diameter tip and expressed in pounds (lb); the data were multiplied by 4.44 to be expressed in Newton (N). The stored figs were taken from the refrigerator to put them on the penetrometer so that they could be analyzed on the equatorial part of the fruit.
The fruit juice was extracted using a centrifuge to measure the TSS and TTA. The TSS content was measured with a handheld refractometer using a drop of juice and expressed as °Brix. The TTA, expressed as % citric acid, was measured using 100 mL of the solution (10 mL of juice + 90 mL distilled water) that were immediately titrated with 0.1 N NaOH using three drops of phenolphthalein as an indicator.
The weight loss of the stored figs was analyzed using the relative weight variation (%) of two figs per replicate. These figs also represented a storage period of 16 d. The fruits were weighed at harvest and at every storage period, and the weight loss was also determined. The fruit weights were determined using an electronic balance.
Experiment design
The experiment was carried out as a completely randomized factorial design with three replicates and two figs per replicate. Thus, a 22 factorial arrangement was used: (1) 1-MCP concentration with four levels (0, 5, 10, and 20 µg L-1) and (2) storage period at five levels (0, 4, 8, 12, and 16 d).
Statistical analysis
The data were submitted to analysis of variance (ANOVA) using the Sisvar Statistical Program, version 5.3 (Ferreira, 2010). When treatment effects were detected with ANOVA, multiple comparisons for the mean were done using the Tukey test at a 5% probability level. Regression analysis was used when a qualitative parameter expressed significant effect.
RESULTS AND DISCUSSION
Firmness
The 1-MCP treatments showed no influence on the firmness of the fig fruits. However, there were significant effects of the 1-MCP treatments on the fig firmness over the different storage periods (Tab. 1). The 20 µg L-1 1-MCP treatment showed the best firmness result after 12 d in storage (26,8 N) although the 5 and 10 µg L-1 of 1-MCP showed similar results to 20 µg L-1 of 1-MCP after 8 and 16 storage days.
Table 1 Effects of the 1-MCP treatments and storage periods on the firmness of ripe figs.
Values followed by the same uppercase letter in a row and lowercase letter in a column are not significantly different from each other according to the Tukey test (P≤ 0.05).
The apparent increase in fruit firmness observed during storage could be related to the fresh weight loss. Fruit shriveling may hamper penetration of a penetrometer tip into figs because weight loss causes laxity and elasticity. Pectins act as a cementing material and are found mainly in the cell wall, being responsible for fruit firmness (Álvarez-Herrera et al., 2016). Thus, the 1-MCP promoted less pectin solubilization in the treated figs, which probably caused delay of the ethylene effect on the pectinase enzymes present in the cell wall; consequently, fuit firmness was promoted (Blum and Ayub, 2009; Oliveira et al., 2005; Terra et al., 2014).
Ribeiro et al. (2012) observed that the resistance grape peels increased when they shriveled in their final ripening stage. Brunini et al. (2011) also reported that weight loss in pitaya fruit resulted in peel laxity, causing difficulty for penetrometer tip penetration. Brackmann and Saquet (1995) and Jeronimo et al. (2007) also studied the firmness of persimmons and mangos, respectively, and suggested that fruit firmness may increase because of fruit shriveling.
Total soluble solids
The results of the present study showed that the 1-MCP treatment influenced the TSS concentration. The °Brix degree reduced with increasing 1-MCP doses, until 12.2 µg L-1 (Fig. 1). According to Blum and Ayub (2009), 1-MCP exhibits ethylene biosynthesis and consequently affects the rate of starch degradation, which is responsible for increasing the concentration of TSS. In the present study, 1-MCP likely exhibited ethylene production mainly from the dose of 12.2 µg L-1. Sisler et al. (1996) showed that exposure to 1-MCP for 6 h at 500 mL L-1 made bananas insensitive to ethylene for 12 days and carnations for 24 h, but mature-green tomatoes became insensitive to ethylene for 8 days at 5000 mL L-1 of 1-MCP. Freiman et al. (2012) showed that preharvest untreated and 1-MCP-treated figs showed different concentrations of SS, implying that the level of SS in untreated figs increased significantly (about 22%), but not in treated figs.
Values followed by the same uppercase letter in a row and lowercase letter in a column are not significantly different from each other according to the Tukey test (P≤ 0.05).
The storage periods also demonstrated a significant effect on the SS concentration. The SS levels increased as the 1-MCP doses increased (Fig. 2). Blum and Ayub (2009) also studied the effect of 1-MCP applied on persimmon and observed that SS levels increased for 20 d. The increase of TSS in fruits can be explained by the hydrolysis of various polysaccharides, such as starch, pectins, and other oligosaccharides in the cell wall, which, when solubilized in the aqueous phase, become part of the cellular juice, as well as by starch accumulation during fruit maturation, which degrades into sugars through the enzymatic action of a-amylase, p-amylase, and starch phosphorylase, increasing TSS concentration (Deaquiz et al., 2014). There was no interaction effect of the levels of 1-MCP treatment and storage time.
Total titratable acidity
The acidity of the figs was affected by the interaction between the 1-MCP treatment and storage time (Tab. 2). Higher acidity was observed in the 1-MCP-untreated figs stored for 8 d and in the figs treated with 20 µg L-1 of 1-MCP stored for 12 d. In general, little increase in acidity was observed during the initial of storage period and, thereafter, it decreased during the final of storage period for all treatments, indicating an apparent acidity establishment. Candan et al. (2011) treated plums with 1-MCP and observed that Royal Zee cultivar fruits showed similar values of TTA in both control and in treated fruits. According to the authors, maintained TTA levels might affect a fruit's final sensory quality. Paula et al. (2007) also observed increased acidity initially, and, later, decreasing acidity in figs treated with fungicide and calcium chloride, which they attributed to fruit quality improvement through mainly lower weight loss and higher firmness.
Weight loss
As expected, the fruit weight loss was very prominent as the storage period was prolonged (Fig. 3). Paula et al. (2007) also showed similar results. The 1-MCP did not significantly influence the fruit weight loss; neither did the interaction between storage time and 1-MCP. However, Ozkaya et al. (2014) showed that a 1-MCP treatment inhibited ethylene evolution and decreased fruit respiration in 'Bursa Siyahi' figs, which likely resulted in less weight loss in the 1-MCP treated fruit. They highlighted the fact that weight loss is one of the most important causes of fruit quality deterioration.
Figs are highly perishable fruits, in which events such as respiration, dehydration, and degradation happen very fast (Gözlekçi et al., 2008; Ozkaya et al., 2014). In addition to this, figs are morphologically interesting fruits that have a particular structure called ostiole that increases water loss through evaporation by exudation of a syrupy liquid through this orifice (Campos, 1994; D'Aquino et al., 2003; Freiman et al., 2012). It is possible that the ostiole is one factor masking 1-MCP treatments of fig. For this reason, it may be interesting to investigate the effects of higher 1-MCP-doses and longer treatment periods combined with other techniques, such as bagging or wrapping, on the conservation and quality of Roxo-de-Valinhos figs.
CONCLUSION
The 1-MCP treatment improved the quality (firmness and acidity) of ripe Roxo-de-Valinhos figs during storage. Testing doses higher than 20 /µg L-1 of 1-MCP would be interesting because it was observed that the soluble solids tended to increase from 10 to 20 /µg L-1. On the order hand, the 1-MCP treatment was not able to reduce the high weight loss in the Roxo-de-Valinhos fresh ripe figs.
