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Introduction
In Colombia at 2013, the lulo culture (Solanum quitoense) reported an increase in its production of 1.87% (MADR, 2014). However, the production growth is accompanied by significant losses at harvest stage, caused by factors such as inadequate practices at harvesting, defective storage infrastructure, high perishability and problems of handling, transport and marketing.
Cooling and packaging are the common strategies deployed to preserve climacteric fruits like lulo. The temperature handling has been the dominant factor for extending the postharvest life of any collected product (Shao-Wei et al., 2017). An increase of 10°C in storage temperature represents an increase in the respiratory activity by a factor of two or three (Silip et al., 2015). Different researches (Nabati et al., 2017; Olivares-Tenorio et al., 2017; Rodoni et al., 2016; Krarup, 1993) show that for some horticultural Solanaceae, temperature and relative humidity are the most important factors in their conservation, suggesting an appropriate storage temperature of 10°C and a relative humidity greater than 90% for eggplant (Solanum melongena), cucumber fruit (Cucumis sativus), red pepper (Capsicum annuum) and tomato (Solanum lycopenicum). In uchuva (Physalis peruviana),García et al. (2014) mention the positive effect of cooling reducing the maturation process. Other case reported by Rugkong et al. (2011), who stored tomato fruits, indicate that cooling reduced ethylene production, color change and firmness loss. According to Balaguera-López et al. (2014), less weight loss and lower color change were observed by dipping lulo (Solanum quitoense Lam.) for 10 min in a CaCl2 solution before storage at 8°C.
Regarding the packing use, Rudra et al. (2013) used active packaging (with microbial and ethylene scrubbers) to store peach and plum at a temperature of 32±2°C. In their investigation an increased shelf life, marketability and retailer profitability through the use of this type of packaging was determined. Other reports involving the use of agro-industrial waste as raw material for packaging have presented satisfactory results: for example, Cavalcante et al. (2015) used waste of minimally processed carrots as edible films. They found that these films have beneficial properties to the consumer such as the antioxidant activity.
In Colombia, the use of different plant leaves as packaging is frequent due to its degradability and its ability to protect fresh or processed products (Díaz, 2012). The bijao leaf (Heliconia bihai) used to commercialize guava paste is an example of this kind of packaging. In addition, this package provides a pleasant flavor and odor characteristic (Prada et al., 2006). "Envuelto de maiz" (a typical food of Colombian Caribbean coast and Panama), is a mass of ground corn/ cob beans, cheese and sugar wrapped in corn leaves and cooked in water (Rodríguez et al., 2008). "Casabe" is bread made from cassava starch, native of the Amazon. In the Department of Bolivar (Colombia), packages of this bread into banana leaves was reported aswell (Hoyos, 2002). The use of banana leaf as primary fruit packaging was explored (Forero et al., 2014) due to its degradability, easy access, and its potential to preserve the fruit harvested quality. However, the effect on postharvest characteristics of this package for different storage temperatures has not been explored in depth.
Lulo is a tropical fruit characterized by its refreshing and intense aroma, attracting domestic and international markets (Forero et al., 2015). Several studies interested in maintaining the qualities of lulo after harvest have been developed (Galvis and Herrera, 1999; Huyskens-Keil et al., 2001; Franco et al., 2002; Casierra-Posada et al., 2004; Forero et al., 2014) using parameters such as weight, color and texture to determine the effects of diverse procedures on the fruits. Some of them affect the pulp taste as titratable acidity, total soluble solids and pH. Monitoring these parameters allows quality control on fruits and vegetables and, as a whole, keeps the nutritional, culinary and commercial value of products reflected on the external appearance (Chacon, 2006).
Thus, this research was aimed to study banana leaf as primary packaging to minimize the loss of quality of lulo stored at different temperatures.
Materials and methods
Fruit material, acquisition and selection
The variety quitoense lulo fruit used in the experiment were grown in San Bernardo, Cundinamarca, Colombia, with geographical coordinates 4°10'44" N and 74°25'20" W. 500 kg of lulo were brought to the laboratory immediately after harvesting procedure. Color uniformity, size and absence of injuries and odors were the criterion considered to fruit selection. Banana leaves were harvested the same day of lulo fruit harvest. To choose the banana leaves, color uniformity and the absence of radiation damage and pest attack were considered. Data of initial characterization as follow: firmness: 18±7.07 N, total soluble solids: 7.14±0.68%, weight: 120±40 g, state at harvest (according with Corpoica, 2001): between 3 and 4 (25-50% of surface was yellow).
Packaging
The distribution of fruit and banana leaves as primary packaging inside the plastic containers was as follows: first, the banana leaves were pretreated by heating in an oven at 80°C during three minutes in order to avoid the spread of fungi of banana leaves. In plastics fruit crates of 80x60x20 cm a first layer of banana leaves was placed to protect lulos and avoid contact with the container surface. After this, a first layer of fruit was placed over the banana leaves. Later, interwoven layers of fruit and banana leaves were added until completing three layers of lulo. This packaging configuration led approximately 220 g of banana leaves used to pack 25 kg of lulo fruit. Then, two treatment groups were studied: lulo fruits packed with banana leaf (PBL) and lulo fruits without packaging (NPBL).
Storage conditions
Before the storage, samples were subjected to impacts to simulate the conditions that often make part of the postharvest process. Ordered fruit in baskets were submitted to mechanical damage for 5, 10, 15, 20 or 25 times (depending on treatment). The beatings were carried out by dropping the basket to a height of 80 cm from the ground. Then, the containers were stored at a temperature of 7°C (RH 90±5%), 10.25°C (RH 85±5%), 13.5°C (RH 60±5%), 16.75°C (RH50±5%) and 20°C (RH 40±10%).
Experimental design
This study was based on a central composite rotational experimental design (CCRD) with two independent factors (or variables): temperature (between 7 and 20°C) and number of impacts suffered (NI) (between 5 and 25). This design helped to form a factorial planning 2k (k = 2 independent variables) with three central points (level 0), four factorial points (levels ±1) and four axial points (levels ± a). In short, 11 experimental trials were packed with banana leaf (PBL) and 11 trials without packaging (NPBL) (Tab. 1). The a-value equal to 1.4142 was used according to a ≈ (2k)1/4.
A second-order model was generated from a CCRD experimental design, wherein the value of the dependent variables is the function of the independent variables, as described in Eq. (1).
This design was chosen because is possible to obtain significant and predictive models and allowed to generate a response surface to optimize the process (Barros Neto et al., 2002).
Fruit quality measurements
The parameters of fruit quality were evaluated in the postharvest laboratory of Civil and Agricultural Engineering Department at Universidad Nacional de Colombia, immediately after harvest, until observe fruit overripe. Four lulos (repetitions) for each treatment were monitored, except for texture test, where twenty lulos at the beginning and twenty at the end of each treatment were evaluated.
The weight was determined by an electronic scale (Precisa Gravimetrics AG, model XT 220±0.0001 g, Dietikon, Switzerland). Weight loss was calculated as:
Where WL was the weight loss expressed as a percentage, W0 was the weight at initiation of treatment for each lulo, and Wi was the weight of lulo for i-day of treatment.
The skin color changes were registered by a digital colorimeter (Konica Minolta, CR-410, Ramsey, USA), using coordinates of CIELAB standard. The total difference between each coordinate obtained was modeled as:
Total titratable acidity (TTA) was obtained by titration with NaOH (1 N), in a sample of the fruit juice (between 0.9 and 1.7 g) and expressed as a percentage of citric acid.
Where, Vf was the final volume of NaOH recorded, Vi meant the initial volume of NaOH recorded, Wm was the weight of the fruit juice sample and the constant 0.64 is the equivalence for citric acid. Change in TA was obtained as,
Where i and f subscripts refer to the initial and final measure, respectively.
The textural properties, defined by Barreiro and Ruiz-Altisent (1996) as firmness and Young's modulus, were evaluated and determined by Magness Taylor test with the use of a texture analyzer (CT3 Brookfield, model CT3-4500, 0 to 4500 g, Middleboro, USA). A ratio to compare the change of Young's modulus from baseline was established. Young's modulus ratio between final and initial value was designated as equation 8.
Where Sf was the final Young's modulus value and Si was initial Young's modulus value.
Statistical analysis
Statistical package STATISTICA software 9.0 - 2001 (Statsoft, Tulsa, OK, USA) was used for ANOVA analysis (significance level α 0.05). ANOVA analysis allowed determinate the effect of the factors and their interactions on the response variables. Statistical models with its corresponding correlation coefficient (R 2), graphs of the response surfaces and contour curves were obtained. Models allowed estimate the optimum storage condition.
Results and discussion
Weight Loss
As shown in figure 1, to stored fruit at room temperature (20°C) and subjected to 15 strokes, average weight losses at 15 d were 8.7% for NPBL fruit and 6.6% to PBL fruit. Forero et al. (2014) reported weight losses of 7.3% for lulo fruits stored in plastic crates and 3.8% for fruits with the proposed packaging. Damage generated by impacts resulted in a higher percentage of weight loss for this assay. Balaguera-López et al. (2014) reported for lulo weight loss of 12.9% at the same storage time and storage temperature.
FIGURE 1 Monitoring of the weight loss in storage time for treatments with NI = 15 impacts. Storage temperature at: (A) 7°C; (B) 13.5°C, and (C) 20°C.
Reina et al. (1998) who worked with lulo stored at 12°C, reported weight losses of 7.7% at 18 d. In this research, similar temperature storage was used (at 13.5°C). To this condition, fruit subjected to 15 impacts and stored by 20 days, the weight losses were 10.9% for NPBL fruits and 9.4% for PBL fruits. The difference between Reina's results and the weight losses reported in this research may be caused by the storage temperature difference (difference by 1.5°C) and physical damage generated. This postharvest stress caused accelerated metabolism in fruit which resulted in greater weight and fruit quality losses.
As shown the figure 1, lower weight loss for fruits stored at temperatures above 13.5°C was found to PBL configuration compared to NPBL configuration. This may be a result of the characteristics of banana leaf to protect the lulo fruit to weight loss caused by fruit transpiration. In the first days of storage, the moisture content of the banana leaf decreased ("weight loss banana leaf") and then this leads to weight loss of lulo by transpiration (Forero et al., 2014).
Moreover, for fruit stored at 7.5°C, Forero et al. (2014) reported weight losses of 5.8% at 18 d of storage. For this research, the lowest storage temperature was 7°C and this condition caused weight losses of 2.5% to NPBL fruit and 2.6% for PBL fruits at 17 d of storage. For this case, the temperature was the main factor in weight losses and there were not significant differences for the two packaging configurations studied (P>0.05).
Compared to fruit packaging used conventionally such as cardboard and wood crates, Bonilla (2010) reported weight losses of 6.8% with wood crates and 7.1% in cardboard packaging for lulos unrefrigerated (20±5°C) by 10 d. For this investigation, average weight losses of 7.2% for NPBL fruits and 5.3% for lulos PBL were recorded to day 13.
Similarly, Bonilla (2010) performed the same test for refrigerated (8±1°C) fruit and he reported weight losses of 1.9% for wood crates and 4.2% for carton packaging, both results for ten days of storage. For this study, the average weight loss for lulos stored at 7°C, at the 11th day was 1.6%.
The proposed packaging configuration yielded comparable results to conventional packaging reports for lulo. The banana leaf besides showing that fulfills its function of reducing weight losses by transpiration, also offers other advantages such as its low cost, its ability to be individually wrapped and especially its biodegradability, comparatives advantages over other materials used in food packaging (Bonilla, 2010).
Color change
A model to describe the change of the variables L* a* b* was not obtained for NPBL fruits (for all the factors P>0.05). However, for PBL fruits the models of equation 9, 10 and 11 were obtained:
Where T is storage temperature. For all three models, the temperature factor (quadratic) was significantly different (P = 0.0103 for change of L*, P = 0.0122 for change of a* and P = 0.0068 for change of b*). The change in b* represents the blue-yellow changes and their model showed a better fit. Mejía et al. (2012) reported that this variable is the key to track the increase in the characteristic orange hue of lulo, and in the case of this fruit is postulated as a variable to summarize the changes in the external appearance of fruits harvested and stored.
The combination of storage temperature and packaging plays an important role in the degradation of pigments in the lulo. This combination can decelerate the changes of color as a result of the ripening process (Salinas et al., 2010). The packaging besides protecting the fruit as we deduced in the analysis of weight loss, also reduces the amount of oxygen that available for the lulo, what it brings consequently the delay to manifest the characteristic yellow-orange hue of a mature lulo.
Total titratable acidity (TTA)
Initial TTA in all samples were similar (P>0.05). Temperature has an important effect on this variable. Significant differences were found between treatments stored at 7 and 20°C. Similar results were found by Díaz and Manzano (2002), who worked with lulo treated with sodium hypochlorite 2% for 5 min and stored at 5, 10 and 15°C for 21 d.
A model for change in TTA for NPBL fruits (Fig. 2) was obtained. This model responded to the number of impacts (P≤0.05) and an adjustment (R 2 0.483) was determined. The resulting model is described by equation 12, where NI is number of impacts.
FIGURE 2 A. Response surface for the change of TTA; B. Contour curves for the change of TTA; C. Tracking of TTA values during the storage time for PBL fruits and D. NPBL fruits, at NI = 15 impacts (central NI value).
When the impacts received by the fruit increased, the difference between the initial and final value of TTA increased. This is probably caused due to the increase in respiratory rate and then the degradation of organic acids reserves in the fruit as a result of respiration process, as Balaguera-López et al. (2016) found for uchuva (Physalis peruviana) storaged at 18°C throughout its ripening process. The greatest variation was found when the fruit receives fifteen impacts. For PBL fruits, an average value of 0.672% in the change in TTA of was obtained. A model was not obtained on the basis of the factors considered in this study and no statistical evidence of the significance of these factors on the change of TA value (P>0.05) was found.
Considering figures 2C and 2D during the first 10 d after the harvest of lulo, the reduction in TTA values was more evident in NPBL fruits than PBL fruits. Then, using banana leaves as packaging for lulo we can expect a steady performance of this variable to a storage period not more than 10 days.
Firmness
The average values for firmness in this research are observed in table 2. Firmness values between 13 and 23N were obtained for freshly harvested lulos, between 4.9 and 19 N, and between 5.4 and 11.6 N, for NPBL and PBL in ripe lulos, respectively. Galvis and Herrera (1999) reported values of 42.26 N for half-ripe fruits and 17.8 N for ripe fruits. Ospina et al. (2007) also found values of 15.7 N for unripe and 14.4 N for ripe fruits.
TABLE 2 Results of maximum strength (firmness) and modules of Young's modulus (stiffness) for fruit PBL and for fruit NPBL.
A decrease of the storage temperature and an increase of the CO2 concentration (by use of packaging) result in a reduction of metabolic activity and a decrease in the rate of degradation of organic acids and hence, the evolution of physicochemical and quality properties such as color, firmness and shelf-life (Alam and Goyal, 2006; Gwanpua et al., 2012).
Also, the variation on this parameter corresponds to the anisotropy of the material. Its value will depend of the physiological conditions of the fruit, postharvest time and load conditions. In addition, fracture strength values indicate that freshly harvested lulo resists greater burden than lulos the end of each treatment, similar results are reported by Ospina et al. (2007) and SENA (1999).
A model of this response variable in function of temperature (P≤0.05) for lulos PBL was obtained with an adjustment of R2 0.854. The NI variable did not have the incidence expected. Storage conditions are the factor affecting the degree of mechanical strength. Nevertheless, adequate storage technique must be ensured to protect them from damage during handling postharvest. To NPBL a significant model was not obtained.
Young's modulus
The mean values of the modules of Young's modulus or deformability of the lulo epidermis can be seen in table 2. In general, for lulos PBL this value is higher, which mechanically represents that these fruits will be deformed unless than lulos NPBL under the same compressive stress. Water loss is one of the main causes of commercial and physiological deterioration of fresh produce, in the form of wilting, shriveling, and decrease of stiffness, turgidity and succulence (Rodov et al., 2010). Then, PBL lulos allowed to loss less water and lower reduction on stiffness.
The model of Young's modulus ratio was only obtained for fruit PBL.
The results for NPBL have a greater dispersion because the blow was more random for this treatment. The banana leaf besides of cushion the blow, also keep the order among the layers of fruit.
Number of impacts (NI) did not have influence on final Young's modulus (Linear P = 0.5029 and quadratic P = 0.3783). However, as seen in figure 3, for lulos PBL, a model based on the Young's modulus mean and the temperature (linear P = 0.0152 and quadratic P = 0.0145) was obtained to predict this parameter. The proposed packaging configuration can cushion the blow that receives lulo.
Conclusions
In this research, the banana leaf as primary packaging reduces the damage caused by the impacts. As a result, less deformation and less mechanical damage to lulos packaged under this configuration were obtained. The packaging configuration consisting in lulos packed with banana leaf in plastic crates of 80x60x20 cm is an easy alternative to gets and preserves the quality of lulo fruits for a longer storage time. For PBL configuration of three layers of fruit, temperatures below 13.5°C and strokes not more than 15 are suggested. Analogously for NPBL fruits, strokes below 10 impacts are suggested.
However, to avoid color degradation and changes in the TA value mechanical damage is not recommended. The influence of impacts to the fruit on TA value and pigments was verified.
Regarding the temperature conditions, when a storage temperature of 7°C can be ensure, use of banana leaf is not necessary to reduce weight losses. However, for future research, use of the proposal packaging configuration to protect the fruit of chilling injury caused by low cooling storage temperatures will be interesting.
For further tests, explore this kind of packaging with other tropical fruits is suggested. Also, analyze the fruit-packing-storage interaction by mathematical modeling will be necessary.
