Post-Harvest Conservation of Green Maize in Different Packages

Maize is the second most produced grain in Brazil, which has the potential to produce green maize, which is maize harvested before physiological maturation. Aimed with this work evaluate three packages for postharvest conservation of green maize at room temperature in the conditions of the Brazilian cerrado. The double hybrid AG-1051 were planted. After the cobs were harvested, they were submitted to three treatments, each treatment with three repetitions and each repetition contained 4 cobs. The analyzes carried out were: fresh weight loss, instrumental coloring, titratable acidity content, ascorbic acid content and soluble solids. The results were submitted to the 5% Tukey test and regression. The treatment with polyethylene stood out for the loss of fresh mass, color of the maize cobs as for the levels of titratable acidity, presenting the best results and, for the same parameters, the maize cobs of the treatment with straw showed the lowest results, showing that the conservation straw cobs is the least suitable treatment. As for the content of ascorbic acid and soluble solids, the results showed no statistical difference for any of the treatments showing that they kept the contents unchanged during storage.


Experimental Area
The experiment was conducted in the experimental area of Jataí Federal University. There referred experimental area has climatic classification as tropical savanna (Aw), with rainy season from October to April and dry season from May to September. The medium temperature annual oscillates between 21 °C and 23 °C and the annual rainfall average is the 1700 mm.

Conducting the Experiment
First of all, with the aid of a regulated seeder with 0.45 m, were open grooves in an area of 371 m². Then, the manual sowing of the hybrid AG 1051 was carried out, obeying the 0.3 m spacing between plants in November 2018. The border planting was carried out with the hybrid 2B810 PW following the same spacing of the mounting of the experiment, but with the use of a pneumatic seeder.
After planting, cover fertilization was performed with simple superphosphate, potassium chloride and urea, according to the recommendation for maize culture. Fertilization and planting were carried out manually.
Periodic spraying with herbicides and insecticides was carried out whenever the need was identified. The harvest of the green cross was made in February 2019, 93 days after plantation, in the moment that the grains were in the milky phase, known as "green crop point". The cobs were packed in bags after the harvest, and then were taken to Agronomic Research Nucleus, where they were selected for size, shape, color and injuries. With straw, the cobs were between 20 and 25 cm long, without straw, the values varied between 14 to 19 cm.
A completely randomized design was used, with parcels subdivided over time, with parcels in package [treatment 1: low density polyethylene bags-LDPE with dimensions of 30.5 × 40.5 cm and thickness of 0.02 mm, which were tied; treatment 2: polyvinyl chloride-PVC with dimensions of 28 × 26 cm and thickness of 0.013 mm, were the maize cobs were placed on Styrofoam trays B3 (with dimensions of 18.5 × 23.5 cm, thickness of 16 mm and weight of 5.12 g) and then wrapped with PVC and treatment 3: strawed cobs arranged on Styrofoam tray B3, with nothing covering them] and in the subplots the sampling intervals, with three repetitions and four cobs for repetition, totaling 12 cobs per experiment parcel in each evaluation day.
After the application of the treatments, the packages were placed on benches and were evaluated on days 0; 2; 4 and 5 after storage, at room temperature, without any kind of environmental control, while they were viable for commercialization.

Fresh Weight Loss
Fresh weight loss is determined by gravimetry, considering the difference between the initial weight of the cobs and that obtained in each sampling, carried out though weighing's during all the evaluation days and the results expressed in %.

Coloring
The instrumental coloring of the cobs is given by the coordinates L*, b*, C* and h°, determined with the aid of a digital colorimeter, which measures the reflected light, using a system of cartesian coordinates L*, a* and b*. The coordinate L* refers to the luminosity varying from black to white; the b* coordinate indicates the color variation from blue to yellow, C (chroma) which measures saturation where the value highlights the distance from the luminosity axis and starts at zero in the center and Angle hue (h°) measures the predominance of intensity of the predominant color in the cobs (Kong, Murdoch, Vogels, Sekulovski, & Heynderickx, 2019).

Soluble Solids Content
The content of soluble solids was evaluated in a digital refractometer and the results expressed in °Brix. Titratable acidity was obtained by titrating green maize grains in 0.1 N NaOH, using phenolphthalein as an indicator and the results expressed in citric acid g/100 mL of pulp (Instituto Adolfo Lutz [IAL], 2008).

Ascorbic Acid Content
The ascorbic acid content was evaluated by titrating 2 ml of pulp until a slightly pink color was obtained for 10 seconds with Tillmans reagent [2.6 dichlorophenolindophenol (sodium salt) 0.1%], in which the results were expressed in mg of ascorbic acid/100 g of juice (IAL, 2008). jas.ccsenet.

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Results
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Discussion
Mass loss is an important feature when working with postharvest conservation of green maize, as it is associated with the quality of life during storage and may be associated with water loss caused by metabolic reactions such as sweating and breathing (Mamede et al., 2014).
According to Braz et al. (2006), for fresh vegetables the maximum fresh weight loss must be 10%. However, losses between 3% and 6% are already capable of causing significant loss of quality (Saores, Guimarães, Silva, Durigan, & Silva, 2017). In the treatments with LDPE, PVC and straw, losses of approximately 2%, 4% and 18% were observed, respectively, for the last day of evaluation, demonstrating that the treatments with polyethylene and PVC are below the limit of 10%, being that, in the treatment in which the LDPE was used, the loss of mass did not influence the loss of quality of the green maize, presenting the lowest values of loss of fresh mass during the whole experimental period when compared to the other packages.
Thus, it is observed that the treatment with LDPE stood out among the others and the treatment with PVC showed acceptable results for the standards of vegetables. This is probably due to the fact that the plastic film creates an environment of modified atmosphere that considerably decreases the rate of metabolic reactions keeping the environment inside the tray moist, reducing the loss of water that leads to lower loss of fresh mass (Braz et al., 2006;Sousa, 2013).
In work with minimally processed green maize in a controlled atmosphere, values up to 90.2 were found (Mamede, Fonseca, Soares, Pereira Filho, & Godoy, 2015), indicating that the green maize in this study maintained the brightness of the cobs during the evaluation period for all packages tested with higher values.
The yellow color in green maize, as well as its intensity, may reveal the content of carotenoids present in the cobs. A high content of carotenoids is a very desirable characteristic since it is directly related to the nutritional quality of maize (Mamede et al., 2014). According to Mamede et al. (2015), the carotenoid content is directly related to the b* coordinate value, the same authors observed a maximum b* value of 30.36 in green maize with controlled atmosphere on the day 11 of evaluation under refrigeration.
For the present work, mean values of b* were observed on the 5th evaluation day of 14.99, 14.12 and 13.53 for green maize in LDPE, PVC and straw packages, respectively. Indicating that the modified LDPE and PVC packaging were efficient in maintaining the yellow color of the green maize even under ambient temperature conditions, because when comparing the data with Mamede et al. (2015), the authors used a controlled atmosphere and a temperature of 5 °C.
The C* coordinate is the chromaticity representing the color saturation and, thus, quantifies the intensity of the coloring element so that brighter colors have high saturation, while neutral colors, low saturation (Ferreira & Spricigo, 2017). Soares et al. (2017) found values of Angle hue between 80.7 and 81.4 in post-harvest work with intercropped green maize. The average values for the Angle hue found in this work indicate that the green maize produced in the conditions of the cerrado of Goiás has a more intense yellow color. Once, this angle represents the qualitative of the color, thus, each of the angles 0°, 90°, 180° and 270° represent a color, and the angle of 90° represents the yellow color (Ferreira & Spricigo, 2017).
In work with baby maize production, Von Pinho, Carvalho, Rodrigues, and Pereira (2003) observed values of titratable acidity between 1.50 and 3.50%, indicating that even in the first days, the cops evaluated in the present study had lower levels of titratable acidity, demonstrating the efficiency of the treatments used. Organic acids are present inside the cells in a free form or associated with salts, esters, and glycosides. During the storage, the organic acids can be used as substrates on the respiratory process or be converted into sugars (Souza et al., 2019), which is why on this work there was a decrease in titratable acidity, especially on the treatments with LDPE and PVC since the increase on temperature provided by the packages leads to the increase of metabolic activity (Arruda, Fischer, Jeronimo, Zanette, & Silva, 2011).
The levels of ascorbic acid observed in this work were higher than those found by Von Pinho et al. (2003) and the content of 16.4 mg/100 g described by Franco (2003) as ideal for the production of green maize. This may be a consequence of factors that influence the cobs' ascorbic acid content, such as edaphoclimatic conditions, post-harvest care, among others (Von Pinho et al., 2003).
The levels of soluble solids found in the present study are close to the values found in a similar study, in which the highest content was 5.82 °Brix, a value similar to that found on the first day in the PVC treatment (5.80 °Brix). In comparison with the works found on minimally processed maize and sweet maize, in which the levels vary between 10 and 15 °Brix (Perfeito et al., 2017;Mamede et al., 2015), values higher than those found in the present work.

Conclusions
The use of polyethylene packaging was the most efficient in the postharvest conservation of green maize, providing less loss of fresh mass, retaining the yellowish color of the cobs grains and presenting the smallest decreases in the titratable acidity levels.
The use of cobs in the straw for post-harvest conservation of green maize is impracticable.
All packaging used maintained the levels of vitamin C and soluble solids.