Genetic material

Two genotypes of popcorn maize, one commercial yellow of the North American Yellow Pearl race and one experimental hybrid were used as a result of a cross between a line of North American Yellow Pearl by a line of a population of the native race palomero Toluqueño, resulting in the next genealogy: NAYPP-II × MEX5-77 (Figure 1).

Figure 1 On the left, the genotype NAYPP-II × MEX5-77, on the right commercial popcorn maize of the North American Yellow Pearl race. 

Humidity balance of the seed under modified atmospheres with saline solutions of different concentration

In order to know the concentration of sodium chloride (common salt), to be used to create an atmosphere for the seed to enter hygroscopic equilibrium with the surrounding environment in order to reach the desired moisture content, a test was carried out, based on the proposal of ^{Brenes (2007)}, using solutions of different concentrations of common salt to modify the surrounding atmosphere, taking as a base the concentration of 175 g of sodium chloride dissolved in one liter of water (Table 1), under the premise that a higher salt concentration would generate a low relative humidity, placing 1 kg of grain in storage in blanket bags in hermetically sealed containers for 7 days. A wooden structure with wire mesh was introduced into the plastic containers above the saline solution, keeping the sample out of contact with the solution; fans were placed to homogenize the environment and a HOBO data recorder was placed inside each container to monitor the relative humidity and temperature at regular intervals. The assembly described above is shown in Figure 2.

Figure 2 Airtight structure to generate the modified atmospheres and obtain different equilibrium humidities in the popcorn grain. 

The moisture content of the grain was determined under the different atmospheres modified using the oven drying method at 103 °C, for 17 h according to the protocols described by the ^{ISTA (1996)} in four repetitions per treatment.

Variation factors used to evaluate the busted method in popcorn

For the busted tests, a Daewoo brand microwave oven, Model KOR-164H with the specifications indicated in Table 2 was used, adjusting the oven power to 70%.

Table 2 Specifications of the microwave oven used for the busted tests. 

Using a microwave oven with the specifications described above, the influence of the following variation factors on the busted characteristics was evaluated.

Genotypes: North American Yellow Pearl and NAYPP-II × MEX5-77.

Time of exposure to microwaves: times of 2:15, 2:30, 2:45 and 3:00 min was evaluated.

Grain moisture: grains with different moisture contents generated by modified atmospheres were used (12, 13, 14 and 15%).

Sample size: samples of corn weighing 20, 30 and 40 g were taken.

With the study factors, 96 treatments were generated, which were analyzed under a completely randomized experimental design with a factorial arrangement of 2 × 4 × 4 × 3, with 4 repetitions.

The statistical analysis was carried out with the statistical package SAS (Statistical Analysis System) version 9.0 ^{SAS (2002)}, by means of a factor variance analysis (p≤ 0.05) and Tukey’s mean comparison test (p≤ 0.05; ^{Steel and Torrie, 1996}).

The response variables that were evaluated in the genotypes North American Yellow Pearl and NAYPP-II × MEX5-77 were: volume of expansion in cm³ (VE); type of rosette (TR), where the value of 1 corresponds to the type ‘butterfly’ and the value of 5 corresponds to the type ‘fungus’; pulverized pericarp (P), where the value of 1 corresponds to a completely pulverized pericarp and percentage of non-burst grains (GNR).

Causality relationship between the moisture content of the seed of popcorn maize and its surrounding modified atmosphere

With the results obtained from the different concentrations of common salt that were used in this study, to modify the surrounding atmosphere; a regression equation was generated and a curve was constructed where the different salt concentrations and their corresponding moisture contents of the grain are represented in a range between 10.5 and 16.5% (Figure 3).

Figure 3 Curve for the percentage of moisture in grain depending on different salt concentrations. 

The moisture content of the seed and its physiological quality are jointly influenced by the species and the warehouse environment (^{Brenes, 2007}). The humidity of the seed increases significantly if the storage environment is warm and humid, which is why, at a higher concentration of salt used in this experiment, it generated a lower relative humidity and, therefore, a lower equilibrium moisture content in the seed.

In Figure 3 it can be seen that using a concentration of 186.1 g of NaCl L^-1 of water, a moisture content of 14.5% of the grain was generated, the concentration of 238 g of NaCl produced a moisture content of 13.3%, while the concentration of 292.7 g of NaCl generated a grain moisture of 12.7%.

Genotypes

In the Table 3 shows the means of the variables evaluated by genotype, where it can be seen that the commercial genotype of the North American Yellow Pearl race showed a greater volume of expansion (23.15 cm³) and a lower percentage of non-burst grains (40%), which demonstrates, on the one hand, the long history of genetic improvement in popcorn maize in North America, as mentioned by ^{Ziegler (2003)} and, on the other hand, the lack of attention in the popcorn maize races native to Mexico.

Regarding the type of rosette, the commercial genotype showed a popcorn shape that approximates the ‘butterfly’ type, while the hybrid presented a form that approximates the type ‘fungus’ as shown in Figure 4. And in the variable pericarp sprayed, the commercial genotype exhibited a pericarp that fragmented, while the hybrid presented a pericarp whole and stuck to the popcorn.

Figure 4 Rosette shape obtained in the busted tests. a) rosettes of experimental hybrid corn and b) rosettes of commercial corn of the North American Yellow Pearl race. 

The importance of the genotype in the VE and in the GNR is widely known and mentioned by several authors (^{Gökmen, 2004}; ^{Soylu and Tekkanat, 2007}; ^{Ertaş et al., 2009}). These results coincide with those previously published by ^{Gökmen (2004)}, where it mentions that commercial varieties of popcorn maize produce a higher VE and lower GNR, than the creole varieties; and that this may be due to the fact that the landraces have large grains but they are not uniform.

Exposure time to microwaves

The comparison of means for the times of exposure to microwaves is presented in Table 4. In which it is observed that as the time increases, the volume of expansion increases and the percentage of non-burst grain decreases.

In this study, it is important to point out that when the time of 3:00 min was used in the microwave, there were burns in the popcorn, such as those observed in Figure 5. Therefore, it is not advisable to use this time exposition.

Figure 5 Popcorn produced at a time of exposure to microwaves at 3:00 min. 

There are very few studies where different exposure times to microwaves are tested. For its part, ^{Gökmen (2004)} evaluated the VE and GNR of different popcorn maize genotypes using a microwave of 230 V and 1200 W and used between 2.5 and 3 min of exposure to microwaves.

Grain humidity

In Table 5 it can be seen that a greater volume of expansion is obtained at a moisture 12% in grain and with that same humidity the lowest percentage of non-burst grain was shown. Most authors agree that the range of 11% to 15.5% moisture in the grain produces the greatest volume of expansion (^{Allred-Coyle et al., 2000}; ^{Shimoni et al., 2002}; ^{Gökmen 2004}; ^{Ertaş et al., 2009}); however, the optimum content depends on the variety of corn, the method of busted and the size of the grain (^{Vázquez et al., 2011}).

Sample size

In the comparison of means by sample size (Table 6), it can be perceived that the greater the sample size, the greater the percentage of non-burst grains, and presents a type of rosette that is closer to the ‘butterfly’ type.

Considering that the primary objective of the study is the standardization of a busted method to optimize grain expansion by manipulating the main factors such as moisture in grain, time of exposure to microwaves, sample size and genotype, which affect the bursting of the same, it is necessary to take into account the individual observations resulting from the combinations of the different levels and factors involved; thus, in Tables 7 and 8, the comparison of the means of expansion volume of popcorn maize is presented, simultaneously considering the variation factors: genotype*humidity*time, using a standard sample size of 30 g.

Analyzing the information in Tables 7 and 8, it is considered convenient to provide specific recommendations for each genotype, so it is considered that the best combination to obtain a greater expansion volume in the commercial genotype is exposing the popcorn maize to 2:45 min in the microwave and containing a humidity of 12%, considering a sample size of 30 g. And for the hybrid genotype, the best results were observed, when using the time of 2:30 min, with a humidity of 13%, considering the same sample size.