Response variables in tomato quality

To determine the response variables of fruit quality, samplings were carried out in three parts of the plant: the first, in the 1^st cluster; the second, in the 5^th cluster and the third, in the 10^th cluster. Four fruits were selected per treatment and firmness, total soluble solids content (^oBrix), titratable acidity, pH and vitamin C content were determined.

Firmness: it was measured in the equatorial zone of the fruits, using a digital texturometer (Universal Force Five), with a scale of 0.1 up to 0.32% strength and a conical strut of 0.8 mm registering the reading in Newton (N) of the applied force until the penetration of the prop.

Total soluble solids (ºBrix): were determined in the juice of the fruit by means of a digital refractometer, Atago brand with scale from 0 to 32% and were expressed in ºBrix.

The pH: the 10 g of pulp were crushed with 50 mL of deionized water, filtered to remove the remains of plant tissue and in an aliquot of 5 mL, the pH was determined with a potentiometer.

Titratable acidity: it was evaluated according to the methodology of ^{AOAC (1990)}, for which 10 g of pulp were homogenized in 50 mL of deionized water. A 10 mL aliquot was taken, which was neutralized with 0.1 N (NaOH) and phenolphthalein with indicator. The results were reported as percentage of citric acid, using the equation.

Where: N= normality; VT= total volume (mL of deionized water plus fruit weight); Meq= Miliequivalents of the acid found in greater proportion (citric acid= 0.064).

Vitamin C (ascorbic acid): determined according to the methodology of ^{AOAC (1990)}. 20 g of fresh tissue were homogenized in 30 mL of oxalic acid solution (0.5%), an aliquot of 5 mL was taken and titrated with tilma solution (0.01%) until a visible pink color remained for 1 min. The concentration was expressed in mg 100 g^-1 using ascorbic acid as a standard.

Total soluble solids-titratable acidity ratio: an indicator was obtained, which is the quotient between total soluble solids (SST or ºBrix) and titratable acidity (AT). It was calculated with the following equation:

Where: SST/AT= ratio total soluble solids / titratable acidity; ºBrix= Brix degree of tomato juice; titulable acidity= expressed in citric acid

Evaluation of yield and number of fruits per plant

After the transplant, seven plants of each treatment were selected, to evaluate yield and number of fruits per plant, up to the tenth cluster. Once the fruits were harvested, they were counted and weighed on a scale, to calculate the yield (kg pl^-1) and number of fruits per plant.

Classification of fruit size

In each cut fruits were classified by size, in this work, the fruit was elongated (saladette) in the following categories (large, medium, small and very small), according to the minimum and maximum equatorial diameter, as mentioned Mexican Standard NMX-FF-031-1997 (Table 1).

Statistical analysis of comparison of means

To determine the significant difference of the variables evaluated for the design of blocks at random, a comparison analysis of means was performed, with the Tukey test with a level of significance of 95%, with the statistical package MINITAB.

Total number of fruits

In the Table 2 presents the total number of fruits, which were obtained in the repetitions of each treatment, where on average 62, 78 and 84 fruits were obtained per plant for T1, T2 and T3, respectively. Significant statistical differences were found in the mean values of the total number of fruits in the three treatments. The results obtained from this research are similar to the results reported by ^{Villegas et al. (2004)} with 47.1 tomato fruits variety Gabriela with planting density of 3.8 pl m^-2 grown under greenhouse conditions.

^{Villegas et al. (2004)} mentions that as the density of plantation per square meter increases, the total number of fruits per plant increases, but the size decreases, with size being an important variable for the selection and classification of fruits for export purposes.

Classification of fruit size

The Figure 1 presents a classification of the size of fruits harvested by treatment. The treatment of a stem (T1) was the best in terms of size refers to 69, 23, 8 and 1% of fruits category (large, medium, small and very small) per plant, respectively. The results obtained in this work are similar to those reported by ^{Rodríguez et al. (2008)}, in tomato under greenhouse, since they found that 60% corresponded to extra-large size, 20% to first, 10% to second and 10% to loss.

In the treatment of two stems (T2) were obtained 49, 33, 17 and 1% fruits of size (large, medium, small and very small) per plant, respectively. The results obtained in this work are similar to that reported by ^{Quintana-Baquero et al. (2010)}, in tomato under greenhouse they reported that 9% of extra-large size, 52% of first, 27% of second, 11% of third and 2% of fourth. For the treatment of three stems (T3), 37, 39, 23 and 2% of fruits of size (large, medium, small and very small) were obtained per plant, respectively. No literature review of performance, size and quality of this treatment was found.

Figure 1 Classification of fruit size in the three treatments. 

Tomato quality response variables

The results of the total soluble solids (ºBrix) were found to be that T3 (three stems) in the secondary stem 1 had the highest value with 4.6, 4.65 and 4.83 for the 1^st, 5^th and 10^th clusters, respectively. No significant statistical differences were found in the values of ºBrix in the three treatments or in the clusters Table 3. The results of this work are similar to that reported by ^{Casierra-Posada and Aguilar-Aventaño (2008)}, in tomato, of 4 ºBrix grown under greenhouse conditions.

^{Renquist and Reld (1998)}, mentions that when the fruits have very low total soluble solids, it is because they are harvested very early, without reaching their degree of development; these authors explain the behavior of the tomatoes in some kind of dilution, by which water intake during the development of the fruit exceeds the production of sugars and organic acids.

It was observed that carbohydrates, as shown in Table 3, underwent biochemical changes by increasing the number of stems per plant, since the degradation of the polysaccharides of cell membranes, made an important contribution to the increase in the content of sugars is why the smaller fruits that are produced in T3 (three stems) presented better flavor and vitamin C content and are used mainly for preparation of puree and sauce.

For the firmness variable, T1 presented the highest value with 4.7, 4.43 and 475 in the 1^st, 5^th and 10^th clusters, respectively. Due to the fact that this treatment had a lower leaf area index, the rays of solar radiation reached to the place where the fruits were, which is why they developed a thicker and stronger cuticle to protect the direct sun damage. In addition, protection of the developed cuticle served the fruits to increase their shelf life. In contrast, T3 presented its thinnest cuticle, because its leaf area index was greater than 6, protecting the entry of direct solar rays towards the fruits (Table 4). The result obtained in this work is similar to that reported by ^{Casierra-Posada and Aguilar-Aventaño (2008)}.

Tomato fruits are composed predominantly of parenchyma cells and microfibrils of cells suspended in a matrix of glycoproteins, water, pectin and hemicellulose polysaccharides (^{Scheible and Pauly, 2004}). These compounds give consistency to the tissues and thereby acquire a greater resistance to the penetrometer; therefore, when the fruits are harvested in early degrees of maturation, the enzymatic activity that denatures the compounds that give rigidity to the fruits is lower than in those fruits harvested in late stages of development.

Total soluble solids- titratable acidity ratio: (^{Ballinger and Maness, 1970}, ^{Marshall and Spiers, 2002}), found that a low ratio of total soluble solids/titratable acidity (SST/AT) is associated with low fruit quality, which it is related to weather variables such as cloudy days, excess radiation, temperature and high fruit transpiration.

In this work an average SST/AT ratio of 10.57, 9.98 and 11.29 was found for T1, T2 and T3, which are the values suitable for tomato harvesting since it allows to lengthen shelf life, a variable that is directly related to firmness of the fruit (Table 5). When the fruits have a firmness value of 4.5, their shelf life can range between 15 and 25 days at room temperature of 18 ºC. The results obtained in this work are similar to those reported by ^{San Martín-Hernández et al. (2012)} of 11.66 in tomato grown in hydroponics under protected conditions in different granulometry of tezontle as substrate.

Some researchers have suggested that SST/AT is important to define quality differences between varieties, others indicate that the quality of the fruits can be improved by increasing the total content of sugars and acids. The best way to define the maturity index in a fruit is by estimating the total soluble solids-titratable acidity ratio, this parameter indicates the content of sugars in relation to the lower amount of acidity present in the fruit (^{Kushman and Ballinger 1967}).

^{Galleta et al. (1970)} mentions that the SST/AT ratio is a parameter that determines the resistance of the fruit to detach from the plant, as long as this characteristic is not taken in hours of high temperature or luminous intensity. An adequate ratio of total soluble solids-titratable acidity is a preponderant measure that is intimately related to the quality of the fruit to be transported to great distances (Kushman and Ballinger, 1970).

Regarding vitamin C, it was found that T1 presented 18.38 mg 100 g-1 of vitamin C content in the 1^st cluster, in the 5^th cluster, T2 presented 17.78 mg 100 g^-1 and in the 10^th cluster, T3 presented the highest Vitamin C content with 18.47 mg 100 g^-1. Significant statistical differences were found in the mean values of vitamin C in the clusters (Figure 2). These results are similar to that reported by ^{Ceballos-Aguirre et al. (2012)}, in fruits of cherry type L., cultivated in field with values from 29 to 85 mg 100 g^-1.

Figure 2 Content of vitamin C (ascorbic acid) in the juice of tomato fruits in the three treatments. 

The disadvantage of T3 is that only 37% of the fruits were large for export, 63% were medium-sized fruits, small and very small. The advantage from the nutritional point of view for humans is 63% of the fruits that were smaller had higher vitamin C content. End consumers prefer this size for fresh consumption such as smoothies, salary preparation, as a condiment (puree), since for them these fruits have better aroma and flavor. Finally, all the fruits produced by the plant are used and waste is avoided because it is smaller.

For pH, T1 presented the highest value in the 10th cluster of 4.47 compared to the T3 that presented the same value of 4.4 (Figure 3). These data are similar to that reported by ^{Gomes and Camelo (2002}), in tomato with pH 4.36. This implies that the fruits harvested before ripening fail to perform the biochemical processes that lead to organic acids being transformed into other compounds that determine the flavor and aroma of the fruits. Significant statistical differences were found in the pH values between the clusters.

Figure 3 pH values in the juice of tomato fruits in the three treatments. 

For the variable titratable acidity, the highest value was presented in the T1 with 0.5% in the 10^th cluster, compared to the T3 that presented the lowest value of 0.38% in the 5^th cluster. No significant statistical differences were found in the values (%) of citric acid between the clusters, nor in the three treatments (Figure 4). Results similar to those found in the present experiment were reported by ^{Gomes and Camelo (2002}), in tomatoes stored in controlled atmospheres of 0.45%.

Figure 4 Titratable acidity, expressed as the percentage of citric acid in the juice in the three treatments. 

Performance per plant

The yield obtained per plant for each of the treatments, the highest value was obtained in T1, with 6.55 kg pl^-1, followed by T2, with 5.91 kg pl^-1 and the lowest was T3, with 5.45 kg pl^-1. The data obtained in this work are similar to those reported by ^{Flores et al. (2007)}, of 4.65 kg pl^-1 (20 kg m^-2) in tomato saladette, tequila variety with planting density of 4.3 pl m^-2 with the same geometric characteristics of the greenhouse, same weather conditions and irrigation management and nutrition.