Vegetal material

The fruits of xocotuna, tuna and xoconostle were used (Table 1) fresh in physiologically mature according to the commercially available visual parameters of fruit size and filling, uniformity and color intensity characteristic for each cultivar; as well as the absence of defects and decay. The experimental determinations were made from fruits harvested in the months of June, August and October of 2013. The plant material came from the Orito, Zacatecas, Mexico. The experimental design was completely random, with four replicates. An experimental unit was considered a fruit. The evaluation comprised 24 days of fruit storage under ambient temperature conditions, determinations were performed every 3 days from zero storage day.

Table 1 List of tunas, xocotunas and xoconostles evaluated. 

Parameters evaluated

The weight loss was assessed using a digital scale, so that weight loss percentage data were obtained by difference between the initial weight and the final weight of each evaluation day, using the formula: (%) PP= (Pi-Pf)/Pi*100, where (%) PP= weight loss (%), Pi initial weight and Pf= final weight of each evaluation period. For color, the hue or hue angle was determined at three opposite sites of the middle part of each fruit, the values were obtained directly with a spectrophotometer (X X-Rite SP62); the readings were taken in the pulp. The thickness of the shell was measured in the equatorial zone of the fruit with a vernier and the result was expressed in mm. The total soluble solids were determined in the juice by a Atago digital refractometer, the results were expressed in °Brix at 20 °C. The titratable acidity was titrated (^{AOAC, 2003}), whereby 5 g of pulp were homogenized with 25 mL of distilled water, then a 5 mL aliquot was taken, which was titrated with NaOH 0.1N and phenolphthalein as indicator; the results were expressed as percent citric acid. For the statistical analysis were performed, an analysis of variance (Anava) and comparison of means Tukey (α= 0.05) using the Statistical Analysis System, ver. 9.0 (^{SAS, 2002}).

Weightloss

According to the percentage of weight loss ‘Rojo Pelón’ was the one that presented the smallest loss during the whole evaluated period. It was also observed that the ‘Cuaresmeño Blanco’ xoconostle had the greatest weight loss (12.59%) at 24 days (Table 2). It is known that a weight loss greater than 5% may be enough to decrease the quality in different fruits, however for cactus fruits it is considered that a loss of around 8% negatively affects their appearance and texture (^{Wills et al., 2007}). This can be closely related to the loss of water through transpiration, which results in weight losses over time (^{Ávila et al., 2007}). The above results indicated that there is no exclusive behavior of tuna, xocotuna and xoconostle to avoid greater weight loss.

Table 2 Percentage of weight loss evaluated in fruits of tuna, xocotuna and xoconostle cultivars.  

Tone or hue angle

The hue or hue angle indicates the coloration, the cultivars evaluated in the present study ranged from light green to purple depending on their typical hue (Table 3), resulting in angles greater than 12 and less than 100°, presenting the ‘Cuaresmeño Blanco’ xoconostle the highest value (95.9 at 15 days) and the ‘Coral’ xocotuna the lowest (6.09 at 9 days).

Table 3 Tone angle (°h) of pulp evaluated in fruits of tuna, xocotuna and xoconostle (am= yellow; vc= light green; rj= red; pu= purple; rs= pink).  

The studied cultivars also showed levels of angle of tone similar to those found for ‘Gavia’ (107), ‘Cardón’, (24.2), ‘Rastrero’ (25.3), ‘Duraznillo Rojo’ (29.4), ‘Tapón’ (24.3), ‘Amarillo’ (103), ‘Pelón’ (26.6) and ‘Duraznillo’ (89.3) (Chávez-Santoscoy et al., 2009). The cultivars of tuna and xoconostle showed similar values and those of xocotuna had the lowest values since the beginning of the evaluation due to the particular characteristics of each of these.

Thickness of the shell

The variability was observed in relation to the thickness of the shell in such a way that the cultivar ‘Cuaresmeño’ showed the greatest thickness (14.28 mm at 6 days) as opposed to that of ‘Forage Slab’ with the lowest value (1.11 mm at the 15 days). It was also observed that the cultivars of xoconostle had the greatest shell thickness, followed by the xocotunas and finally the tunas during the evaluation period (Table 4). This makes them attractive because the shell is the usable portion of these fruits (^{Gallegos-Vázquez et al., 2014}). On the other hand, an important reduction in the thickness of the shell was appreciated as the evaluation period progressed. This reduction was possibly due to the loss of moisture of the shell, which may be one of the main causes of the decrease in the thickness of the shell, as it was verified in the fruit (Table 2).

Table 4 Shell thickness (mm) evaluated in fruits of tuna, xocotuna and xoconostle cultivars.  

Another reason for the reduction of the thickness is due to the possible hydrolysis of the starch in the fruit ripening process, which is not very convenient because they influence the postharvest storage potential insofar as they behave as a front barrier to the desiccation, chemical attack, mechanical lesions and microbial attack (^{Lara et al., 2014}).

Total soluble solids

In this respect, it was found that the initial total soluble solids contents had close values between the cultivars of tuna (12.47 and 14.52 °Brix) and xocotuna (8.12 and 12.92 °Brix), unlike the cultivars of xoconostle that showed the lowest levels (between 4.52 and 4.92 °Brix) (Table 5). The sugar content of cacti fruits is essentially determined by the harvest time, because the post-harvest changes become relatively small, so the accumulation of sugars during ripening of the fruits of Opuntia ficus-indica is related to a contribution of the metabolism of the shell and not with a decrease in the content of starch and mucilages of the pulp as happens in the fruits of pitahaya (^{Barrera and Nobel, 2004}).

Table 5 Total soluble solids content (°Brix) in fruits of tuna, xocotuna and xoconostle cultivars.  

Titratable acidity

The titratable acidity values for tuna and xocotuna were in a range that was 0.04-1.06%, while for xoconostle was 1.68-4.23% (Table 6). The acid titratable levels found were similar to those reported by ^{Ávalos-Andrade et al. (2006)} with average values for the species O. amiclaea, O. oligacantha and O. matudae of 0.07, 0.43 and 0.47% respectively, and those shown by ^{Sedki et al. (2013)} for the ‘Hawara’, ‘Imimkorn’, ‘Achefri’, ‘Aissa’ and ‘Moussa’ ecotypes with titratable acidity values of 0.04 to 0.07%, in turn mentioned that this parameter is inversely proportional to the sugar content in the fruit. And that changes in acidity may vary according to maturity and storage temperature (^{Pinedo-Espinoza et al., 2010}).

Table 6 Titratable acidity values (%) of citrus pulp evaluated in fruits of tuna, xocotuna and xoconostle cultivars.