Introduction

Prunus persica (L.) Batsch is widely grown in temperate climates, has great standards in the international market, is the third most important temperate climate fruit behind apple and pear. More than 90% of the world’s production is intended fresh. The fruits are characterized by their content of vitamin C, carbohydrates, such as fructose, sorbitol, glucose and sucrose, as well as oxalic, citric, isocitric, malic, fumaric and shikimate acids (^{Nowicka et al., 2019}). Peach is a climacteric fruit, it means that once the fruits are harvested, they continue with metabolic activity, related to the maturation process where energy is produced by the oxidation of sugars and organic acids, which leads to the formation of water and CO₂ (^{Martínez-González et al., 2017}).

In addition to containing plant hormones such as ethylene, it regulates the main molecular, biochemical and physiological processes during fruit maturation, reflected in the increase in degrees Brix, color changes at the level of epidermis and pulp, decrease in acidity and firmness of the fruit, at the level of carbohydrates, in the conversion of starch to soluble sugars, which increases the sweet flavor of the fruits, there is also degradation of polymeric carbohydrates, starch and cellulose (^{Hiwasa-Tanase and Ezura, 2014}).

In addition to the above, in the fruit, the firmness and total soluble solids (TSS) cause changes in the organoleptic variables as time passes and senescence begins, which generates the greatest postharvest losses, due to the weakening of the structure of the fruit that is reflected in a lower firmness and causes a greater susceptibility to mechanical damage and the attack of pathogens (^{Farias et al., 2017}). ^{Abbasi et al. (2019}) point out that firmness is the most used indicator to determine the potential shelf life of fruits at postharvest and for decision-making in selection, packaging and transport operations. ^{Buitrago-Guacaneme et al. (2015}) mention that the TSS are also an important quality criterion since the determination of their accumulation in the fruit together with the titratable acidity (TA) allow estimating the time of harvest; whereas the ratio of TSS/TA is an indicator of the flavor of the fruit during maturation (^{Nowicka et al., 2019}).

In Mexico, the main peach-producing states are: Zacatecas, Michoacán, State of Mexico and Tlaxcala, the latter ranks ninth nationally with an area of 1 160 ha planted, the largest production is concentrated in the northwest of the state, the fruits are intended for the local and regional market. One of the important cultivars is semi-freestone Escarcha, of intermediate maturation in early July (^{Fernández-Montes et al., 2011}), on the other hand are Amarillo criollo and Lucero, of late maturation in September.

The varieties are of great importance for their organoleptic characteristics, the production in 2020 was 3 979 t with a price of $ 25 t^-1 (^{SIAP, 2020}), in addition, they are varieties that have adapted to the local climate, which encourages the producer to develop an adequate management that guarantees a greater volume and quality (^{Fernández-Montes et al., 2011}).

For the sale of the fruits, characteristics that allow them to last on the shelf are required. Most producers are unaware of their product information when harvesting, while the canning industry requires cultivars with specific characteristics of texture, firmness and pulp color (^{Abbasi et al., 2019}). Therefore, harvesting fruits with ideal commercial values is an economic guarantee for the producer.

The demanded characteristics of maturity are: firmness and flavor, which allow estimating the acceptance it will have in the market (^{Altube et al., 2017}). Therefore, the objective of this research was to evaluate the physical and chemical characteristics in fruits of three varieties of peach in the state of Tlaxcala, which will allow determining their postharvest disposition, under the hypothesis that there are differences between the varieties.

Materials and methods

The Escarcha variety is from the locality of Concepción Hidalgo, located in the municipality of Altzayanca, Tlaxcala (19° 23’ 30.61” north latitude and 97° 49’ 35.84” west longitude), with an average temperature of 14 °C and a rainfall of 600 mm. The varieties Lucero and Amarillo criollo are produced in the locality of San José Xicohténcatl, Huamantla, Tlaxcala (19° 21’ 27.8” north latitude and 97° 49’ 52. 26” west longitude), at 16 °C and 700 mm of rainfall (^{INEGI, 2009}) (Table 1). The soils of both sites have a loamy sandy texture, bulk density of 1.11 to 1.25 g cm^-3, neutral pH and an organic matter content less than 1.01% (^{García-Gallegos et al., 2020}).

Since the maturation of the varieties differs, the harvest of the fruits for Escarcha was in July; while, for Lucero and Amarillo criollo in September. Four trees per variety were randomly selected and from each tree, 10 fruits without mechanical damage and in good phytosanitary conditions were collected early in the morning, in total 40 peaches per variety, deposited in cardboard pulp trays for immediate transfer to the laboratory.

Within the physical characteristics, the following were determined: weight of each fruit was measured with a digital triple beam balance (Adam^®), the size was measured with a RoHS^® digital Vernier based on the major diameter (polar diameter) and the minor diameter (equatorial diameter) by the NMX-FF-009-SCFI-1982 (^{DOF, 1982}), the firmness was measured with a texture meter (TAXT Plus^®), coupled to the Stable Micro Systems software, with a tip of 3 mm in diameter, which corresponded to the maximum force in g that was converted to kg and then multiplied by the acceleration constant (9.866 m s^-2) resulting in the force in Newton (N) based on the NMX-FF-060-SCFI-2009 (^{DOF, 2009}).

For the chemical characteristics, the fruits were removed the epicarp and immersed in water at 100 °C for 40 s, then in cold water at 5 °C and thus inhibit enzyme activity; subsequently, the endocarp was removed and the mesocarp was crushed with an electrical processor (Hand Blender^®), the juice was extracted by vacuum filtration.

The pH and electrical conductivity (EC) were determined in juice in a ratio 1:1 v/v (water/juice), with a potentiometer (Conductronic^®) and a conductivity meter (ExStik^® II model EC500) (^{AOAC, 1990}). The TSS were quantified with a digital benchtop refractometer (Atago^® model 1T), a refractive index and a value in °Brix were obtained, it was previously calibrated with 2 drops of distilled water, the TA was determined in a ratio 1:1 v/v (water/juice) by titration, titrated with NaOH 0.1 N and phenolphthalein as an indicator, the formula by ^{López and Argaiz (1993}) was used, they refer to the data as acidity (% citric acid) = (V_NaOH x N_NaOH x meq citric acid/V) x 100. Where: V_NaOH= volume of NaOH in the titration; N_NaOH= normality NaOH (0.1 N); meq citric acid= 0.064 g citric acid; V= volume of the sample; in addition, the maturity index (MI) of the fruits was estimated by dividing the values of the TSS (°Brix) by the percentage of citric acid that corresponds to the TA (^{Abbasi et al., 2019}).

The data obtained were subjected to the tests of normality of Shapiro-Wilks (p> 0.05) and homogeneity of variances of Levene (p> 0.05), once the assumptions were met, the Anova was performed under the linear model: Y_ij = µ+(_i + (_ij. Where: Y_ij= response of the j-th repetition of the i-th treatment; (_i = effect of treatment i; (_ij= random error associated with observation Y_ij. Then a Tukey test (p≤ 0.05) to identify differences between varieties. A Pearson correlation analysis (p≤ 0.05) was performed, and a principal component analysis (PCA) was performed to determine the degree of association of fruit characteristics and varieties. This using the statistical software Info Stat version 2008 (^{Di Rienzo et al., 2008}).

Results and discussion

In the weight of the fruit, differences were observed between varieties (p≤ 0.05), the Lucero variety presented the highest weight (Table 2) and was the one that had a yield per tree of 10.5 kg ha^-1 and 99 fruits tree^-1, compared to the varieties Escarcha and Amarillo criollo (Table 1). However, the values were lower than that reported for fruits of the Dorado variety (181 g), although it is important to consider that the weight decreases at postharvest due to the loss of moisture and temperature during storage or transport (^{Africano et al., 2016}). ^{Sun et al. (2018}) report a reduction of 2.43% in the varieties Sugar Top and Okubao, once harvested the weight loss at 28 days was 6.4, 7.6, 8.4 and 8.8% stored at 0, 2, 5 and 8 °C, respectively, which can happen with the varieties analyzed.

Regarding the polar diameter, there were significant differences between the varieties, the same pattern as in the weight occurred (p≤ 0.05) (Table 2). According to the classification by size indicated by the NMX-FF-060-SCFI-2009 (^{DOF, 2009}), the fruits evaluated in this study are optimal for fresh consumption. ^{Díaz-Martínez et al. (2017}) report, for landrace-type peach of Santiago Papasquiaro, Durango, a polar diameter of 54.92 mm, a value higher than those obtained; whereas the equatorial diameter of 52.48 mm was lower than Escarcha and Lucero.

As for the values of firmness, it was obtained that the Lucero cultivar was significantly higher, followed by Amarillo criollo and Escarcha (p≤ 0.05) (Table 2). Nonetheless, these values are lower than indicated by the NMX-FF-060-SCFI-2009, which establishes an overall firmness of 34.32 N, so the marketing time of the varieties evaluated could be restricted. ^{Africano et al. (2016}) report, for the Dorado variety, a firmness of 12.81 N, higher than the Escarcha variety and lower than the other two varieties.

For the Flordaking variety, a firmness of 36.35 N at postharvest is reported and it decreases to 8.73 N at the sixth week. ^{Abbasi et al. (2019}) point out that the firmness of peach fruits tends to decrease as maturation progresses. The softening of the fruit occurs as a process of modifications in the structure of the cell wall, where the depolymerization of glucans and solubilization of pectin occurred; likewise, the decrease in firmness can be related to a high concentration of nitrogen in the fruit (^{Santiago-Mejía et al., 2008}), which will depend on the variety and management provided by the producer (Table 1).

In juice, the pH of the Escarcha variety was more acidic with respect to the values obtained in Amarillo criollo and Lucero (p≤ 0.05) (Table 3). ^{Miranda et al. (2019}) report, for the Chimarrita variety, a value of 3.2. The NMX-F-034-1982 (^{DOF, 1982}) establishes a pH of 3.5 as a minimum and 4.2 as a maximum for the preparation of peaches in syrup, therefore, the varieties analyzed are not suitable for that purpose; however, the standard allows citric acid to be added as an acidulant when it is required to reach the required pH.

In the electrical conductivity (EC), the average values were different between the varieties, the highest was obtained by Escarcha (p≤ 05) (Table 3), even so, lower compared to what was reported by ^{Bonazzola et al. (2007}) for the varieties Flordarking and Forastero (5.10 and 5.40 mS cm^-1). One of the most abundant elements in peach pulp is K, followed by Ca, Mg and Na, but its concentration may decrease during fruit maturation (^{Dabbou et al., 2017}).

For titratable acidity (TA), the values showed that the Escarcha variety is significantly higher with respect to Lucero (p≤ 0.05) (Table 3). The three varieties analyzed had values lower than those evidenced by ^{Africano et al. (2016}) for the Dorado variety, with a percentage of 0.87%, while in the Flavorcrest variety, it went from 0.96 in the first harvest to 0.74% in the last of 2010, and from 0.47 to 0.5% in 2012 (^{Altube et al., 2017}).

In the same variety, ^{Abbasi et al. (2019}) report an acidity percentage of 1.09 at the time of harvest and it decreased to 0.63% at the sixth week of storage, higher than what Tlaxcala fruits had. The predominant organic acids in the fruits of Prunus persica (L.) Batsch are malic and citric acids, which decrease with maturation, these acids represent 65% of the total content of organic acids in the fruit, in addition to oxalic, fumaric and isocitric acid (^{Nowicka et al., 2019}).

Regarding TSS, the value of the Lucero variety was significantly higher with respect to Escarcha and Amarillo criollo (p≤ 0.05) (Table 3). The sugar content in the fruits is within what is established by the NMX-FF-060-SCFI-2009 (^{DOF, 2009}), which, as a maturity specification, stipulates that the minimum TSS content in fruits of all peaches must be 10 °Brix. The Lucero variety had a higher amount of sugars than reported by ^{Africano et al. (2016}) for the Dorado variety, with 11.6 °Brix; ^{Farias et al. (2017}) report values of 11.45 °Brix for the Cascata 1513 genotype, increasing to 12.66 °Brix after 10 days of storage, similar to the Escarcha variety.

In mature fruits, soluble solids include: oligosaccharides, polysaccharides, organic acids, pigments and tannins, among others, but their content is usually higher in fruits of intense color (^{Nowicka et al., 2019}), that when the degradation process occurs, soluble sugars are generated, which increases the TSS in the fruit (^{Buitrago-Guacaneme et al., 2015}).

In the case of climacteric fruits, the respiratory rate increases at maturation to produce the energy necessary for the hydrolysis of starch that leads to the synthesis of monosaccharides such as glucose and fructose between 30 and 50%, in less quantity sucrose, which represented 1.5 and 5% of the total weight of the fruit (^{Agustí, 2013}). When establishing the TSS:AT ratio to obtain the maturity index, the Lucero variety had a significant increase, as a result of the reduction of the TA and increase in the TSS (Table 3).

The fruits of the Escarcha variety had an index of 33.67, this relationship is an indicator of fruit maturity, this variety is of intermediate maturation and due to its semi-freestone condition, the time of harvest of the fruit could be earlier than that established by the producer. ^{Marques-Costa et al. (2008}) point out that a maturity index for peaches ranges from 16.5 to 36; nevertheless, values of 25 or less indicate an optimal maturity for harvesting, according to this value, the fruits of the three varieties were harvested after the optimal point.

^{Leonel et al. (2014}) report a maturity index of 12.5 to 21.4 for climacteric fruits such as peach. ^{Africano et al. (2016}) obtained a maturity index of 12.41 for the Dorado variety, values lower than that reported in this study for the varieties of Tlaxcala. In three-year-old trees of the August Flame variety, a maturity index of 1.4 is reported, increasing as the storage time progresses to 2.76 at the fourth week (^{Ceccarelli et al., 2019}).

The differences between the varieties (Tables 2 and 3) are the result of genotype, but it is also important to mention that agroecological factors such as moisture, temperature, management (Table 1) and possible effects of pests and diseases influence the characteristics evaluated and the productive potential and quality of the fruit (size, shape, sugar content, acidity and postharvest life) (^{Fernández-Montes et al., 2011}). On the other hand, ^{Nowicka et al. (2019}) indicate that the TSS:AT ratio can also be used to classify peach fruits based on their flavor, thus a maturity index of 5 to 7 they are acidic, 17 to 24 bittersweet and 31 to 98 sweet, based on this classification, the fruits of the three varieties analyzed are considered sweet, although the Lucero variety exceeded the highest value of the classification by 10.27 units, this may indicate that it is a fruit harvested after the optimal time and therefore, its postharvest life will be affected.

The correlation analysis shows that, the greater the polar and equatorial diameter of the fruit, the greater the weight. An increase in the firmness of the fruit resulted in a higher electrical conductivity and therefore an increase in the pH value (Table 4). At a lower acidity, an increase in the maturity index, which coincides with ^{Buitrago-Guacaneme et al. (2015}), who report that this TSS:AT ratio shows an increase during fruit maturation, resulting from reduced acidity and increased TSS, which is reflected in the Lucero variety.

The principal component analysis (PCA) shows that the total variability is explained by the components PC1 (77%) and PC2 (23%), these two components explain 100% of the total variation of the data. The characteristics that indicate a greater weight in the PC1 maturity index, TSS, TA and firmness. The PC2 is explained by equatorial diameter and pH (Table 5 and Figure 1).

Figure 1 Biplot of the physicochemical characteristics of the fruit in three varieties of Prunus persica (L.) Batsch in Tlaxcala. 

Conclusions

Based on the size, the three varieties meet the standards of Mexican regulations, the firmness indicates that the harvest times of the fruits are not the optimal of the producer. Due to the chemical characteristics, the fruits are suitable for preparing preserves. With the PCA, it was determined that the size, maturity index and firmness for the Lucero variety are determinants for its postharvest quality and in the case of the Escarcha variety, it was the titratable acidity. Therefore, it is important that producers consider these variables to establish the optimal times for harvesting the fruit, as well as the management in the plots (fertilization, irrigation, control of weeds, pests and diseases).