Introduction

In Mexico, the potato is the seventh most important for the value of crop production, annually it occurs between 1.7 and 1.8 million tons, of which most (75%) is concentrated in six states in the north (Sonora, Sinaloa, Nuevo Leon) and center of the country (Veracruz, State of Mexico, Puebla). The main destination of production is consumed fresh (56%), 29% goes to industry and the remaining 15% is used as seed for planting (^{CONPAPA, 2013}). The potato production of the high valleys of Central Mexico currently faces several problems for marketing because of its low quality, is mainly intended for fresh consumption because most of the production does not meet the parameters requested by industries processing (^{Vazquez-Carrillo et al., 2013a}).

Features such as skin color and pulp, the size, shape and depth of the eyes are important in the decision to purchase by the consumer in the fresh market. In addition, other parameters such as dry matter content, starch and reducing sugars are monitored because of its importance for processing (^{Vázquez-Carrillo et al., 2013}). The dry matter content is the main characteristic appreciated by the industry, as the performance of the products obtained (flour, mashed potatoes, chips, french fries, starch), texture and oil absorption depend on this parameter. Furthermore, the snack industry (chips and french fries) requires tubers with a low content of reducing sugars, since a high content of these compounds produces a dark colouration that entails a distortion of taste (bitter) plus that contribute to the formation of acrylamide (^{Pedreschi, 2009}).

^{Herrera and Scott (1993)} report that in Latin America, there are many factors that affect the production, quality and use of potato tubers, the most important factors biotic and abiotic stresses. Climatic factors such as drought, frost and hail affecting foliage which consequently influences the yield and quality of tubers because the process of photosynthesis and starch synthesis is interrupted (^{Vazquez-Carrillo et al., 2013a}). Another factor affecting the quality of the tubers in the producing areas of the country and especially in the high valleys of central Mexico is the presence of pests and diseases, mainly purple top and potato late blight. Purple top potato generates an internal breakdown of the tuber so these tubers are rejected for fresh consumption and for production of chips (^{Munyaneza, 2012}). Additionally, stress generated on the ground induces the accumulation of various compounds as a defense mechanism, including reducing sugars and phenolic compounds both undesirable for fresh consumption and snack industry (^{Friedman, 1997}; ^{Rodriguez-Saona and Wrolstad, 1997}).

The phenols are chemicals product of secondary metabolism of plants, contribute to the astringency, color, taste and smell of processed products (^{Naczk and Shahidi, 2004}). These compounds have antioxidant, anti-inflammatory and anticancer properties, which are demanded in the food industry and consumers (^{Friedman, 1997}). The presence of phenolic compounds in free form, such as chlorogenic acid and caffeic acid, have been reported in potato and has recently demonstrated its high antioxidant activity (^{Reddivari et al., 2007}).

Therefore the National Potato Program of the National Institute for Forestry, Agriculture and Livestock (INIFAP) has developed genotypes adapted to the growing conditions of the high valleys of Central Mexico with high agricultural production (^{Rubio-Covarrubias et al., 2013}) where it is necessary to evaluate its quality.

The objective of this research was to evaluate the effect of location and genotype in the characteristics of quality, content of reducing sugars, phenols and antioxidant activity in tubers of six genotypes of potato (Solanum tuberosum L).

Materials and methods

Vegetal material. Six genotypes of potato (Solanum tuberosum L.) were used: a commercial variety (Fianna) and five clones (Nau, 5-10, 8-29, 99-38 and 99-39), whose main characteristics are shown in Table 1.

The six genotypes occurred in the spring-summer 2012 cycle in two locations in the State of Mexico Metepec and Raíces. Geographical and environmental characteristics of each location are shown in Table 2.

In the town Raíces planting was held on May 15, 2012, the formula fertilization was fue 200-300-200 (N-P-K) kg ha^-1, additionally applied 50 kg ha^-1 of micronutrients and 1 t ha^-1 chicken manure; fungicides and insecticides applied to control pests and diseases, the date of application to the foliage desiccant was on 21 september and harvest took place on 03 october 2012. In Metepec seeded the june 11, 2012, the dose was 200-200-200 fertilization (NPK) kg ha^-1, plus 50 kg ha^-1 of micronutrients and 1 t ha^-1 of chicken manure, fungicides and insecticides were also applied to control late blight and the date of application foliage desiccant was on september 13, while the harvest took place on october 4, 2012.

Once harvested, the tubers were transported at room temperature (25 °C) Campo Experimental Valle thede Mexico the INIFAP for analysis.

Tuber quality. Determinations were made in six tubers of each genotype at random, tubers were selected uniform in size, shape, color, and free of pathogens and physical damage, were washed and identified.

In each of these tubers was evaluated the degree of internal staining caused by the disease of potato purple top (PMP). All tubers were cut in half to rate the degree of staining on a scale of 0 to 5, where 0 is a tuber without staining and 5 indicates an intensely stained tubercle (^{Rubio-Covarrubias et al., 2013}).

The color of the pulp was evaluated with Hunter Lab colorimeter MiniScan XE Plus Model 45/0-L^® (Reston, VA, USA) in CieLab scale with illuminant D/65 and an angle of 10°. In Hunter Lab colorimeter lightness values (L*), a* and b*; were obtained; the values of a* and b* were used to determine the angle of pitch "hue" and the color purity "chroma" variables that locate the color of the material in the plane in different shades of red, yellow, green and blue (^{Vázquez-Carrillo et al., 2013}).

Susceptibility to enzymatic browning (SPE) was evaluated according to the methodology described by ^{Vazquez-Carrillo et al. (2013)}, each tuber was transected and immediately determined the color using a colorimeter Hunter Lab Scan Mini XE Plus (Model 45/0-L, USA) taking into account only the value of L*. The operation was repeated 30 minutes after completing the cut. The susceptibility to enzymatic browning was taken as the difference between the two readings (ΔL*=L*_0min-L*_30min), the higher is the value of ΔL* greater susceptibility to enzymatic browning.

The specific gravity was determined by the method of saline solutions of known density described by ^{Gould (1999)}. The NaCl solutions in distilled water with a specific gravity were prepared from 1040-1140, tubers were put in each solution and moved from one solution to another to find the solution in which the tuber floated, the specific gravity of the solution was taken as the same tuber. The solids content was determined by difference between the moisture content (AOAC method of 920.151 (2000) and the total solids.

Chemical components. The total starch content was determined using the kit Megazyme^® (Megazyme Internacional Ireland, Ltd. Wicklow, Ireland) based method 76-13.01 of the AACC (^{AACC International 2000}). Extraction and quantification of reducing sugars (glucose and fructose) and sucrose was carried by the enzymatic method described by ^{Castañeda-Saucedo et al. (2012)}. Soluble sugars were extracted by incubation for tuber 10 min ethanol (80% v/v). The extracts were evaporated at 50 °C, resuspended in distilled water and stored at -20 °C until analysis. Glucose, fructose and sucrose were quantitated after sequential addition of hexokinase (EC 2.7.1.1), phosphoglucose isomerase (EC 5.3.1.9) and invertase (EC 3.2.1.26). Readings were taken in a microplate reader Multiskan (Ascent^®, Thermo Electron Co., Finland) at 340 nm against standard curves of glucose, fructose and sucrose.

Extraction and quantification of phenolic compounds. The 1 g of dry flour was taken and was placed in an Erlenmeyer flask to which was added 30 mL of grade methanol HPLC 80% (v/v). The mixture was sonicated for 10 min (Bath sonicator, Branson model 2510^®; Danbury, USA), prior to stirring for 90 min on a horizontal shaker model G10^® (New Jersey, USA) at room temperature in the dark. Each sample was centrifuged at 3 500 rpm for 15 min in a Hettich centrifuge zentrifugen® Mod. Universal 32 (Germany). The supernatant was saved and the residue was subjected to a second extraction with the methodology described. Supernatants were combined and filtered with Whatman no. 4, then concentrate to dryness on a rotary evaporator Buchi® Mod R-215 (Switzerland), at 42 °C. Samples were re-suspended in 5 mL of methanol 80% (v/v) and then stored in amber vials under conditions colored frozen until further analysis.

The quantification was made from the methanolic extract obtained as described above, by the Folin-Ciocalteu (^{Singleton et al., 1999}). A 100 µL of extract 125 µL of Folin-Ciocalteu reagent 1N (1:2 dilution of the commercial reagent 2N) was added and stirred and reacted for 6 min, then neutralize the reaction with the addition of 1 250 µL of carbonate sodium (Na₂CO₃) to 19% (p/v); then he gauged to 3 mL with distilled water, stirred and allowed to stand at room temperature in the dark for 90 min. The samples were centrifuged at 14 000 rpm for 10 min to remove turbidity. Absorbance was measured with Perkin Elmer Lambda one 25^® UV/Vis (USA) 760 nm. The chlorogenic acid a standard curve was developed to express the content of total phenols in function of this acid.

Antioxidant activity. The antioxidant capacity of the phenolic compounds present in the potato samples was determined using 60 μM of DPPH dissolved in methanol solution at 80%, with the method reported by ^{Brand-Williams et al. (1995)} using the radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) (Sigma Aldrich Co).

Statistical analysis. The results were analyzed under an asymmetric 6 x 2 factorial design with an allocation scheme completely randomized with two repetitions. The first factor was the genotype (Fianna, Nau, 5-10, 8-29, 99-39 and 9939) and the second factor was the location (Metepec and Raíces). To determine differences between treatments for multiple comparisons of treatment means were performed using Tukey's test. The analysis of results was performed using the SAS statistical package for Windows, version 9.0 (^{SAS, 2002}).

Results and discussion

The significant effects (p< 0.01) were found in the interaction G x L in all variables evaluated (Table 3), differences due to agro-climatic conditions (temperature, fertilization, altitude) of the two locations and the genetic variability among the six genotypes.

Tuber quality

The interaction G x L had significant effects (p< 0.01) in the variables staining index, specific gravity, moisture content, dry matter and starch, which means that both factors influenced the variables. The results are shown in Table 4.

Table 4 Physical and chemical characteristics of six genotypes of potatoes grown in two locations. 

Variable	Nau		Fianna		5-10		8-29		99-38		99-39
	Metepec	Raíces	Metepec	Raíces	Metepec	Raíces	Metepec	Raíces	Metepec	Raíces	Metepec	Raíces
índice de manchado	0.6	0.0	2.4	0.0	0.6	0.0	1.0	0.0	1.3	0.0	2.0	0.0
Luminosidad (L*)	78.7	80.4	72.6	78.5	78.3	78.9	76.4	78.8	78.3	79.6	78.9	77.7
Hue	88.73	90.12	85.18	89.68	87.61	89.31	88.72	90.5	88.73	90.38	90.17	90.47
Croma)	27.34	25.76	24.6	29.9	36.41	37.06	36.33	37.32	31.26	34.64	22.74	24.51
SPE (ΔL*)	1.69	0.92	1.84	0.11	0.68	0.48	1.57	0.4	1.91	1.61	1.32	0.46
Gravedad específica	1.065	1.082	1.06	1.095	1.075	1.082	1.075	1.077	1.072	1.092	1.062	1.082
Humedad (%)	81.1	78.2	83	75.2	79.5	79.2	80.3	80.1	81.5	77.2	83.3	77.7
Materia seca (g/100g)	18.9	21.8	17	24.8	20.5	20.8	19.7	19.9	18.5	22.8	16.7	22.3
Almidón (g/100g)	14.4	15.5	13	19.8	16.5	17.6	15	16.9	12.9	18.7	13	17
Azúcares reductores (g/100g fw)	0.04	0.13	0.59	0.02	0.22	0.02	0.09	0.32	0.26	0.05	0.17	0.58
Azúcares totales (g/100g fw)	0.2	0.32	0.95	0.27	0.63	0.19	0.21	0.51	0.44	0.2	0.24	0.77
Fenoles (mg/100 g fw)	371.3	139.3	387.3	162.8	305.6	114.1	283.5	247.2	255.3	274.5	277	251.3

ΔL*= L*_0min-L*_{30 min}; SPE= susceptibilidad al pardeamiento enzimático.

Regarding the resistance against internal staining of the tubers caused by the disease purple top potato, it was observed that in the town with the lowest altitude (Metepec) rates higher spotted were taken, while the higher and lower temperatures (Table 1) (Raíces) any tuber presented stained internal (Table 4) location. According to ^{Rubio-Covarrubias et al. (2011)} the population of the insect vector Bactericera cockerelli and symptoms of purple top potato decrease with height, they found that more than heights 3 200 snm no significant problems arose from this disease which is due to a by now the low temperatures do not allow the development of this insect or cause the population is low. The clones that showed the lowest rate of Metepec were spotted in Nau and 5-10, behavior that previously was reported by ^{Vázquez-Carrillo et al. (2013)} and ^{Rubio-Covarrubias et al. (2013)} and confirm your tolerance to internal staining of tubers caused by purple top potato; in contrast the variety Fianna showed increased susceptibility to this disease as has been reported by other authors (^{Rubio-Covarrubias et al. 2013}). The internal staining of the tubers is an important industry snaks parameter due to frying, these symptoms become more pronounced and chips or french fries infected tubers show blemishes and very dark stripes, which makes them commercially unacceptable (^{Munyaneza, 2012}). In that sense, all genotypes grown in Raíces are of acceptable quality, either for the production of chips or french fries or domestic process.

As for the color of the pulp, most genotypes except clone 99-39, had a brighter pulp when they were grown in Raíces than those grown in Metepec (Table 4). The Nau clone grown in Raíces showed the highest brightness values (80.4), while the Fianna variety grown in Metepec introduced less clear flesh color (L*= 72.6). This is related to the internal staining of the tubers caused by purple top potato which was more pronounced in the Fianna variety in clone Nau as previously noted (Table 4). The hue values between 85 and 91, and chroma between 22 and 38 indicate that tuber color is in the light yellow tone, which corresponds to the visual description of them (Table 1).

Another factor affecting the quality of tubers mainly for fresh consumption is the enzymatic browning. Darkening or enzymatic browning occurs on the fabric surface of the tubers are peeled or when these cut and exposed to atmospheric oxygen (^{Stark and Love, 2003}). The results of the evaluation of susceptibility to enzymatic browning by the difference in brightness showed significant differences due to the interaction G x L (Table 4). The tubers grown in Metepec genotypes showed a greater decrease in pulp brightness that grown in Raíces, which means that these tubers had a greater darkening when exposed for 30 min to oxygen in the air (Table 4). In Metepec clone 10.05 was the least susceptible to darkening, introducing the smallest decrease brightness, while Raíces was the variety Fianna (Table 4).

^{Vázquez-Carrillo et al. (2013)} also found that the clone 10.05 was the least susceptible to enzymatic browning between 39 genotypes in the town of Roots in the springsummer cycle 2009. The 99-38 clone grown in both locations was highly susceptible to darkening what is observed with a greater decrease in pulp brightness. The phenolic compounds are associated with the color of raw potatoes and are partly responsible for the coloration of processed products produced. When tubers are cut phenols quickly become melanins due to oxidation of these compounds by the enzyme phenolase causing enzymatic browning (^{Álvarez and Canet, 2009}). Phenolic compounds are closely related to the defense of plants to pests and diseases, therefore a significant correlation between staining index, caused by the disease purple top potato, and susceptibility was observed enzymatic browning (r= 0.65, p< 0.01) between the staining index and total phenol content (r= 0.61, p< 0.01), and between susceptibility to enzymatic browning and phenolic compounds (r= 0.63, p< 0.01) (Figure 1).

Figure 1 Relationship between susceptibility of tubers to enzymatic browning and content of phenols. 

The genotypes locally grown in Raíces showed higher values of specific gravity (> 1.077), and dry matter (> 19.9%) and starch (> 15.5%) compared with genotypes grown in Metepec (Table 4) . It was noted that among all genotypes, the Fianna variety grown in Raíces showed the highest values of specific gravity, dry matter and starch, while the same variety grown in Metepec presented the lowest values for the same variables. Several factors influence the accumulation of starch and dry matter in tubers including weather, rain, irrigation and soil moisture, soil type, planting date and harvesting, the type and amount of fertilizer used, and control of pests and diseases.

In this experiment the differences in starch content, dry matter and specific gravity are mainly due to the temperature, precipitation, planting date and the amount of phosphorus used in fertilization. ^{Álvarez and Canet (2009)} state that at low temperatures, such as those presented in Raíces, the respiration rate is lower than the rate of photosynthesis, resulting in a greater accumulation of carbohydrates in tubers and increased severity specific. The same authors claim that a higher moisture content in the soil and early planting influence greater dry matter accumulation in the town of Raíces a month before seeded and presented greater precipitation and moisture content in the soil in Metepec (Table 2).

The fourth factor explaining these differences is the supply of phosphorus during fertilization, in Raíces the soil has a high binding capacity and phosphorus fertilizer applied dose had a higher proportion of phosphorus. According to ^{Stark and Love (2003)} phosphorus (P) tends to increase starch synthesis, promotes the rapid formation of tubers and root growth, improves the resistance to low temperatures, increases the efficiency of use of water, contributes to disease resistance and hastens maturity, so deficient plants produced tubers phosphor with lower specific gravity compared to those with proper nutrition phosphorus.

^{Stark and Love (2003)} state that for most products (chips, french fries, starch) which are made from potato processors require tubers with a starch content of 13% or higher content solid or dry matter of 20% or higher, and a specific gravity of 1.08 or higher so the variety Fianna and Nau, 5-10, 99-38 and 99-39 clones locally grown in Raíces are suitable for prosecution.

The specific gravity showed a positive correlation with the dry matter content (r= 0.92, p< 0.01) and starch (r= 0.87, p< 0.01) this is because the specific gravity is influenced by the amount of compounds solids present, including the amount of starch; Similar correlations were reported by ^{Vázquez-Carrillo et al. (2013)} for potato clones developed for the high valleys of Mexico.

Phenols and antioxidant activity

The content of total phenolic compounds showed significant differences between genotypes, locations and the effect of the interaction G x L (Table 3). The total phenolic content varied between 114.1 and 387.3 mg/100 g fresh weight of the tuber. In general, the genotypes grown in Metepec had higher phenol content than those grown in Raíces, being Fianna variety cultivated Metepec in the largest content (387.3 mg/100 g fw) and the lower clone 5-10 (114.1 mg/100 g fw). Significantly, the Fianna variety grown in the town of Metepec also had the highest rate of internal staining caused by disease purple top potato with what is found that the production of phenols is a defense mechanism of plants against this disease, as suggested ^{Navarre et al. (2009)}.

In the Figure 2 the antioxidant activity of raw potato tubers shown, that activity ranged from 15.08% to 36.47%.

Figure 2 Ability DPPH radical inhibition of fresh potato tubers six genotypes grown in two locations in the State of Mexico. 

Several compounds in potato have antioxidant activity, including the carotenoids and phenolic compounds, for this reason the capacity of inhibition of DPPH radical was higher in tubers genotypes grown in Metepec since these showed higher phenolic content because of the incidence of the disease purple top of potatoes. ^{Lewis et al. (1998)} found that in potato tubers ill effect of late blight increased significantly the concentration of flavonoids and phenolic acids both skin and pulp because it is known that diseases stimulate the route of the phenylpropanoids producing an increase of these metabolites. Among the tubers grown in Raíces lower inhibition capacity of radical DPPH showed the Nau clone which is due to its color from yellow skin and creamy flesh, which means it has a low content of carotenoids, compounds that influence antioxidant activity. The ability of DPPH radical inhibition was positively correlated with phenol content (r= 0.87, p< 0.01) (Figure 3), confirming the involvement of these compounds in antioxidant activity as alleged ^{Reddivari et al. (2007)}.

Figure 3 Relationship between phenol content and antioxidant activity of potato genotypes grown in two locations in the State of Mexico. 

Conclusions

All physicochemical variables were affected by genotype x location interaction, differences were mainly caused by the effect of altitude, temperature, fertilization and the presence of disease purple top. The genotypes locally grown in Raíces showed higher specific weight, high dry matter and starch, had a clearer pulp and were less susceptible to enzymatic browning, genotypes grown in Metepec. For tuber quality, locality in Raíces it has advantages for growing genotypes for the fresh market. The Fianna variety and 5-10 clone grown in Raíces presented the best physical and chemical characteristics and the lower content of reducing sugars so they are suitable for the production of snacks. The highest concentration of phenolic compounds and high antioxidant activity in genotypes grown in Metepec, was associated with the incidence of the disease purple top.