INTRODUCTION

In 2000-2011 period, the value of vegetable production in Mexico reached 467 283.5 million pesos. Of the 158 different types of produced vegetables, red tomato, green tomato, white onion, green chili and asparagus made up the largest portion (^{SAGARPA, 2012}). Consumer longevity seems to increase with caloric reduction and high levels of antioxidants in their diets. Antioxidants can reduce mitochondrial degradation, cell metabolism and oxygen consumption. It has been observed that antioxidant levels reduce with aging, mainly in blood and some organs. These degenerative immune system changes can induce development of cataracts, Alzheimer, Parkinson or cardiovascular disorders (^{Zorrilla, 2002}; ^{De La Fuente, 2002}). Generally, plants with high antioxidant potential contain phenolic compounds that act as reducing agents, hydrogen donors and reactive oxygen species quenchers (^{Javanmardi et al., 2003}). Consuming fruits and vegetables is of great importance in the treatment and prevention of aging, cancer, chronic-degenerative diseases, among other disorders caused by oxidative stress which generates reactive oxygen species (ROS) in the body. ROS action is quenched by the content and capacity of antioxidant compounds through donation of one of their own electrons, protecting cells from oxidative damage (^{Soto et al., 2012}). ROS are mainly formed endogenously in cellular respiration (mitochondria) and, due to the effect of pollutants, exogenously. These molecules react quickly and damage other biomolecules, competing for electrons in order to reach stability (Figure 1) (^{Rodríguez et al., 2006}).

Figure 1: Oxidative stress effects on human health. 

Consumer demand for organic products is increasing and is reflected in a significant increment of 11% in the USA market, the largest organic market in the world. Farmers are producing organically on more organic certified land and 179 countries report organic agriculture (^{Willer and Lernoud, 2017}). This study has the objective of determining the nutritional value, through the antioxidant capacity, of four groups of vegetables available to Baja California Sur consumers: tomatoes, peppers, citrus fruits and leafy vegetables.

MATERIALS AND METHODS

This study was carried out in May 2018 at Universidad Autónoma de Baja California Sur (UABCS), located in La Paz, Baja California Sur, Mexico. According to Robles (1998¹), the city of La Paz is located at the northeastern tip of the La Paz Valley 24° 10’ N and 110° 19’ W at 18.5 m altitude. The fruits and vegetables analyzed were obtained from local markets of the city and were organized into four groups: tomatoes, peppers, citrus and leafy vegetables and subgrouped into conventional, wild and organically produced. The obtained tomatoes were round (conventional, hydroponic), cherry (conventional), saladette (conventional), grape (organic). The obtained peppers were California (conventional), güerito (conventional), poblano (conventional), habanero (conventional), jalapeño (conventional) and bell (organic). The citrus fruits obtained were naranjita (wild), orange (conventional), grapefruit (conventional) and lime (conventional). Finally, the leafy vegetables obtained were lettuce (conventional), spinach (conventional), chard (conventional), purslane (conventional) and quelite (wild).

The evaluated parameter was the antioxidant power of leaves, stem and fruits of the samples mentioned above (vide supra). One-hundred grams of the sample were finely ground with 100 mL of distilled water and then carefully vacuum filtered in order to obtain a clear mixture. The obtained mixture was used for measurement of total soluble solids (ºBrix) using a digital refractometer (Hanna, HI 96801). Likewise, the mixture was used to measure pH and electric conductivity using a pH/EC/TDS/Temp tester (Hanna, HI98130). To measure antioxidant capacity, 1 mL serial dilutions of the juice were prepared in methanol. These dilutions were then mixed with 1 mL of a DPPH solution in methanol at 4 mg 100 mL^-1 concentration. The mixture was then left to stand for 30 min in the dark, and absorbance was obtained in a spectrophotometer (ThermoFisher Scientific, Genesys 10S) at 517 nm (^{Xie and Schaich, 2014}).

The obtained measurements were then compared to a trolox calibration curve constructed using the same DPPH solution mention above and trolox solutions at different concentrations (Figure 2). Results are expressed in milligrams of trolox equivalent g^-1 fresh fruit or vegetable biomass.

Figure 2: Trolox calibration curve. 

A randomized blocks experimental assay was used and multiple mean comparisons were performed using Tukey HSD P = 0.05. The statistical analysis was carried out using a statistics program from the Agronomy Department at the Universidad Autónoma de Nuevo León (^{Olivares, 2012}).

RESULTS AND DISCUSSION

DPPH is a stable free radical, commonly used to evaluate the antioxidant potential of foods (^{Floegel et al., 2011}). The higher the ability of a fruit or vegetable sample to turn DPPH purple color to yellow, due to the formation of 1,1-Diphenyl-2-picrylhydrazine, the higher its antioxidant power (Figure 3).

Figure 3: DPPH reaction with natural antioxidants. 

As shown in Table 1, among the group of tomatoes, certified organic grape tomatoes had the highest antioxidant capacity with 1.273 mg trolox equivalents g^-1 fresh biomass. Round and saladette tomatoes grown conventionally had the lowest antioxidant capacity. Regarding the group of peppers, certified organic bell pepper showed the highest antioxidant capacity with 1.119 mg trolox equivalents per g fresh biomass. All other tested peppers (güerito, California, habanero, poblano and jalapeño) showed similar antioxidant capacity that was lower than the organically produced bell pepper. From the citrus group, wild naranjita had 1.623 mg trolox equivalents per gram of fresh biomass, and lime showed the lowest antioxidant capacity. Finally, among the leafy vegetables wild quelite had the highest antioxidant capacity with 1.313 mg trolox equivalents per gram of fresh biomass. Chard and purslane obtained conventionally showed the lowest antioxidant power. All data obtained agree with the studies carried out by ^{De Oliveira et al. (2016)}, who found that organic crops contain higher amounts of phenolic compounds and higher antioxidant capacity than conventional crops.

Interestingly, the content if soluble solids of a given group correlates with the antioxidant capacity, possibly because of water soluble antioxidants like vitamin C (^{Thaipong et al., 2006}). However, among tomato group, the organic grape variety shows higher antioxidant power than conventional cherry which has more total soluble solids. This case could be an example of vegetables with other more potent antioxidants like distinctive phenolic compounds and carotenoids (^{Lindsay and Astley, 2002}). A healthy and balanced diet based on fruits and vegetables that contain high amounts of compounds with antioxidant properties can help reduce cell death in sick patients (^{Terry et al., 2001}) and most importantly, consumption of these fruits and vegetables can prevent development of chronic-degenerative diseases (^{Hu, 2003}; ^{McCullough et al., 2003}). A study carried out by ^{Baudry et al. (2018)} showed an important decrease in cancer risk of French participants that had higher consumption of organic food. This study conducted from 2009 to 2016 considered nearly 70 000 people. An important fact to consider is that organic products are pesticide and synthetic fertilizer free; therefore, consumers are not exposed to possible carcinogenic chemicals such as malathion, parathion, etc., which have been linked to increased cancer risk (^{Gray et al., 2017}; ^{Mostafalou and Abdollahi, 2013}).

CONCLUSIONS

- Organic certified grape tomato with 1.273 mg trolox equivalent per g of sample presented the highest antioxidant power. Additionally, certified organic bell pepper had the highest antioxidant power with 1.119 mg trolox equivalent per g of sample. Regarding the group of citrus fruits, we found that wild naranjitas had higher antioxidant power with 1.623 mg trolox equivalent per g of sample. Finally, as far as the leafy vegetables group, wild quelites showed the highest antioxidant power with 1.313 mg trolox equivalent per g of sample.

- The results obtained in this study give evidence that the production method has a very important effect on nutritional value. Wild and organic production increases antioxidant capacity of fruits and vegetables as shown the analysis performed in this investigation. Wild production is an interesting source of fruits and vegetables, especially those from regions where stressful climate conditions promote metabolic changes in plants, which protect themselves by producing antioxidant compounds. Agriculture practiced in extreme conditions, such as those in Baja California Sur, could be a strategy to increase the nutritional value of fruits and vegetables. Consumption of these products is undoubtedly a natural strategy that can help to reduce problems related to aging and chronic-degenerative diseases.