Nopal (Opuntia ficus)

Nopal (Opuntia ficus) is a plant of the Cactaceae family native to the American continent and characterized by a green color, presence of thorns and a branched structure whose extension can reach up to 5 meters high. Nearly 300 species of the genus Opuntia are known. However, there are only 10 or 12 species so far used by man, whether for fruit production and nopalitos for human consumption, forage or cochineal to obtain dye. Succulent and articulated stems or cladodes, commonly called penca, have ovoid or elongated racket shape, reaching 60-70 cm in length, depending on the moisture and available nutrients (^{Sudzuki et al., 1993}).

When its measures are about 10-12 cm they are tender and can be consumed as vegetables. In México the nopal is an element present in the national symbolism whose representation dates back from pre-hispanic cultures being part of various mythologies (^{Chávez-Moreno et al., 2009}). There is evidence on the knowledge, use and consumption of nopal by the first settlers of México, for example, in the excavations of Tamaulipas and Tehuacán, Puebla there were found fossilized seeds, tuna shells and nopal fibers with an antiquity of seven thousand years (^{Saenz, 2006}).

In Mexico, Opuntia ficus-indica is the cultivated species of major commercial importance for the production of nopalito (^{Pimienta, 1997}). Within Mexico’s gastronomy the consumption of nopal is a daily custom due to the benefits related to its consumption. The nopal has proven to be a source of manganese in the diet as well as an efficient source of minerals with 13% of the daily value, values increase with the maturity degree of the plant (^{Hernández-Urbiola et al., 2011}). Also it is known that nopal consumption is a source of digestive fiber and its nutritional value can help in treating diabetes and hyperglycemia (^{Bacardi-Gascon et al., 2007}).

The nopal cultivation in Mexico

The nopal cultivation in México is based on a traditional system and a micro tunnels system. The traditional system has planting densities of 15 000 to 40 000 plants per hectare by planting the mature pencas (2 to 3 years old) in rows at a distance of 30 to 40 cm between plants and 80 to 100 cm between rows. The plants height goes from 1 to 1.5 m. On the other hand, the micro tunnel system employs planting densities of 120 000 to 160 000 plants per hectare using beds of 1.2 to 2 m wide and 40 to 47 m long with a separation of 1 to 1.5 m (^{Flores-Valdéz, 2003}). The micro tunnels system involves to cover the plantations which increases yields in cold climates. Yields in the traditional system vary from 30 to 80 t ha^-1 (^{Pimienta, 1997}), while in the intensive system, 179-263 t ha^-1 have been recorded (^{Blanco-Macías et al., 2002}).

The traditional system uses cattle manure as a fertilizer (over 100 t ha^-1) every two or three years, which is complemented by mineral fertilizers such as urea or ammonium sulfate. In contrast the micro tunnels system uses cattle manure, nitrogen and phosphorus in proportions of 100 to 200 t ha^-1, 100 to 200 kg ha^-1 and 80 to 100 kg ha^-1, respectively (^{Pimienta, 1997}; ^{Flores-Valdez, 1999}).

Commercial plantations of nopal in México are located in regions with an average rainfall of 600 to 800 mm in the summer (^{Orona-Castillo et al., 2004}), and harvesting is performed mostly manually using knives to cut the base of the penca (^{Cantwell, 1999}). The cutting of the plant and the fruit must be carried out under suitable climatic conditions in order to avoid a high acidity content, the harvest should also be performed when the plant has reached a length of 20-25 cm and weigh about 90 to 100 g (^{FAO-OMS, 1993}). Cladodes are classified as small when they have an average length of 12 cm or less, and as large when they reach 30 cm. The final use of the plant depends on its characteristics based on its quality, the commercialization can be done for direct consumption or destined to the elaboration of minimally processed nopal (^{Rodríguez-Felix, 2002}). The criterion used to separate the nopalitos in the different quality degrees is based on its appearance, mainly the presence of defects in shape and color and mechanical damage. The classification is based on the size (length) of the nopalito, establishing five categories. These include cladodes from 9 cm up to 30 cm in length (^{FAO-OMS, 1993}). In order for the vegetable nopal to be commercialized in international markets, compliance with additional quality standards is required in order to generate an adequate degree of competitiveness in contrast to other higher consumption vegetables (^{Caplan, 1995}).

In regional terms, the Milpa Alta delegation in Mexico City ranks as the main producer of vegetable nopal with an annual average of 350 000 t and an average annual yield of 90 t ha^-1 (^{SAGARPA, 2016}). Regarding to the marketing and transportation of nopal in Milpa Alta, two alternatives are shown. The nopal for local consumption is sold in baskets weighing no more than 5 kg on the other hand, for consumption outside the region the nopal is marketed in bales of 1 m in diameter and 1.7 m in height containing approximately 3 000 pencas and weigh from 250 to 300 kg. Bales are transported to wholesale markets in México City. For markets distant from the production areas, they are packed in 20 kg wooden boxes or in carton boxes of 5 to 10 kg and transported in cooled trucks at 10 °C (^{Flores, 1992}; ^{Cantwell, 1999}).

When the nopal reaches the commercialization stage, the period for consumption and expiration depends on variables such as the type of crop, procedure and characteristic of packaging, as well as temperature and humidity in storage (^{Nerd et al., 1997}). Cladodes that show a damage in its base must be marketed in a short time and avoid storage or transport to distant markets since a loss by rots is generated up to 53% (^{Ramayo et al., 1978}). The damage to the base is mainly caused by the harvest technique, which when performed manually involves a high risk of affecting the quality of the crop (^{Rodríguez-Felix and Villegas-Ochoa, 1997}). Consequently, it is possible to have a high amount of product (nopal) that does not meet the quality standards necessary to be marketed in the different consumer markets, a situation that implies the searching for an alternative to better use the cladodium.

Nopal mucilage

On the other hand, it is known that mucilage is a byproduct obtained from cladodes of nopal, commonly referred to as polysaccharide pectin, it is a characteristic of the cactus family members. This compound is found in cladodes, skin and pulp of the fruit, although in different proportions (^{Medina-Torres et al.,2003}). The chemical composition of Ofi nopal mucilage, has been the subject of several studies, ^{Cardenas et al. (1997)} found that the mucilage was a neutral polysaccharide containing arabinose, rhamnose, galactose and xylose. Furthermore, ^{Paulsen and Lund (1979)} reported that Opuntia ficus-indica extract was a mixture of a neutral glucan, glycoproteins and an acid polysaccharide made of L- arabinose, D-galactose, L-rhamnose, D-xylose and D-galacturonic acid. According to ^{Trachtenberg and Mayer (1981)}, mucilage is a polysaccharide, containing 10% uronic acid, arabinose, galactose, rhamnose and xylose.

The composition of nopal mucilage was quite similar to ^{Madjdoub et al. (2001)} composed mainly of L-arabinose, D-galactose, D-xylose and 19.4% of uronic acid. ^{McGarvie and Parolis (1981)} found that the nopal mucilage is composed of a family of highly branched polysaccharides by a structure of α-D-galacturonic acid units attached 1-2 to units of β-L-rhamnose linked 1-4 with branching at C-4, galactose oligosaccharide branches which carry L-arabinose and D-xylose as substituents.

Use and application of nopal mucilage

Nowadays, nopal mucilage finds multiple applications in fields such as the food, cosmetics and pharmaceutical, construction and environmental industries, among others, which arise as research on its properties advances. The application of by-products that use mucilage as the main component has demonstrated practical results in different industries. Nopal mucilage has been used in aspects of environmental conservation as an element for the treatment of leachates in landfills ^{Castellanos et al. (2012)}, also in the food industry nopal mucilage has been used to achieve a browning during banana dehydration ^{Aquino et al. (2009)} as well as to accelerate the enzymatic activity in apples (^{Martínez, 2012}). Another application of the nopal mucilage can be seen as a fiber reinforcer used for packaging made from biopolymers (^{Rivera, 2015}). However, an application that has gained importance for the nopal mucilage, is its use in the construction industry. The mucilage, given its characteristics as a thickening or modifying polysaccharide, makes it possible to increase the plasticity degree, as well as the decrease in the permeability of liquids in cement and lime pastes, as well as the property to form films (^{Muñoz, 2016}).

Experimental nopal cultivation

The cladodium cultivation process was carried out under the climatic conditions and with the soil characteristics of the region in order to establish a representative model of the nopal produced in the entire Milpa Alta delegation. The cultivation activities were carried out by means of a programmed control of fertilization using different products to be able to identify a greater yield according to the type of fertilization system. For the nopal production process, five types of treatment were determined: manure, compost, chemical fertilizer, leachate and water; a) the treatment is placed in the middle of the rows and these are those who receive such treatment directly; b) the chemical fertilizer, corresponds to the mara Yara Mix + urea, applied in the soil; and c) the leachate to be applied in the row 10 and 11 is divided into two, since the row 10 is applied to the soil and in the row 11 is foliarly applied. (Figure 1).

Figure 1 Experimental plot by treatment type. 

Mucilage characterization

Of the types of treatment used, two were selected: compost and manure, because these two types of fertilization systems are currently used in the region, leachate and chemical fertilizer are not generally applied. The obtained mucilage must be representative of the nopal produced in the region and grown under the same conditions performed by farmers of Milpa Alta (Figure 2-15).

Figure 2 Pruning of the remains of nopal pencas in order to establish an adequate space to proceed with the nopal sowing. 

Figure 3 Determination of the area destined for the experimental plot in San Lorenzo Tlacoyucan, Milpa Alta delegation of 500 m². 

Figure 4 Plantation of pencas with a separation of 30 cm each. 

Figure 5 Separation between furrows of 150 cm. 

Figure 6 The nopal planting with a distribution of 255 nopal pencas. 

Figure 7 The division through labels that allow to identify the treatment type is shown. 

Figure 8 Harvesting by manual cutting. The last proportion was used for the mucilage extraction process. 

Figure 9 Selection of adult or larger and with thicker thorns nopales. 

Figure 10 The selected nopales underwent a process of manual cutting to reduce its volume to samples of 1 cm³. 

Figure 11 The mixture of nopal and water was subjected to a heating process using thermoline laboratory equipment in order to reach an optimum temperatura for the use of the carbohydrates in the mixture. 

Figure 12 The cactus and water mixture was subjected to a heating process using thermoline laboratory equipment in order to reach an optimum temperature for the use of the mixture carbohydrates. 

Figure 13 Temperature monitoring was performed until reaching 50 °C (Hanna thermocouple), for one hour, to avoid reduction of viscosity and functionality in the mucilage. 

Figure 14 Separation process by liquid decantation by density. When the separation process was carried out, it was also carried out a filtering process with a 500 microns mesh to remove residues. 

Figure 15 Storage in glass containers in order to avoid chemical interaction or properties alteration due to the packaging material.