General characteristics of soursop fruit

The soursop plant is a tree from 3 to 10 m tall, branched, conical, leafy, with elliptical oval leaves of 2 to 6 cm wide by 6 to 12 cm long, with axillary buds. The root is pivotal with strong branching anchorage, the highest percentage is found in the first 30 cm of depth. The flowers are hermaphrodite, distributed along the stem and axillar; fruits constitute of a berry product of multi ovaries (^{Méndez, 2003}; ^{Miranda et al., 1998}).

The soursop fruit is classified as multiple of oblong conical form, similar to a heart or of irregular shape, the last one is due to an improper development of the carpel or voids produced by insects; the fruit reaches 10 to 30 cm in length with a weight between 1 to 5 kg, with a dark green shell that possesses several small, soft and fleshy spines. When the fruit is ripe the peel shows a dull green color and acquires a soft consistency with a vertical appearance. The pulp is white, creamy, aromatic, juicy and smooth, adhered to the cuticle, but it easily separates into segments and completely covers the black seeds that have average dimensions of 1 to 2 cm long, each fruit can have up to 200 seeds (^{Morton, 1987}; ^{Méndez, 2003}; ^{Blench and Dendo, 2007}; ^{Janick and Paull, 2008}). The pulp contains 80-83% water, 1% protein, 14-18% carbohydrate, 3.43% titratable acidity, 24.5% non-reducing sugars and B1, B2, and C vitamins (^{Morton, 1987}; ^{Onimawo, 2002}).

Soursop production in Mexico

In México during 2013 it was grown in approximately 2 724 ha, with an average yield of 8.5 t ha^-1 and a value of the total production of around 105 million pesos (SIAP, 2014). The main producing states are: Nayarit, Colima, Jalisco, Michoacán, Guerrero, Oaxaca, Tabasco, Campeche, Yucatán, Puebla, Veracruz and Morelos. At the national level, Nayarit is the largest soursop producer with an established area of 73% (SIAP, 2014). The soursop production is regularly exported throughout the year and there has been an increase in the average values of consumption in the fresh market (^{Lima and Alves, 2011}).

Importance of soursop fruit

Generally the fruit is consumed fresh, directly as dessert, the pulp has an immense potential for its processing due to its pleasant taste and the high yield per fruit (up to 85.5%) which makes it an attractive raw material for preparing beverages, puree, juice, jam, jelly, candy bars and as a basis for making ice creams, sorbets, jellies. Utilization and improved marketing of the soursop fruit is due, from the food point of view, to the nutritional components (vitamins, minerals etc) it contains (^{Ojeda et al., 2007}).

In the area of pharmacology soursop has begun to gain strength due to the fact that its stem, leaves and seeds have been historically used in traditional medicine by indigenous peoples given its anti-tumor, fungicidal and anti-diarrheal capabilities (^{Solís-Fuentes et al., 2010}). The soursop leaves are traditionally used in Brazil for liver problems (Solis-Fuentes et al., 2010), they are also used as suppurative (against mucus, secretions or discharge) and antipyretic (^{Badrie and Schauss, 2010}). ^{Vieira et al. (2010)} performed an analysis of the anti-inflammatory capacity of the ethanolic extract of soursop leaves obtaining confirmation of its possible therapeutic use; however, they recommend studies on the side effects that may occur. Regarding the soursop seeds, the oil may contain physicochemical characteristics that increase its use in the food, pharmaceutical and cosmetics industries (Solis-Fuentes et al., 2010). Traditionally these seeds are used as insecticide, astringent and fishing bait (^{Badrie and Schauss, 2010}).

Harvest of soursop fruit

The soursop fruits are harvested at the point of physiological maturity, which coincides with its maximum size, with the stiffness loss of the stylistic rudiments and change in the epidermis tonality, going from a dark green to a lighter green (mate). Meanwhile ^{Worrell et al. (1994)} report that the harvest index is given 160 days after the anthesis when the fruit acquires the light green or yellowish color. The cutting of the fruit is done manually with a pruning shear, leaving 2 to 3 cm of the peduncle attached to the fruit, it is recommended to use stairs to collect the fruits of the high parts of the tree, without affecting the branches.

It is not recommended to let the fruits ripen in the tree as it can suffer damage, decreasing its quality. On the other hand, if the fruits are harvested before the physiological maturity, it does not ripe well and the pulp can have a bitter taste. It is recommended to harvest in the early hours of the morning to avoid dehydration of the fruit. The soursop presents a long cycle (120-180 days) from the flowering until the fruit formation in its harvest index; therefore, it requires a large investment of resources to obtain a quality product that must be kept in harvest and postharvest practices (^{Ramírez et al., 1998}).

Post-harvest management of soursop fruit

The postharvest is the stage of the agroindustrial process that involves all the activities aimed at offering fruits of excellent quality to the consumer. The conservation of perishable agricultural products constitutes a guarantee for the safety of the population’s food supply (^{Bernal and Díaz, 2003}). The high post-harvest losses (25-35%) for soursop fruits, generally due to inadequate practices, make this product a very important plant material to be studied (^{Ramírez, 2008}). The initial quality of the harvested fruit can not be improved by applying technologies during the postharvest period, but it is possible to maintain it using conservation systems; such as adequate packaging, refrigeration systems, controlled or modified atmospheres, among others (^{Gutiérrez and López, 1999}).

The fruits must be handled under ideal temperature conditions, relative humidity (85-90%), packing and storage, in order to prolong its useful life since these are living organisms that after being harvested are susceptible to physical, chemical and microbiological damages. Among the normal post-harvest handling operations of soursop fruits, the following operations are highlighted: collection, selection, disposal of organic waste, sorting, disinfection, weighing, pre-cooling between 12 to 15 °C, drying of residual moisture, waxing (optional), storage and transport (^{Morton, 1987}; ^{Lima et al., 2003}).

Physiological changes of guanabana fruit

The guanabana is a compound fruit, constituted by the added development of multiple ovaries. In the 1970s it was reported that atypical behavior, in terms of the respiratory pattern, could be attributed to differences in the physiological ages of the tissues of the different fertilized ovaries (^{Paull, 1982}). ^{Bruinsma and Paull (1984)} reported that the initial respiratory increase is due to the increase in mitochondrial respiration promoted by the increase in the supply of carboxylated substrates, probably induced by the harvest act.

Due to its respiration rate, soursop fruits have been classified as climacteric, such fruits are often harvested at physiological maturity and ripen after harvest, the respiration rate and ethylene production are high, up to 150 mg kg^-1 h^-1 and 100 ml kg^-1 h^-1 of CO₂, respectively at 24.5 °C, which makes this fruit extremely perishable (^{Paull, 1996}; ^{Bruinsma and Paull, 1984}). As the fruit matures, there is a slight pallor in the intense green shell, changing to a yellowish-green coloration (^{Worrell et al., 1994}; ^{Paull, 1982}) reflecting a chlorophyll degradation, in the last stage of maturation shell changes from green to dark brown (^{Paull et al., 1983}), this possibly due to chloroplasts breaking, which causes release of the polyphenol oxidase enzyme (PPO) that causes oxidation and the phenols polymerization.

The formation of volatile compounds (methyl hexanoate and methyl (E)-2-hexenoate) is parallel to the production of ethylene and reaches the maximum production of these compounds five days after harvest; likewise, a maximum formation of sugars and organic acids, and the development of the consumption characteristics of the fruit such as color, firmness, acidity, total soluble solids and fragrance of fruit consumption can be observed (^{Paull et at., 1983}). After this stage occurs a decrease in the production of the main aroma constituents and volatiles appear, such as butanoic acid, hexanoic acid, and γ-butyrolactone, to which the fermented odor of the over-ripened fruit is attributed, the same trend can be seen in relation to sugars and organic acids (^{Paull et al., 1983}; ^{Márquez-Cardozo et al., 2011}).

Enzymatic activity

Most of the biochemical changes that occur in living tissues are caused by enzymes, with a very large number of enzyme systems that have been discovered in plant tissues, where they play an important role in its composition and rigidity; but also the enzymes present in fruits are responsible for the maturation and formation of the sensorial characteristics that are known. However, after harvesting, enzymes are responsible for senescence and undesirable changes occurring in the fruits which causes them to be discarded and not used for its transformation.

The enzymatic activity of polyphenol oxidase (EC 1.14.18.1; PPO) and peroxidase (EC 1.11.1.7; POD) causes one of the most significant impact and deterioration reactions in the sensorial quality of fruits and vegetables, PPO Catalyzes the conversion of o-diphenols into o-quinones. ^{Oliveira et al. (1994)} studied the PPO activity in the soursop fruit as a function of pH and the concentration of polyphenols during ripening. It was observed that the optimum activity was at a pH of 7, for fully mature fruits and 7.5 for immature fruits

It is important to note that PPO inhibition studies were carried out on the extract of the enzyme at its optimum pH but not on the plant tissue at its natural pH, this fact invalidates the application of inhibition conditions to fruits and plant tissues that may have a different pH. ^{Bora et al. (2004)} studied the enzymatic activity of PPO in semipurified enzyme extracts of mature soursop fruits and compared the efficiency of the thermal and chemical inhibition conditions on the PPO activity, having as results pH and optimum temperatures for the PPO activity of 7.5 and 32 °C.

The activity of the PPO enzyme is involved in the oxidative browning of plant tissues. ^{Lima et al. (2002)} reported the greatest increase in PPO enzyme activity in soursop fruit occurred from the first to the second day, when the activity increased from 243 to 400 g EAU^-1 min^-1. Studies by ^{Oliveira et al. (1994)} showed that the PPO activity decreased as the guanabana fruit matured. However, the authors expressed activity based on the protein content (specific activity), so that any variation would affect the activity of this enzyme.

The POD enzyme, in turn, plays a limited role in the enzymatic browning, because it depends on the availability of hydrogen peroxide (^{Robards et al., 1999}). However, phenolic substrates can be oxidized in the presence of small amounts of hydrogen peroxide, and various compounds are susceptible to oxidation by these enzymes. ^{Lima et al. (2002)} reported that POD activity has an initial increase of 1 273 UAE g^-1 min^-1, followed by a sharp decline until day 4, the highest variation was recorded from third to fourth day. At the end of the period, activity doubled, although it remained below the initial one.

These same researchers indicate that, compared with the PPO activity, it seems that POD, in addition to having higher levels, showed more pronounced variations; however, the lowest enzyme activity was observed from the second day, this may be associated with reduced susceptibility to browning during soursop maturation, as indicated by Lima de ^{Oliveira et al. (1994)}.

Variations during the life of the fruit, genetic differences represent different levels of susceptibility to oxidative browning, indicating a complex interaction between the activity and the amount of the enzyme and the types of phenolic compounds (^{Robards et al., 1999}). The enzyme pectinmethylesterase (EC 3.1.1.11; PME) is related to the degradation of the pectic substances of the cell’s middle lamella, a component of the cell wall that acts as a cementing agent or ligand between cells and can also control movements of soluble materials (^{King, 1990}; ^{Proctor and Miesle, 1991}). ^{Aziz and Yusof (1994)} reported activity of the PME enzyme in soursop fruits.

On the other hand, ^{Lima et al. (2006)} quantified the activity of PME, indicating a considerable increase of this enzyme in short periods, the activity showed by PME was 23 times greater in the fruit at consumption maturity with respect to the fruit at physiological maturity. There have been some studies in order to purify and characterize the PME in the soursop fruit, which is present in two isoforms (PE I and PE II) (^{Arbaisah et al., 1996}; ^1997a; ^1997b). It has been suggested that the function of the PME is to promote de-esterification of galacturonans in order to allow the action of PGs (^{Giovane et al., 1990}; ^{Kays, 1997}).

PGs are pectolytic enzymes which are identified as endo-PG (EC 3.2.1.15) and exo-PG (EC 3.2.1.67); the endo-PG catalyzes the random hydrolytic cleavage of the α-(1-4) bonds of the galacturonanes; the exo-PG is hydrolyzed by liberating galacturonic acid (^{Seymour and Gross, 1996}; ^{Kays, 1997}). PG activity has been observed in several fruits such as mango and durian (Durio zibethinus) (^{Abu-Sarra and Abu-Goukh, 1992}; ^{Ketsa and Daengkanit, 1999}), where an increase during softening can be verified, possibly caused by the production of ethylene. The enzyme promotes the degradation of the middle lamella of parenchymal cells, resulting in softening.

Furthermore, it may be involved in the autocatalytic release of uronic acids, pectic polymers degradation solubilized during maturation and polyuronides depolymerization (^{Redgwell et al., 1992}). ^{Aziz and Yusof (1994)} reported a sudden increase in PG activity in soursop fruits, which coincided with the climacteric of the fruit. ^{Lima et al. (2006)} observed that, after the increase, the low enzymatic activity coincides with a sudden decrease in pectin content and with a higher solubility of pectin, indicating that most of the PG substrates were used immediately at the time of maximum activity.

Post-harvest conservation technologies

Among the technologies that allow to prolong the conservation period and to maintain the own quality characteristics of the fruit, the refrigeration is one of the most effective and of general use; however, the soursop fruits are sensitive to cold; that is, they suffer physiological damage when stored at temperatures from 4 to 18 °C of refrigeration, observing superficial damages (discolouration and depressions), maintenance or increase of the pulp firmness, loss of the capacity to mature, browning of the pulp, loss of taste and accelerated senescence (^{Alves et al., 1997}).

It has been observed that the soursop storage at 15 °C allows a three days delay in the time necessary for maturation, on the other hand, it has been reported that the soursop fruits could not be maintained at temperatures between 12 and 14 °C for more than six days (^{Silva et al., 2001}). The tropical nature of the soursop makes its useful life limited by the presence of cold damage when it is stored in refrigeration. Some investigators indicate that this physiopathy is induced at temperatures below 15 °C (^{Reginato and Lizana, 1980}; ^{Luchsinger and Artés, 2000}).

^{Castillo-Ánimas et al. (2005)} evaluated the tolerance to cold damage and post-harvest life at different temperatures (12 to 14 and 16 to 18 °C) in soursop fruits; the soursops were harvested in two maturity stages (light green and dark green), these were treated with waxes and growth regulators (gibberellic acid and isopropyl ester of 2,4-dichlorophenoxyacetic acid), resulting in dark green fruits showed damaged by cold at 12 to 14 and 16 to 18 °C, on the other hand the light green fruits only showed this physiopathy at 12 and 14 °C.

The use of modified and controlled atmospheres with low levels of ethylene and oxygen, or high levels of CO₂ and supplemented with cooling significantly contributed to extend the shelf life of fruits and vegetables, while maintaining its quality (^{Kader, 1995}). Meanwhile, there are few studies in these areas with soursop fruits and the available information is still scarce to allow greater flexibility in marketing and reach more distant markets (^{Alves et al., 1997}). ^{Silva et al. (2001)} observed that soursop fruits individually packed in polystyrene trays coated with a flexible polyethylene film at 12 °C and 14 °C maintain its quality for up to 22 days.

It is also possible that the fruit ripening is delayed by the use of production inhibitors and ethylene action (^{Abdi et al., 1998}). In this regard, some studies have been conducted with the application of 1-methylcyclopropene (1-MCP) in soursop at concentrations of 200 nL L^-1 wich has allowed a delay in the loss of firmness and increased soluble solids content, occurring in the ripening of soursop; same response was observed in the same study with the application of wax or the association of wax + 1-MCP (^{Lima et al., 2002}; ^{Lima et al., 2004}; ^{Lima and Alves, 2010}).

Furthermore ^{Tovar-Gómez et al. (2011)} evaluated the application of emulsions and 1-MCP, reporting that these combinations did not delay the ripening of the soursop, but did decrease weight loss compared to the control; however, they observed that using 1-MCP at 1 000 nL L^-1 for 12 h combined with emulsions based on carnauba wax with silicone oils or candelilla at storage temperature of 13 ±2 ºC extended shelf life of the soursop, since fruits reached maturity consumption at 15 days of storage with three more days to be commercialized.

^{Moreno-Hernandez et al. (2014)} evaluated the effect of 1-MCP and waxes emulsions on composition, vitamin C, polyphenols, and antioxidant capacity of soursop fruits stored at 25 and 16 °C, reporting that fruits stored at 16 °C without 1-MCP showed visible symptoms of cold damage and fruits treated with 1-MCP combined with emulsions maintained to a greater extent their vitamin C content, dietary fiber, total phenolic compounds content and antioxidant activity, concluding that the interaction between these two compounds can be used for post-harvest handling as it helps to preserve the nutritional composition of soursop fruits.

Similarly ^{Montalvo-González et al. (2014)} evaluated the application of 1-MCP (1 500 nL L^-1, 12 h) in combination with emulsions based on candelilla wax or beeswax diluted with water, on fruits stored at 25 to 16 ºC, reporting that the fruits stored at 16 ºC with and without emulsions showed cold damage and did not ripen; in fruits with the application of 1-MCP alone or combined with emulsions, in any of the dilutions, no symptoms of cold damage were observed in the pulp. On the other hand, they observed that the combination of 1-MCP and beeswax-based emulsion at 15:85 v/v dilution preserved soursop fruits for 14-15 days compared to fruits stored at 25 °C (6 days).