ABSTRACT
Production data for most new and expanding tropical fruits is rarely available. Available evidence from individual countries suggests that Dragon Fruit (Selenicereus spp.) is expanding. Significant production is occurring and expanding in many countries including: Vietnam, China, Mexico, Colombia, Nicaragua, Ecuador, Thailand, Malaysia, Indonesia, Australia and the United States. Species from two former genera (Hylocereus, Selenicereus) are being grown with Hylocereus being regarded as having three-ribbed shoots and four or more ribbed species being in Selenicereus. Recent molecular studies have shown that Hylocereus falls under Selenicereus and implies a single common ancestor to the species in these former two genera. This reclassification removes some of the previous confusion about the overlapping characteristics of the many named species in this genus. This reclassification implies that fewer species and wider crosses are possible. The current limited selections used commercially have major consumer long-term appeal issues if the crop is to be moved more widely into global markets and beyond speciality stores and local markets. The expanded market possibilities have been spurred in part by the acceptance of a fruit irradiation protocol by the United States in 2012 to meet fruit fly disinfestation requirements. Consumers are intrigued by the exotic nature of dragon fruit with their bright red skin and greenish scales, and white or sometimes red flesh, however, a common feedback from consumers is that though it has a delicate flavor and has antioxidants benefit, the taste is sometimes bland and not sweet. This limitation has led to importation of sweeter yellow dragon fruit from Ecuador and Colombia into North America and Europe. Another limitation to the expansion of markets is the need to have fruits that are consistent in terms of quality and in quantities to meet the market demand. Consistency in quality bring to the forefront the need to improve aspects of harvest maturity, handling and packing that minimizes mechanical injury and ensures a safe and optimum storage and shipping environment.
Keywords: Production, taxonomy, global marketing, postharvest handling, cultivars
INTRODUCTION
In this discussion for convenience, I will use the common name Dragon fruit in reference to the species in the current two that are commercially grown vine climbing cacti (Hylocereus Berger) Britton and Rose, and Selencereus (Berger) Britton and Rose) (Figure 1). The common Spanish names for these fruits are: pitaya, pitajaya or pitahaya, cuauhnochtli in the Nahuatl language, cierge lèzard, poire de chardon in French and night blooming cereus, strawberry pear and dragon fruit in English. Many columnar cacti in the genus Stenocereus are also called pitaya or pitahaya or pitajaya in Latin America. Both genera (Hylocereus, Selenocereus) are members of the tribe Hylocereeae which are found in Central American-Mexico and northern South America and are epiphytic, hemi-epiphytic and climbing cacti (Mizrahi et al., 1997; Korotkova et al., 2017). The genus Selencereus has more edible fruits, though Hylocereus spp. has been spread more widely around the world as a fruit crop (Le Bellec et al., 2006). In a number of countries, Hylocereus undatus is regarded as an invasive species (https://www.cabi.org/isc/datasheet/27317).
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Production data for most new and expanding tropical fruit is rarely available. Available evidence from individual countries suggests that Dragon fruit production is expanding. Significant production is occurring and expanding in many countries including: Vietnam, China, Mexico, Colombia, Nicaragua, Ecuador, Thailand, Malaysia, Indonesia, Australia and United States.
Vietnam, the leading exporter of dragon fruit in the world, has almost 40,000 ha devoted to dragon fruit with a volume of production reaching about 1 million metric tons (MT) (Australian Department of Agriculture and Water Resources 2017) valued at US$ 895.70 million (VNA 2016). Vietnam fruit yield averagees 22-35 MT/ha/year (Nguyen et al. 2015). The majority of dragon fruit in the Chinese market comes from Vietnam. The import volume reached 533 MT in 2017 and exceeded 500 MT per year in the last five years. This imported fruit was valued at $390 M (Kubo and Sakata, 2018). Malaysia had 1,641 ha in production in 2013 and produced of 11,000 MT (Kek Hoe 2017) with acreage reported to be increasing. In Indonesia, commercial production started in 2000 and is reported to be 4,300 ha, mainly in Banyuwangi, East Java and East Kalimantan. From the 2,300 ha in Banyuwangi, 117,700 MT was harvested (Riska 2016; Hamidah et al., 2017). Dragon fruit is the fifth most imported tropical fruit from Asia exported to China after lychee, longan, banana, and mango. PR China is also expanding its own production in the last few years and possibly exceeds 40,000 ha (https://www.freshplaza.com/article/2199051/2018-dragon-fruit-production-...). This report does point out that the Vietnamese fruit have white flesh and new varieties being planted in Guangxi have red flesh and are regarded as being preferred to the Vietnam imports in flavor, ripeness, and freshness. In the Philippines, the area planted has been increased in the last six years, from 182 hectares (ha) in 2012 to 450 ha in 2018 and a total production 1,463 metric tons (MT) (Eusebio and Alaban, 2018). Production in the United States is limited to Florida, California and Hawaii (Merten 2002: Lobo et al., 2013). The acreage is limited but increasing with California having up to 150 ha, Florida 160 ha and with about 80 ha in Hawaii (Lobo et al., 2013; Anon, 2018). US consumers have been mainly Asian and Latin Americans and sales until now is restricted to speciality stores and farmers markets.
TAXONOMY
The definition of a plant species and a genus has always presented a level of confusion (Rieseberg and Willis, 2007: Naomi, 2011), with barriers to genetic exchange that develop during evolution, as propounded by Mayr (1942) being a common criteria. However, like most things in biology there are exceptions with reproductive isolation or a lack of genetic exchange not being the only possibility Mayden (1997) presents 22 concepts of a species that have been grouped in various ways (Rieseberg and Willis, 2007: Naomi, 2011). Plants can vary in mating systems, ploidy levels, mode of dispersal and life history than adds another layer of complexity compared to animals as to what is a species. We have tended to focus on self-incompatability mechanisms that enforce out-crossing in many hermaphrodite plants such as cactus, as we attempt to carry out our breeding programs. Polyploidy, full genome duplication either as a hybrid between to species (allopolyploidy) or autopolyploids not due to hybrid origin, have seemly occurred in dragon fruit (Lichtenzveig et al., 2000; Tel-Zur et al., 2004a, b, c; Plume et al., 2013). Hybrids for example, have been produced between H. undatus and H. costaricensis to overcome self-incompatibility. In the Hylocereus, vegetative characteristics such as color of the stems, whether the aeroles have spines and their length, nature of the stem margins and stem joints, and fruit shape are used to separate species (Britton and Rose, 1963; Le Bellec et al., 2006). These vegetative traits have significant variation and not unique markers as environmental factors and genetic makeup can alter these traits. Molecular markers have been used to categorize Mexican and Colombia that show 92.54% polymorphism among, within and between populations (Tel-Zur et al., 2004; Cisneros & Tel-Zur 2013; Pagliaccia et al., 2015). The Colombian selection is related to the majority of Mexican selections, suggesting a common origin and wide dispersal precontact. We do need a definition of a species to carry out our research and breeding programs irrespective of the theoretical problems with the understanding that a species, because of evolution, is not fixed. A species may have considerable variation in morphology, physiology, biochemistry and genetics.
At the next level of biological classification, the genus, a definition of species adds additional confusion for taxonomists. The current criteria include i) “monophyly” with all species in a genus being descendants of an ancestral species, ii) the genus is reasonably compact, and, iii) the genus is distinct with evolutionarily relevant criteria (morphology, biogeography, DNA sequences) that are a consequence of evolutionary divergence. These criteria often lead to assumptions and arbitrary assignments. Attempts are being made to bring more exactness to plant classification to reduce confusion as genetic markers cast doubt on current genera assignment of species and on what is a separate species (Hawkesbury, 2010). The International Plant Name Index (IPNI) is a step to reduce some of the confusion (https://www.ipni.org/).
The composition of the tribe Hylocereeae has been in constant flux from when it was created as a subtribe by Britton and Rose (1920) and as a tribe by Barthlott and Hunt (1993). This tribe is an example of varying genetic limits and excessive splitting with most of the genera not being well defined (Korotkova et al., 2017). The species in the two genera (Hylocereus, Selenicereus) are being grown commercially. Hylocereus is regarded as having three-ribbed shoots and those with four or more ribbed species being in Selenicereus (Mizrahi, 2015). This vegetative criterion is variable trait possible cause by a modification of a few genes or a single developmental control point. Differences between the members of these two genera from a taxonomic perspective do give morphological reason for separate genera (Table 1).
Table 1. Comparison of Selenicereus spp. and other Hylocereus clones (Red) used in commercial production of dragon fruit. The genera are similar with scrambling or climbling cacti with large, usually white, nocturnal flowers. From Mizrahi et al., 1997; Mizrahi (2015) and Korotkova et al. (2017)
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In addition, Selenicereus does not have a clear taxonomic concept often being an assemblage of species that could be placed in different genera, while Hylocereus is much more consistent in its boundaries. Studies with Arabidopsis indicate that single mutation can lead to significant changes in stem and leaf color (Shirley et al., 1995) and stem morphology and anatomy (Kim et al., 2005). Recent molecular studies (plastid & nuclear DNA) support the conclusion that Hylocereus falls under Selenicereus and implies a single common ancestor to the species in these former two genera (Plume et al., 2013; Korotkova et al., 2017). Morphogical and anatomical comparisons reach a similar conclusion (Gomez-Hinostrosa et al., 2014). This reclassification removes some of the previous confusion about the overlapping characteristics of the many named species in these two genera. The overall number of genera in the tribe Hylocereeae has declined with this realignment from 17 to eight (Korotkova et al., 2017). Because Selenicereus has already been published (Hunt, 2017), it is regarded as having priority over Hylocereus though from an economic perspective Hylocereus has an international market and is relevant to CITES and trade (Korotkova et al., 2017). In the new classification of the genus Selenicereus, the Hylocereus species specific epithet is retained. For example; Selenicereus costaricensis (F.A.C. Weber) S. Arias & N. Korotkova replaces Hylocereus costaricensis (F.A.C. Weber) Britton & Rose, Selenicereus guatemalensis (Eichlam ex Weingart) D.R. Hunt replaces Hylocereus guatemalensis (Eichlam ex Weingart) Britton & Rose, Selenicereus megalanthus (K. Schumann ex Vaupel) Moran for Hylocereus megalanthus (K. Schumann ex Vaupel) Ralf Bauer, and Selenicereus undatus (Haworth) D.R. Hunt for Hylocereus undatus (Haworth) Britton & Rose. This reclassification implies that they are fewer species with wider variation in each species and gives greater clarity in breeding program.
EXPANDING MARKETS
Consumers are intrigued by the exotic nature of dragon fruit with their bright red skin and greenish scales, and white or sometimes red flesh, nutritional value and antioxidant content (Chang and Yen, 1997; Paull, 2002, Mahattanatawee et al., 2006, Bellec et al., 2006; Jamilah et al. 2011; Ortiz-Hernandez & Carrillo-Salazar, 2012; Mizrahi, 2015; Hamidah et al., 2017; Perween et al., 2018). However, a common refrain from consumers is that though it has a delicate flavour and has antioxidants benefits (Mahattanatawee et al., 2006), the taste is sometimes bland and not sweet. This has led to importation of the sweeter yellow dragon fruit from Ecuador and Colombia into North America and Europe. The current limited number of varietal selections, used commercially, has major consumer long-term appeal issues if the crop is to be moved more widely into global markets and beyond speciality stores and local markets. The expanded market possibilities have been spurred in part by the acceptance of a fruit fly irradiation protocol by the United States in 2012.
Fruit sweetness is often evaluated as total soluble solids by refractive index (RI) with widely different values being reported from less than 10% to more than 18% for different species and varieties (Table 2). The sugar profile also is widely variable, in some cases with little or no sucrose being reported. The differences in part are due to firstly that RI measures measure the total dissolved solids in solution that includes soluble pectins released during ripening (Liaotrakoon et al., 2013), and, secondly the levels of invertase in the fruit (Wu and Chen, 1997) can cause rapid hydrolysis (inversion) of sucrose to glucose and fructose if care is not taken to inactivate the invertase before homogenization.
Table 2. Comparson TSS and sugar component analysis
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Refractive index of a solution such as a fruit juice is expressed as percentage total soluble solids (TSS) (Magwaza and Opara, 2015). This reading varies with temperature with modern instrument have a temperature compensation circuity to adjust the soluble solids reading (Reid, 2003; Magwaza and Opara, 2015). Generally, the instrument requires calibration with distilled water. Sugars are the major soluble solids in fruit juice, however other soluble compounds including organic and amino acids, soluble pectins and salts and have there own refractive Index (Table 3). TSS gives you a value that compounds are in solution but not what is its composition. The TSS value cannot be directly equated to sugar or sucrose content but in most circumstances give a useful approximation of sugar content. Brix is a measure of sugar concentration using hydrometers (specific gravity) in the brewing and candy industries with pure sucrose as grams of sucrose per 100 grams solution in water. For a sucrose solution refractive index varies from 1.333 at 0% to 1.504 with a 80% solution, most refractive index instruments can read from 1.3 to 1.7.
Table 3. Refractive index and relative sweetness of common substances. Sweetness index from Moskowitz (1970)
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Sucrose, the frequently the most abundant sugar in fruit and is about three times sweeter than maltose; while fructose and glucose are, respectively, 5-fold and 2-fold sweeter than maltose (Moskowitz, 1970). However, these comparisons are for pure solutions and in mixtures with other cell components the organileptic evaluation by sensory panels may indicate less or greater sweetness. In addition, high-performance liquid chromatography (HPLC) sugar quantification may not agree with the sugar content of the cellular sap measured by refractive index (RI) (Table 2).
POSTHARVEST HANDLING AND QUALITY
Another limitation to expansion of markets is the need to have fruit that is consistent in quality and in quantities to meet a markets demand. Consistency in quality bring to the forefront the need to improve aspects of harvest maturity, handling and packing that minimizes mechanical injury and ensure a safe and optimum storage and shipping environment. Immediately after harvest when fresh, the fruit has a bright red appearance though on the retail shelf the fruit has lost a lot of its appeal being often slighy shriveled with less gloss and the scales showing sever dehydration and browning (Mizrahi, 2015). The scales and the fruit body also often show signs of mechanical abrasion and impact injury.These mechanical injury is suffered during harvest, accumulation, transportation and packing.
The availability of an approved fruit fly disinfestation treatment approved for USA imports in 2012, increases the possibilities for market expansion (Wall and Khan, 2008). An alternate, heat treatments, has not been approved and can cause injury at the higher temperature normally used for these disinfestation treatments (Hoa et al., 2006).
Skin color is the most frequently used criteria to judge fruit maturity (Nerd et al., 1999). Other harvest indices also include: color, total soluble solids, titratable acidity, and days from flowering (minimum 32 days). Skin color begins to change 25 to 30 days from flowering in both S. undatus and S. polyhizus, at about the same time the flesh firmness approaches a minimum, and eating quality approaches a maximum 33 to 37 days after flowering (Nerd et al., 1999). Fruit can be harvested from 25 to 45 days from flowering, Israeli workers recommend 32-35 days (Nerd et al., 1999). Final fruit size is dependent upon seed number (Weiss et al., 1994). As the fruit matures, acidity reaches a peak just as the skin color change occurs, then declines 25 to 30 days after flowering (Nerd et al., 1999; Le et al., 2000a). At this stage soluble solids contents increases, to about 14% (Nerd et al., 1999; Le et al., 2000a). A soluble solids/acidity ratio of 40 has been suggested as a harvest index.
There are no US or international grade standards and this can lead to problems in international trade. Fruit are generally graded by size and color. Size grades suggested for Vietnam are: Extra large fruit >500 g, large 380-500 g, regular 300-380 g, medium 260-300 g, small <260 g (Le et al., 2000a). Fruit exported from Israel to Europe are graded by number of fruits (6, 8, 10, 12, 14, and 16) per 4 kg cardboard box.
Chilling injury, mechanical injury and water loss are the three major disorders observed (Paull 2014). Mechanical injury (impact and abrasion) leads to the development of sunken areas and loss of scale appearance. More mature fruit has a higher susceptibility to mechanical injury (Le et al., 2000a). Splitting is a problem in fruit older than 35 days from flowering, that has received rainfall or excessive irrigation during ripening (Le et al., 2000a).
CONCLUSION
Dragon fruit has shown dramatic increases in production with most of the early research work on the crop’s biology and production technology being from the 1970s in Israel (Mizrahi, 2015). The research is now being carried out widely in Asia and leading to improvement in production practices suited for particular areas. The species naming and its confusion has recently been brought more in focus with newer taxonomic approaches that has led to the vine cactus now being classified in the consolidated genus Selenocereus. Newer varieties and selection that are more consistent in sweetness and taste should greatly assist in the industries expansion. Another major improvement is needed in postharvest handing to avoid mechanical injury that causes breaks in the cuticle leading to greater water loss and potential for disease development. These improvements can be expected to increase consumer appeal and expand demand.
ACKNOWLEDGEMENT
The research was supported in part by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under an agreement #58‐2040‐5‐010 through the Agriculture Research Service and Hatch Project H862 to Robert E. Paull.
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Date submitted: September 17, 2019
Reviewed, edited and uploaded: October 18, 2019 |
Overall Dragon Fruit Production and Global Marketing
ABSTRACT
Production data for most new and expanding tropical fruits is rarely available. Available evidence from individual countries suggests that Dragon Fruit (Selenicereus spp.) is expanding. Significant production is occurring and expanding in many countries including: Vietnam, China, Mexico, Colombia, Nicaragua, Ecuador, Thailand, Malaysia, Indonesia, Australia and the United States. Species from two former genera (Hylocereus, Selenicereus) are being grown with Hylocereus being regarded as having three-ribbed shoots and four or more ribbed species being in Selenicereus. Recent molecular studies have shown that Hylocereus falls under Selenicereus and implies a single common ancestor to the species in these former two genera. This reclassification removes some of the previous confusion about the overlapping characteristics of the many named species in this genus. This reclassification implies that fewer species and wider crosses are possible. The current limited selections used commercially have major consumer long-term appeal issues if the crop is to be moved more widely into global markets and beyond speciality stores and local markets. The expanded market possibilities have been spurred in part by the acceptance of a fruit irradiation protocol by the United States in 2012 to meet fruit fly disinfestation requirements. Consumers are intrigued by the exotic nature of dragon fruit with their bright red skin and greenish scales, and white or sometimes red flesh, however, a common feedback from consumers is that though it has a delicate flavor and has antioxidants benefit, the taste is sometimes bland and not sweet. This limitation has led to importation of sweeter yellow dragon fruit from Ecuador and Colombia into North America and Europe. Another limitation to the expansion of markets is the need to have fruits that are consistent in terms of quality and in quantities to meet the market demand. Consistency in quality bring to the forefront the need to improve aspects of harvest maturity, handling and packing that minimizes mechanical injury and ensures a safe and optimum storage and shipping environment.
Keywords: Production, taxonomy, global marketing, postharvest handling, cultivars
INTRODUCTION
In this discussion for convenience, I will use the common name Dragon fruit in reference to the species in the current two that are commercially grown vine climbing cacti (Hylocereus Berger) Britton and Rose, and Selencereus (Berger) Britton and Rose) (Figure 1). The common Spanish names for these fruits are: pitaya, pitajaya or pitahaya, cuauhnochtli in the Nahuatl language, cierge lèzard, poire de chardon in French and night blooming cereus, strawberry pear and dragon fruit in English. Many columnar cacti in the genus Stenocereus are also called pitaya or pitahaya or pitajaya in Latin America. Both genera (Hylocereus, Selenocereus) are members of the tribe Hylocereeae which are found in Central American-Mexico and northern South America and are epiphytic, hemi-epiphytic and climbing cacti (Mizrahi et al., 1997; Korotkova et al., 2017). The genus Selencereus has more edible fruits, though Hylocereus spp. has been spread more widely around the world as a fruit crop (Le Bellec et al., 2006). In a number of countries, Hylocereus undatus is regarded as an invasive species (https://www.cabi.org/isc/datasheet/27317).
Production data for most new and expanding tropical fruit is rarely available. Available evidence from individual countries suggests that Dragon fruit production is expanding. Significant production is occurring and expanding in many countries including: Vietnam, China, Mexico, Colombia, Nicaragua, Ecuador, Thailand, Malaysia, Indonesia, Australia and United States.
Vietnam, the leading exporter of dragon fruit in the world, has almost 40,000 ha devoted to dragon fruit with a volume of production reaching about 1 million metric tons (MT) (Australian Department of Agriculture and Water Resources 2017) valued at US$ 895.70 million (VNA 2016). Vietnam fruit yield averagees 22-35 MT/ha/year (Nguyen et al. 2015). The majority of dragon fruit in the Chinese market comes from Vietnam. The import volume reached 533 MT in 2017 and exceeded 500 MT per year in the last five years. This imported fruit was valued at $390 M (Kubo and Sakata, 2018). Malaysia had 1,641 ha in production in 2013 and produced of 11,000 MT (Kek Hoe 2017) with acreage reported to be increasing. In Indonesia, commercial production started in 2000 and is reported to be 4,300 ha, mainly in Banyuwangi, East Java and East Kalimantan. From the 2,300 ha in Banyuwangi, 117,700 MT was harvested (Riska 2016; Hamidah et al., 2017). Dragon fruit is the fifth most imported tropical fruit from Asia exported to China after lychee, longan, banana, and mango. PR China is also expanding its own production in the last few years and possibly exceeds 40,000 ha (https://www.freshplaza.com/article/2199051/2018-dragon-fruit-production-...). This report does point out that the Vietnamese fruit have white flesh and new varieties being planted in Guangxi have red flesh and are regarded as being preferred to the Vietnam imports in flavor, ripeness, and freshness. In the Philippines, the area planted has been increased in the last six years, from 182 hectares (ha) in 2012 to 450 ha in 2018 and a total production 1,463 metric tons (MT) (Eusebio and Alaban, 2018). Production in the United States is limited to Florida, California and Hawaii (Merten 2002: Lobo et al., 2013). The acreage is limited but increasing with California having up to 150 ha, Florida 160 ha and with about 80 ha in Hawaii (Lobo et al., 2013; Anon, 2018). US consumers have been mainly Asian and Latin Americans and sales until now is restricted to speciality stores and farmers markets.
TAXONOMY
The definition of a plant species and a genus has always presented a level of confusion (Rieseberg and Willis, 2007: Naomi, 2011), with barriers to genetic exchange that develop during evolution, as propounded by Mayr (1942) being a common criteria. However, like most things in biology there are exceptions with reproductive isolation or a lack of genetic exchange not being the only possibility Mayden (1997) presents 22 concepts of a species that have been grouped in various ways (Rieseberg and Willis, 2007: Naomi, 2011). Plants can vary in mating systems, ploidy levels, mode of dispersal and life history than adds another layer of complexity compared to animals as to what is a species. We have tended to focus on self-incompatability mechanisms that enforce out-crossing in many hermaphrodite plants such as cactus, as we attempt to carry out our breeding programs. Polyploidy, full genome duplication either as a hybrid between to species (allopolyploidy) or autopolyploids not due to hybrid origin, have seemly occurred in dragon fruit (Lichtenzveig et al., 2000; Tel-Zur et al., 2004a, b, c; Plume et al., 2013). Hybrids for example, have been produced between H. undatus and H. costaricensis to overcome self-incompatibility. In the Hylocereus, vegetative characteristics such as color of the stems, whether the aeroles have spines and their length, nature of the stem margins and stem joints, and fruit shape are used to separate species (Britton and Rose, 1963; Le Bellec et al., 2006). These vegetative traits have significant variation and not unique markers as environmental factors and genetic makeup can alter these traits. Molecular markers have been used to categorize Mexican and Colombia that show 92.54% polymorphism among, within and between populations (Tel-Zur et al., 2004; Cisneros & Tel-Zur 2013; Pagliaccia et al., 2015). The Colombian selection is related to the majority of Mexican selections, suggesting a common origin and wide dispersal precontact. We do need a definition of a species to carry out our research and breeding programs irrespective of the theoretical problems with the understanding that a species, because of evolution, is not fixed. A species may have considerable variation in morphology, physiology, biochemistry and genetics.
At the next level of biological classification, the genus, a definition of species adds additional confusion for taxonomists. The current criteria include i) “monophyly” with all species in a genus being descendants of an ancestral species, ii) the genus is reasonably compact, and, iii) the genus is distinct with evolutionarily relevant criteria (morphology, biogeography, DNA sequences) that are a consequence of evolutionary divergence. These criteria often lead to assumptions and arbitrary assignments. Attempts are being made to bring more exactness to plant classification to reduce confusion as genetic markers cast doubt on current genera assignment of species and on what is a separate species (Hawkesbury, 2010). The International Plant Name Index (IPNI) is a step to reduce some of the confusion (https://www.ipni.org/).
The composition of the tribe Hylocereeae has been in constant flux from when it was created as a subtribe by Britton and Rose (1920) and as a tribe by Barthlott and Hunt (1993). This tribe is an example of varying genetic limits and excessive splitting with most of the genera not being well defined (Korotkova et al., 2017). The species in the two genera (Hylocereus, Selenicereus) are being grown commercially. Hylocereus is regarded as having three-ribbed shoots and those with four or more ribbed species being in Selenicereus (Mizrahi, 2015). This vegetative criterion is variable trait possible cause by a modification of a few genes or a single developmental control point. Differences between the members of these two genera from a taxonomic perspective do give morphological reason for separate genera (Table 1).
Table 1. Comparison of Selenicereus spp. and other Hylocereus clones (Red) used in commercial production of dragon fruit. The genera are similar with scrambling or climbling cacti with large, usually white, nocturnal flowers. From Mizrahi et al., 1997; Mizrahi (2015) and Korotkova et al. (2017)
In addition, Selenicereus does not have a clear taxonomic concept often being an assemblage of species that could be placed in different genera, while Hylocereus is much more consistent in its boundaries. Studies with Arabidopsis indicate that single mutation can lead to significant changes in stem and leaf color (Shirley et al., 1995) and stem morphology and anatomy (Kim et al., 2005). Recent molecular studies (plastid & nuclear DNA) support the conclusion that Hylocereus falls under Selenicereus and implies a single common ancestor to the species in these former two genera (Plume et al., 2013; Korotkova et al., 2017). Morphogical and anatomical comparisons reach a similar conclusion (Gomez-Hinostrosa et al., 2014). This reclassification removes some of the previous confusion about the overlapping characteristics of the many named species in these two genera. The overall number of genera in the tribe Hylocereeae has declined with this realignment from 17 to eight (Korotkova et al., 2017). Because Selenicereus has already been published (Hunt, 2017), it is regarded as having priority over Hylocereus though from an economic perspective Hylocereus has an international market and is relevant to CITES and trade (Korotkova et al., 2017). In the new classification of the genus Selenicereus, the Hylocereus species specific epithet is retained. For example; Selenicereus costaricensis (F.A.C. Weber) S. Arias & N. Korotkova replaces Hylocereus costaricensis (F.A.C. Weber) Britton & Rose, Selenicereus guatemalensis (Eichlam ex Weingart) D.R. Hunt replaces Hylocereus guatemalensis (Eichlam ex Weingart) Britton & Rose, Selenicereus megalanthus (K. Schumann ex Vaupel) Moran for Hylocereus megalanthus (K. Schumann ex Vaupel) Ralf Bauer, and Selenicereus undatus (Haworth) D.R. Hunt for Hylocereus undatus (Haworth) Britton & Rose. This reclassification implies that they are fewer species with wider variation in each species and gives greater clarity in breeding program.
EXPANDING MARKETS
Consumers are intrigued by the exotic nature of dragon fruit with their bright red skin and greenish scales, and white or sometimes red flesh, nutritional value and antioxidant content (Chang and Yen, 1997; Paull, 2002, Mahattanatawee et al., 2006, Bellec et al., 2006; Jamilah et al. 2011; Ortiz-Hernandez & Carrillo-Salazar, 2012; Mizrahi, 2015; Hamidah et al., 2017; Perween et al., 2018). However, a common refrain from consumers is that though it has a delicate flavour and has antioxidants benefits (Mahattanatawee et al., 2006), the taste is sometimes bland and not sweet. This has led to importation of the sweeter yellow dragon fruit from Ecuador and Colombia into North America and Europe. The current limited number of varietal selections, used commercially, has major consumer long-term appeal issues if the crop is to be moved more widely into global markets and beyond speciality stores and local markets. The expanded market possibilities have been spurred in part by the acceptance of a fruit fly irradiation protocol by the United States in 2012.
Fruit sweetness is often evaluated as total soluble solids by refractive index (RI) with widely different values being reported from less than 10% to more than 18% for different species and varieties (Table 2). The sugar profile also is widely variable, in some cases with little or no sucrose being reported. The differences in part are due to firstly that RI measures measure the total dissolved solids in solution that includes soluble pectins released during ripening (Liaotrakoon et al., 2013), and, secondly the levels of invertase in the fruit (Wu and Chen, 1997) can cause rapid hydrolysis (inversion) of sucrose to glucose and fructose if care is not taken to inactivate the invertase before homogenization.
Table 2. Comparson TSS and sugar component analysis
Refractive index of a solution such as a fruit juice is expressed as percentage total soluble solids (TSS) (Magwaza and Opara, 2015). This reading varies with temperature with modern instrument have a temperature compensation circuity to adjust the soluble solids reading (Reid, 2003; Magwaza and Opara, 2015). Generally, the instrument requires calibration with distilled water. Sugars are the major soluble solids in fruit juice, however other soluble compounds including organic and amino acids, soluble pectins and salts and have there own refractive Index (Table 3). TSS gives you a value that compounds are in solution but not what is its composition. The TSS value cannot be directly equated to sugar or sucrose content but in most circumstances give a useful approximation of sugar content. Brix is a measure of sugar concentration using hydrometers (specific gravity) in the brewing and candy industries with pure sucrose as grams of sucrose per 100 grams solution in water. For a sucrose solution refractive index varies from 1.333 at 0% to 1.504 with a 80% solution, most refractive index instruments can read from 1.3 to 1.7.
Table 3. Refractive index and relative sweetness of common substances. Sweetness index from Moskowitz (1970)
Sucrose, the frequently the most abundant sugar in fruit and is about three times sweeter than maltose; while fructose and glucose are, respectively, 5-fold and 2-fold sweeter than maltose (Moskowitz, 1970). However, these comparisons are for pure solutions and in mixtures with other cell components the organileptic evaluation by sensory panels may indicate less or greater sweetness. In addition, high-performance liquid chromatography (HPLC) sugar quantification may not agree with the sugar content of the cellular sap measured by refractive index (RI) (Table 2).
POSTHARVEST HANDLING AND QUALITY
Another limitation to expansion of markets is the need to have fruit that is consistent in quality and in quantities to meet a markets demand. Consistency in quality bring to the forefront the need to improve aspects of harvest maturity, handling and packing that minimizes mechanical injury and ensure a safe and optimum storage and shipping environment. Immediately after harvest when fresh, the fruit has a bright red appearance though on the retail shelf the fruit has lost a lot of its appeal being often slighy shriveled with less gloss and the scales showing sever dehydration and browning (Mizrahi, 2015). The scales and the fruit body also often show signs of mechanical abrasion and impact injury.These mechanical injury is suffered during harvest, accumulation, transportation and packing.
The availability of an approved fruit fly disinfestation treatment approved for USA imports in 2012, increases the possibilities for market expansion (Wall and Khan, 2008). An alternate, heat treatments, has not been approved and can cause injury at the higher temperature normally used for these disinfestation treatments (Hoa et al., 2006).
Skin color is the most frequently used criteria to judge fruit maturity (Nerd et al., 1999). Other harvest indices also include: color, total soluble solids, titratable acidity, and days from flowering (minimum 32 days). Skin color begins to change 25 to 30 days from flowering in both S. undatus and S. polyhizus, at about the same time the flesh firmness approaches a minimum, and eating quality approaches a maximum 33 to 37 days after flowering (Nerd et al., 1999). Fruit can be harvested from 25 to 45 days from flowering, Israeli workers recommend 32-35 days (Nerd et al., 1999). Final fruit size is dependent upon seed number (Weiss et al., 1994). As the fruit matures, acidity reaches a peak just as the skin color change occurs, then declines 25 to 30 days after flowering (Nerd et al., 1999; Le et al., 2000a). At this stage soluble solids contents increases, to about 14% (Nerd et al., 1999; Le et al., 2000a). A soluble solids/acidity ratio of 40 has been suggested as a harvest index.
There are no US or international grade standards and this can lead to problems in international trade. Fruit are generally graded by size and color. Size grades suggested for Vietnam are: Extra large fruit >500 g, large 380-500 g, regular 300-380 g, medium 260-300 g, small <260 g (Le et al., 2000a). Fruit exported from Israel to Europe are graded by number of fruits (6, 8, 10, 12, 14, and 16) per 4 kg cardboard box.
Chilling injury, mechanical injury and water loss are the three major disorders observed (Paull 2014). Mechanical injury (impact and abrasion) leads to the development of sunken areas and loss of scale appearance. More mature fruit has a higher susceptibility to mechanical injury (Le et al., 2000a). Splitting is a problem in fruit older than 35 days from flowering, that has received rainfall or excessive irrigation during ripening (Le et al., 2000a).
CONCLUSION
Dragon fruit has shown dramatic increases in production with most of the early research work on the crop’s biology and production technology being from the 1970s in Israel (Mizrahi, 2015). The research is now being carried out widely in Asia and leading to improvement in production practices suited for particular areas. The species naming and its confusion has recently been brought more in focus with newer taxonomic approaches that has led to the vine cactus now being classified in the consolidated genus Selenocereus. Newer varieties and selection that are more consistent in sweetness and taste should greatly assist in the industries expansion. Another major improvement is needed in postharvest handing to avoid mechanical injury that causes breaks in the cuticle leading to greater water loss and potential for disease development. These improvements can be expected to increase consumer appeal and expand demand.
ACKNOWLEDGEMENT
The research was supported in part by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under an agreement #58‐2040‐5‐010 through the Agriculture Research Service and Hatch Project H862 to Robert E. Paull.
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Reviewed, edited and uploaded: October 18, 2019