Postharvest Biology and Technology 17 (1999) 39–45 Ripening and postharvest behaviour of fruits of two Hylocereus species (Cactaceae) Avinoam Nerd a , Feiga Gutman a , Yosef Mizrahi a,b, * a The Institutes for Applied Research, Ben-Gurion Uni6ersity of the Nege6, P.O. Box 653 , Beer-She6a 84105 , Israel b Department of Life Science, Ben-Gurion Uni6ersity of the Nege6, P.O. Box 653 , Beer-She6a 84105 , Israel Received 10 February 1999; accepted 26 May 1999 Abstract Fruit growth and ripening, and the effect on fruit quality of various storage temperatures, were studied with Hylocereus undatus and Hylocereus polyrhizus plants growing in Beer-Sheva (Israeli Negev desert) under greenhouse conditions. Fruit growth was sigmoidal with a strong decline in growth rate after the onset of peel colour change. The first change in peel colour was recorded 24–25 days after anthesis in H. undatus and 26– 27 days in H. polyrhizus.In both species, the peel turned fully red 4–5 days after the first colour change (mean temperature for the study period was 26.6 9 2.1°C). The slow growth phase was characterised by a decrease in the proportion of peel and concomitant increase in that of pulp, increase in the concentration of soluble solids and soluble sugars and a decline in firmness and the concentration of starch and mucilage. The surge in acidity prior to colour change indicated the beginning of the ripening processes. For H. polyrhizus, which has a red–violet pulp, the increase in pulp pigment paralleled the development of peel colour. Fruits were non-climacteric, and when harvested at close to full colour, they retained market quality for at least 2 weeks at 14°C or 1 week at 20°C. Storage at 6°C is not recommended, because transfer from that temperature to room conditions caused fruits to lose their firmness and flavour rapidly. © 1999 Elsevier Science B.V. All rights reserved. Keywords : Hylocereus undatus; Hylocereus polyrhizus; Chemophysical changes; Gas production; Pitaya; Storage www.elsevier.com/locate/postharvbio 1. Introduction Several species of climbing cacti of the genus Hylocereus have recently been developed as fruit crops (Barbeau, 1990; Reyes-Ramos, 1995; Mizrahi et al., 1997). The fruit of these species, known as red pitaya in Latin America, is a medium– large berry bearing large green or red scales (Nerd and Mizrahi, 1997). The peel is usu- ally red, and the pulp varies from purple –red to white. The pulp is delicate and juicy and contains numerous small soft seeds. The plants are grown in the open in tropical areas, but must be pro- tected from intense solar radiation and subfreez- ing temperatures when cultivated under subtropical conditions such those prevailing in Israel (Mizrahi et al., 1997; Raveh et al., 1997). * Corresponding author. Tel.: + 972-7-6461966; fax:+972- 7-6472984. E-mail address : mizrahi@bgumail.bgu.ac.il (Y. Mizrahi) 0925-5214/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0925-5214(99)00035-6 A. Nerd et al. / Posthar6est Biology and Technology 17 (1999) 39 – 45 40 Fruits of cultivated species of Hylocereus are picked at various stages of peel colour develop- ment, from first appearance to full colour (Bar- beau, 1990; Reyes-Ramos, 1995). However little research has been done on fruit development or ripening and on the behaviour of the fruit during or after storage. Studies with yellow pitaya (Se- lenicereus megalanthus), a related climbing cactus, show that the duration of fruit development de- pends on seasonal temperatures and that the fruits reach the optimal flavour close to full colour stage (Nerd and Mizrahi, 1998). This stage is also the best time for harvest for short- or long-distance markets (Nerd and Mizrahi, 1999). With the objective of determining ripening criteria for Hylocereus undatus and Hylocereus polyrhizus, we examined flavour and a number of physicochemical parameters during the last stage of fruit development. In addition, the effect of storage temperature on fruit quality was deter- mined for fruits harvested close to full colour. 2. Materials and methods 2 . 1 . Plant material and growth conditions Studies were conducted in 1997 and 1998 in an orchard of climbing cacti grown in a ventilated greenhouse in Beer-Sheva (northern Israeli Negev desert). The plants had been established from cuttings in 1992. Clone B of H. undatus (red peel and white pulp) and clone C of H. polyrhizus (red peel and red–purple pulp; Weiss et al., 1994) were used for the study. Each clone sample consisted of nine plants. Spaces were 1.5 m within the row and 2.5 m between rows. Plants were trained on a 1.5 m-high trellis system for support, and the soil was a loamy loess. Since these plants cannot grow in Israel under full sunlight, about 50% shade was provided by black net. Average monthly tempera- ture and relative humidity during the study peri- ods (July–October 1997 and the same months in 1998) ranged from 25 to 30°C and 49 –87% RH, respectively. Plants were drip-irrigated once a week, to provide5lofwater per plant in the summer and 2.5 l in the winter. Fertiliser contain- ing 23% N, 3% P, 20% K at a concentration of 70 mg l −1 was applied with the irrigation water. Since both clones were self-incompatible (Weiss et al., 1994), the nocturnal flowers, which stayed open for 1 night only, were hand cross-pollinated in the night with pollen of the other species. Pollinated flowers were tagged to allow calcula- tion of the number of days elapsed from anthesis to various stages of fruit growth and development. 2 . 2 . Fruit growth and ripening Length and mid-length diameter were measured with a caliper during fruit development in four fruits of each clone, set at the beginning of Au- gust 1997. Physicochemical parameters and flavour were determined in fruits of both clones set between mid-July and mid-August of the same year. Fruits were harvested at random at 3 –4-day intervals between 25 and 41 days after anthesis (DAA) for a total of six fruits per clone at each date. At the first date the fruits were either green or on the verge of first colour; 8 –10 days before the last date they were all fully red. 2 . 3 . Storage Fruits were picked in September 1998, either at first colour stage or 2 –3 days after appearance of first colour (close to full colour). They were washed with tap water to remove the sugars ex- creted from the scales during fruit development, dried with tissue paper and distributed randomly among three dark chambers held at 69 1.6°C, 149 1.4°C or 20 9 1°C, and at 65–75% RH. Sixty fruits of each species were stored at each tempera- ture. Physicochemical parameters and flavour were examined once a week, until the appearance of visible disorders such as scale wilting. Fruits stored at 6°C were also analysed after 1 week of further storage at 20°C. Five fruits were analysed at each sampling date. 2 . 4 . Fruit analysis Colour values of peel and pulp at the equator of the fruit were determined with a Minolta Chroma Meter CR-200 (Ramsey, NJ) and ex- pressed as hue angles (McGuire, 1992). Fruit firm- A. Nerd et al. / Posthar6est Biology and Technology 17 (1999) 39 – 45 41 ness was determined at two opposite points on the equator of each fruit with a penetrometer (Faccini, Alfonsine, Italy) fitted with a 2.5-mm plunger. Peel and pulp were separated and weighed, and a sample of each was weighed both when fresh and after drying at 70°C for determination of water content. The concentra- tion of red pigment in H. polyrhizus pulp which belongs to the betacyanin group (Forni et al., 1992), was determined in 1g samples obtained from the inner part of the pulp. Tissue was ho- mogenised in 80% ethanol and filtered through Whatman paper no. 41. The filtrate was mixed with 0.1 M acetate buffer at pH 4.5, and ab- sorbance determined at 538 nm (u max ) with a Uvikon 810 spectrophotometer (Kontron, Mi- lano, Italy). The concentration of pulp pigments was expressed as betanin using A 538 (1%)= 1120 (Forni et al., 1992). Acid concentration was de- termined from 10 g of pulp macerated in dis- tilled water and titrated with 0.05 M NaOH to pH 8.0. Ascorbic acid concentration was deter- mined in extracts of fresh pulp by titration with 2,6-dichloro-indophenol (AOAC, 1990). Soluble solids concentration (SSC) was measured with a refractometer (PR-100, Atago, Japan) in sap pressed from the pulp. The concentrations of total soluble sugars, starch, and mucilage were determined in dry samples of pulp according to a procedure previously reported (Nerd and Mizrahi, 1999). Flavour of fruits was assessed by a panel of ten on a hedonic scale with 1 indicating extreme dislike and 10 indicating strong liking. 2 . 5 . Ethylene and CO 2 production rates Fruits picked at first colour and 3 and 6 days later (three per species at each date) were weighed and then enclosed individually in jars held at 20°C and under a light regime of 12 h fluorescent light/12 h dark. A continuous air flow at approximately 5 ml min −1 was passed through the jar. Ethylene and CO 2 concentra- tions in the effluent air stream were analysed with a gas chromatograph (Varian 3300, Sugar- land, TX) once a day. 3. Results and discussion 3 . 1 . Fruit growth and ripening In both H. undatus and H. polyrhizus, growth of fruits attached to the vine exhibited a sigmoid pattern (Fig. 1). Peel colour was developed during the last phase which was characterised by a slow growth rate. First colour (area between the scales turned faint red) appeared 24–25 DAA in H. undatus and 26 –27 DAA in H. polyrhizus. Fruits of both species turned fully red 4–5 days after first colour. Like the fruit dimensions, fruit fresh weight reached 80% of the final maximum weight, before appearance of first colour (Fig. 2(A)). Significant increase in pulp content (percent of fruit fresh weight) occurred several days before, and throughout colour change (Fig. 2(B)), while fruit firmness dropped to less than 1 kg cm −2 in that time (Fig. 2(C)). During the final stage, percentage of water in the peel decreased (more markedly in H. undatus) while remaining almost constant in the pulp (Fig. 2(D)). Calculations based on data Fig. 1. Growth (length and diameter) during fruit development on the vine in H. undatus and H. polyrhizus. Values are means9SE (n=4). A. Nerd et al. / Posthar6est Biology and Technology 17 (1999) 39 – 45 42 Fig. 2. Whole fruit fresh weight (A), percentage of peel or pulp in fruit by weight (B), firmness (C), and water percent (FW basis) (D), during the last stage of fruit development in H. undatus and H. polyrhizus. Values are means9 SE (n=6). Fig. 3. Colour changes in H. undatus (red peel and white pulp) and H. polyrhizus (red peel and red-violet pulp) during the last stage of fruit development. Values are means9 SE (n=6). of colour change was low in fruits of both species, ranging from 7.0 to 11.4 mg 100 g −1 fresh weight. Changes in the concentration of soluble solids and a number of non-structural carbohydrate compo- nents of the pulp occurred during fruit develop- ment. Both soluble solids and soluble sugars concentrations increased significantly during colour development, reaching 16–17 and 8–9%, respectively, at full colour stage (Fig. 5(A, B)). Since changes in the two parameters were highly correlated, SSC appears to be a reliable indicator for maturity in the investigated species. The con- centrations of starch and mucilage decreased in relation to the accumulation of soluble sugars presented in Fig. 2 show that during the slow growth period (25– 41 DAA), pulp dry weight increased significantly from 36 to 62 g in H. undatus and from 21 to 48 g in H. polyrhizus, while peel dry weight (around 18 g in both species at 25 DAA) decreased slightly by4ginthat time. Hence the peel although accounting for a high proportion of fruit weight 20–25 DAA, could not have been an important source of assimilates to the developing pulp which presumably received assimilates from the stem. The decreasing values of the peel hue angle during maturation (Fig. 3) reflect development of peel colour. In H. polyrhizus, which has a red –vi- olet pulp, the development of peel colour was accompanied by an increase in the content of water-soluble pigment in the pulp (Fig. 3). The maximum concentration of pigment (expressed as betanin) in fruits with fully red peel was 0.24 mg g −1 fresh weight. Titratable acidity of pulp showed a short-lived surge at the start of colour change, followed by a decline later on to  30 mmol H + kg −1 in H. undatus and  45 mmol H + kg −1 in H. polyrhizus (equivalent to 0.22 and 0.32% citric acid, respectively; Fig. 4). Ascorbic acid concentration of the pulp during the period Fig. 4. Concentration of titratable acidity (FW basis) during the last stage of fruit development in H. undatus and H. polyrhizus. Values are means9 SE (n=6). A. Nerd et al. / Posthar6est Biology and Technology 17 (1999) 39 – 45 43 Fig. 5. Concentration (FW basis) of soluble solids (SSC) (A), soluble sugars (B), starch (C), and mucilage (D), during the last stage of fruit development in H. undatus and H. polyrhizus. Values are means9 SE (n=6). and SSC and soluble sugars were high. The ratio of percentage of soluble sugars to acidity (in terms of citric acid) in such fruits was  40 in H. undatus and  22 in H. polyrhizus. Thus for both red pitayas, chemical changes during ripening, colour development and palatability were found to coincide, as reported for the yellow pitaya S. megalanthus (Nerd and Mizrahi, 1998). In the cactus pear (Opuntia ficus indica), however, opti- mum eating quality was reached at first colour stage (Nerd and Mizrahi, 1997). 3 . 2 . Gas production Production of ethylene and CO 2 in fruits har- vested at various colour stages was low and did not peak during 6 days at 20°C. The production rate of ethylene ranged from 0.025 to 0.091 ml kg −1 h −1 and that of CO 2 from 0.52 to 0.78 ml kg −1 h −1 . Hence H. undatus and H. polyrhizus may be defined as non-climacteric, similar to other cactus fruit crops that have been investi- gated, namely cactus pears (Opuntia species) and yellow pitaya (S. megalanthus; Nerd and Mizrahi, 1997, 1998). 3 . 3 . Storage Fruits were picked close to full colour and examined at the end of 3 consecutive weeks in storage at different temperatures. Distinct disor- ders were observed at the end of week 2 for fruits held at 20°C and at the end of week 3 for fruits held at 14°C. Symptoms consisted of extreme softening, difficulty in separation of peel from the pulp, and wilting and browning of scales (these phenomena appeared usually together). The con- centration of soluble solids and soluble sugars remained fairly constant throughout storage at all the storage temperatures (Table 2). High storage temperatures resulted in lower fruit firmness, wa- ter content, acidity and flavour. The two species responded to storage in a similar manner, though fruits of H. polyrhizus tended to maintain a higher level of acidity at 14 and 20°C. This higher acidity may explain the higher preference rating of H. polyrhizus recorded under these conditions. Some of the fruits stored at 6°C were transferred at (Fig. 5(C, D)). Maximal concentration of starch was less than half that of the soluble sugars in ripe fruit, and accumulation of soluble sugars during ripening cannot be therefore attributed to starch degradation alone. The two species differed with regard to timing of changes in chemical constituents and other maturation parameters which coincided with the timing of first colour appearance. Organoleptic testing indicated that fruits were most palatable 33 –37 DAA (Table 1). At that stage they had turned fully red, acidity was low Table 1 Flavour rating of fruits of H. undatus and H. polyrhizus harvested between 25 and 41 DAA a DAA Flavour (rating) H. undatus H. polyrhizus 25 1.3 d b 1.4 d 3.3 c3.7 c29 5.7 b33 7.0 a 37 6.5 a 6.4 a 5.8 b 5.3 b41 a Flavour was assessed by a panel of ten according to a hedonic scale with 1 indicating extreme dislike and 10 indicat- ing strong liking. b Mean separation within column at P50.05 by Duncan’s multiple range test. A. Nerd et al. / Posthar6est Biology and Technology 17 (1999) 39 – 45 44 Table 2 Effect of storage temperatures on fruit attributes in H. undatus and H. polyrhizus a Storage Water loss SSCFirmness Soluble sugars Acidity mmol H + Flavour (%)(kg cm −2 )kg −1 (%) (rating)(%) Weeks°C H. undatus 14.0 a 6.6 a2.4 a b 93 aInitial 6.2 a 6 2.4 a1 1.8 c 13.6 a 6.6 a 62 b 5.2 a 2.3 a 2.8 c 13.7 a2 6.7 a6 61 b 5.9 a 36 0.3 b 5.8 a 13.7 a 6.7 a 65 b 5.9 a 2.4 a 2.7 c 13.3 a 6.0 a 37 c14 5.9 a1 2.0 a 4.4 b 12.2 a2 5.4 a14 17 d 2.8 b 20 1.8 a1 4.2 b 13.4 a 6.2 a 16 d 2.6 b H. polyrhizus Initial 13.2 bcd2.5 a 6.3 ab 93 ab 6.1 a 2.7 a 2.2 d 12.7 cd1 5.2 b6 100 a 5.8 a 2.4 a 4.0 c 13.0 bcd6 6.3 ab2 84 abc 6.4 a 0.4 c 5.4 a 14.5 a3 5.4 b6 81 bc 5.6 ab 114 1.8 b 2.5 d 13.4 bc 6.8 a 71 cd 6.9 a 1.5 b 4.5 bc 12.0 d2 5.9 ab14 51 e 5.3 ab 120 1.7 b 4.9 ab 13.9 ab 6.8 a 58 de 4.4 b a Fruits were harvested close to full colour and analysed prior to appearance of distinct disorders such as extreme softening and scale wilting. Values are means9 SE (n=5). b Mean separation within column at P50.05 by Duncan’s multiple range test. different times to 20°C and their attributes exam- ined 1 week later (data not shown). Fruits trans- ferred at the end of week 1 were similar to those stored immediately after harvest for 1 week at 20°C (Table 2). However fruits transferred to 20°C at the end of week 2 or 3 at 6°C became soft (B 0.3 kg cm −2 ), had high water loss (8–11%), low acidity (B 25 mol H + kg −1 ) and poor flavour (B 2.5). Fruits of H. undatus developed chilling injury symptoms such as wilting and darkening of the scales and browning of the outer layer of the pulp upon transfer to 20°C after 2 weeks at 6°C. To summarise, fruit harvested close to full colour kept their visual acceptance, i.e. marketing quality for at least 3 weeks at 6°C, 2 weeks at 14°C or 1 week at 20°C. Fruits stored at 6°C maintained their eating quality (flavour) for a longer period but lost quality rapidly when transferred to room tempera- ture. The effect of harvesting at an earlier ripening stage (first colour) on fruit quality during or after storage should be examined with a view to extend- ing the shelf life of these pitayas. Acknowledgements The authors thank the Fleischer Foundation and the Israel Ministry of Agriculture for a partial financial support, Dorot Imber for editing the manuscript and Josef Mouyal and Eyal Naus for their technical assistance. References AOAC, 1990. Official Methods of Analysis, 15th ed. Associa- tion of Official Analytical Chemist, Arlington, VA. Barbeau, G., 1990. La pitahaya rouge, un nouveau fruit exotique. Fruits 45, 141–147. Forni, G., Polesello, A., Montefiori, D., Maestrelli, A., 1992. High-performance liquid chromatographic analysis of the pigments of blood-red prickly pear (Opuntia ficus indica). J. Chromatogr. 593, 177–183. 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Flowering behavior and pollination requirements in climbing cacti with fruit crop potential. HortScience 29, 1487–1492. . . Postharvest Biology and Technology 17 (1999) 39–45 Ripening and postharvest behaviour of fruits of two Hylocereus species (Cactaceae) Avinoam Nerd a , Feiga. end of week 2 for fruits held at 20°C and at the end of week 3 for fruits held at 14°C. Symptoms consisted of extreme softening, difficulty in separation of peel from the pulp, and wilting and. proportion of peel and concomitant increase in that of pulp, increase in the concentration of soluble solids and soluble sugars and a decline in firmness and the concentration of starch and mucilage.