Suitable Drying Temperature for Preserving Cucurbitacins in Fruit of Wild Cucumber and Wild Watermelon

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  • 1 1Limpopo Agro-Food Technology Station, University of Limpopo, Private Bag X1106, Sovenga 0727, South Africa
  • 2 2Green Technologies Research Centre, University of Limpopo, Private Bag X1106, Sovenga 0727, South Africa

The thermostable cucurbitacin A and B from mature fruit of wild cucumber (Cucumis myriocarpus) and wild watermelon (Cucumis africanus), respectively, are used in product development for various industries. Mature fruit from wild cucumber and wild watermelon suffer from high incidents of postharvest decays. Drying fruit at the recommended temperatures of 30 to 40 °C for medicinal plants resulted in molds developing on the material, with optimum temperature to prevent decays being at 52 °C. The influence of 52 °C and higher temperatures on active ingredients in the two fruit had not been documented. The objective of this study, therefore, was to determine the relative effects of increasing drying temperatures above the 52 °C standard on concentrations of cucurbitacin A and B in fruit of wild cucumber and wild watermelon. Fruit pieces were oven-dried at 52, 60, 70, 80, 90, and 100 °C for 72 hours. Relative to 52 °C, higher temperatures resulted in 25% to 92% less cucurbitacin compared with the maximum produced at 60 °C. In contrast, relative to 52 °C, higher temperatures reduced concentrations of cucurbitacin B by 47% to 86%. In conclusion, the compromise temperature of 52 °C for preserving fruit pieces in wild cucumber and wild watermelon from decay should also be viewed as the optimum temperature for preserving cucurbitacin A and B.

Abstract

The thermostable cucurbitacin A and B from mature fruit of wild cucumber (Cucumis myriocarpus) and wild watermelon (Cucumis africanus), respectively, are used in product development for various industries. Mature fruit from wild cucumber and wild watermelon suffer from high incidents of postharvest decays. Drying fruit at the recommended temperatures of 30 to 40 °C for medicinal plants resulted in molds developing on the material, with optimum temperature to prevent decays being at 52 °C. The influence of 52 °C and higher temperatures on active ingredients in the two fruit had not been documented. The objective of this study, therefore, was to determine the relative effects of increasing drying temperatures above the 52 °C standard on concentrations of cucurbitacin A and B in fruit of wild cucumber and wild watermelon. Fruit pieces were oven-dried at 52, 60, 70, 80, 90, and 100 °C for 72 hours. Relative to 52 °C, higher temperatures resulted in 25% to 92% less cucurbitacin compared with the maximum produced at 60 °C. In contrast, relative to 52 °C, higher temperatures reduced concentrations of cucurbitacin B by 47% to 86%. In conclusion, the compromise temperature of 52 °C for preserving fruit pieces in wild cucumber and wild watermelon from decay should also be viewed as the optimum temperature for preserving cucurbitacin A and B.

Fruit of wild cucumber and wild watermelon are used in medicinal systems, nutrition, pharmaceutical, cosmetic, and pesticidal industries (Lee et al., 2010; Mashela et al., 2011; Thies et al., 2010; Van Wyk and Wink, 2012; Van Wyk et al., 2002). Fruit of wild cucumber and wild watermelon contain cucurbitacin A (C32H46O9) and cucurbitacin B (C32H46O8), respectively (Chen et al., 2005; Jeffrey, 1978). The two thermostable chemical compounds (Krieger, 2001) are classified as triterpenoids (Chen et al., 2005; Van Wyk and Wink, 2012). Cucurbitacin A, which is soluble in water (Jeffrey, 1978), is unstable and readily oxidises to cucumin (C27H40O9) and leptodermin (C27H38O8), whereas the insoluble cucurbitacin B is stable (Jeffrey, 1978). Fruit of wild cucumber and wild watermelon are seasonal, but cannot be stored in fresh form due to the high incidents of postharvest decays. Mphahlele et al. (2012) identified the causal agent as the acid-loving fungus Penicillium simplicissimum. Usually, fungal decay promotes losses of constituents of the affected organs. Fungi digest food outside its cells by secreting acids and powerful hydrolytic enzymes that decompose complex molecules into simpler compounds that the fungus can absorb and metabolize (Campbell, 1990). Fungal decays have been ameliorated through drying, which had been successfully used to preserve active ingredients in most organs used in various medicinal systems (Danso-Boateng, 2013; Diaz-Maroto et al., 2002; Mudau and Ngezimana, 2014; Rocha et al., 2011).

The recommended drying temperature range for various organs in medicinal plants is 30 to 40 °C (Müller and Heindl, 2006). However, when fruit pieces of the wild cucumber and wild watermelon were dried within the recommended range, most of the cucurbitacin materials were lost to decay, with a blue, bluish-green, or olive green colors, surrounded by white mycelium and a band of water-soaked tissues that characterize P. simplicissimum infection. A preliminary optimum drying temperature to prevent growth of mycelia and, therefore, subsequent decay, was established at 52 °C (Mashela, 2002). However, there was no information on the impact of this and higher temperatures on cucurbitacin A or B concentrations in fruit of wild cucumber and wild watermelon. The objective of this study was to determine the relative effects of increasing drying temperatures above 52 °C on concentrations of cucurbitacin A and B in fruit of wild cucumber and wild watermelon.

Materials and methods

Study site and raising wild cucumber and wild watermelon.

The study was conducted at the Green Technologies Research Center, University of Limpopo, South Africa (lat. 23°53′10″S, long. 29°44′15″E). Soil at the site comprised Hutton sandy loam (65% sand, 30% clay, 5% silt) containing 1.6% organic carbon, with electrical conductivity of 0.148 dS·m−1 and pH of 6.5. The hot and dry summers usually have day maximum temperatures ranging from 28 to 38 °C, with mean annual rainfall below 500 mm. Seedlings of wild cucumber and wild watermelon were raised in adjacent separate fields, containing five plots (1 × 1 m), each plot with four plants. One experiment evaluated cucurbitacin A from fruit of wild cucumber, whereas the second experiment only evaluated cucurbitacin B from wild watermelon fruit. Each plant was fertilized once using 3 g of 6.3N–9.4P–6.3K fertilizer (Omnia, Bryanston, South Africa) and plots irrigated weekly using sprinklers.

Experimental design and treatments.

Sixty fruit from each plot were harvested at fruit maturity (Shadung et al., 2015), chopped into pieces, and divided equally into six portions. Each portion per plot was randomly assigned to one of the six forced-air drying ovens (EcoTherm; Labotech, Cape Town, South Africa). The drying treatment ovens were set at 52, 60, 70, 80, 90, or 100 °C and arranged in a complete randomized design, with five replications. Each drying treatment ran for 72 h and afterward the samples were ground in a Wiley mill to pass through a 1-mm sieve. Before extraction, samples were stored in hermetically sealed plastic bottles at room temperature.

Extraction and cucurbitacins quantification.

A representative subsample of 4 g dried crude extracts of fruit from each treatment were extracted in closed conical flasks containing 100 mL methanol and dichloromethane at 1:1 (v/v) solution inside a rotary evaporator (Rotavapor model R-205; Buchi Labortechnik, Essen, Germany) set at 60 rpm at 40 °C for 4 h. After extraction, subsamples were concentrated by reducing the volume to 30 mL under reduced pressure on a rotary evaporator and then 1 mL aliquot centrifuged at 2422 gn for 10 min before filtering through 0.22-µm filter (Miller; Sigma-Aldrich, Johannesburg, South Africa). Concentrations of cucurbitacin were quantified using the isocratic elution high-performance liquid chromatography (Prominence model LC-10 AD VP; Shimadzu, Kyoto, Japan) with detection using a diode array detector (CTO-20A; Shimadzu). Quantification was performed in a wide pore reverse phase C18 (25 cm × 4.0 mm, 5 µm) column (Sigma-Aldrich, Milan, Italy) using methanol and deionized water at 2:3 (v/v) solution that served as a mobile phase at a flow rate of 1.0 mL·min−1 in an oven at 35 °C, with wavelengths monitored at 230 nm for 43 min. Quantification of cucurbitacin A and B was accomplished by comparing the retention times and peak areas of subsamples to those of pure (about 98%) cucurbitacin A and B standards (Wuhan ChemFaces Biochemical Co., Wuhan, China). Standards were dissolved in methanol and prepared in serial dilutions of 0.02, 0.04, 0.06, 0.08, and 1.0 µg·mL−1.

Data analysis.

Cucurbitacin A and B data were subjected to analysis of variance procedure using SAS software (version 9.2; SAS Institute, Carry, NC). When treatments were significant, the sums of squares were partitioned to determine the percentage contribution of sources of variation to the total treatment variation (TTV) in concentration of cucurbitacins. Mean separation was achieved using Waller–Duncan multiple range test. Unless otherwise stated, only treatment means significant at the probability level of 5% were discussed.

Results

Treatment effects.

Increasing oven-drying temperatures had highly significant (P ≤ 0.01) effects on concentrations of cucurbitacin A and B (Table 1). Increasing temperatures contributed 65% and 71% in TTV of cucurbitacin A and B concentrations, respectively.

Table 1.

Responses of sum of squares (SS) for cucurbitacin A and B concentrations from fruit of wild cucumber and wild watermelon, respectively, to different oven-drying temperatures (n = 30).

Table 1.

Relative impact.

The highest concentration of cucurbitacin A occurred in fruit dried at 60 °C. The higher temperatures reduced cucurbitacin A by 25% to 92%. In contrast, temperatures above 52 °C reduced cucurbitacin B by 28% to 86%.

Generated models.

A quadratic relationship was observed between cucurbitacin A and B concentrations and drying temperature with an R2 of 0.94 and 0.95, respectively (Fig. 1).

Fig. 1.
Fig. 1.

Relationship between cucurbitacin A and B concentrations from fruit of wild cucumber and wild watermelon, respectively, over increasing drying temperatures at 72-h exposure time; (1.8 × °C) + 32 = °F, 1 µg·mL−1 = 1 ppm.

Citation: HortTechnology hortte 26, 6; 10.21273/HORTTECH03400-16

Discussion

Concentrations of cucurbitacin A and B triterpenoids (Chen et al., 2005) were inversely related to drying temperature. Others (Du et al., 2003; Hwang et al., 2014) observed that concentrations of ginsenoside (another triterpenoids) from ginseng roots decreased when dried at 40, 55, or 70 °C. Similar findings were noted with the pyrethrins—the monoterpenoids (Morris et al., 2006). Rosmarinic acid and sinenselin (phenolic compounds) from misai kucing (Orthosiphon staminiues), increased with increasing temperature below 40 °C, but decreased when dried at 40, 55, or 70 °C (Abdullah et al., 2011). Drying bush tea (Athrixia phylicoides) between 45 and 65 °C reduced total phenolic content when compared with freeze- and shade-drying (Mudau and Ngezimana, 2014).

The reduction of chemical compounds with increasing drying temperature has been attributed to the accelerated degradation of the compounds (Phillips et al., 1960), which depends on the chemical bonds within the chemical compounds (Phillips et al., 1960). In essential oils, for example, drying temperature for oregano (Origanum vulgare ssp. hirtum) was optimized at 40 °C for 72 h (Novák et al., 2011), whereas at higher temperatures most essential oils were volatilized (Faridah et al., 2010; Radünz et al., 2003). Cucurbitacins are thermostable, with boiling temperatures of cucurbitacin A and B being at 731 and 699 °C, respectively, at 760 mm Hg (Krieger, 2001). The decrease in cucurbitacin with increasing temperature agreed with observations in other chemical compounds such as ginsenosides and pyrethrins (Du et al., 2003; Hwang et al., 2014; Morris et al., 2006).

Drying fruit from wild cucumber and wild watermelon at 52 °C should be viewed as a compromise temperature for preserving cucurbitacins from P. simplicissimum postharvest decay losses. Optimizing temperature for the retention of cucurbitacins appeared to be above 52 °C for cucurbitacin A and below 52 °C for cucurbitacin B, where fruit are sensitive to decay. However, at 52 °C it is still necessary to establish the suitable exposure period since the drying periods are inversely proportional to the drying temperatures (Barbieri et al., 2004; Gregory et al., 2005; Hallström and Wimmerstedt, 1983). The proposed optimization of the drying period at 52 °C could reduce potential losses through degradation and volatilization.

The cucurbitacin concentrations vs. increasing drying temperatures had density-dependent growth (DDG) patterns (Liu et al., 2003; Salisbury and Ross, 1992). Apparently, should drying temperatures start from 25 to 100 °C, cucurbitacin concentrations would go through the three stages of DDG patterns, namely, stimulation, neutral, and inhibition (Mashela et al., 2015). In our study and those of others (Abdullah et al., 2011; Du et al., 2003; Morris et al., 2006), inhibition ranges were exhibited since the drying temperatures were already above the optimization temperatures for the variables. At temperature from 40 to 70 °C, Abdullah et al. (2011) observed that the phenolic compounds tested increased with increasing temperatures, which was a reflection of the stimulation stage (Liu et al., 2003).

Conclusion

The drying temperature for fruit of wild cucumber and wild watermelon should be retained at 52 °C as a compromise against decay at lower drying temperatures. However, it would be necessary to optimize the exposure period for drying fruit of wild cucumber and wild watermelon to ensure optimum retention of cucurbitacin A and B.

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Literature cited

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  • Barbieri, S., Elustondo, M.E. & Urbicain, M. 2004 Retention of aroma compounds in basil dried with low pressure superheated steam J. Food Eng. 65 109 115

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  • Chen, J.C., Chiu, M.H., Nie, R.L., Cordell, G.A. & Qiu, S.X. 2005 Cucurbitacins and cucurbitane glycosides: Structures and biological activities Nat. Prod. Rpt. 22 386 399

    • Search Google Scholar
    • Export Citation
  • Danso-Boateng, E. 2013 Effects of drying methods on nutrient quality of basil leaves cultivated in Ghana Intl. Food Res. J. 20 1569 1573

  • Diaz-Maroto, M.C., Pérez-Coello, M.S. & Cabezudo, M.D. 2002 Effect of drying method on the volatilities in bay leaf (Laurus nobilis L.) J. Agr. Food Chem. 50 4520 4524

    • Search Google Scholar
    • Export Citation
  • Du, X.W., Wills, R.B.K. & Stuart, D.L. 2003 Changes in neutral and malonyl ginsenosides in American ginseng (Panax quinquefolium) during drying, storage and ethanolic extraction Food Chem. 86 155 159

    • Search Google Scholar
    • Export Citation
  • Faridah, Q.Z., Abdelmageed, A.H.A., Nor, H.A.N. & Muhamad, Y. 2010 Comparative study of essential oil composition of leaves and rhizomes of Alpinia conchigera Griff. at different post-harvest drying periods J. Med. Plants Res. 4 2700 2705

    • Search Google Scholar
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    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
  • Morris, S.E., Davies, N.W., Brown, P.H. & Groom, T. 2006 Effect of drying conditions on pyrethrins content Ind. Crops Prod. 23 9 14

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    • Search Google Scholar
    • Export Citation
  • Mudau, F.N. & Ngezimana, W. 2014 Effect of different drying methods on chemical composition and antimicrobial activity of bush tea (Athrixia phylicoides) Intl. J. Agr. Biol. 16 1011 1014

    • Search Google Scholar
    • Export Citation
  • Müller, J. & Heindl, A. 2006 Drying of medicinal plants, p. 237−252. In: R.J. Bogers, L.E. Craker, and D. Lange (eds.). Medicinal and aromatic plants. Springer, Dordrecht, The Netherlands

  • Novák, I., Sipos, L., Kókai, Z., Szabó, K., Pluhár, Z.S. & Sárosi, S.Z. 2011 Effect of the drying method on the composition of Origanum vulgare L. ssp. hirtum essential oil analysed by GC-MS and sensory profile method Acta Aliment. 40 130 138

    • Search Google Scholar
    • Export Citation
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  • Radünz, L.L., Melo, E.C., Berbert, P.A., Barbosa, L.C.A., Santos, R.H.S. & Rocha, R.P. 2003 Influence of drying air temperature on the amount of essential oil extracted from guaco (Mikania glomerata Sprengel) J. Storage 28 41 45

    • Search Google Scholar
    • Export Citation
  • Rocha, R.P., Melo, E.C. & Radünz, L.L. 2011 Influence of drying process on the quality of medicinal plants: A review J. Med. Plants Res. 5 7076 7084

  • Salisbury, F.B. & Ross, C.W. 1992 Plant physiology. 4th ed. Wadsworth, Belmont, CA

  • Shadung, K.G., Mashela, P.W., Mulaudzi, V.L., Mphosi, M.S. & Ncube, I. 2015 Optimum harvest time of Cucumis africanus fruit using concentration of cucurbitacin B as a maturity standard J. Agr. Sci. 7 181 186

    • Search Google Scholar
    • Export Citation
  • Thies, J.A., Ariss, J.J., Hassell, R.L., Olson, S., Kousik, C.S. & Levi, A. 2010 Grafting for management of southern root-knot nematode, Meloidogyne incognita, in watermelon Plant Dis. 94 1195 1199

    • Search Google Scholar
    • Export Citation
  • Van Wyk, B., Heerden, F. & Outshoorn, B. 2002 Poisonous plants of South Africa. Briza Publ., Pretoria, South Africa

  • Van Wyk, B.E. & Wink, M. 2012 Medicinal plants of the world: An illustrated scientific guide to important medicinal plants and their uses. Briza Publ., Pretoria, South Africa

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Contributor Notes

Corresponding author. E-mail: kagiso.shadung@ul.ac.za.

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    Relationship between cucurbitacin A and B concentrations from fruit of wild cucumber and wild watermelon, respectively, over increasing drying temperatures at 72-h exposure time; (1.8 × °C) + 32 = °F, 1 µg·mL−1 = 1 ppm.

  • Abdullah, S., Ahmad, M.S., Shaari, A.R., Johar, H.M. & Noor, N.F.M. 2011 Drying characteristics and herbal metabolites composition of misai kucing (Orthosiphon staminues Benth.) leaves Intl. Conf. Food Eng. Biotechnol. 9 305 309

    • Search Google Scholar
    • Export Citation
  • Barbieri, S., Elustondo, M.E. & Urbicain, M. 2004 Retention of aroma compounds in basil dried with low pressure superheated steam J. Food Eng. 65 109 115

    • Search Google Scholar
    • Export Citation
  • Campbell, N.A. 1990 Biology. Benjamin/Cummings, Redwood City, CA

  • Chen, J.C., Chiu, M.H., Nie, R.L., Cordell, G.A. & Qiu, S.X. 2005 Cucurbitacins and cucurbitane glycosides: Structures and biological activities Nat. Prod. Rpt. 22 386 399

    • Search Google Scholar
    • Export Citation
  • Danso-Boateng, E. 2013 Effects of drying methods on nutrient quality of basil leaves cultivated in Ghana Intl. Food Res. J. 20 1569 1573

  • Diaz-Maroto, M.C., Pérez-Coello, M.S. & Cabezudo, M.D. 2002 Effect of drying method on the volatilities in bay leaf (Laurus nobilis L.) J. Agr. Food Chem. 50 4520 4524

    • Search Google Scholar
    • Export Citation
  • Du, X.W., Wills, R.B.K. & Stuart, D.L. 2003 Changes in neutral and malonyl ginsenosides in American ginseng (Panax quinquefolium) during drying, storage and ethanolic extraction Food Chem. 86 155 159

    • Search Google Scholar
    • Export Citation
  • Faridah, Q.Z., Abdelmageed, A.H.A., Nor, H.A.N. & Muhamad, Y. 2010 Comparative study of essential oil composition of leaves and rhizomes of Alpinia conchigera Griff. at different post-harvest drying periods J. Med. Plants Res. 4 2700 2705

    • Search Google Scholar
    • Export Citation
  • Gregory, M.J., Menary, R.C. & Davies, N.W. 2005 Effect of drying temperature and air flow on the production and retention of secondary metabolites in saffron J. Agr. Food Chem. 53 5969 5975

    • Search Google Scholar
    • Export Citation
  • Hallström, A. & Wimmerstedt, R. 1983 Drying of porous granular materials Chem. Eng. Sci. 38 1507 1516

  • Hwang, C.R., Lee, S.H., Jang, G.Y., Hwang, I.G., Kim, H.Y., Woo, K.S., Lee, J. & Jeong, H.S. 2014 Changes in ginsenoside composition and antioxidant activities of hydroponic cultured ginseng roots and leaves heating temperature J. Ginseng Res. 38 180 186

    • Search Google Scholar
    • Export Citation
  • Jeffrey, C. 1978 Cucurbitaceae, p. 115−117. In: V.H. Heywood (ed.). Flowering plants of the world. Oxford Univ. Press, Oxford, UK

  • Krieger, R. 2001 Handbook of pesticides toxicology. Academic Press, San Diego, CA

  • Lee, D.H., Iwanski, G.B. & Thoennissen, N.H. 2010 Cucurbitacin: Ancient compound shedding new light on cancer treatment Sci. World J. 10 413 418

  • Liu, D.L., An, M., Johnson, I.R. & Lovett, J.V. 2003 Mathematical modelling of allelopathy. III. A model for curve-fitting allelochemical dose responses Nonlinearity Biol. Toxicol. Med. 1 37 50

    • Search Google Scholar
    • Export Citation
  • Mashela, P.W. 2002 Ground wild cucumber fruit suppress numbers of Meloidogyne incognita on tomato in microplots Nematropica 32 13 19

  • Mashela, P.W., Dube, Z.P. & Pofu, K.M. 2015 Managing the phytotoxicity and inconsistent nematode suppression in soil amended with phytonematicides, p. 147−173. In: M.K. Meghvansi and A. Varma (eds.). Organic amendments and soil suppressiveness in plant disease management. Springer Intl. Publ., Heidelberg, Germany

  • Mashela, P.W., De Waele, D. & Pofu, K.M. 2011 Use of indigenous Cucumis technology as alternative to synthetic nematicides in management of root-knot nematodes in low input agricultural farming system: A review Sci. Res. Essays 6 6762 6789

    • Search Google Scholar
    • Export Citation
  • Morris, S.E., Davies, N.W., Brown, P.H. & Groom, T. 2006 Effect of drying conditions on pyrethrins content Ind. Crops Prod. 23 9 14

  • Mphahlele, R.R., Mashela, P.W. & Pofu, K.M. 2012 Post harvesting fruit decay–inducing pathogen in medicinally important Cucumis species indigenous to South Africa Afr. J. Agr. Res. 6 3786 3791

    • Search Google Scholar
    • Export Citation
  • Mudau, F.N. & Ngezimana, W. 2014 Effect of different drying methods on chemical composition and antimicrobial activity of bush tea (Athrixia phylicoides) Intl. J. Agr. Biol. 16 1011 1014

    • Search Google Scholar
    • Export Citation
  • Müller, J. & Heindl, A. 2006 Drying of medicinal plants, p. 237−252. In: R.J. Bogers, L.E. Craker, and D. Lange (eds.). Medicinal and aromatic plants. Springer, Dordrecht, The Netherlands

  • Novák, I., Sipos, L., Kókai, Z., Szabó, K., Pluhár, Z.S. & Sárosi, S.Z. 2011 Effect of the drying method on the composition of Origanum vulgare L. ssp. hirtum essential oil analysed by GC-MS and sensory profile method Acta Aliment. 40 130 138

    • Search Google Scholar
    • Export Citation
  • Phillips, R.C., Chamberlain, D.L. & Ferguson, F.A. 1960 High-temperature synthesis of new thermally-stable chemical compounds. Wright-Patterson Air Base, Dayton, OH

  • Radünz, L.L., Melo, E.C., Berbert, P.A., Barbosa, L.C.A., Santos, R.H.S. & Rocha, R.P. 2003 Influence of drying air temperature on the amount of essential oil extracted from guaco (Mikania glomerata Sprengel) J. Storage 28 41 45

    • Search Google Scholar
    • Export Citation
  • Rocha, R.P., Melo, E.C. & Radünz, L.L. 2011 Influence of drying process on the quality of medicinal plants: A review J. Med. Plants Res. 5 7076 7084

  • Salisbury, F.B. & Ross, C.W. 1992 Plant physiology. 4th ed. Wadsworth, Belmont, CA

  • Shadung, K.G., Mashela, P.W., Mulaudzi, V.L., Mphosi, M.S. & Ncube, I. 2015 Optimum harvest time of Cucumis africanus fruit using concentration of cucurbitacin B as a maturity standard J. Agr. Sci. 7 181 186

    • Search Google Scholar
    • Export Citation
  • Thies, J.A., Ariss, J.J., Hassell, R.L., Olson, S., Kousik, C.S. & Levi, A. 2010 Grafting for management of southern root-knot nematode, Meloidogyne incognita, in watermelon Plant Dis. 94 1195 1199

    • Search Google Scholar
    • Export Citation
  • Van Wyk, B., Heerden, F. & Outshoorn, B. 2002 Poisonous plants of South Africa. Briza Publ., Pretoria, South Africa

  • Van Wyk, B.E. & Wink, M. 2012 Medicinal plants of the world: An illustrated scientific guide to important medicinal plants and their uses. Briza Publ., Pretoria, South Africa

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