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Storage response of cactus pear fruit following hot water brushing
Postharvest Biology and Technology 38 (2005) 145–151
Storage response of cactus pear fruit following hot water brushing Lydakis Dimitris a,∗ , N. Pompodakis a , E. Markellou b , S.M. Lionakis a a b
Technological Education Institute of Crete, GR-71 100 Heraklion, Crete, Greece Benaki Phytopathological Institute, 8 St. Delta Street, GR-145 61 Athens, Greece Received 1 April 2005; accepted 21 June 2005
Abstract The storage response of cactus pear [Opuntia ficus-indica Miller (L.)] following hot water brushing was investigated. Fruit were simultaneously brushed for spine removal and sprayed with water. Ranges of temperature (60–70 ◦ C) and treatment time intervals (10–30 s) were evaluated. All tested treatments were found not to significantly affect respiration rate, total soluble solids or acid concentrations. Treatments at 60 and 65 ◦ C were found to reduce water loss and the incidence of rusty-brown spots on fruit peel. At higher temperatures, some negative effects were observed, namely increased electrolyte leakage and extended light brown areas on the fruit peel, indicating heat damage to the fruit. All the treatments effectively controlled decay in fruit stored at 6 ± 1 ◦ C for 4 weeks followed by 1 week at 20 ± 1 ◦ C. The effectiveness of treatments to control decay was increased with temperature and treatment time. Based on the results of this study, treatments at 60 ◦ C for 30 s or 65 ◦ C for 20 s reduced the decay incidence by 86–91% without compromising fruit quality. These treatments can be easily applied in commercial practice with slight modification of the despining facilities in use. © 2005 Elsevier B.V. All rights reserved. Keywords: Cactus pear; Decay control; Heat treatment; Hot water brushing; Opuntia
1. Introduction Cactus pear [Opuntia ficus-indica Miller (L.)] is cultivated for fruit production worldwide, either in the subsistence agriculture of dry land areas or as a cash crop (Inglese et al., 1995). Cactus pear trees can be found wild or cultivated in all the Mediterranean countries. ∗ Corresponding author. Tel.: +30 2810 379408; fax: +30 2810 318204. E-mail address: [email protected] (L. Dimitris).
0925-5214/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2005.06.006
Commercial plantations, however, are grown mainly in Italy, particularly, in Sicily (Barbera, 1995). In Greece, they can be found in all the Aegean Islands and in some areas of the southern mainland (Lionakis et al., 2001). Due to increasing demand for a diversified diet and the use of unusual products, the consumption of cactus pear has been increasing steadily over the last few years throughout Europe. One of the major problems with cactus pear is the presence of glochids (spines) on the fruit peel. Because of the glochids, cactus pears are difficult
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to harvest and handle after harvest. Consumers do not prefer fruit with glochids. Therefore, postharvest operations for fruit destined for the market involve glochid removal. Glochids can be eliminated by spreading the fruit on grass or on open mesh tables and brushing with brooms. In packing houses, fruit are passed over a series of brushes with the application of water sprays or air suction to trap and remove spines. Cactus pears are characterized as highly perishable fruit. Storage at room temperature favours decay, fruit weight loss, wilting, softening and off-flavor development, while storage at temperatures below 8–10 ◦ C promotes physiological breakdown. Physical damage to the peel induced during spine removal predisposes cactus pear to attack by decay-causing pathogens. Loose glochids may also damage the surface of harvested fruit, resulting in small rusty-brown discoloured areas (Cantwell, 1995). Storability of fruit can be improved by postharvest heat treatments. Treatments in the form of hot water, hot air or vapour heat have been developed to control postharvest decay, insect infestation and alleviate storage disorders in a wide range of fresh produce (Barkai-Golan and Phillips, 1991; Lurie, 1998; Ferguson et al., 2000; Fallik et al., 2000). The response of cactus pear to prestorage heat treatments has been tested in different studies. Treatments at 38 ◦ C for 24 h (Schirra et al., 1997) and hot water dips at 55 ◦ C for 2 min (Berger et al., 2002) reduced rot development and decline in overall appearance during storage. Dips in hot thiabendazole solutions with reduced doses of fungicide (150 mg/L) at 52 ◦ C for 3 min suppressed fruit decay by 89% without compromising fruit quality (Schirra et al., 2002). Hot water brushing has been successfully used for several commodities, such as peppers (Fallik et al., 1999), melons (Fallik et al., 2000), oranges (Porat et al., 2000), etc. In mango fruit, hot water brushing has been found effective in controlling Alternaria alternata (Prusky et al., 1999), which is one of the most frequently found pathogens in cactus pear grown on Crete. The objective of this study was to investigate the effects of hot water sprays at various temperature levels and time intervals during the despining process of cactus pear on fruit quality and decay incidence during storage.
2. Materials and methods 2.1. Plant material Ripe cactus pear fruit [O. ficus-indica Miller (L.)] of a yellow-flesh local genotype, were harvested from an orchard located in the south-eastern region of Crete, Greece. Fruit were cut with a small part of the mother cladode attached and transferred the same day to the laboratory. 2.2. Treatment conditions Spines were removed using a purpose-built laboratory device (Laboratory of Mechanical Engineering, Technological Education Institute, Crete, Greece). The device was equipped with a brush revolving at a speed of 90 rpm over the fruit placed on two parallel plastic rollers. The brush was made of soft plastic bristles 10 cm long and 1 mm diameter. Fruit were simultaneously brushed and sprayed with water at 20, 60, 65 and 70 ◦ C for 10, 20 and 30 s. 2.3. Measurement of water loss Six fruit per treatment were used. After treatment, fruit were stored at 20 ± 1 ◦ C for 14 days. Water loss was determined by weighing them every other day. 2.4. Total soluble solids (TSS) and total acids For the determination of TSS and acid levels, samples of six fruit of the same size and maturity level per treatment were selected. TSS and total acids were measured before and 7 days after treatments. Each fruit was crushed and the pulp was homogenized in a blender (Brown AG, Frankfurt/M, Germany) at a speed of 2000 rpm for 1 min and then filtered through a filter paper. The TSS was determined by placing 2–3 drops on a temperature-compensated bench top digital refractometer (Atago Co. Ltd., Tokyo, Japan). The total acids were measured in aliquots of 5 mL of juice by titration with 0.1N NaOH to pH 8.1, using an automatic titration system (Metrohm, Model 719 S, Herisau, Switzerland). Results were expressed as citric acid equivalents (g dm−3 ).
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2.5. Respiration
2.9. Statistics
Respiration rate was measured using a CO2 analyser (M & C, Bleiswijk, The Netherlands). Four replicates consisting of six fruit per treatment were enclosed in airtight transparent boxes and CO2 was measured at 8 h intervals. Untreated fruit (not despined) were used as controls. Following measurements, boxes containing the fruit were fully ventilated and resealed. Results were expressed as mg CO2 kg (fresh weight) h−1 .
Analysis of variance (ANOVA model one-way) was performed on all the data and Fisher’s LSD method was used to establish differences among the means at P < 0.05. Data were tested using STATISTICA for Windows, StatSoft Inc. (1996). All the experiments were conducted many times over the 2-year (2003–2004) experimental period. Only selective results are shown, although these are representative of the typical results found in all experiments.
2.6. Determination of relative electrolyte leakage
3. Results and discussion
Pieces of peel (1 cm × 2 cm) were taken from the equatorial zone of six fruit per treatment using a surgical blade. Fruit were then subjected to hot water brushing and similar peel pieces were taken from the opposite side of the fruit. All the pieces were placed in glass beakers and were washed three times with distilled water. The peel sections were then covered with 10 mL of 0.4 M mannitol solution and incubated for 4 h at 20 ◦ C on a flask shaker (Thermolyne, IA, USA) at 200 rpm. Electrical conductivity of the solution bathing the peel pieces was determined using a conductivity meter (AGB, London, England) after 4-h incubation. Relative electrolyte leakage was calculated for each fruit as the percent of two peel sections.
3.1. Water loss
2.7. Evaluation of external appearance Samples consisting of 10 fruit per treatment were used in this experiment. After treatment, fruit were stored at 20 ± 1 ◦ C for 7 days. To evaluate external appearance, the number of discoloured areas on the fruit peel was counted and their size was measured as millimeter of diameter. 2.8. Decay incidence
Heat treatments have been found to cause either an increase or decrease in water loss depending on the treatment and the product (Williams et al., 1994; Lydakis and Aked, 2003). In cactus fruit sprayed with hot water at 60 and 65 ◦ C for 10–30 s, a significant reduction in water loss was observed compared to the control (Fig. 1). In fruit treated at 70 ◦ C for 20 and 30 s, however, water loss was found to be the same or slightly higher than the control. In a similar study, Williams et al. (1994) found that ‘Valencia’ oranges treated in hot water at 45 ◦ C for 42 min lost significantly less moisture, while fruit immersed in 53 ◦ C for 12 min showed the greatest weight loss. It has been shown that heat treatments can cause cuticular wax to melt slightly and fill in surface micro-cracks and stomata (Schirra et al., 2000). In the present study, it is likely that the hot sprays at 60 and 65 ◦ C caused the wax to melt and this, combined with brushing which would help spread the wax uniformly, may well help to reduce transpiration. However, in treatments at higher temperature levels, epicuticular waxes melt completely and may be removed. This may explain the higher weight loss observed in fruit treated at 70 ◦ C for 20–30 s in the present study. 3.2. Chemical characteristics
Fruit were simultaneously brushed and spayed with water at 20, 60, 65 and 70 ◦ C for 10, 20 and 30 s. Sixty fruit per treatment were used. After treatment, fruit were kept for 4 weeks at 6 ± 1 ◦ C followed by 1-week shelf life at 20 ± 1 ◦ C. Decay was monitored at weekly intervals.
Before treatment, TSS and titratable acidity measured in cactus pears used in this study ranged from 11.5 to 13.30 Brix and from 0.5 to 0.7 g dm−3 , respectively. A slight decrease of 1–3% in both TSS and acidity was measured at the end of the 1-week storage
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Fig. 2. Respiration rate of cactus pear not-despined (䊉) and following hot water brushing at 20 (), 60 (), 65 () and 70 ◦ C () for 20 s. Fruit were stored at 20 ± 1 ◦ C in airtight boxes and respiration rate was measured at 8 h intervals. Following measurements boxes were thoroughly ventilated and resealed.
with time in storage (Cantwell, 1995). Respiration rate of the fruit used in this study before treatments was 21 mg CO2 kg−1 h−1 . A significant rise in respiration rate was observed immediately after brushing (Fig. 2), due to the great number of slight injuries induced during the despining process. However, this rise was not affected by the hot water spays at 60–70 ◦ C. Respiration rate in despined fruit remained high until the end of the second day after treatment and then declined to the pre-treatment levels. 3.4. Electrolyte leakage
Fig. 1. Water loss of cactus pear following hot water brushing at 20 (䊉), 60 (), 65 (), and 70 ◦ C (), for 10 (A), 20 (B) and 30 s (C). Water loss was determined by weighting every other day the fruit stored at 20 ± 1 ◦ C for 14 days.
period at 20 ± 1 ◦ C. No statistically significant differences were found between heat-treated and control fruit. It is concluded that these short time treatments did not significantly raise the internal temperature of the fruit and therefore, the chemical composition was not affected. 3.3. Respiration rate Cactus pears are non-climacteric fruit and under normal conditions, respiratory activity declines slowly
Damage to cell membranes after heat treatment was tested by relative electrolyte leakage measurements, based on the principle that damage to cell membranes results in enhanced leakage of solutes into the apoplastic water (Linden et al., 2000). In fruit treated at 60 ◦ C, the ratio (b/a) of the rate of electrolyte leakage of the two peel sections after treatment (a) to before treatment (b) was very close to 1 (Table 1), suggesting that treatment at this temperature level did not affect cell membrane integrity. This ratio increased to 1.09–1.10 in fruit treated at 65 ◦ C for 20 s and 70 ◦ C for 10 s, indicating some negative effects on the fruit skin. Nevertheless, in fruit treated at 65 ◦ C for 30 s and 70 ◦ C for 20–30 s, this rate increased to 1.20 and 1.31–1.61, respectively, showing that these treatments were having a detrimental effect on membrane integrity.
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Table 1 Index of relative electrolyte leakage (b/a) of cactus fruit peel sections
3.6. Decay incidence
Treatment
Relative electrolyte leakage (b/a)
60 ◦ C × l0 s 60 ◦ C × 20 s 60 ◦ C × 30 s 65 ◦ C × l0 s 65 ◦ C × 20 s 65 ◦ C × 30 s 70 ◦ C × l0 s 70 ◦ C × 20 s 70 ◦ C × 30 s
1.04 a 1.04 a 1.06 a 1.05 a 1.09 b 1.20 c 1.10b 1.31 d 1.61 e
At least four genera of fungi namely Alternaria sp., Fusarium sp., Botrytis cinerea and Penicillium sp. were isolated from the exterior tissue layer of fruit. In some cases, soft fruit with an unpleasant smell and sticky exudates were obtained indicating that bacteria or yeasts might be present, but no further tests were
Electrolyte leakage was measured in two equatorial sections 1 cm × 2 cm of the same fruit before (a) and after (b) hot water brushing at 60–70 ◦ C for 10–30 s. Different letters (a–e) in the same column indicate statistically significant differences (* P < 0.05).
3.5. External appearance The despining process causes tiny wounds on the peel. This results in pitting and dark-brown spots on the peel surface (Cantwell, 1995). In fruit treated at 60 and 65 ◦ C for 10–30 s, both the number and size of brown spots were decreased (Table 2). Similar effects were observed in cactus fruit following hot water dips at 55 ◦ C for 2 min (Berger et al., 2002). Conversely, higher temperature (70 ◦ C) induced heat damage in the form of extended light-brown coloured areas.
Table 2 Number and average diameter of brown areas (spots and zones) measured 7 days after hot water brushing at 20, 60, 65 and 70 ◦ C on the peel of cactus pear (10 fruit per treatment) Treatment
Number of brown areas
Average diameter (mm)
20 ◦ C × l0 s 20 ◦ C × 20 s 20 ◦ C × 30 s 60 ◦ C × l0 s 60 ◦ C × 20 s 60 ◦ C × 30 s 65 ◦ C × l0 s 65 ◦ C × 20 s 65 ◦ C × 30 s 70 ◦ C × l0 s 70 ◦ C × 20 s 70 ◦ C × 30 s
50 a 37 ab 43 a 33 b 33 b 23 c 25 c 23 c 20 c 25 c 27 c 20 c
5.3 a 4.7 a 4.6 a 3.5 b 4.2 ab 3.1 b 2.7 b 2.8 b 3.1b 6.7 c 9.9 d 9.3 d
Following treatment, fruit were stored at 20 ± 1 ◦ C. Different letters (a–d) in the same column indicate statistically significant differences (* P < 0.05).
Fig. 3. Decay incidence of cactus pear following hot water brushing at 20 (䊉), 60 (), 65 (), and 70 ◦ C () for 10 (A), 20 (B), and 30 s (C). After treatment fruit were stored at 6 ± 1 ◦ C for 4 weeks followed by 1 week self life at 20 ± 1 ◦ C. Fruit with decay lesions detectable by eye were monitored at weakly intervals.
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carried out. Physical damage to the peel during harvesting and brushing predispose cactus pears to attack by decay-causing pathogens. In the present study, the incidence of decay in control fruit treated at 20 ◦ C was found to be 15% higher at the end of the storage period compared to non-treated fruit (data not shown). All the treatments at 60, 65 and 70 ◦ C were found effective in controlling decay (Fig. 3). The effectiveness of the treatments initially increased with the temperature and time. The most effective treatments were those at 60 ◦ C for 30 s, 65 ◦ C for 20 s and 70 ◦ C for 10 s, which reduced the decay incidence by 86–91% compared to control. Prolonged treatments at 65 ◦ C for 30 s or at 70 ◦ C for 20–30 s, however, did not further increase the control of decay. The effectiveness of hot water brushing in controlling decay in stored fruit is associated with various factors. Hot water brushing cleans effectively and removes fungal spores from the fruit surface. Treatments at temperature levels used in this study are effective in killing spores or may cause a direct inhibition of fungal germination and growth (Schirra and D’hallewin, 1997; Fallik et al., 1999; Fallik, 2004). Furthermore, the partial melting of the epicutilar wax layer in combination with brushing leads to occlusion of possible entry points for wound pathogens, such as Penicillium sp. and B. cinerea (Schirra et al., 1999; Porat et al., 2000).
4. Conclusions Based on the results of this study, heat treatments at 60 ◦ C for 30 s or 65 ◦ C for 20 s could be used safely in controlling decay in cactus pear. The same treatments beneficially affected external fruit appearance, minimizing the number and the size of brown spots on the fruit skin. The success of a heat treatment in commercial use depends on the existence of a sufficient difference between the heat tolerance of the host and the pathogens. Despite the fact that Opuntia plants are known to be extremely tolerant of high temperatures (Nobel, 1995), treatments at 70 ◦ C induced detrimental effects on cactus fruit. These effects were manifested as an increase in weight loss and electrolyte leakage, as well as by the presence of light-brown coloured zones on the fruit skin. Therefore, treatments at this temperature level should be avoided.
The proposed treatment can easily be adopted in commercial practice, since only a minor modification of the despining facilities in use is involved. In addition, this physical treatment is in accordance with the consumer demand for non-chemical alternatives for postharvest treatment of fruit, especially where cactus pear can be grown organically. References Barbera, G., 1995. Economic and agro-ecological importance. In: Barbera, G., Inglese, P., Pimienta Barios, E., Jr. (Eds.), Agrogeology, Cultivation and Uses of Cactus Pear. Publ. Div. FAO, Rome, pp. 1–11. Barkai-Golan, R., Phillips, D.J., 1991. Postharvest heat treatments of fresh fruit and vegetables for decay control. Plant Dis. 75, 1085–1089. Berger, H., Mitrovic, L., Galletti, L., Oyarzun, J., 2002. Effect of hot water and wax applications on storage life of cactus pear [Opuntia ficus-indica Miller (L.)] fruits. Acta Hortic. 581, 211–220. Cantwell, M., 1995. Postharvest management of fruits and vegetable stems. In: Barbera, G., Inglese, P., Pimienta Barios, E., Jr. (Eds.), Agrogeology, Cultivation and Uses of Cactus Pear. Publ. Div. FAO, Rome, pp. 120–136. Fallik, E., Grinberg, S., Alkalai, S., Yekutieli, O., Wiseblum, A., Regev, R., Beres, H., Bar-Lev, E., 1999. A unique rapid hot water treatment to improve storage quality of sweet pepper. Postharvest Biol. Technol. 15, 25–32. Fallik, E., Aharoni, Y., Copel, A., Rodov, R., Tuvia-Alkalai, S., Horev, B., Yekutieli, O., Wiseblum, A., Regev, R., 2000. A short hot water rinse reduces postharvest losses of ‘Galia’ melon. Plant Pathol. 49, 333–338. Fallik, E., 2004. Prestorage hot water treatments (immersion, rinsing and brushing). Postharvest Biol. Technol. 32, 125–134. Ferguson, I.B., Ben-Yehoshua, S., Mitcham, E.J., McDonald, R.E., Lurie, S., 2000. Postharvest heat treatments: introduction and workshop summary. Postharvest Biol. Technol. 21, 1–6. Inglese, P., Barbera, G., Lamantia, T., 1995. Research strategies for the improvement of cactus pear (Opuntia ficus-indica) fruit quality and production. J. Arid Environ. 29, 455–468. Linden, L., Palonen, P., Linden, M., 2000. Relating freeze-induced electrolyte leakage measurements to lethal temperature in red raspberry. J. Am. Soc. Hortic. Sci. 125, 429–435. Lionakis, S., Loxu, V., Lydakis, D., 2001. Study of plant development and fruit ripening of Opuntia genotypes grown in Chanea Crete. In: Proceedings of the 20th Congress of the Greek Soc. Hortic. Sci, pp. 262–265 (in Greek). Lurie, S., 1998. Postharvest heat treatments. Postharvest Biol. Technol. 14, 257–269. Lydakis, D., Aked, J., 2003. Vapour heat treatment of Sultanina table grapes II: effects on postharvest quality. Postharvest Biol. Technol. 27, 117–126. Nobel, P.S., 1995. Environmental biology. In: Barbera, G., Inglese, P., Pimienta Barios, E., Jr. (Eds.), Agrogeology, Cultivation and Uses of Cactus Pear. Publ. Div. FAO, Rome, pp. 120–136.
L. Dimitris et al. / Postharvest Biology and Technology 38 (2005) 145–151 Porat, R., Daus, A., Weiss, B., Cohen, L., Fallik, E., Droby, S., 2000. Reduction of postharvest decay in organic citrus fruit by a short water brushing treatment. Postharvest Biol. Technol. 18, 151–157. Prusky, D., Fuchs, Y., Kobiler, I., Roth, I., Weksler, A., Shalom, Y., Fallik, E., Zauberman, G., Pesis, E., Akerman, M., Ykuteli, O., Weisblum, A., Regev, R., Artes, L., 1999. Effect of hot water brushing, prochloraz treatment and waxing on the incidence of black spot decay caused by Alternaria alternata in mango fruits. Postharvest Biol. Technol. 15, 165–174. Schirra, M., D’hallewin, G., 1997. Storage performance of Fortune mandarins following hot water dips. Postharvest Biol. Technol. 10, 229–238. Schirra, M., Barberra, G., D’hallewin, G., Inglese, P., La Mantia, T., 1997. Storage response of cactus pear fruit to CaCl2 preharvest spray and postharvest heat treatment. J. Hortic. Sci. 72, 371– 377.
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Schirra, M., D’hallewin, G., Inglese, P., La Mantia, T., 1999. Epicutilar changes and storage potential of cactus pear [Opuntia ficus-indica (L.) Mill Cv Gialla] fruit following gibberellic acid preharvest sprays and postharvest heat treatment. Postharvest Biol. Technol. 17, 79–88. Schirra, M., D’hallewin, G., Ben-Yehoshua, S., Fallik, E., 2000. Host–pathogen interactions modulated by heat treatment. Postharvest Biol. Technol. 21, 71–85. Schirra, M., Brandolini, V., Cabras, P., Angioni, A., Inglese, P., 2002. Thiabendazole uptake and storage performance of cactus pear [Opuntia ficus-indica (L.) Mill Cv Gialla] fruit following postharvest treatments with reduced doses of fungicide at 52 degrees C. J. Agric. Food Chem. 13, 739–743. Williams, M.H., Brown, M.A., Vesk, M., Brady, C., 1994. Effect of postharvest heat treatments on fruit quality, surface structure, and fungal disease in Valencia oranges. Aust. J. Exp. Agric. 34, 1183–1190.
Storage response of cactus pear fruit following hot water brushing Lydakis Dimitris a,∗ , N. Pompodakis a , E. Markellou b , S.M. Lionakis a a b
Technological Education Institute of Crete, GR-71 100 Heraklion, Crete, Greece Benaki Phytopathological Institute, 8 St. Delta Street, GR-145 61 Athens, Greece Received 1 April 2005; accepted 21 June 2005
Abstract The storage response of cactus pear [Opuntia ficus-indica Miller (L.)] following hot water brushing was investigated. Fruit were simultaneously brushed for spine removal and sprayed with water. Ranges of temperature (60–70 ◦ C) and treatment time intervals (10–30 s) were evaluated. All tested treatments were found not to significantly affect respiration rate, total soluble solids or acid concentrations. Treatments at 60 and 65 ◦ C were found to reduce water loss and the incidence of rusty-brown spots on fruit peel. At higher temperatures, some negative effects were observed, namely increased electrolyte leakage and extended light brown areas on the fruit peel, indicating heat damage to the fruit. All the treatments effectively controlled decay in fruit stored at 6 ± 1 ◦ C for 4 weeks followed by 1 week at 20 ± 1 ◦ C. The effectiveness of treatments to control decay was increased with temperature and treatment time. Based on the results of this study, treatments at 60 ◦ C for 30 s or 65 ◦ C for 20 s reduced the decay incidence by 86–91% without compromising fruit quality. These treatments can be easily applied in commercial practice with slight modification of the despining facilities in use. © 2005 Elsevier B.V. All rights reserved. Keywords: Cactus pear; Decay control; Heat treatment; Hot water brushing; Opuntia
1. Introduction Cactus pear [Opuntia ficus-indica Miller (L.)] is cultivated for fruit production worldwide, either in the subsistence agriculture of dry land areas or as a cash crop (Inglese et al., 1995). Cactus pear trees can be found wild or cultivated in all the Mediterranean countries. ∗ Corresponding author. Tel.: +30 2810 379408; fax: +30 2810 318204. E-mail address: [email protected] (L. Dimitris).
0925-5214/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2005.06.006
Commercial plantations, however, are grown mainly in Italy, particularly, in Sicily (Barbera, 1995). In Greece, they can be found in all the Aegean Islands and in some areas of the southern mainland (Lionakis et al., 2001). Due to increasing demand for a diversified diet and the use of unusual products, the consumption of cactus pear has been increasing steadily over the last few years throughout Europe. One of the major problems with cactus pear is the presence of glochids (spines) on the fruit peel. Because of the glochids, cactus pears are difficult
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L. Dimitris et al. / Postharvest Biology and Technology 38 (2005) 145–151
to harvest and handle after harvest. Consumers do not prefer fruit with glochids. Therefore, postharvest operations for fruit destined for the market involve glochid removal. Glochids can be eliminated by spreading the fruit on grass or on open mesh tables and brushing with brooms. In packing houses, fruit are passed over a series of brushes with the application of water sprays or air suction to trap and remove spines. Cactus pears are characterized as highly perishable fruit. Storage at room temperature favours decay, fruit weight loss, wilting, softening and off-flavor development, while storage at temperatures below 8–10 ◦ C promotes physiological breakdown. Physical damage to the peel induced during spine removal predisposes cactus pear to attack by decay-causing pathogens. Loose glochids may also damage the surface of harvested fruit, resulting in small rusty-brown discoloured areas (Cantwell, 1995). Storability of fruit can be improved by postharvest heat treatments. Treatments in the form of hot water, hot air or vapour heat have been developed to control postharvest decay, insect infestation and alleviate storage disorders in a wide range of fresh produce (Barkai-Golan and Phillips, 1991; Lurie, 1998; Ferguson et al., 2000; Fallik et al., 2000). The response of cactus pear to prestorage heat treatments has been tested in different studies. Treatments at 38 ◦ C for 24 h (Schirra et al., 1997) and hot water dips at 55 ◦ C for 2 min (Berger et al., 2002) reduced rot development and decline in overall appearance during storage. Dips in hot thiabendazole solutions with reduced doses of fungicide (150 mg/L) at 52 ◦ C for 3 min suppressed fruit decay by 89% without compromising fruit quality (Schirra et al., 2002). Hot water brushing has been successfully used for several commodities, such as peppers (Fallik et al., 1999), melons (Fallik et al., 2000), oranges (Porat et al., 2000), etc. In mango fruit, hot water brushing has been found effective in controlling Alternaria alternata (Prusky et al., 1999), which is one of the most frequently found pathogens in cactus pear grown on Crete. The objective of this study was to investigate the effects of hot water sprays at various temperature levels and time intervals during the despining process of cactus pear on fruit quality and decay incidence during storage.
2. Materials and methods 2.1. Plant material Ripe cactus pear fruit [O. ficus-indica Miller (L.)] of a yellow-flesh local genotype, were harvested from an orchard located in the south-eastern region of Crete, Greece. Fruit were cut with a small part of the mother cladode attached and transferred the same day to the laboratory. 2.2. Treatment conditions Spines were removed using a purpose-built laboratory device (Laboratory of Mechanical Engineering, Technological Education Institute, Crete, Greece). The device was equipped with a brush revolving at a speed of 90 rpm over the fruit placed on two parallel plastic rollers. The brush was made of soft plastic bristles 10 cm long and 1 mm diameter. Fruit were simultaneously brushed and sprayed with water at 20, 60, 65 and 70 ◦ C for 10, 20 and 30 s. 2.3. Measurement of water loss Six fruit per treatment were used. After treatment, fruit were stored at 20 ± 1 ◦ C for 14 days. Water loss was determined by weighing them every other day. 2.4. Total soluble solids (TSS) and total acids For the determination of TSS and acid levels, samples of six fruit of the same size and maturity level per treatment were selected. TSS and total acids were measured before and 7 days after treatments. Each fruit was crushed and the pulp was homogenized in a blender (Brown AG, Frankfurt/M, Germany) at a speed of 2000 rpm for 1 min and then filtered through a filter paper. The TSS was determined by placing 2–3 drops on a temperature-compensated bench top digital refractometer (Atago Co. Ltd., Tokyo, Japan). The total acids were measured in aliquots of 5 mL of juice by titration with 0.1N NaOH to pH 8.1, using an automatic titration system (Metrohm, Model 719 S, Herisau, Switzerland). Results were expressed as citric acid equivalents (g dm−3 ).
L. Dimitris et al. / Postharvest Biology and Technology 38 (2005) 145–151
147
2.5. Respiration
2.9. Statistics
Respiration rate was measured using a CO2 analyser (M & C, Bleiswijk, The Netherlands). Four replicates consisting of six fruit per treatment were enclosed in airtight transparent boxes and CO2 was measured at 8 h intervals. Untreated fruit (not despined) were used as controls. Following measurements, boxes containing the fruit were fully ventilated and resealed. Results were expressed as mg CO2 kg (fresh weight) h−1 .
Analysis of variance (ANOVA model one-way) was performed on all the data and Fisher’s LSD method was used to establish differences among the means at P < 0.05. Data were tested using STATISTICA for Windows, StatSoft Inc. (1996). All the experiments were conducted many times over the 2-year (2003–2004) experimental period. Only selective results are shown, although these are representative of the typical results found in all experiments.
2.6. Determination of relative electrolyte leakage
3. Results and discussion
Pieces of peel (1 cm × 2 cm) were taken from the equatorial zone of six fruit per treatment using a surgical blade. Fruit were then subjected to hot water brushing and similar peel pieces were taken from the opposite side of the fruit. All the pieces were placed in glass beakers and were washed three times with distilled water. The peel sections were then covered with 10 mL of 0.4 M mannitol solution and incubated for 4 h at 20 ◦ C on a flask shaker (Thermolyne, IA, USA) at 200 rpm. Electrical conductivity of the solution bathing the peel pieces was determined using a conductivity meter (AGB, London, England) after 4-h incubation. Relative electrolyte leakage was calculated for each fruit as the percent of two peel sections.
3.1. Water loss
2.7. Evaluation of external appearance Samples consisting of 10 fruit per treatment were used in this experiment. After treatment, fruit were stored at 20 ± 1 ◦ C for 7 days. To evaluate external appearance, the number of discoloured areas on the fruit peel was counted and their size was measured as millimeter of diameter. 2.8. Decay incidence
Heat treatments have been found to cause either an increase or decrease in water loss depending on the treatment and the product (Williams et al., 1994; Lydakis and Aked, 2003). In cactus fruit sprayed with hot water at 60 and 65 ◦ C for 10–30 s, a significant reduction in water loss was observed compared to the control (Fig. 1). In fruit treated at 70 ◦ C for 20 and 30 s, however, water loss was found to be the same or slightly higher than the control. In a similar study, Williams et al. (1994) found that ‘Valencia’ oranges treated in hot water at 45 ◦ C for 42 min lost significantly less moisture, while fruit immersed in 53 ◦ C for 12 min showed the greatest weight loss. It has been shown that heat treatments can cause cuticular wax to melt slightly and fill in surface micro-cracks and stomata (Schirra et al., 2000). In the present study, it is likely that the hot sprays at 60 and 65 ◦ C caused the wax to melt and this, combined with brushing which would help spread the wax uniformly, may well help to reduce transpiration. However, in treatments at higher temperature levels, epicuticular waxes melt completely and may be removed. This may explain the higher weight loss observed in fruit treated at 70 ◦ C for 20–30 s in the present study. 3.2. Chemical characteristics
Fruit were simultaneously brushed and spayed with water at 20, 60, 65 and 70 ◦ C for 10, 20 and 30 s. Sixty fruit per treatment were used. After treatment, fruit were kept for 4 weeks at 6 ± 1 ◦ C followed by 1-week shelf life at 20 ± 1 ◦ C. Decay was monitored at weekly intervals.
Before treatment, TSS and titratable acidity measured in cactus pears used in this study ranged from 11.5 to 13.30 Brix and from 0.5 to 0.7 g dm−3 , respectively. A slight decrease of 1–3% in both TSS and acidity was measured at the end of the 1-week storage
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Fig. 2. Respiration rate of cactus pear not-despined (䊉) and following hot water brushing at 20 (), 60 (), 65 () and 70 ◦ C () for 20 s. Fruit were stored at 20 ± 1 ◦ C in airtight boxes and respiration rate was measured at 8 h intervals. Following measurements boxes were thoroughly ventilated and resealed.
with time in storage (Cantwell, 1995). Respiration rate of the fruit used in this study before treatments was 21 mg CO2 kg−1 h−1 . A significant rise in respiration rate was observed immediately after brushing (Fig. 2), due to the great number of slight injuries induced during the despining process. However, this rise was not affected by the hot water spays at 60–70 ◦ C. Respiration rate in despined fruit remained high until the end of the second day after treatment and then declined to the pre-treatment levels. 3.4. Electrolyte leakage
Fig. 1. Water loss of cactus pear following hot water brushing at 20 (䊉), 60 (), 65 (), and 70 ◦ C (), for 10 (A), 20 (B) and 30 s (C). Water loss was determined by weighting every other day the fruit stored at 20 ± 1 ◦ C for 14 days.
period at 20 ± 1 ◦ C. No statistically significant differences were found between heat-treated and control fruit. It is concluded that these short time treatments did not significantly raise the internal temperature of the fruit and therefore, the chemical composition was not affected. 3.3. Respiration rate Cactus pears are non-climacteric fruit and under normal conditions, respiratory activity declines slowly
Damage to cell membranes after heat treatment was tested by relative electrolyte leakage measurements, based on the principle that damage to cell membranes results in enhanced leakage of solutes into the apoplastic water (Linden et al., 2000). In fruit treated at 60 ◦ C, the ratio (b/a) of the rate of electrolyte leakage of the two peel sections after treatment (a) to before treatment (b) was very close to 1 (Table 1), suggesting that treatment at this temperature level did not affect cell membrane integrity. This ratio increased to 1.09–1.10 in fruit treated at 65 ◦ C for 20 s and 70 ◦ C for 10 s, indicating some negative effects on the fruit skin. Nevertheless, in fruit treated at 65 ◦ C for 30 s and 70 ◦ C for 20–30 s, this rate increased to 1.20 and 1.31–1.61, respectively, showing that these treatments were having a detrimental effect on membrane integrity.
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Table 1 Index of relative electrolyte leakage (b/a) of cactus fruit peel sections
3.6. Decay incidence
Treatment
Relative electrolyte leakage (b/a)
60 ◦ C × l0 s 60 ◦ C × 20 s 60 ◦ C × 30 s 65 ◦ C × l0 s 65 ◦ C × 20 s 65 ◦ C × 30 s 70 ◦ C × l0 s 70 ◦ C × 20 s 70 ◦ C × 30 s
1.04 a 1.04 a 1.06 a 1.05 a 1.09 b 1.20 c 1.10b 1.31 d 1.61 e
At least four genera of fungi namely Alternaria sp., Fusarium sp., Botrytis cinerea and Penicillium sp. were isolated from the exterior tissue layer of fruit. In some cases, soft fruit with an unpleasant smell and sticky exudates were obtained indicating that bacteria or yeasts might be present, but no further tests were
Electrolyte leakage was measured in two equatorial sections 1 cm × 2 cm of the same fruit before (a) and after (b) hot water brushing at 60–70 ◦ C for 10–30 s. Different letters (a–e) in the same column indicate statistically significant differences (* P < 0.05).
3.5. External appearance The despining process causes tiny wounds on the peel. This results in pitting and dark-brown spots on the peel surface (Cantwell, 1995). In fruit treated at 60 and 65 ◦ C for 10–30 s, both the number and size of brown spots were decreased (Table 2). Similar effects were observed in cactus fruit following hot water dips at 55 ◦ C for 2 min (Berger et al., 2002). Conversely, higher temperature (70 ◦ C) induced heat damage in the form of extended light-brown coloured areas.
Table 2 Number and average diameter of brown areas (spots and zones) measured 7 days after hot water brushing at 20, 60, 65 and 70 ◦ C on the peel of cactus pear (10 fruit per treatment) Treatment
Number of brown areas
Average diameter (mm)
20 ◦ C × l0 s 20 ◦ C × 20 s 20 ◦ C × 30 s 60 ◦ C × l0 s 60 ◦ C × 20 s 60 ◦ C × 30 s 65 ◦ C × l0 s 65 ◦ C × 20 s 65 ◦ C × 30 s 70 ◦ C × l0 s 70 ◦ C × 20 s 70 ◦ C × 30 s
50 a 37 ab 43 a 33 b 33 b 23 c 25 c 23 c 20 c 25 c 27 c 20 c
5.3 a 4.7 a 4.6 a 3.5 b 4.2 ab 3.1 b 2.7 b 2.8 b 3.1b 6.7 c 9.9 d 9.3 d
Following treatment, fruit were stored at 20 ± 1 ◦ C. Different letters (a–d) in the same column indicate statistically significant differences (* P < 0.05).
Fig. 3. Decay incidence of cactus pear following hot water brushing at 20 (䊉), 60 (), 65 (), and 70 ◦ C () for 10 (A), 20 (B), and 30 s (C). After treatment fruit were stored at 6 ± 1 ◦ C for 4 weeks followed by 1 week self life at 20 ± 1 ◦ C. Fruit with decay lesions detectable by eye were monitored at weakly intervals.
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carried out. Physical damage to the peel during harvesting and brushing predispose cactus pears to attack by decay-causing pathogens. In the present study, the incidence of decay in control fruit treated at 20 ◦ C was found to be 15% higher at the end of the storage period compared to non-treated fruit (data not shown). All the treatments at 60, 65 and 70 ◦ C were found effective in controlling decay (Fig. 3). The effectiveness of the treatments initially increased with the temperature and time. The most effective treatments were those at 60 ◦ C for 30 s, 65 ◦ C for 20 s and 70 ◦ C for 10 s, which reduced the decay incidence by 86–91% compared to control. Prolonged treatments at 65 ◦ C for 30 s or at 70 ◦ C for 20–30 s, however, did not further increase the control of decay. The effectiveness of hot water brushing in controlling decay in stored fruit is associated with various factors. Hot water brushing cleans effectively and removes fungal spores from the fruit surface. Treatments at temperature levels used in this study are effective in killing spores or may cause a direct inhibition of fungal germination and growth (Schirra and D’hallewin, 1997; Fallik et al., 1999; Fallik, 2004). Furthermore, the partial melting of the epicutilar wax layer in combination with brushing leads to occlusion of possible entry points for wound pathogens, such as Penicillium sp. and B. cinerea (Schirra et al., 1999; Porat et al., 2000).
4. Conclusions Based on the results of this study, heat treatments at 60 ◦ C for 30 s or 65 ◦ C for 20 s could be used safely in controlling decay in cactus pear. The same treatments beneficially affected external fruit appearance, minimizing the number and the size of brown spots on the fruit skin. The success of a heat treatment in commercial use depends on the existence of a sufficient difference between the heat tolerance of the host and the pathogens. Despite the fact that Opuntia plants are known to be extremely tolerant of high temperatures (Nobel, 1995), treatments at 70 ◦ C induced detrimental effects on cactus fruit. These effects were manifested as an increase in weight loss and electrolyte leakage, as well as by the presence of light-brown coloured zones on the fruit skin. Therefore, treatments at this temperature level should be avoided.
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