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Effects of chitosan coating on shelf life of cold-stored litchi fruit at ambient temperature
ARTICLE IN PRESS
LWT 38 (2005) 757–761 www.elsevier.com/locate/lwt
Effects of chitosan coating on shelf life of cold-stored litchi fruit at ambient temperature Yueming Jianga,b,, Jianrong Lib, Weibo Jiangc a
College of Food Science, Biotechnology and Environmental Engineering, Zhejiang Gongshang University, Hangzhou 310035, PR China b South China Botanic Garden, The Chinese Academy of Sciences, Guangzhou, LeYiJu 510650, PR China c School of Food Science, China Agricultural University, P.O. Box 204, Qinghua Donglu, Haidian, Beijing 100083, PR China Received 4 May 2004; received in revised form 2 September 2004; accepted 14 September 2004
Abstract Postharvest pericarp browning of litchi fruit results in an accelerated loss in shelf life and a reduced commercial value. Visual quality was lost in only 6 h at ambient temperature when fruit were removed from storage at 2 1C, due to browning. The experiment was conducted to test the role of chitosan coating in inhibiting skin browning and extending shelf life of cold-stored litchi fruit at ambient temperature. Litchi fruit were treated with 2 g chitosan/100 g solution and then stored for 20 days at 2 1C and 90–95% relative humidity (RH), prior to shelf life evaluation at 25 1C and 80–90% RH. Changes in polyphenol oxidase (PPO) activity, anthocyanin concentration, colour index, eating quality and concentrations of total soluble solids and titratable acidity were measured. The effects of chitosan coating on disease incidence were also evaluated. Application of chitosan coating delayed the decrease in anthocyanin content, the increase in PPO activity and the changes in colour index and eating quality, reduced the decrease in concentrations of total soluble solids and titratable acidity, and partially inhibited decay. The results suggested that treatment with chitosan coating exhibited a potential for shelf life extension at ambient temperature when litchi fruit were removed from cold storage. r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Chitosan; Coating; Fruit; Litchi; Shelf life
1. Introduction Litchi (Litchi chinensis Sonn.) is a tropical and subtropical fruit of high commercial value for its white, translucent aril and attractive red colour (Holcroft & Mitcham, 1996). Although litchi fruit can be stored for about 20 days at a low temperature range of 2–5 1C, the fruit can deteriorate rapidly due to peel browning when removed from the cold storage (Jiang & Li, 2003). Thus, the major limitation in litchi marketing is the rapid lose of the red color after harvest (Nip, 1988; Jiang, Yao, Corresponding author. Department of Plant Resources, South China Institute of Botany, Guangzhou, LeYiJu 510650, PR China. Tel.: +86 20 37252525; fax: +86 20 37252831. E-mail address: [email protected] (Y. Jiang).
Lichter, & Li, 2003). Postharvest browning of litchi fruit was generally thought to be a rapid degradation of anthocyanins caused by polyphenol oxidase (PPO), producing brown by-products (Lee & Wicker, 1991; Jiang, 2000). Postharvest treatments, such as sulphur fumigation and acid dip can effectively inhibited PPO activity and thus delay loss of red skin colour of litchi fruit (Zauberman et al., 1991). However, alternative chemicals for colour control without toxic effects in harvested litchi fruit are needed because of concerns for food safety and restrictions in the use of chemicals (Jiang et al., 2003). Application of semi-permeable coatings with modified atmosphere of CO2 and O2 under small storage environment conditions has been shown to improve the storability of perishable crops (Cisneros-Zevallos &
0023-6438/$30.00 r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2004.09.004
ARTICLE IN PRESS 758
Y. Jiang et al. / LWT 38 (2005) 757–761
Krochta, 2002; Lee, Park, Lee, & Choi, 2003). For example, coating treatments delayed fruit ripening of cherimoya, banana and pear through modifying the internal atmospheres (Banks, 1984; Amarante, Banks, & Ganesh, 2001; Yonemoto, Higuchi, & Kitano, 2002). Similar results were obtained in pears and apples using edible coatings (Amarante et al., 2001; Bai, Hagenmaier, & Baldwin, 2003). Chitosan (a high molecular weight cationic polysaccharide) is soluble in dilute organic acids and could theoretically be used as a preservative coating material for fruits. The coating is also safe (Hirano et al., 1990) and shows antifungal activity against several fungi (El Ghaouth, Arul, Grenier, & Asselin, 1992; Li & Yu, 2001; Romanazzi, Nigro, Ippolito, Di Venere, & Salerno, 2002). Previous studies indicated that chitosan coating had the potential to prolong storage life and control decay of many fruits, such as strawberry, peach and table grape (Du, Gemma, & Iwahori, 1997; El Ghaouth, Arul, Ponnamapalam, & Boulet, 1991; Romanazzi, Nigro, & Ippolito, 2003). Zhang and Quantick (1997) and Jiang and Li (2000) reported that application of 2 g chitosan/100 g solution was the most effective in delaying browning when litchi and longan fruits were stored at low temperature. However, the effect of the chitosan coating on shelf life at ambient temperature of litchi fruit removed from cold storage has not been investigated. The objective of this study was to examine the effects of the treatment with 2 g chitosan/100 g solution on the shelf life of the cold-stored litchi fruit at ambient temperature.
control. The treated and control fruit were packaged in plastic boxes (18 26 32 cm; 300 fruit/box), then overwrapped with plastic bags, and finally stored for 20 days at 271 1C and 90–95% relative humidity (RH). After 20 days of storage, the fruit were removed from cold storage and then held for 18 h under ambient conditions (25 1C, 80–90% RH) for evaluation of fruit quality, concentrations of total soluble solids and titratable acidity, PPO activity and anthocyanin content. The experiments were conducted in sequential 2 years. Similar results were obtained from the two experiments. The data from the experiment in 2002 were presented. 2.2. Fruit quality evaluation Appearance was assessed by measuring the extent of the total browned area on each fruit pericarp, using 300 fruit during shelf life evaluation, on the following scale: 1=no browning (excellent quality); 2=slight browning; 3=o1/4 browning; 4=1/4–1/2 browning; 5=41/2 browning (poor quality). The browning index was calculated as S (browning scale percentage of corresponding fruit within each class). Fruit at higher than 2.0 (browning index) was considered unacceptable for marketing. Disease incidence was monitored by randomly collecting 300 fruit and then recording the percentage of fungal growth or bacterial lesions on the fruit surface. Eating quality of fruit pulp was assessed hedonically using a trained six-member panel. At each withdrawal, 30 fruit were randomly selected and rated on the scale of 1=poor to 9=excellent. Fruit at higher than a scale of 6.0 was considered for consumer acceptance.
2. Materials and methods 2.1. Plant materials and treatments Fruit of litchi (Litchi chinensis Sonn.) cv. Huaizhi at the commercially mature stage were harvested from an orchard in Guangzhou. Fruit were selected for uniformity, shape, colour, and size, and any blemished or diseased fruit discarded. The fruits (50 kg) were randomly distributed into two groups prior to treatments. To prepare 3000 ml of 2 g chitosan/100 g solution, 60.0 g of chitosan (Crab shell chitosan, Sigma Chemicals) was dispersed in 2500 ml of distilled water to which 150 ml of glacial acetic acid was added to dissolve the chitosan. The pH of the solution was adjusted to pH 5.0 with 1 mol/l NaOH and the solution was made up to 3000 ml. Fruit were allowed to dry for 4 h at 25 1C after 1 min dipping. In this study, 2 g chitosan/100 g solution was used because the concentration was the most effective in delaying browning and extending storage life when litchi and longan fruits were stored at low temperature (Zhang & Quantick, 1997; Jiang & Li, 2000). Fruits dipped in the acid solution without chitosan, pH 5.0, were used as
2.3. Measurements of total soluble solids and titratable acidity The concentrations of total soluble solids and titratable acidity were analysed during shelf life evaluation. Visible fugal mycelia on fruit surface were removed carefully using a rod with a small cotton ball. Pulp (20 g) from 15 fruit was homogenized in a grinder and then centrifuged at 15,000 g (Beckman J20-2) for 20 min. The supernatant phase was collected to analyse for: total soluble solids, 1Brix, using a hand refractometer (J1-3A, Guangdong Scientific Instruments) and titratable acidity, g citric acid on a fresh weight (FW) basis, determined by titration with 0.1 M NaOH. 2.4. Enzyme assay and protein determination Visible fugal mycelia on fruit surface were removed carefully using a rod with a small cotton ball. Peel (6.0 g) from 15 fruit was homogenized in 30 ml of 0.02 M phosphate buffer (pH 6.8) containing 0.6 g of polyvinylpyrrolidone (insoluble) in an ice bath. The
ARTICLE IN PRESS Y. Jiang et al. / LWT 38 (2005) 757–761
2.5. Anthocyanin assay Visible fugal mycelia on fruit surface were removed carefully using a rod with a small cotton ball. Litchi pericarp (10 g) from 15 fruit was finely sliced and extracted with 200 ml of 0.1% HCl-methanol (1 ml HCl in 99 ml methanol) for 2 h, according to the method of Jiang (2000). The extract was filtered and diluted, and absorbance measured at 530 nm using a spectrophotometer (Beckman DU-7). Anthocyanin concentration was expressed as absorbance at 530 nm/g fresh weight.
6
3. Results and discussion 3.1. Fruit quality The chitosan-untreated (control) and chitosan-treated fruit stored for 20 days at 2 1C had red colour and no disease development on the fruit surface. Removal from cold storage, the fruit browned and eating quality decreased gradually (Fig. 1A and C). However, no diseases on fruit surface developed within 6 h. When shelf life evaluation was extended to 12 h, the initial bright colour of control fruit had largely disappeared and about 14% of the fruit had begun to rot (Fig. 1B). However, the pulp of the control fruit was edible by the end of the 18-h shelf life evaluation, with an eating
Browning index
4 3 2 1 0 (B)
30 25 20 15 10 5 0
(C)
10
2.6. Data handling
8 Eating quality
In all experiments, fruit were arranged in a completely randomized design, and each treatment comprised of 300 replicate fruit for evaluations of appearance and disease incidence, six replicates for assessments of eating quality and concentrations of total soluble solids and titratable acidity, or three replicates for measurements of PPO activity and anthocyanin concentration. Data were subject to analysis of variance (ANOVA) using Genstat Version 5. Data were presented in figures as means 7 standard errors.
(A)
5
Disease incidence (%)
homogenate was centrifuged at 19,000 g (Beckman J20-2) for 20 min at 4 1C and then the supernatant collected as the crude enzyme extract. PPO activity was assayed by the method of Zauberman et al. (1991). The assay of PPO activity was performed using 1.0 ml of 0.1 M phosphate buffer (pH 6.8), 0.5 ml of 0.1 M 4-methylcatechol, and 0.5 ml of enzyme solution. The increase in absorbance at 410 nm at 25 1C was automatically recorded for 3 min (Beckman DU-7). One unit of enzyme activity was defined as the amount, which caused a change of 0.001 in absorbance per minute. The protein content was determined by the method of Bradford (1976) with albumin bovine serum as the standard.
759
6 4 2 0 0
6 12 Shelf time (hours)
18
Fig. 1. Effects of chitsoan coating on browning index, disease incidence and eating quality of cold-stored litchi fruit during shelf life evaluation at ambient temperature. Each point indicates the mean and standard error for browning index and disease incidence and for eating quality. Fruits were stored for 20 days at 2 1C and then held for 18 h at 25 1C and 80–90% RH. K, Control; ’, 2 g chitosan/100 g solution.
quality of 7.2 (Fig. 1C). Thus, the shelf life of the control fruit was about 6 h, in terms of skin browning and disease development. Treatment with chitsoan coating delayed the browning, with a browning index being 1.9 after 12 h of shelf life evaluation, while the browning index of the control fruit was 2.5. Furthermore, the chitosan-treated fruit did not rot until 12 h of shelf life evaluation. Chitosan coating has shown to inhibit fungal growth on some fruits (El Ghaouth et al., 1992; Li & Yu, 2001;
ARTICLE IN PRESS Y. Jiang et al. / LWT 38 (2005) 757–761
3.2. Concentrations of total soluble solids and titratable acidity The concentrations of titratable acidity and total soluble solids decreased over time during shelf life evaluation (Fig. 3), which was consistent with the decline in eating quality (Fig. 1C). Treatment with chitosan coating reduced the decrease in concentrations of total soluble solids and titratable acidity. Du et al. (1997) reported that application of chitosan coating inhibited respiration rates of peach, Japanese pear and kiwifruit. In this study, a lower level of respiration rate of the chitosan-treated litchi fruit after 20 days of storage at 2 1C was observed (data not shown). Thus, the higher levels of total soluble solids and titratable acidity in the pulp of the litchi fruit coated with chitosan may be due to protective O2 barrier or reduction of oxygen supply on the fruit surface which inhibited respiration (Jiang & Li, 2000; Yonemoto et al., 2002). The impact of the chitosan on O2 and CO2 concentrations in relation to fruit respiration requires investigation. 3.3. PPO activity Lee and Wicker (1991) and Jiang (2000) reported that litchi PPO involved in the degradation of anthocyanins and the oxidation of phenolics, resulting in the formation of brown by-products. Browning has been studied in other fruits, and discoloration correlated well with PPO activity (Coseteng & Lee, 1987). The treatment with chitosan coating markedly reduced PPO activity during shelf life evaluation (Fig. 2A), which was associated with delayed skin browning of litchi fruit (Fig. 1A). The inhibition of the PPO activity by chitosan coating was also observed with longan fruit and freshcut Chinese water chestnut stored at low temperature (Jiang & Li, 2000; Peng & Jiang, 2003), and it may be partially attributed to enhanced antioxidant activity of skin tissue (Jiang et al., 2003) but merits investigation (Fig. 3). 3.4. Anthocyanin content Bright red colour of litchi fruit has mainly been attributed to anthocyanin content in skin (Jiang, 2000). In this study, skin anthocyanin content decreased gradually during evaluation (Fig. 2B). The chitsoancoated fruit had higher anthocyanin content. Zhang,
16
(A)
14 Total soluble solids (˚Brix)
Romanazzi et al., 2002). As attack by pathogens is a major factor causing discoloration of harvested litchi fruit (Jiang & Li, 2003), the delay in the skin browning of the cold-stored litchi fruit at ambient temperature by chitosan coating could be partially due to decay control in this study (Fig. 1A and B).
12 10 8 6 4 2 0 (B)
0.7 0.6 Titratable acidity (g/100 gFW)
760
0.5 0.4 0.3 0.2 0.1 0.0 0
6 12 Storage time (h)
18
Fig. 2. Effects of chitsoan coating on concentrations of total soluble solids and titratable acidity of cold-stored litchi fruit during shelf life evaluation at ambient temperature. Each point indicates the mean and standard error. Fruit were stored for 20 days at 2 1C and then held for 18 h at 25 1C and 80–90% RH. K, Control; ’, 2 g chitosan/100 g solution.
Pang, Ji, and Jiang (2001) reported that PPO involved in anthocyanin degradation system. The maintenance of the skin colour of the litchi fruit by chitsoan coating may be accounted for the higher level of anyhocyanin content in the skin resulting from the inhibition of the PPO activity (Figs. 1A and 2A). In conclusion, the shelf life of the litchi fruit after 20 days of storage at 2 1C was about 6 h at ambient temperature, in terms of skin browning and disease development. Application of chitosan coating delayed skin browning and maintained higher concentrations of total soluble solids and titratable acidity, with a shelf life of 12 h, which suggested that the treatment with chitosan coating exhibited a potential for shelf life extension at ambient temperature when litchi fruit were removed from cold storage. In view of effects of chitosan coating on protective O2 barrier on fruit surface, more researches including application of different concentrations of chitosan and the impact of the chitosan on O2 and CO2 concentrations in relation to fruit respiration are needed to conduct in the future. Furthermore, the mechanism of the inhibition of the PPO activity by chitosan coating requires investigation to improve response.
ARTICLE IN PRESS Y. Jiang et al. / LWT 38 (2005) 757–761
1.8
(A)
PPO activity (x103 unit/mg protein)
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 (B)
Anthocyanin content (530 nm/g FW)
1.4 1.2
S h e lf t im e (h)
1.0 0.8 0.6 0.4 0.2 0.0 0
6
12
18
Storage time (h) Fig. 3. Effects of chitsoan coating on PPO activity and anthocyanin content of cold-stored litchi fruit during shelf life evaluation at ambient temperature. Each point indicates the mean and standard error. Fruit were stored for 20 days at 2 1C and then held for 18 h at 25 1C and 80–90% RH. K, Control; ’, 2 g chitosan/100 g solution.
Acknowledgements This work was supported by the International Foundation for Science (Grant No. E2265/3F) and the National Natural Science Foundation of China (Grant No. 39900102). References Amarante, C., Banks, N. H., & Ganesh, S. (2001). Effects of coating concentration, ripening stage, water status and fruit temperature on pear susceptibility to friction discolouration. Postharvest Biology and Technology, 21, 283–290. Bai, J., Hagenmaier, R. D., & Baldwin, E. A. (2003). Coating selection for ‘Delicious’ and other apples. Postharvest Biology and Technology, 28, 381–390. Banks, N. H. (1984). Some effects of TAL Pro-Long coating on ripening bananas. Journal of Experimental Botany, 35, 127–137. Bradford, M. M. (1976). A rapid and sensitive method for the quantitation of microgram quantities of protein utilising the principle of protein-dye binding. Analytical Biochemistry, 72, 248–254. Cisneros-Zevallos, L., & Krochta, J. M. (2002). Internal modified atmospheres of coated fresh fruits and vegetables: understanding relative humidity effects. Journal of Food Science, 67, 1990–1994. Coseteng, M. Y., & Lee, C. Y. (1987). Changes in polyphenol oxidase and polyphenol concentrations in relation to the degree of browning. Journal of Food Science, 52, 985–989.
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Du, J. M., Gemma, H., & Iwahori, S. (1997). Effects of chitosan coating on the storage of peach, Japanese pear, and kiwifruit. Journal of the Japanese Society for Horticultural Science, 66, 15–22. El Ghaouth, A., Arul, J., Ponnamapalam, R., & Boulet, M. (1991). Chitosan coating effect on storability and quality of fresh strawberries. Journal of Food Science, 56, 1618–1620. El Ghaouth, A., Arul, J., Grenier, J., & Asselin, A. (1992). Antifungal activity of chitosan on two postharvest pathogens of strawberry fruits. Phytopathology, 82, 398–402. Hirano, S., Itakura, C., Seino, H., Akiyama, Y., Nonata, I., Kanbara, N., & Kawahami, T. (1990). Chitosan as an ingredient for domestic animal feeds. Journal of Agriculture and Food Chemistry, 38, 1214–1217. Holcroft, D. M., & Mitcham, E. J. (1996). Postharvest physiology and handling of litchi (Litchi chinensis Sonn.). Postharvest Biology and Technology, 9, 265–281. Jiang, Y. M. (2000). Role of anthocyanins, polyphenol oxidase and phenols in lychee pericarp browning. Journal of the Science of Food and Agriculture, 80, 305–310. Jiang, Y. M., & Li, Y. B. (2000). Effects of chitosan coating on postharvest life and quality of longan fruit. Food Chemistry, 73, 139–143. Jiang, Y. M., & Li, Y. B. (2003). Effects of low temperature acclimation on browning of litchi fruit in relation to shelf life. Journal of Horticultural Science and Biotechnology, 78, 437–440. Jiang, Y. M., Yao, L. H., Lichter, A., & Li, J. R. (2003). Postharvest biology and technology of litchi fruit. Journal of Food, Agriculture and Environment, 1(2), 76–81. Lee, H. S., & Wicker, J. (1991). Quantitative changes in anthocyanin pigments of lychee fruit during refrigerated storage. Food Chemistry, 40, 263–270. Lee, J. Y., Park, H. J., Lee, C. Y., & Choi, W. Y. (2003). Extending shelf-life of minimally processed apples with edible coatings and antibrowning agents. Food Science and Technology, 36, 323–329. Li, H., & Yu, T. (2001). Effect of chitosan on incidence of brown rot, quality and physiological attributes of postharvest peach fruit. Journal of the Science of Food and Agriculture, 81, 269–274. Nip, W. K. (1988). Handling and preservation of lychee (Litchi chinensis Sonn.) with emphasis on colour retention. Tropical Science, 28, 5–11. Peng, L. L., & Jiang, Y. M. (2003). Effects of chitosan coating on shelf life and quality of fresh-cut Chinese water chestnut. LebensmittelWissenschaft und Technologie, 36, 359–364. Romanazzi, G., Nigro, F., Ippolito, A., Di Venere, D., & Salerno, M. (2002). Effects of pre- and postharvest chitosan treatments to control storage grey mold of table grapes. Journal of Food Science, 67, 1862–1867. Romanazzi, G., Nigro, F., & Ippolito, A. (2003). Short hypobaric treatments potentiate the effect of chitosan in reducing storage decay of sweet cherries. Postharvest Biology and Technology, 29, 73–80. Yonemoto, Y., Higuchi, H., & Kitano, Y. (2002). Effects of storage temperature and wax coating on ethylene production, respiration and shelf-life in cherimoya fruit. Journal of the Japanese Society for Horticultural Science, 71, 643–650. Zauberman, G., Ronen, R., Akerman, M., Weksler, A., Rot, I., & Fuchs, Y. (1991). Postharvest retention of the red colour of litchi fruit pericarp. Scientia Horticulturae, 47, 89–97. Zhang, D. L., & Quantick, P. C. (1997). Effects of chitosan coating on enzymatic browning and decay during postharvest storage of litchi (Litchi chinensis Sonn.) fruit. Postharvest Biology and Technology, 12, 195–202. Zhang, Z. Q., Pang, X. P., Ji, Z. L., & Jiang, Y. M. (2001). Role of anthocyanin degradation in litchi pericarp browning. Food Chemistry, 75, 217–221.
LWT 38 (2005) 757–761 www.elsevier.com/locate/lwt
Effects of chitosan coating on shelf life of cold-stored litchi fruit at ambient temperature Yueming Jianga,b,, Jianrong Lib, Weibo Jiangc a
College of Food Science, Biotechnology and Environmental Engineering, Zhejiang Gongshang University, Hangzhou 310035, PR China b South China Botanic Garden, The Chinese Academy of Sciences, Guangzhou, LeYiJu 510650, PR China c School of Food Science, China Agricultural University, P.O. Box 204, Qinghua Donglu, Haidian, Beijing 100083, PR China Received 4 May 2004; received in revised form 2 September 2004; accepted 14 September 2004
Abstract Postharvest pericarp browning of litchi fruit results in an accelerated loss in shelf life and a reduced commercial value. Visual quality was lost in only 6 h at ambient temperature when fruit were removed from storage at 2 1C, due to browning. The experiment was conducted to test the role of chitosan coating in inhibiting skin browning and extending shelf life of cold-stored litchi fruit at ambient temperature. Litchi fruit were treated with 2 g chitosan/100 g solution and then stored for 20 days at 2 1C and 90–95% relative humidity (RH), prior to shelf life evaluation at 25 1C and 80–90% RH. Changes in polyphenol oxidase (PPO) activity, anthocyanin concentration, colour index, eating quality and concentrations of total soluble solids and titratable acidity were measured. The effects of chitosan coating on disease incidence were also evaluated. Application of chitosan coating delayed the decrease in anthocyanin content, the increase in PPO activity and the changes in colour index and eating quality, reduced the decrease in concentrations of total soluble solids and titratable acidity, and partially inhibited decay. The results suggested that treatment with chitosan coating exhibited a potential for shelf life extension at ambient temperature when litchi fruit were removed from cold storage. r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Chitosan; Coating; Fruit; Litchi; Shelf life
1. Introduction Litchi (Litchi chinensis Sonn.) is a tropical and subtropical fruit of high commercial value for its white, translucent aril and attractive red colour (Holcroft & Mitcham, 1996). Although litchi fruit can be stored for about 20 days at a low temperature range of 2–5 1C, the fruit can deteriorate rapidly due to peel browning when removed from the cold storage (Jiang & Li, 2003). Thus, the major limitation in litchi marketing is the rapid lose of the red color after harvest (Nip, 1988; Jiang, Yao, Corresponding author. Department of Plant Resources, South China Institute of Botany, Guangzhou, LeYiJu 510650, PR China. Tel.: +86 20 37252525; fax: +86 20 37252831. E-mail address: [email protected] (Y. Jiang).
Lichter, & Li, 2003). Postharvest browning of litchi fruit was generally thought to be a rapid degradation of anthocyanins caused by polyphenol oxidase (PPO), producing brown by-products (Lee & Wicker, 1991; Jiang, 2000). Postharvest treatments, such as sulphur fumigation and acid dip can effectively inhibited PPO activity and thus delay loss of red skin colour of litchi fruit (Zauberman et al., 1991). However, alternative chemicals for colour control without toxic effects in harvested litchi fruit are needed because of concerns for food safety and restrictions in the use of chemicals (Jiang et al., 2003). Application of semi-permeable coatings with modified atmosphere of CO2 and O2 under small storage environment conditions has been shown to improve the storability of perishable crops (Cisneros-Zevallos &
0023-6438/$30.00 r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2004.09.004
ARTICLE IN PRESS 758
Y. Jiang et al. / LWT 38 (2005) 757–761
Krochta, 2002; Lee, Park, Lee, & Choi, 2003). For example, coating treatments delayed fruit ripening of cherimoya, banana and pear through modifying the internal atmospheres (Banks, 1984; Amarante, Banks, & Ganesh, 2001; Yonemoto, Higuchi, & Kitano, 2002). Similar results were obtained in pears and apples using edible coatings (Amarante et al., 2001; Bai, Hagenmaier, & Baldwin, 2003). Chitosan (a high molecular weight cationic polysaccharide) is soluble in dilute organic acids and could theoretically be used as a preservative coating material for fruits. The coating is also safe (Hirano et al., 1990) and shows antifungal activity against several fungi (El Ghaouth, Arul, Grenier, & Asselin, 1992; Li & Yu, 2001; Romanazzi, Nigro, Ippolito, Di Venere, & Salerno, 2002). Previous studies indicated that chitosan coating had the potential to prolong storage life and control decay of many fruits, such as strawberry, peach and table grape (Du, Gemma, & Iwahori, 1997; El Ghaouth, Arul, Ponnamapalam, & Boulet, 1991; Romanazzi, Nigro, & Ippolito, 2003). Zhang and Quantick (1997) and Jiang and Li (2000) reported that application of 2 g chitosan/100 g solution was the most effective in delaying browning when litchi and longan fruits were stored at low temperature. However, the effect of the chitosan coating on shelf life at ambient temperature of litchi fruit removed from cold storage has not been investigated. The objective of this study was to examine the effects of the treatment with 2 g chitosan/100 g solution on the shelf life of the cold-stored litchi fruit at ambient temperature.
control. The treated and control fruit were packaged in plastic boxes (18 26 32 cm; 300 fruit/box), then overwrapped with plastic bags, and finally stored for 20 days at 271 1C and 90–95% relative humidity (RH). After 20 days of storage, the fruit were removed from cold storage and then held for 18 h under ambient conditions (25 1C, 80–90% RH) for evaluation of fruit quality, concentrations of total soluble solids and titratable acidity, PPO activity and anthocyanin content. The experiments were conducted in sequential 2 years. Similar results were obtained from the two experiments. The data from the experiment in 2002 were presented. 2.2. Fruit quality evaluation Appearance was assessed by measuring the extent of the total browned area on each fruit pericarp, using 300 fruit during shelf life evaluation, on the following scale: 1=no browning (excellent quality); 2=slight browning; 3=o1/4 browning; 4=1/4–1/2 browning; 5=41/2 browning (poor quality). The browning index was calculated as S (browning scale percentage of corresponding fruit within each class). Fruit at higher than 2.0 (browning index) was considered unacceptable for marketing. Disease incidence was monitored by randomly collecting 300 fruit and then recording the percentage of fungal growth or bacterial lesions on the fruit surface. Eating quality of fruit pulp was assessed hedonically using a trained six-member panel. At each withdrawal, 30 fruit were randomly selected and rated on the scale of 1=poor to 9=excellent. Fruit at higher than a scale of 6.0 was considered for consumer acceptance.
2. Materials and methods 2.1. Plant materials and treatments Fruit of litchi (Litchi chinensis Sonn.) cv. Huaizhi at the commercially mature stage were harvested from an orchard in Guangzhou. Fruit were selected for uniformity, shape, colour, and size, and any blemished or diseased fruit discarded. The fruits (50 kg) were randomly distributed into two groups prior to treatments. To prepare 3000 ml of 2 g chitosan/100 g solution, 60.0 g of chitosan (Crab shell chitosan, Sigma Chemicals) was dispersed in 2500 ml of distilled water to which 150 ml of glacial acetic acid was added to dissolve the chitosan. The pH of the solution was adjusted to pH 5.0 with 1 mol/l NaOH and the solution was made up to 3000 ml. Fruit were allowed to dry for 4 h at 25 1C after 1 min dipping. In this study, 2 g chitosan/100 g solution was used because the concentration was the most effective in delaying browning and extending storage life when litchi and longan fruits were stored at low temperature (Zhang & Quantick, 1997; Jiang & Li, 2000). Fruits dipped in the acid solution without chitosan, pH 5.0, were used as
2.3. Measurements of total soluble solids and titratable acidity The concentrations of total soluble solids and titratable acidity were analysed during shelf life evaluation. Visible fugal mycelia on fruit surface were removed carefully using a rod with a small cotton ball. Pulp (20 g) from 15 fruit was homogenized in a grinder and then centrifuged at 15,000 g (Beckman J20-2) for 20 min. The supernatant phase was collected to analyse for: total soluble solids, 1Brix, using a hand refractometer (J1-3A, Guangdong Scientific Instruments) and titratable acidity, g citric acid on a fresh weight (FW) basis, determined by titration with 0.1 M NaOH. 2.4. Enzyme assay and protein determination Visible fugal mycelia on fruit surface were removed carefully using a rod with a small cotton ball. Peel (6.0 g) from 15 fruit was homogenized in 30 ml of 0.02 M phosphate buffer (pH 6.8) containing 0.6 g of polyvinylpyrrolidone (insoluble) in an ice bath. The
ARTICLE IN PRESS Y. Jiang et al. / LWT 38 (2005) 757–761
2.5. Anthocyanin assay Visible fugal mycelia on fruit surface were removed carefully using a rod with a small cotton ball. Litchi pericarp (10 g) from 15 fruit was finely sliced and extracted with 200 ml of 0.1% HCl-methanol (1 ml HCl in 99 ml methanol) for 2 h, according to the method of Jiang (2000). The extract was filtered and diluted, and absorbance measured at 530 nm using a spectrophotometer (Beckman DU-7). Anthocyanin concentration was expressed as absorbance at 530 nm/g fresh weight.
6
3. Results and discussion 3.1. Fruit quality The chitosan-untreated (control) and chitosan-treated fruit stored for 20 days at 2 1C had red colour and no disease development on the fruit surface. Removal from cold storage, the fruit browned and eating quality decreased gradually (Fig. 1A and C). However, no diseases on fruit surface developed within 6 h. When shelf life evaluation was extended to 12 h, the initial bright colour of control fruit had largely disappeared and about 14% of the fruit had begun to rot (Fig. 1B). However, the pulp of the control fruit was edible by the end of the 18-h shelf life evaluation, with an eating
Browning index
4 3 2 1 0 (B)
30 25 20 15 10 5 0
(C)
10
2.6. Data handling
8 Eating quality
In all experiments, fruit were arranged in a completely randomized design, and each treatment comprised of 300 replicate fruit for evaluations of appearance and disease incidence, six replicates for assessments of eating quality and concentrations of total soluble solids and titratable acidity, or three replicates for measurements of PPO activity and anthocyanin concentration. Data were subject to analysis of variance (ANOVA) using Genstat Version 5. Data were presented in figures as means 7 standard errors.
(A)
5
Disease incidence (%)
homogenate was centrifuged at 19,000 g (Beckman J20-2) for 20 min at 4 1C and then the supernatant collected as the crude enzyme extract. PPO activity was assayed by the method of Zauberman et al. (1991). The assay of PPO activity was performed using 1.0 ml of 0.1 M phosphate buffer (pH 6.8), 0.5 ml of 0.1 M 4-methylcatechol, and 0.5 ml of enzyme solution. The increase in absorbance at 410 nm at 25 1C was automatically recorded for 3 min (Beckman DU-7). One unit of enzyme activity was defined as the amount, which caused a change of 0.001 in absorbance per minute. The protein content was determined by the method of Bradford (1976) with albumin bovine serum as the standard.
759
6 4 2 0 0
6 12 Shelf time (hours)
18
Fig. 1. Effects of chitsoan coating on browning index, disease incidence and eating quality of cold-stored litchi fruit during shelf life evaluation at ambient temperature. Each point indicates the mean and standard error for browning index and disease incidence and for eating quality. Fruits were stored for 20 days at 2 1C and then held for 18 h at 25 1C and 80–90% RH. K, Control; ’, 2 g chitosan/100 g solution.
quality of 7.2 (Fig. 1C). Thus, the shelf life of the control fruit was about 6 h, in terms of skin browning and disease development. Treatment with chitsoan coating delayed the browning, with a browning index being 1.9 after 12 h of shelf life evaluation, while the browning index of the control fruit was 2.5. Furthermore, the chitosan-treated fruit did not rot until 12 h of shelf life evaluation. Chitosan coating has shown to inhibit fungal growth on some fruits (El Ghaouth et al., 1992; Li & Yu, 2001;
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3.2. Concentrations of total soluble solids and titratable acidity The concentrations of titratable acidity and total soluble solids decreased over time during shelf life evaluation (Fig. 3), which was consistent with the decline in eating quality (Fig. 1C). Treatment with chitosan coating reduced the decrease in concentrations of total soluble solids and titratable acidity. Du et al. (1997) reported that application of chitosan coating inhibited respiration rates of peach, Japanese pear and kiwifruit. In this study, a lower level of respiration rate of the chitosan-treated litchi fruit after 20 days of storage at 2 1C was observed (data not shown). Thus, the higher levels of total soluble solids and titratable acidity in the pulp of the litchi fruit coated with chitosan may be due to protective O2 barrier or reduction of oxygen supply on the fruit surface which inhibited respiration (Jiang & Li, 2000; Yonemoto et al., 2002). The impact of the chitosan on O2 and CO2 concentrations in relation to fruit respiration requires investigation. 3.3. PPO activity Lee and Wicker (1991) and Jiang (2000) reported that litchi PPO involved in the degradation of anthocyanins and the oxidation of phenolics, resulting in the formation of brown by-products. Browning has been studied in other fruits, and discoloration correlated well with PPO activity (Coseteng & Lee, 1987). The treatment with chitosan coating markedly reduced PPO activity during shelf life evaluation (Fig. 2A), which was associated with delayed skin browning of litchi fruit (Fig. 1A). The inhibition of the PPO activity by chitosan coating was also observed with longan fruit and freshcut Chinese water chestnut stored at low temperature (Jiang & Li, 2000; Peng & Jiang, 2003), and it may be partially attributed to enhanced antioxidant activity of skin tissue (Jiang et al., 2003) but merits investigation (Fig. 3). 3.4. Anthocyanin content Bright red colour of litchi fruit has mainly been attributed to anthocyanin content in skin (Jiang, 2000). In this study, skin anthocyanin content decreased gradually during evaluation (Fig. 2B). The chitsoancoated fruit had higher anthocyanin content. Zhang,
16
(A)
14 Total soluble solids (˚Brix)
Romanazzi et al., 2002). As attack by pathogens is a major factor causing discoloration of harvested litchi fruit (Jiang & Li, 2003), the delay in the skin browning of the cold-stored litchi fruit at ambient temperature by chitosan coating could be partially due to decay control in this study (Fig. 1A and B).
12 10 8 6 4 2 0 (B)
0.7 0.6 Titratable acidity (g/100 gFW)
760
0.5 0.4 0.3 0.2 0.1 0.0 0
6 12 Storage time (h)
18
Fig. 2. Effects of chitsoan coating on concentrations of total soluble solids and titratable acidity of cold-stored litchi fruit during shelf life evaluation at ambient temperature. Each point indicates the mean and standard error. Fruit were stored for 20 days at 2 1C and then held for 18 h at 25 1C and 80–90% RH. K, Control; ’, 2 g chitosan/100 g solution.
Pang, Ji, and Jiang (2001) reported that PPO involved in anthocyanin degradation system. The maintenance of the skin colour of the litchi fruit by chitsoan coating may be accounted for the higher level of anyhocyanin content in the skin resulting from the inhibition of the PPO activity (Figs. 1A and 2A). In conclusion, the shelf life of the litchi fruit after 20 days of storage at 2 1C was about 6 h at ambient temperature, in terms of skin browning and disease development. Application of chitosan coating delayed skin browning and maintained higher concentrations of total soluble solids and titratable acidity, with a shelf life of 12 h, which suggested that the treatment with chitosan coating exhibited a potential for shelf life extension at ambient temperature when litchi fruit were removed from cold storage. In view of effects of chitosan coating on protective O2 barrier on fruit surface, more researches including application of different concentrations of chitosan and the impact of the chitosan on O2 and CO2 concentrations in relation to fruit respiration are needed to conduct in the future. Furthermore, the mechanism of the inhibition of the PPO activity by chitosan coating requires investigation to improve response.
ARTICLE IN PRESS Y. Jiang et al. / LWT 38 (2005) 757–761
1.8
(A)
PPO activity (x103 unit/mg protein)
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 (B)
Anthocyanin content (530 nm/g FW)
1.4 1.2
S h e lf t im e (h)
1.0 0.8 0.6 0.4 0.2 0.0 0
6
12
18
Storage time (h) Fig. 3. Effects of chitsoan coating on PPO activity and anthocyanin content of cold-stored litchi fruit during shelf life evaluation at ambient temperature. Each point indicates the mean and standard error. Fruit were stored for 20 days at 2 1C and then held for 18 h at 25 1C and 80–90% RH. K, Control; ’, 2 g chitosan/100 g solution.
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