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Effects of chitosan coating on mass transfer during osmotic dehydration of papaya
Food Research International 43 (2010) 1656–1660
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Food Research International journal homepage: www.elsevier.com/locate/foodres
Effects of chitosan coating on mass transfer during osmotic dehydration of papaya M. García *, R. Díaz, Y. Martínez, A. Casariego Pharmacy and Food Institute, University of Havana, Ave. 23 No. 21425, Havana, CP 13600, Cuba
a r t i c l e
i n f o
Article history: Received 11 March 2010 Accepted 10 May 2010
Keywords: Papaya Carica papaya L. Osmotic dehydration Chitosan coating
a b s t r a c t The aim of this work was to evaluate the influence of chitosan coatings in the osmotic dehydration of scalded cut papaya var. Red Maradol in two ripening stages (green and ripped). Papaya cubic cuts (1 cm3) were divided into three groups depending on the treatments: without chitosan coatings; with chitosan coatings at 1% (w/v) in lactic acid 1% (v/v) and Tween 80 at 0.1% (v/v); and with chitosan coatings at 1% (w/v) in lactic acid 1% (v/v), Tween 80 at 0.1% (v/v) and oleic acid at 2% (v/v). The study of dehydration kinetics and mass transfer was carried out with osmotic solution of sucrose (40°Brix) in a ratio fruit/solution of 1:60, and weight reduction, water loss and solids gain were measured. Chitosan coatings improved the efficiency of osmotic dehydration process in both ripening stages, increasing the water loss and decreasing the solids gain. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction In developing countries, postharvest losses can reach between 25% and 50% (FAO, 2002) due to physiological disorders and lack of adequate food storage technologies, which generates that food industry, is researching on alternative preservation methods for vegetables, including the osmotic dehydration process. Regarding the practical development of osmotic process, various scientific works have described the behavior of different products during the treatment. This behavior is variable from one product to another, according to their composition and their ultra-structural organization. In this respect, the main process variables largely studied, and which control the transfer mechanism between the product and the osmotic solution are the composition and concentration of the osmotic solution (Kwang Sup & Yong Hee, 1995; Raoult-Wack, Botz, Guilbert, & Ríos, 1991), the molar mass of solutes in the solution (Lenart, 1992; Saurel, Raoult-Wack, Rios, & Guilbert, 1994), the specific surface of the sample and the temperature of treatment (Raoult-Wack et al., 1991). Osmotic dehydration is widely used to remove water from fruits and vegetables by immersion in aqueous solution of sugars and/or salts at high concentration. This process is usually used to partially remove water from vegetable tissues obtaining stabilization without acidification or pasteurization treatments. Moreover, osmotic dehydration is used as a pre-treatment prior to freezing, freeze drying, vacuum drying and air drying (Dixon & Jen, 1977; Hawkes & Flink, 1978; Nanjundaswamy, Radhakrishnaiah, Balachandran, Saroja, & Murthy, 1978; Ponting, 1973). The effects of process variables such as temperature, solution concentration, type of osmotic agent, * Corresponding author. Tel.: +53 7 271 6389. E-mail address: [email protected] (M. García). 0963-9969/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2010.05.002
size and shape, rate of agitation, solid–solution mass fraction and level of vacuum on water loss and solid gain of food have been reported (Dixon & Jen, 1977; Giangiacomo, Torreggiani, & Abbo, 1987; Lerici, Pinnavaia, Dalla-Rosa, & Bartolucci, 1985). Besides, it was reported that the use of chitosan coatings can improve the efficiency of osmotic dehydration process (Díaz, 2003). Chitosan, a linear polymer of 2-amino-2-deoxy-b-D-glucan, is a deacetylated form of chitin, a naturally occurring cationic biopolymer (Airoldi, 2008; Lin & Zhao, 2007). It occurs as a shell component of crustaceans, as the skeletal substance of invertebrates, and as the cell wall constituent of fungi and insects. Applications of chitosan include as flocculating agent, clarifier, thickener, gasselective membrane, coating material, promoter of plant disease resistance, wound-healing factor agent, and antimicrobial agent (Dong et al., 2000; Garcia, 2008; Garcia et al., 2008). Chitosan has recently been approved by the authorities for its use in pharmaceutical forms, and a monograph relating to chitosan hydrochloride was included in the European Pharmacopoeias (2005). The objective of the study was to evaluate the influence of chitosan coatings in the mass transfer during osmotic dehydration of papaya. The process variable considered in the study is the nature of used coatings from chitosan-Tween 80 solution and chitosan lipid emulsion. The main goal of the work has then been to differentiate material transfer during treatment through measurement of water loss, weight loss and solubles exchange. 2. Materials and methods 2.1. Raw material Papayas (Carica papaya L.) var. Red Maradol selected on the basis of two ripening degree, green and ripped, and commercial
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sucrose were purchased in local market. After washing, papayas were cut using a knife into a cubic shape (1 cm3). All samples had a weight (mean (standard deviation)) of 2.4 (0.6) and 2.2 (0.5) g for green and ripped papayas, respectively. Then fruit samples were scalded in calcium chloride solution at 0.4% to 65 °C for 60 min using solution/samples mass relation of 1/5. 2.2. Preparation of films and coatings-forming solutions Materials used for preparing chitosan films and coatings were: chitosan (Table 1) obtained at Medication Research and Development Center (Cuba), lactic acid (Merck, Germany), Tween 80 (Acros Organics, Belgium), and distilled water. Chitosan (1%, w/v) was dispersed in an aqueous solution of lactic acid (1%, v/v), at 40 °C, since chitosan is only soluble in an acidic medium. Then, Tween 80 at 0.1% (v/v) was added for improving wettability. The resulting mixture was stirred vigorously with heating using a magnetic stirrer during 60 min until chitosan was dissolved. Besides, chitosan lipid emulsion was prepared by addition of oleic acid in the previously mixture to reach a final concentration of 2% (v/v). These mixtures were emulsified at 13,500 rpm for 4 min (Vargas, Albors, Chiralt, & González-Martínez, 2006). Both, chitosan-Tween 80 solution and chitosan lipid emulsion, were stirred and then cast onto a glass plate. The films were dried in an oven at 35 °C during 8 h. Dried films were peeled from the plate and cut in circles with 5.5 cm of diameter, approximately, for property testing. 2.2.1. Film solubility The film solubility in water was determined according to the method reported by Cuq, Gontard, Cuq, and Guilbert (1997). It was defined by the content of dry matter solubilized after 24 h of immersion in water. The initial dry matter content of each film was determined by drying to constant weight in an oven at 105 °C. Two disks of film (5.5 cm, diameter) were cut, weighed, and immersed in 50 mL of water. After 24 h of immersion at 20 °C with occasional agitation, the pieces of films were taken out and dried to constant weight in an oven at 105 °C, to determine the weight of dry matter which was not solubilized in water. 2.2.2. Film solids gain from syrup The initial dry matter content of each film was determined by drying to constant weight in an oven at 105 °C. Two disks of film (5.5 cm, diameter) were cut, weighed, and immersed in 50 mL of syrup. After 24 h of immersion in syrup (40°Brix) at 20 °C with occasional agitation, the pieces of films were taken out and dried to constant weight in an oven at 105 °C, to determine the final weight of dry matter. Then, the film solids gain in syrup was evaluated as the difference between initial and final dry matter content of each film. 2.3. Coatings Coatings were applied by double immersion of papaya samples in the films-forming solutions, chitosan-Tween 80 solution or
Table 1 Characteristics of chitosan. Parameters
Percentage (dry basis)
Total solids Insoluble material Amino groups Deacetylation degree
98.9 0.99 7.5 83.2
chitosan lipid emulsion, depending on treatments, during 30 s and coatings were dried by forced convection for 30 min at 25 °C and it was continued in an oven at 40 °C during 30 min. Both papaya samples, green and ripened, were divided, for submitting to osmotic dehydration process, into three groups depending on the treatments: (i) without chitosan coatings; (ii) with chitosan coatings at 1% (w/v) in lactic acid 1% (v/v) and Tween 80 at 0.1% (v/v); and (iii) with chitosan coatings at 1% (w/v) in lactic acid 1% (v/v), Tween 80 at 0.1% (v/v) and oleic acid at 2% (v/v). Osmotic solutions were prepared by dissolving an amount of sucrose in distilled water to obtain a concentration of 40°Brix. A solution: samples mass relation of 60:1 was used to avoid change in osmotic solution concentration during the treatment time. The experiments, carried out in glass beaker, were performed at 25 °C. For each treatment, product samples were maintained under immersion for 1, 2, 3, 4, 5, 24 and 25 h. After immersion time, the dehydrated papaya samples were recuperated on a strainer and washed with tap water for few seconds to remove the adhering osmotic solution and gently blotted with tissue paper. Recuperation of samples and draining of excess water were carried out in a maximum time of 3 min, in order to minimize exchanges between the samples and the ambient air. Water loss (WL), solids gain (SG) and weight reduction (WR) were calculated by the following equations (Beristain, Azuara, Cortés, & García, 1990):
ðM t Þðxw;t Þ ðM 0 Þðxw;0 Þ M0 Mt M0 WR ¼ M0 WL ¼
SG ¼
ðMt ÞðxS;t Þ ðM 0 ÞðxS;0 Þ M0
where M0 is the initial weight of sample (g); Mt is the weight of sample at time t (g); xw,0 is the mass fraction of water initial content; xw,t is the mass fraction of water content at time t; xS,0 is the mass fraction of soluble solids initial content; and xS,t is the mass fraction of soluble solids content at each sampling times. Soluble solids were measured (AOAC, 2003) and moisture content was determined gravimetrically by drying 2.5 g of osmotically dehydrated papaya samples in an oven at 105 °C till a constant weight was measured (AOAC, 2003). All treatments were carried out in duplicate and the average values were used. 2.4. Statistical analysis Analyses of Variance were performed using STATISTICS software (STATISTICS, 1998) and the Duncan’s multiple range tests were used to compare differences among mean values. Mean values were reported, and the significance was defined at p 6 0.05. 3. Results and discussion 3.1. Characterization of fresh papaya Table 2 shows the physico-chemical characteristics of the fresh green and ripened papaya. Chavarro-Castrillón, Ochoa, and AyalaAponte (2006) reported, for green papaya var. Maradol, similar results for soluble solids, acidity, pH and humidity. Also, others authors informed soluble solids values between 9.9 and 12.5°Brix for ripped papaya (Díaz, 2003; Fagundes & Yamanishi, 2001; Leyva, 2002). The difference between these values could be due to that the evaluated papaya samples were not completely homogeneous or presented differences in the ripening degree. On the other hand, the observed difference in these values for the two ripening degree, is according with the physiologic changes that suffer the fruit during its ripening (Gómez, Lajolo, & Cordenunsi, 2002). In this way,
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Table 2 Characterization of fresh papaya (n = 3). Parameters
Table 4 Means for osmotic dehydration of papaya from Duncan’s multiple range tests. Ripening degree
Soluble solids (°Brix) Acidity (% citric acid, w/w) pH Humidity (%, w/w) Penetration distance (1/10 mm) Ripening index
Green
Ripped
3.7 (0.2) 0.1 (0) 5.61 (0.02) 93.2 (0.4) 24 (2) 58 (3)
9.2 (0.6) 0.13 (0) 5.77 (0.04) 89 (1) 122.3 (0.6) 72 (4)
Ripening degree
Treatments
Weight reduction
Solids gain
Water loss
Green
Control Chitosan-Tween 80 Emulsion
0.121941 a 0.217057 b 0.202294 b
0.145354 b 0.097924 a 0.082654 a
0.276123 a 0.336876 b 0.315266 b
Ripped
Control Chitosan-Tween 80 Emulsion
0.076356 a 0.132082 b 0.132182 b
0.157541 c 0.136812 b 0.126726 a
0.256902 a 0.293196 b 0.294240 b
Mean (standard deviation). Different letters (a–c), within a same ripening degree, differ significantly (p 6 0.05).
Mean (standard deviation).
the increase of the penetrated distance is the reflection of the loss of stability that accompanies the fruits ripening process in general. 3.2. Characterization of chitosan films Table 3 shows the humidity, solubility and solids gain values of elaborated films from chitosan-Tween 80 solution and chitosan lipid emulsion, respectively. Prodpran, Benjakaul, and Artharn (2007) studied the effect of the film composition on its solubility and found that the chitosan addition in films-forming solutions based on proteins, decrease the film solubility in certain way when the chitosan concentration was increased, independently of presence or not of oil, which could due to interactions protein/chitosan or between chitosan molecules (Giancone, 2006). Oil addition in the preparation of chitosan lipid emulsion increased the film solubility due to oil drops can insert into matrix film, decreasing the crossed links between proteins or interactions between chitosan molecules (Prodpran et al., 2007). On the other hand, the solids gain from syrup was similar in both films. 3.3. Kinetics of material transfer Combining the resulting values from different times for each variable in every treatment and comparing the resulting average values, it is possible to know if the kind of treatment could affect the obtained results. Table 4 shows the results of Duncan’s multiple range tests for treatments carried out for the evaluation of the kinetics of material transfer during osmotic dehydration process of papaya samples. The results were quality similar for both papaya ripening degrees, observing the same differences between treatments. 3.3.1. Effect of process time The general behavior of material transfer shows (Figs. 1 and 2) the predominance of water removal, as compared to soluble exchanges. The kinetics of material transfer can be divided into three phases: (i) a starting phase (0–2 h) during which exchange rates grow until 2 h; (ii) an acceleration phase (2–5 h) during which the material exchange reaches its maximum value; and (iii) a decreasing phase where exchange values decrease.
Table 3 Characteristics of elaborated films from chitosan (n = 3). Parameters
Weight (g) Humidity (%, w/w) Water solubility (%, w/w) Soluble solids gain (%, w/w) Mean (standard deviation).
Treatments Chitosan-Tween 80
Emulsion
0.207 (0.004) 28.1 (0.8) 30.6 (0.1) 1.2 (0.1)
0.291 (0.004) 11.8 (0.5) 55.7 (0.3) 1.1 (0.3)
Fig. 1. Variation of water loss of papaya samples during osmotic dehydration in sucrose solution 40°Brix (n = 4). (a) Green papaya and (b) ripped papaya.
The observed behavior could be due to that in the initial stage it exists more difference between the chemical potential of the fruit and osmotic solution and, for consequence, it exists more water loss and solids gain, permitting that the diffusion of the molecules is quicker. The decreasing rates could be attributed to a decrease in the concentration gradient and structural changes that occur in the tissues, slowing the diffusion process. The used solution:samples mass relation avoid changes in osmotic solution concentration during the dehydration process, therefore, changes in concentration gradient are due to the changes in the fruit. 3.3.2. Process efficiency The main characteristic of an osmotic dehydration process is the loss of water; however, the solids gain is a parameter to consider, because the process efficiency depends on these two parameters. The weight reduction is also considered an important parameter in order to measure the efficiency of the osmotic process. 3.3.2.1. Water loss. Fig. 1 shows the changes in the water loss during the osmotic dehydration process of green and ripped papaya
M. García et al. / Food Research International 43 (2010) 1656–1660
Fig. 2. Variation of solid gain of papaya samples during osmotic dehydration in sucrose solution 40°Brix (n = 4). (a) Green papaya and (b) ripped papaya.
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Fig. 3. Variation of weight reduction of papaya samples during osmotic dehydration in sucrose solution 40°Brix (n = 4). (a) Green papaya and (b) ripped papaya.
samples with and without chitosan coatings. During the process it was observed a similar behavior in the water loss values in all evaluated treatments, observing an increment in the values of this parameter in the time, reaching the high rate of water loss during the first three hours of the process, in correspondence with Barbosa and Vega (2000), who concluded that the high water loss of in food occurs in the first 6 h of the process, being the two initial hours, those of high rate of water removal. This kinetic tendency was also reported by Nowakunda, Andrés, and Fito (2004) in osmodehydrated banana slices. Green papaya presented higher values of water loss than ripped papaya, what it could be due to that the green fruit presents a higher porosity respect the ripped one, permitting that the contained water inside the cells can be transferred through the pores easily (Chavarro-Castrillón et al., 2006). In both ripening degree, it is observed that uncoated fruit lost less water than coated samples during the osmotic process, according to Díaz (2003).
The gain of solubles by the samples is comparable to weight loss, particularly in the first phase of transfer phenomenon (0– 2 h). In the second phase (2–5 h), the weight loss becomes higher than gain of solubles. As weight loss is the balance between water removal and soluble gain, it could be concluded that, gain of solubles is compensated by the water removal.
3.3.2.2. Soluble solids gain. Fig. 2 shows the percentage of soluble solids gain during the osmotic dehydration process of papaya samples. All evaluated treatments for green and rippened papayas, presented an increment in the values of soluble solids gain. It was observed that coated papaya samples with emulsion presented a minor soluble solids gain than papaya samples without coating. In the case of the treatments with coated papaya, the solids accumulation on the coatings surface, limited its penetration inside the fruits, which it did not happen in samples without coatings, where a great amount of solubles penetrated inside the fruits (Díaz, 2003). The solid accumulation, together with the use of coatings, may create a crust which constitutes a barrier to mass transfer, limiting the dehydration regime and consequently the solubles gain. The above assumptions could explain the difference of dehydration regimes and material transfer between the coated and uncoated papaya samples.
3.3.2.4. Process efficiency index. The values of the process efficiency index (Pr = WL/SG) are used for evaluation of efficiency of osmotic dehydration process (Fig. 4), due to their easy interpretation, because if Pr increases it could mean one of this three possibilities: (i) the process is favoring the water loss and solids gain, but mostly the water loss; (ii) the process limits the solids gain; and (iii) the process favors the water loss. It was observed, in general, that the treatments with coatings presented higher values of Pr during the osmotic dehydration process, what could be due to that in these treatments the water loss was favored, while the soluble solids gain was limited. At the beginning of the process, the changes in the Pr value can be attributable mainly to the water loss, and when the process time increases, the solids gain has more influence on the Pr value. This indicates that upon designing an osmotic dehydration process for this fruit, the time of contact will be defined in function of the pursued objective.
3.3.2.3. Weight reduction. Fig. 3 shows the evolution of weight reduction of the coated and uncoated papaya samples during dehydration process. All treatments, independently of the ripening degree, showed a similar behavior for weight reduction, observing, of general way, that coated papaya samples presented higher values for this parameter than uncoated fruits, which it could due to that the fruits treated with coatings lost more water (Fig. 1) and won less solids than fruits without coatings (Fig. 2). It is agree with the results obtained by Díaz (2003) and Argaiz, Leyva, and Jiménez (2003).
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Fig. 4. Variation of efficiency index during osmotic dehydration of papaya samples in sucrose solution 40°Brix (n = 4). (a) Green papaya and (b) ripped papaya.
4. Conclusions The study showed that chitosan coatings improved the efficiency of osmotic dehydration process, increasing the water loss and decreasing the solids gain. In both ripening stages, the water loss was higher in coated fruits. Emulsified films were more soluble than that elaborated from chitosan solution and Tween 80. Green papaw presented higher values of water loss than the ripped papaya. Chitosan coatings improved the efficiency of osmotic dehydration process in both ripening stages, increasing the water loss and decreasing the solids gain.
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deshidratación osmótica de papaya (Carica papaya L. var., Maradol). Ciência e Tecnologia de Alimentos, 26(3), 596–603. Cuq, B., Gontard, M., Cuq, J., & Guilbert, S. (1997). Functional properties of myofibrillar protein-based biopackaging as affected by film tickness. Journal of Food Science, 61(3), 580–584. Díaz, M. (2003). Evaluación preliminar de la vida de anaquel de papaya tratada osmóticamente con películas de quitosano. Tesis en opción al título académico Ingeniero de Alimentos, Universidad de las Américas, Puebla. Dixon, G. M., & Jen, J. J. (1977). Changes of sugar and acid of osmovac dried apple slices. Journal of Food Science, 42, 1126–1131. Dong, Y., Yuan, Y., Qiu, W., Wu, Y., Wang, M., & Xu, C. (2000). Application of chitin/ chitosan and their derivates in food industry. Food Science and Technology, 5, 28–31. European Pharmacopoeia (2005). European Pharmacopoeia (5th ed.). France: Council of Europe. pp. 1248–1249. Fagundes, G., & Yamanishi, O. (2001). Características físicas e químicas de frutos de mamoeiro do grupo ‘Solo’ comercializados em quatro estabelecimentos de Brasília-df. Brasileira de Fruticultura, 23(3), 345–350. FAO. Manual para el mejoramiento del manejo de frutas y hortalizas [en línea]. Depósito de Documentos de la FAO. Departamento de Agricultura. Fecha de actualización: 2002. Fecha de consulta: 2007. Disponible en:. Garcia, M. (2008). Películas y cubiertas de quitosana en la conservación de vegetales. **Cienc. Tecnol. Alim., 18(1), 71–76. Garcia, M., Diaz, R., Puerta, F., Beldarrain, T., Castillo, A., Gonzalez, J., et al. (2008). Envasado activo de embutidos. Cienc. Tecnol. Alim., 18(2), 1–7. Giancone, T. (2006). Hydrocolloid based edible films: Composition–srtucture– properties relationship. Trabajo de grado, Doctorado en Ciencia y Tecnología Agroalimentaria, Universidad de Estudios de Nápoles, Italia. Giangiacomo, R., Torreggiani, D., & Abbo, E. (1987). Osmotic dehydration of fruit. Part 1: Sugar exchange between fruit and extracting syrup. Journal of Food Processing and Preservation, 11, 183–195. Gómez, M., Lajolo, F., & Cordenunsi, B. (2002). Evolution of soluble sugars during ripening of papaya fruit and its relation to sweet taste. Journal of Food Science, 67(1), 442–447. Hawkes, J., & Flink, J. (1978). Osmotic Concentration on fruit slice prior to freeze dehydration. Journal of Food Processing and Preservation, 2, 265–284. Kwang Sup, S. Y., & Yong Hee, H. C. (1995). Mass transfer characteristics in the osmotic dehydration of carrots. Korean Journal of Food Science and Technology, 27(3), 387–393. Lenart, A. (1992). Mathematical modelling of osmotic dehydration of apple and carrot. Acta Alimentaria Polonica, 42(1), 33–44. Lerici, C., Pinnavaia, G., Dalla-Rosa, M., & Bartolucci, L. (1985). Osmotic dehydration of fruit: Influence of osmotic agents on drying behaviour and product quality. Journal of Food Science, 50, 1217–1219. Leyva, M. (2002). Utilización de las películas hidrofóbicas de quitosano en la deshidratación osmótica de papaya y su efecto en la vida de anaquel. Trabajo de grado. Universidad de Puebla, México. Lin, D., & Zhao, Y. (2007). Innovations in the development and application of edible coatings for fresh and minimally processed fruits and vegetables. Food Science and Food Safety, 6, 60–75. Nanjundaswamy, A. M., Radhakrishnaiah, G., Balachandran, C., Saroja, S., & Murthy, K. B. S. (1978). Studies on development of new categories of dehydrated product from indigenous fruit. Indian Food Packer, 22, 91–93. Nowakunda, K., Andrés, A., & Fito, P. (2004). Osmotic dehydration of banana slices as a pretreatment for drying processes. São Paulo, Brazil. In Proceedings of the 14th international drying symposium. Ponting, J. D. (1973). Osmotic dehydration of fruits – Recent modifications and applications. Process Biochemistry, 8, 18–20. Prodpran, T., Benjakaul, S., & Artharn, A. (2007). Properties and microstructure of protein-based film from round scad (Decapterus maruadsi) muscle as affected by palm oil and chitosan incorporation. International Journal of Biological Macromolecules, 41(5), 605–614. Raoult-Wack, A., Botz, O., Guilbert, S., & Ríos, G. (1991). Simultaneous water and solute transportation in shrinking media-Part 1. Simultaneous dewatering and impregnation soaking process analysis osmotic dehydration. Drying Technololgy, 9(3), 589–612. Saurel, R., Raoult-Wack, A. L., Rios, G., & Guilbert, S. (1994). Mass transfer phenomena during osmotic dehydration of apple. I: Fresh plant tissue. International Journal of Food Science and Technology, 29(5), 531–542. STATISTICA for Windows release 5.1. StatSoft, Inc. Copyright 1998. Vargas, M., Albors, A., Chiralt, A., & González-Martínez, C. (2006). Quality of coldstored strawberries as affected by chitosan–oleic acid edible coatings. Postharvest Biology Technology, 41, 164–171.
Contents lists available at ScienceDirect
Food Research International journal homepage: www.elsevier.com/locate/foodres
Effects of chitosan coating on mass transfer during osmotic dehydration of papaya M. García *, R. Díaz, Y. Martínez, A. Casariego Pharmacy and Food Institute, University of Havana, Ave. 23 No. 21425, Havana, CP 13600, Cuba
a r t i c l e
i n f o
Article history: Received 11 March 2010 Accepted 10 May 2010
Keywords: Papaya Carica papaya L. Osmotic dehydration Chitosan coating
a b s t r a c t The aim of this work was to evaluate the influence of chitosan coatings in the osmotic dehydration of scalded cut papaya var. Red Maradol in two ripening stages (green and ripped). Papaya cubic cuts (1 cm3) were divided into three groups depending on the treatments: without chitosan coatings; with chitosan coatings at 1% (w/v) in lactic acid 1% (v/v) and Tween 80 at 0.1% (v/v); and with chitosan coatings at 1% (w/v) in lactic acid 1% (v/v), Tween 80 at 0.1% (v/v) and oleic acid at 2% (v/v). The study of dehydration kinetics and mass transfer was carried out with osmotic solution of sucrose (40°Brix) in a ratio fruit/solution of 1:60, and weight reduction, water loss and solids gain were measured. Chitosan coatings improved the efficiency of osmotic dehydration process in both ripening stages, increasing the water loss and decreasing the solids gain. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction In developing countries, postharvest losses can reach between 25% and 50% (FAO, 2002) due to physiological disorders and lack of adequate food storage technologies, which generates that food industry, is researching on alternative preservation methods for vegetables, including the osmotic dehydration process. Regarding the practical development of osmotic process, various scientific works have described the behavior of different products during the treatment. This behavior is variable from one product to another, according to their composition and their ultra-structural organization. In this respect, the main process variables largely studied, and which control the transfer mechanism between the product and the osmotic solution are the composition and concentration of the osmotic solution (Kwang Sup & Yong Hee, 1995; Raoult-Wack, Botz, Guilbert, & Ríos, 1991), the molar mass of solutes in the solution (Lenart, 1992; Saurel, Raoult-Wack, Rios, & Guilbert, 1994), the specific surface of the sample and the temperature of treatment (Raoult-Wack et al., 1991). Osmotic dehydration is widely used to remove water from fruits and vegetables by immersion in aqueous solution of sugars and/or salts at high concentration. This process is usually used to partially remove water from vegetable tissues obtaining stabilization without acidification or pasteurization treatments. Moreover, osmotic dehydration is used as a pre-treatment prior to freezing, freeze drying, vacuum drying and air drying (Dixon & Jen, 1977; Hawkes & Flink, 1978; Nanjundaswamy, Radhakrishnaiah, Balachandran, Saroja, & Murthy, 1978; Ponting, 1973). The effects of process variables such as temperature, solution concentration, type of osmotic agent, * Corresponding author. Tel.: +53 7 271 6389. E-mail address: [email protected] (M. García). 0963-9969/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2010.05.002
size and shape, rate of agitation, solid–solution mass fraction and level of vacuum on water loss and solid gain of food have been reported (Dixon & Jen, 1977; Giangiacomo, Torreggiani, & Abbo, 1987; Lerici, Pinnavaia, Dalla-Rosa, & Bartolucci, 1985). Besides, it was reported that the use of chitosan coatings can improve the efficiency of osmotic dehydration process (Díaz, 2003). Chitosan, a linear polymer of 2-amino-2-deoxy-b-D-glucan, is a deacetylated form of chitin, a naturally occurring cationic biopolymer (Airoldi, 2008; Lin & Zhao, 2007). It occurs as a shell component of crustaceans, as the skeletal substance of invertebrates, and as the cell wall constituent of fungi and insects. Applications of chitosan include as flocculating agent, clarifier, thickener, gasselective membrane, coating material, promoter of plant disease resistance, wound-healing factor agent, and antimicrobial agent (Dong et al., 2000; Garcia, 2008; Garcia et al., 2008). Chitosan has recently been approved by the authorities for its use in pharmaceutical forms, and a monograph relating to chitosan hydrochloride was included in the European Pharmacopoeias (2005). The objective of the study was to evaluate the influence of chitosan coatings in the mass transfer during osmotic dehydration of papaya. The process variable considered in the study is the nature of used coatings from chitosan-Tween 80 solution and chitosan lipid emulsion. The main goal of the work has then been to differentiate material transfer during treatment through measurement of water loss, weight loss and solubles exchange. 2. Materials and methods 2.1. Raw material Papayas (Carica papaya L.) var. Red Maradol selected on the basis of two ripening degree, green and ripped, and commercial
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sucrose were purchased in local market. After washing, papayas were cut using a knife into a cubic shape (1 cm3). All samples had a weight (mean (standard deviation)) of 2.4 (0.6) and 2.2 (0.5) g for green and ripped papayas, respectively. Then fruit samples were scalded in calcium chloride solution at 0.4% to 65 °C for 60 min using solution/samples mass relation of 1/5. 2.2. Preparation of films and coatings-forming solutions Materials used for preparing chitosan films and coatings were: chitosan (Table 1) obtained at Medication Research and Development Center (Cuba), lactic acid (Merck, Germany), Tween 80 (Acros Organics, Belgium), and distilled water. Chitosan (1%, w/v) was dispersed in an aqueous solution of lactic acid (1%, v/v), at 40 °C, since chitosan is only soluble in an acidic medium. Then, Tween 80 at 0.1% (v/v) was added for improving wettability. The resulting mixture was stirred vigorously with heating using a magnetic stirrer during 60 min until chitosan was dissolved. Besides, chitosan lipid emulsion was prepared by addition of oleic acid in the previously mixture to reach a final concentration of 2% (v/v). These mixtures were emulsified at 13,500 rpm for 4 min (Vargas, Albors, Chiralt, & González-Martínez, 2006). Both, chitosan-Tween 80 solution and chitosan lipid emulsion, were stirred and then cast onto a glass plate. The films were dried in an oven at 35 °C during 8 h. Dried films were peeled from the plate and cut in circles with 5.5 cm of diameter, approximately, for property testing. 2.2.1. Film solubility The film solubility in water was determined according to the method reported by Cuq, Gontard, Cuq, and Guilbert (1997). It was defined by the content of dry matter solubilized after 24 h of immersion in water. The initial dry matter content of each film was determined by drying to constant weight in an oven at 105 °C. Two disks of film (5.5 cm, diameter) were cut, weighed, and immersed in 50 mL of water. After 24 h of immersion at 20 °C with occasional agitation, the pieces of films were taken out and dried to constant weight in an oven at 105 °C, to determine the weight of dry matter which was not solubilized in water. 2.2.2. Film solids gain from syrup The initial dry matter content of each film was determined by drying to constant weight in an oven at 105 °C. Two disks of film (5.5 cm, diameter) were cut, weighed, and immersed in 50 mL of syrup. After 24 h of immersion in syrup (40°Brix) at 20 °C with occasional agitation, the pieces of films were taken out and dried to constant weight in an oven at 105 °C, to determine the final weight of dry matter. Then, the film solids gain in syrup was evaluated as the difference between initial and final dry matter content of each film. 2.3. Coatings Coatings were applied by double immersion of papaya samples in the films-forming solutions, chitosan-Tween 80 solution or
Table 1 Characteristics of chitosan. Parameters
Percentage (dry basis)
Total solids Insoluble material Amino groups Deacetylation degree
98.9 0.99 7.5 83.2
chitosan lipid emulsion, depending on treatments, during 30 s and coatings were dried by forced convection for 30 min at 25 °C and it was continued in an oven at 40 °C during 30 min. Both papaya samples, green and ripened, were divided, for submitting to osmotic dehydration process, into three groups depending on the treatments: (i) without chitosan coatings; (ii) with chitosan coatings at 1% (w/v) in lactic acid 1% (v/v) and Tween 80 at 0.1% (v/v); and (iii) with chitosan coatings at 1% (w/v) in lactic acid 1% (v/v), Tween 80 at 0.1% (v/v) and oleic acid at 2% (v/v). Osmotic solutions were prepared by dissolving an amount of sucrose in distilled water to obtain a concentration of 40°Brix. A solution: samples mass relation of 60:1 was used to avoid change in osmotic solution concentration during the treatment time. The experiments, carried out in glass beaker, were performed at 25 °C. For each treatment, product samples were maintained under immersion for 1, 2, 3, 4, 5, 24 and 25 h. After immersion time, the dehydrated papaya samples were recuperated on a strainer and washed with tap water for few seconds to remove the adhering osmotic solution and gently blotted with tissue paper. Recuperation of samples and draining of excess water were carried out in a maximum time of 3 min, in order to minimize exchanges between the samples and the ambient air. Water loss (WL), solids gain (SG) and weight reduction (WR) were calculated by the following equations (Beristain, Azuara, Cortés, & García, 1990):
ðM t Þðxw;t Þ ðM 0 Þðxw;0 Þ M0 Mt M0 WR ¼ M0 WL ¼
SG ¼
ðMt ÞðxS;t Þ ðM 0 ÞðxS;0 Þ M0
where M0 is the initial weight of sample (g); Mt is the weight of sample at time t (g); xw,0 is the mass fraction of water initial content; xw,t is the mass fraction of water content at time t; xS,0 is the mass fraction of soluble solids initial content; and xS,t is the mass fraction of soluble solids content at each sampling times. Soluble solids were measured (AOAC, 2003) and moisture content was determined gravimetrically by drying 2.5 g of osmotically dehydrated papaya samples in an oven at 105 °C till a constant weight was measured (AOAC, 2003). All treatments were carried out in duplicate and the average values were used. 2.4. Statistical analysis Analyses of Variance were performed using STATISTICS software (STATISTICS, 1998) and the Duncan’s multiple range tests were used to compare differences among mean values. Mean values were reported, and the significance was defined at p 6 0.05. 3. Results and discussion 3.1. Characterization of fresh papaya Table 2 shows the physico-chemical characteristics of the fresh green and ripened papaya. Chavarro-Castrillón, Ochoa, and AyalaAponte (2006) reported, for green papaya var. Maradol, similar results for soluble solids, acidity, pH and humidity. Also, others authors informed soluble solids values between 9.9 and 12.5°Brix for ripped papaya (Díaz, 2003; Fagundes & Yamanishi, 2001; Leyva, 2002). The difference between these values could be due to that the evaluated papaya samples were not completely homogeneous or presented differences in the ripening degree. On the other hand, the observed difference in these values for the two ripening degree, is according with the physiologic changes that suffer the fruit during its ripening (Gómez, Lajolo, & Cordenunsi, 2002). In this way,
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Table 2 Characterization of fresh papaya (n = 3). Parameters
Table 4 Means for osmotic dehydration of papaya from Duncan’s multiple range tests. Ripening degree
Soluble solids (°Brix) Acidity (% citric acid, w/w) pH Humidity (%, w/w) Penetration distance (1/10 mm) Ripening index
Green
Ripped
3.7 (0.2) 0.1 (0) 5.61 (0.02) 93.2 (0.4) 24 (2) 58 (3)
9.2 (0.6) 0.13 (0) 5.77 (0.04) 89 (1) 122.3 (0.6) 72 (4)
Ripening degree
Treatments
Weight reduction
Solids gain
Water loss
Green
Control Chitosan-Tween 80 Emulsion
0.121941 a 0.217057 b 0.202294 b
0.145354 b 0.097924 a 0.082654 a
0.276123 a 0.336876 b 0.315266 b
Ripped
Control Chitosan-Tween 80 Emulsion
0.076356 a 0.132082 b 0.132182 b
0.157541 c 0.136812 b 0.126726 a
0.256902 a 0.293196 b 0.294240 b
Mean (standard deviation). Different letters (a–c), within a same ripening degree, differ significantly (p 6 0.05).
Mean (standard deviation).
the increase of the penetrated distance is the reflection of the loss of stability that accompanies the fruits ripening process in general. 3.2. Characterization of chitosan films Table 3 shows the humidity, solubility and solids gain values of elaborated films from chitosan-Tween 80 solution and chitosan lipid emulsion, respectively. Prodpran, Benjakaul, and Artharn (2007) studied the effect of the film composition on its solubility and found that the chitosan addition in films-forming solutions based on proteins, decrease the film solubility in certain way when the chitosan concentration was increased, independently of presence or not of oil, which could due to interactions protein/chitosan or between chitosan molecules (Giancone, 2006). Oil addition in the preparation of chitosan lipid emulsion increased the film solubility due to oil drops can insert into matrix film, decreasing the crossed links between proteins or interactions between chitosan molecules (Prodpran et al., 2007). On the other hand, the solids gain from syrup was similar in both films. 3.3. Kinetics of material transfer Combining the resulting values from different times for each variable in every treatment and comparing the resulting average values, it is possible to know if the kind of treatment could affect the obtained results. Table 4 shows the results of Duncan’s multiple range tests for treatments carried out for the evaluation of the kinetics of material transfer during osmotic dehydration process of papaya samples. The results were quality similar for both papaya ripening degrees, observing the same differences between treatments. 3.3.1. Effect of process time The general behavior of material transfer shows (Figs. 1 and 2) the predominance of water removal, as compared to soluble exchanges. The kinetics of material transfer can be divided into three phases: (i) a starting phase (0–2 h) during which exchange rates grow until 2 h; (ii) an acceleration phase (2–5 h) during which the material exchange reaches its maximum value; and (iii) a decreasing phase where exchange values decrease.
Table 3 Characteristics of elaborated films from chitosan (n = 3). Parameters
Weight (g) Humidity (%, w/w) Water solubility (%, w/w) Soluble solids gain (%, w/w) Mean (standard deviation).
Treatments Chitosan-Tween 80
Emulsion
0.207 (0.004) 28.1 (0.8) 30.6 (0.1) 1.2 (0.1)
0.291 (0.004) 11.8 (0.5) 55.7 (0.3) 1.1 (0.3)
Fig. 1. Variation of water loss of papaya samples during osmotic dehydration in sucrose solution 40°Brix (n = 4). (a) Green papaya and (b) ripped papaya.
The observed behavior could be due to that in the initial stage it exists more difference between the chemical potential of the fruit and osmotic solution and, for consequence, it exists more water loss and solids gain, permitting that the diffusion of the molecules is quicker. The decreasing rates could be attributed to a decrease in the concentration gradient and structural changes that occur in the tissues, slowing the diffusion process. The used solution:samples mass relation avoid changes in osmotic solution concentration during the dehydration process, therefore, changes in concentration gradient are due to the changes in the fruit. 3.3.2. Process efficiency The main characteristic of an osmotic dehydration process is the loss of water; however, the solids gain is a parameter to consider, because the process efficiency depends on these two parameters. The weight reduction is also considered an important parameter in order to measure the efficiency of the osmotic process. 3.3.2.1. Water loss. Fig. 1 shows the changes in the water loss during the osmotic dehydration process of green and ripped papaya
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Fig. 2. Variation of solid gain of papaya samples during osmotic dehydration in sucrose solution 40°Brix (n = 4). (a) Green papaya and (b) ripped papaya.
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Fig. 3. Variation of weight reduction of papaya samples during osmotic dehydration in sucrose solution 40°Brix (n = 4). (a) Green papaya and (b) ripped papaya.
samples with and without chitosan coatings. During the process it was observed a similar behavior in the water loss values in all evaluated treatments, observing an increment in the values of this parameter in the time, reaching the high rate of water loss during the first three hours of the process, in correspondence with Barbosa and Vega (2000), who concluded that the high water loss of in food occurs in the first 6 h of the process, being the two initial hours, those of high rate of water removal. This kinetic tendency was also reported by Nowakunda, Andrés, and Fito (2004) in osmodehydrated banana slices. Green papaya presented higher values of water loss than ripped papaya, what it could be due to that the green fruit presents a higher porosity respect the ripped one, permitting that the contained water inside the cells can be transferred through the pores easily (Chavarro-Castrillón et al., 2006). In both ripening degree, it is observed that uncoated fruit lost less water than coated samples during the osmotic process, according to Díaz (2003).
The gain of solubles by the samples is comparable to weight loss, particularly in the first phase of transfer phenomenon (0– 2 h). In the second phase (2–5 h), the weight loss becomes higher than gain of solubles. As weight loss is the balance between water removal and soluble gain, it could be concluded that, gain of solubles is compensated by the water removal.
3.3.2.2. Soluble solids gain. Fig. 2 shows the percentage of soluble solids gain during the osmotic dehydration process of papaya samples. All evaluated treatments for green and rippened papayas, presented an increment in the values of soluble solids gain. It was observed that coated papaya samples with emulsion presented a minor soluble solids gain than papaya samples without coating. In the case of the treatments with coated papaya, the solids accumulation on the coatings surface, limited its penetration inside the fruits, which it did not happen in samples without coatings, where a great amount of solubles penetrated inside the fruits (Díaz, 2003). The solid accumulation, together with the use of coatings, may create a crust which constitutes a barrier to mass transfer, limiting the dehydration regime and consequently the solubles gain. The above assumptions could explain the difference of dehydration regimes and material transfer between the coated and uncoated papaya samples.
3.3.2.4. Process efficiency index. The values of the process efficiency index (Pr = WL/SG) are used for evaluation of efficiency of osmotic dehydration process (Fig. 4), due to their easy interpretation, because if Pr increases it could mean one of this three possibilities: (i) the process is favoring the water loss and solids gain, but mostly the water loss; (ii) the process limits the solids gain; and (iii) the process favors the water loss. It was observed, in general, that the treatments with coatings presented higher values of Pr during the osmotic dehydration process, what could be due to that in these treatments the water loss was favored, while the soluble solids gain was limited. At the beginning of the process, the changes in the Pr value can be attributable mainly to the water loss, and when the process time increases, the solids gain has more influence on the Pr value. This indicates that upon designing an osmotic dehydration process for this fruit, the time of contact will be defined in function of the pursued objective.
3.3.2.3. Weight reduction. Fig. 3 shows the evolution of weight reduction of the coated and uncoated papaya samples during dehydration process. All treatments, independently of the ripening degree, showed a similar behavior for weight reduction, observing, of general way, that coated papaya samples presented higher values for this parameter than uncoated fruits, which it could due to that the fruits treated with coatings lost more water (Fig. 1) and won less solids than fruits without coatings (Fig. 2). It is agree with the results obtained by Díaz (2003) and Argaiz, Leyva, and Jiménez (2003).
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Fig. 4. Variation of efficiency index during osmotic dehydration of papaya samples in sucrose solution 40°Brix (n = 4). (a) Green papaya and (b) ripped papaya.
4. Conclusions The study showed that chitosan coatings improved the efficiency of osmotic dehydration process, increasing the water loss and decreasing the solids gain. In both ripening stages, the water loss was higher in coated fruits. Emulsified films were more soluble than that elaborated from chitosan solution and Tween 80. Green papaw presented higher values of water loss than the ripped papaya. Chitosan coatings improved the efficiency of osmotic dehydration process in both ripening stages, increasing the water loss and decreasing the solids gain.
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