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Using pullulan-based edible coatings to extend shelf-life of fresh-cut ‘Fuji’ apples
International Journal of Biological Macromolecules 55 (2013) 254–257
Contents lists available at SciVerse ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Using pullulan-based edible coatings to extend shelf-life of fresh-cut ‘Fuji’ apples Shengjun Wu a,∗ , Jinhua Chen b a b
School of Marine Science and Technology, Huaihai Institute of Technology, 59 Cangwu Road, Xinpu 222005, China Lianyungang Ajinomoto Frozen Foods Co., Ltd., Song Jump Industrial Zone, Xinpu 222002, China
a r t i c l e
i n f o
Article history: Received 13 December 2012 Received in revised form 4 January 2013 Accepted 14 January 2013 Available online 31 January 2013 Keywords: Apple slices Glutathione Pullulan Chitooligosaccharides
a b s t r a c t Pullulan is a thickener that can form semipermeable films, and glutathione is an effective reducing agent, while chitooligosaccharide has antibacterial activity. In this study, effect of pullulan-based coatings in combination with antibrowning and antibacterial agents (1% pullulan; 0.8% glutathione + 1% chitooligosaccharides; and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan) on apple slices was investigated during hypothermia storage. Pullulan-coating treatments effectively retarded enzymatic browning, maintained firmness, decreased weight loss, and inhibited microbial growth and respiration rate of apple slices during hypothermia storage compared with that of the control (p < 0.05). Results indicate that using pullulan-based coatings in combination with glutathione and chitooligosaccharides is a promising way to extend the shelf-life of apple slices. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Minimal processing apples (fresh-cut apples) have emerged as popular products for consumers who demand healthy alternatives to conventional snack foods and have been approved for school lunch programs [1]. However, the overall quality and shelf-life of them are reduced by several factors including enzymatic browning, texture deterioration, water loss, microbial growth, senescence processes, and others [2]. Edible coatings have been used to reduce the deleterious effect brought about by minimal processing. The semipermeable barrier provided by edible coatings is aimed to extend shelf life by reducing moisture and solute migration, gas exchange, respiration, and oxidative reaction rates, as well as suppress physiological disorders on fresh-cut fruits [2,3]. In addition, edible coatings can also serve as carriers of food additives, e.g. antibrowning and antimicrobials agents, colorants, flavors, nutrients, and spices [4]. So far, few researches focused on the effect of the combination of pullulan-based edible coatings, antibrowning agents, and antibacterial agents for minimally processed products to extend their shelf-life. Pullulan is a thickener and can form edible films [5]. Glutathione is an effective reducing agent [6], and chitooligosaccharide has antibacterial activity [7]. Therefore, it was of our interest to develop procedures for the use of edible coatings in combination with antibrowning and antibacterial agent to extend the shelf-life of minimally processed
∗ Corresponding author. Tel.: +86 518 85895427; fax: +86 518 85895428. E-mail address: [email protected] (S. Wu). 0141-8130/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2013.01.012
apple slices. The effects of pullulan coatings on browning, firm, weight loss, microbial growth, and respiration rate in fresh-cut apple pieces were studied. 2. Methods and materials 2.1. Materials Apples (Malus domestica cv. Fuji) were purchased from a local supermarket and stored at 4 ◦ C until used. Pullulan, with molecular weight 2.7 × 105 , was purchased from Pharmacopoeia, Japan. Raw chitosan from shrimp shells was obtained from Nantong Biochemical Co. (Jiangsu, China). The molecular weight and degree of deacetylation were 41 × 104 Da and 93.5%, respectively. Glutathione was purchased from Fuchen Chemical Reagents Co. (Tianjin, China). All other chemicals were reagent grade. 2.2. Preparation of chitooligosaccharides The chitooligosaccharides were prepared according to the methods described by Wu with slight modifications [8]. Briefly, the raw chitosan was dissolved in 1% (v/v) aqueous acetic acid (HAc) to a concentration of 1% (w/w) and the pH was adjusted to 5.5 using 1 M NaOH. A 40 mg mass of ␣-amylase was added into a reactor containing 500 mL of chitosan solution, maintained in a thermostatic water bath at 55 ◦ C for 4 h, and then heated to 95 ◦ C for 15 min to terminate the reaction. The hydrolysate was neutralized with 1 M NaOH, filtered, concentrated to ∼16% (w/v), precipitated with 5 volumes of ethanol, and dried at 60 ◦ C for 3 h to yield a white powder.
S. Wu, J. Chen / International Journal of Biological Macromolecules 55 (2013) 254–257
2.3. Preparation of apple slices
2.4. Dipping and storage conditions of apple slices Dipping solutions of glutathione, chitooligosaccharides, and 1% pullulan were: (1) control (deionized water), (2) 1% pullulan, (3) 0.8% glutathione + 1% chitooligosaccharides, and (4) 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan. The apple slices were first dipped into the four dipping solutions fully immersed for 5 min. Residual solutions on the apple slices were dripped off for 1 min, and the apple slices were kept at 5 ◦ C until the excess of water was drained. Then these apple slices were placed on plastic-coated wire racks inside plastic containers at 5 ◦ C for 10 d. 2.5. Color determination The color of apple slices was measured using a Minolta colorimeter (CS-100A, Minolta Co. Ltd., Japan) according to Lu et al. with slight modifications [9]. The degree of browning was expressed by the changes in the lightness (L) value. 2.6. Firmness determination Firmness of the apple wedges was evaluated using a TA-XT2 texture analyzer (Texture Technologies Corp., Scarsdale, N.Y., USA) following a modified procedure described by Rojas-Graü, GrasaGuillem, and Martín-Belloso [10]. Firmness was expressed by the peak force required to penetrate the apple wedges by 10 mm with a 5 mm diameter probe. 2.7. Weight loss The weight of apples slices was determined during storage to evaluate the efficacy of pullulan coatings as moisture barriers. The percent weight loss was calculated by weighing the samples every 2 d. 2.8. Microbiological analysis Microbiological analysis was carried out according to the method of Nirmal and Benjakul with slight modifications [11]. 25 g of apple slices were placed in a Waring blender (JJ-2, Wuxi Woshin Instruments Co., Ltd., Wuxi, China) containing 225 mL of 0.85% saline water. After homogenization, appropriate dilutions were prepared to spread on the plate count agar medium containing 0.5% NaCl for the determination of total viable counts. Total viable counts of psychrophilic bacterial were determined by incubating the agar plates at 4 ◦ C for 7 d. 2.9. Measurement of respiration rate Effect of coatings on respiration rate was measured by analyzing the headspace gas composition. 100 g apple slices were stored in a 100 mL tight-sealed glass container at 25 ◦ C for 24 h. Headspace samples were withdrawn at various time intervals and analyzed for CO2 using a Trace 2000 GC series gas chromatograph and Thermo mass spectrometer. A SGE BPx70 column (60 m × 0.25 mm, 0.25 mm film thickness) was used. Helium was the carrier gas at
80
70
L
Apple slices and treatment were performed according to the methods described by Lu et al. with slight modifications [9]. Apples with uniform size, color, and maturity were washed with tap water, cored, peeled, and cut into eight equal slices using a sharp stainless steel knife.
255
60
50 0
2
4
6
8
10
Time (d) Fig. 1. L-value of minimally processed Fuji apples during storage at 5 ◦ C ((♦), 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan; () 0.8% glutathione + 1% chitooligosaccharides; () 1% pullulan; () control). Values are means of three replicates. Vertical bars represent standard deviation.
35 mL/min. The injector was 50 ◦ C, detector 100 ◦ C, and the column was 50 ◦ C [5]. 2.10. Statistical analysis All data are presented as mean ± S.D. Statistical analysis was performed using Statgraphics Centurion XV Version 15.1.02. A multifactor ANOVA with posterior multiple range test was used to find significant differences among the effects of storage time and dipping condition on color, firmness, and microbiological count. 3. Results and discussion 3.1. Browning of the apple slices during hypothermia storage L values of control, 1% pullulan, 0.8% glutathione + 1% chitooligosaccharides, and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan samples were 55.11, 65.03, 73.60, and 75.01 on 10 d at 5 ◦ C, respectively (Fig. 1). Results showed that pullulanbased edible coatings containing glutathione as antibrowning agent prevented apple slices from browning during 10 d of storage, indicating that glutathione is an effective antibrowning agent that can be incorporated within the edible coatings and pullulan coating is a semipermeable to oxygen and thus reduce the browning in apple slices. It is interesting that the efficacy of 0.8% glutathione + 1% chitooligosaccharides treatment as antibrowning agent was comparable to that of 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan treatment. The efficacy of antibrowning agents incorporated within edible coatings has been reported by some authors [2,3,5,12]. 3.2. Firmness changes of apple slices during hypothermia storage Apple slices tend to soften principally due to acid hydrolysis and also enzymatic degradation [12], and any minimal process technique needs to address this additional problem. The firmness of pullulan-coated slices did not significantly decrease over the 10 d of storage (p < 0.05), and the uncoated slices showed faster softening, indicating coating with pullulan effectively retarded or avoided tissue softening. This may be ascribed to the lower water loss caused by the surface coatings (Fig. 2). As to firmness of apple, calcium cation is well known to increase firmness and delay membrane lipid catabolism in apple fruit [13,14], and calcium chloride has been found to be effective in improving the firmness of fresh-cut apples
S. Wu, J. Chen / International Journal of Biological Macromolecules 55 (2013) 254–257
7.5
7.5
7
7
Microbial population (Log CFU g -1)
Firmness (N)
256
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6
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Time (d)
Time (d) Fig. 2. Firmness of minimally processed Fuji apples during storage at 5 ◦ C ((♦) 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan; () 1% pullulan; () 1% 0.8% glutathione + 1% chitooligosaccharides; () control). Values are means of three replicates. Vertical bars represent standard deviation.
treated with 4-hexylresorcinol [15] and isoascorbic acid [16]. In this study, Pullulan coatings maintained firmness of apple slices, which suggests a promising formula ideal for softening inhibition. 3.3. Weight loss of apple slices during hypothermia storage When the skinless apple slices exposes to an environment with lower relative humidity, substantial weight loss will be happening. After 2 d of storage, control apples lost around 15.1% of their weight, while coated apples lost 11–12% of their weight (p < 0.05). The three formulations prevented water loss by producing high relative humidity at the surface of sliced apples. The best coating to prevent water loss was 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan, which allowed a 11.2% loss compared to 15.1% for control apples at day 2 (p < 0.05). 1% pullulan allowed 12.3% weight loss at day 2, and also showed good water loss prevention. 1% pullulan, 0.8% glutathione + 1% chitooligosaccharides, and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan allowed about 11%, 33%, and 29% less weight loss than the control apple slices, respectively (p < 0.05) at day 10 (Fig. 3). Results indicate that pullulan coatings play important role in water loss prevention.
Fig. 4. The microbiological changes of minimally processed Fuji apples during storage at 5 ◦ C ((♦) control; () 1% pullulan; () 0.8% glutathione + 1% chitooligosaccharides; () 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan). Values are means of three replicates. Vertical bars represent standard deviation.
3.4. Microbial growth in apple slices during hypothermia storage The changes in bacterial count of apple slices during storage are shown in Fig. 4. In the control and 1% pullulan, bacterial count increased steadily in 10 d (Fig. 4). However, bacterial count in the apple slices treated with 0.8% glutathione + 1% chitooligosaccharides, and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan decreased continuously, respectively, compared with the control (p < 0.05). This indicated the antimicrobial activity of chitooligosaccharides towards psychrophilic bacteria in apple slices even during hypothermia storage (Fig. 4). 3.5. Initial respiration rate of apple slices during hypothermia storage The physical damage caused by coring, peeling, and cutting increases respiration rate shortly, thus reducing initial respiration rate is essential for extending shelf-life of minimally processed fruits [5]. The effect of pullulan coatings on initial respiration rate is shown in Fig. 5. 1% pullulan, 0.8% glutathione + 1% chitooligosaccharides, and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan had a significantly greater effect than the control. Oxygen barrier
90
25
85
Respiratory rate (CO2 mg/kg h)
Weight loss (%, w/w)
20
15
10
5
80 75 70 65 60 55 50 45
0
40 0
2
4
6
8
10
Time (d) Fig. 3. Weight loss of minimally processed Fuji apples during storage at 5 ◦ C ((♦) control; () 0.8% glutathione + 1% chitooligosaccharides; () 1% pullulan; () 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan). Values are means of three replicates. Vertical bars represent standard deviation.
0
2
4
6
8
10
Time (d) Fig. 5. The initial respiration rate of minimally processed Fuji apples during storage at 5 ◦ C ((♦) control; () 1% 0.8% glutathione + 1% chitooligosaccharides; () 1% pullulan; () 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan). Values are means of three replicates. Vertical bars represent standard deviation.
S. Wu, J. Chen / International Journal of Biological Macromolecules 55 (2013) 254–257
properties of pullulan coatings and related enzyme activity inhibition by glutathione made them suitable as coating to reduce respiration rate. 4. Conclusions Browning during storage, firmness decreasing, weight loss, microbial growth, and high respiration rate decrease the shelflife of apple slices. Pullulan-based coating treatments effectively inhibited enzymatic browning, retarded tissue softening, inhibited microbial growth, decreased weight loss and respiration rate of the minimally processed apple slices during storage. Therefore, treatment of apple slices with 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan means the extended shelf-life of apple slices. References [1] J.R. Gorny, Fresh Cut 11 (2003) 14–15. [2] M.A. Rojas-Graü, M.S. Tapia, O. Martín-Belloso, LWT – Food Science and Technology 41 (2008) 139–147.
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[3] M.A. Rojas-Graü, M.S. Tapia, F.J. Rodríguez, A.J. Carmona, O. Martin-Belloso, Food Hydrocolloids 21 (2007) 118–127. [4] Y. Pranoto, V. Salokhe, K.S. Rakshit, Food Research International 38 (2005) 267–272. [5] H.P. Qi, W.Z. Hu, A.L. Jiang, M.X. Tian, Y.Q. Li, Innovative Food Science and Emerging Technologies 12 (2011) 62–66. [6] L. El Hosry, L. Auezova, A. Sakr, E. Hajj-Moussa, International Journal of Food Science and Technology 44 (2009) 2459–2463. [7] Y.J. Du, Y.Q. Zhao, S.C. Dai, B. Yang, Innovative Food Science and Emerging Technologies 10 (2009) 103–107. [8] S.J. Wu, Food Chemistry 128 (2011) 769–772. [9] S.M. Lu, Y.G. Luo, E. Turner, H. Feng, Food Chemistry 104 (2007) 824–829. [10] M.A. Rojas-Graü, A. Sobrino-López, M.S. Tapia, O. Martín-Belloso, Journal of Food Science 71 (2005) S59–S65. [11] N.P. Nirmal, S. Benjakul, Journal of Agricultural and Food Chemistry 57 (2009) 3578–3586. [12] O.P. Chauhan, P.S. Raju, A. Singh, A.S. Bawa, Food Chemistry 126 (2011) 961–966. [13] Y. Luo, G.V. Barbosa-Cánovas, Food Science and Technology International 2 (1996) 315–321. [14] G.A. Picchioni, A.E. Watada, W.S. Conway, B.D. Whitaker, C.E. Sams, Journal of Agricultural and Food Chemistry 46 (1998) 2452–2457. [15] Y.G. Luo, S.M. Lu, B. Zhou, H. Feng, LWT – Food Science and Technology 44 (2011) 1621–1625. [16] J.G. Buta, H.E. Moline, D.W. Spaulding, C.Y. Wang, Journal of Agricultural and Food Chemistry 47 (1999) 1–6.
Contents lists available at SciVerse ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Using pullulan-based edible coatings to extend shelf-life of fresh-cut ‘Fuji’ apples Shengjun Wu a,∗ , Jinhua Chen b a b
School of Marine Science and Technology, Huaihai Institute of Technology, 59 Cangwu Road, Xinpu 222005, China Lianyungang Ajinomoto Frozen Foods Co., Ltd., Song Jump Industrial Zone, Xinpu 222002, China
a r t i c l e
i n f o
Article history: Received 13 December 2012 Received in revised form 4 January 2013 Accepted 14 January 2013 Available online 31 January 2013 Keywords: Apple slices Glutathione Pullulan Chitooligosaccharides
a b s t r a c t Pullulan is a thickener that can form semipermeable films, and glutathione is an effective reducing agent, while chitooligosaccharide has antibacterial activity. In this study, effect of pullulan-based coatings in combination with antibrowning and antibacterial agents (1% pullulan; 0.8% glutathione + 1% chitooligosaccharides; and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan) on apple slices was investigated during hypothermia storage. Pullulan-coating treatments effectively retarded enzymatic browning, maintained firmness, decreased weight loss, and inhibited microbial growth and respiration rate of apple slices during hypothermia storage compared with that of the control (p < 0.05). Results indicate that using pullulan-based coatings in combination with glutathione and chitooligosaccharides is a promising way to extend the shelf-life of apple slices. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Minimal processing apples (fresh-cut apples) have emerged as popular products for consumers who demand healthy alternatives to conventional snack foods and have been approved for school lunch programs [1]. However, the overall quality and shelf-life of them are reduced by several factors including enzymatic browning, texture deterioration, water loss, microbial growth, senescence processes, and others [2]. Edible coatings have been used to reduce the deleterious effect brought about by minimal processing. The semipermeable barrier provided by edible coatings is aimed to extend shelf life by reducing moisture and solute migration, gas exchange, respiration, and oxidative reaction rates, as well as suppress physiological disorders on fresh-cut fruits [2,3]. In addition, edible coatings can also serve as carriers of food additives, e.g. antibrowning and antimicrobials agents, colorants, flavors, nutrients, and spices [4]. So far, few researches focused on the effect of the combination of pullulan-based edible coatings, antibrowning agents, and antibacterial agents for minimally processed products to extend their shelf-life. Pullulan is a thickener and can form edible films [5]. Glutathione is an effective reducing agent [6], and chitooligosaccharide has antibacterial activity [7]. Therefore, it was of our interest to develop procedures for the use of edible coatings in combination with antibrowning and antibacterial agent to extend the shelf-life of minimally processed
∗ Corresponding author. Tel.: +86 518 85895427; fax: +86 518 85895428. E-mail address: [email protected] (S. Wu). 0141-8130/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2013.01.012
apple slices. The effects of pullulan coatings on browning, firm, weight loss, microbial growth, and respiration rate in fresh-cut apple pieces were studied. 2. Methods and materials 2.1. Materials Apples (Malus domestica cv. Fuji) were purchased from a local supermarket and stored at 4 ◦ C until used. Pullulan, with molecular weight 2.7 × 105 , was purchased from Pharmacopoeia, Japan. Raw chitosan from shrimp shells was obtained from Nantong Biochemical Co. (Jiangsu, China). The molecular weight and degree of deacetylation were 41 × 104 Da and 93.5%, respectively. Glutathione was purchased from Fuchen Chemical Reagents Co. (Tianjin, China). All other chemicals were reagent grade. 2.2. Preparation of chitooligosaccharides The chitooligosaccharides were prepared according to the methods described by Wu with slight modifications [8]. Briefly, the raw chitosan was dissolved in 1% (v/v) aqueous acetic acid (HAc) to a concentration of 1% (w/w) and the pH was adjusted to 5.5 using 1 M NaOH. A 40 mg mass of ␣-amylase was added into a reactor containing 500 mL of chitosan solution, maintained in a thermostatic water bath at 55 ◦ C for 4 h, and then heated to 95 ◦ C for 15 min to terminate the reaction. The hydrolysate was neutralized with 1 M NaOH, filtered, concentrated to ∼16% (w/v), precipitated with 5 volumes of ethanol, and dried at 60 ◦ C for 3 h to yield a white powder.
S. Wu, J. Chen / International Journal of Biological Macromolecules 55 (2013) 254–257
2.3. Preparation of apple slices
2.4. Dipping and storage conditions of apple slices Dipping solutions of glutathione, chitooligosaccharides, and 1% pullulan were: (1) control (deionized water), (2) 1% pullulan, (3) 0.8% glutathione + 1% chitooligosaccharides, and (4) 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan. The apple slices were first dipped into the four dipping solutions fully immersed for 5 min. Residual solutions on the apple slices were dripped off for 1 min, and the apple slices were kept at 5 ◦ C until the excess of water was drained. Then these apple slices were placed on plastic-coated wire racks inside plastic containers at 5 ◦ C for 10 d. 2.5. Color determination The color of apple slices was measured using a Minolta colorimeter (CS-100A, Minolta Co. Ltd., Japan) according to Lu et al. with slight modifications [9]. The degree of browning was expressed by the changes in the lightness (L) value. 2.6. Firmness determination Firmness of the apple wedges was evaluated using a TA-XT2 texture analyzer (Texture Technologies Corp., Scarsdale, N.Y., USA) following a modified procedure described by Rojas-Graü, GrasaGuillem, and Martín-Belloso [10]. Firmness was expressed by the peak force required to penetrate the apple wedges by 10 mm with a 5 mm diameter probe. 2.7. Weight loss The weight of apples slices was determined during storage to evaluate the efficacy of pullulan coatings as moisture barriers. The percent weight loss was calculated by weighing the samples every 2 d. 2.8. Microbiological analysis Microbiological analysis was carried out according to the method of Nirmal and Benjakul with slight modifications [11]. 25 g of apple slices were placed in a Waring blender (JJ-2, Wuxi Woshin Instruments Co., Ltd., Wuxi, China) containing 225 mL of 0.85% saline water. After homogenization, appropriate dilutions were prepared to spread on the plate count agar medium containing 0.5% NaCl for the determination of total viable counts. Total viable counts of psychrophilic bacterial were determined by incubating the agar plates at 4 ◦ C for 7 d. 2.9. Measurement of respiration rate Effect of coatings on respiration rate was measured by analyzing the headspace gas composition. 100 g apple slices were stored in a 100 mL tight-sealed glass container at 25 ◦ C for 24 h. Headspace samples were withdrawn at various time intervals and analyzed for CO2 using a Trace 2000 GC series gas chromatograph and Thermo mass spectrometer. A SGE BPx70 column (60 m × 0.25 mm, 0.25 mm film thickness) was used. Helium was the carrier gas at
80
70
L
Apple slices and treatment were performed according to the methods described by Lu et al. with slight modifications [9]. Apples with uniform size, color, and maturity were washed with tap water, cored, peeled, and cut into eight equal slices using a sharp stainless steel knife.
255
60
50 0
2
4
6
8
10
Time (d) Fig. 1. L-value of minimally processed Fuji apples during storage at 5 ◦ C ((♦), 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan; () 0.8% glutathione + 1% chitooligosaccharides; () 1% pullulan; () control). Values are means of three replicates. Vertical bars represent standard deviation.
35 mL/min. The injector was 50 ◦ C, detector 100 ◦ C, and the column was 50 ◦ C [5]. 2.10. Statistical analysis All data are presented as mean ± S.D. Statistical analysis was performed using Statgraphics Centurion XV Version 15.1.02. A multifactor ANOVA with posterior multiple range test was used to find significant differences among the effects of storage time and dipping condition on color, firmness, and microbiological count. 3. Results and discussion 3.1. Browning of the apple slices during hypothermia storage L values of control, 1% pullulan, 0.8% glutathione + 1% chitooligosaccharides, and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan samples were 55.11, 65.03, 73.60, and 75.01 on 10 d at 5 ◦ C, respectively (Fig. 1). Results showed that pullulanbased edible coatings containing glutathione as antibrowning agent prevented apple slices from browning during 10 d of storage, indicating that glutathione is an effective antibrowning agent that can be incorporated within the edible coatings and pullulan coating is a semipermeable to oxygen and thus reduce the browning in apple slices. It is interesting that the efficacy of 0.8% glutathione + 1% chitooligosaccharides treatment as antibrowning agent was comparable to that of 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan treatment. The efficacy of antibrowning agents incorporated within edible coatings has been reported by some authors [2,3,5,12]. 3.2. Firmness changes of apple slices during hypothermia storage Apple slices tend to soften principally due to acid hydrolysis and also enzymatic degradation [12], and any minimal process technique needs to address this additional problem. The firmness of pullulan-coated slices did not significantly decrease over the 10 d of storage (p < 0.05), and the uncoated slices showed faster softening, indicating coating with pullulan effectively retarded or avoided tissue softening. This may be ascribed to the lower water loss caused by the surface coatings (Fig. 2). As to firmness of apple, calcium cation is well known to increase firmness and delay membrane lipid catabolism in apple fruit [13,14], and calcium chloride has been found to be effective in improving the firmness of fresh-cut apples
S. Wu, J. Chen / International Journal of Biological Macromolecules 55 (2013) 254–257
7.5
7.5
7
7
Microbial population (Log CFU g -1)
Firmness (N)
256
6.5
6
6.5
6
5.5
5.5
5
5 0
2
4
6
8
2
0
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10
Time (d)
Time (d) Fig. 2. Firmness of minimally processed Fuji apples during storage at 5 ◦ C ((♦) 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan; () 1% pullulan; () 1% 0.8% glutathione + 1% chitooligosaccharides; () control). Values are means of three replicates. Vertical bars represent standard deviation.
treated with 4-hexylresorcinol [15] and isoascorbic acid [16]. In this study, Pullulan coatings maintained firmness of apple slices, which suggests a promising formula ideal for softening inhibition. 3.3. Weight loss of apple slices during hypothermia storage When the skinless apple slices exposes to an environment with lower relative humidity, substantial weight loss will be happening. After 2 d of storage, control apples lost around 15.1% of their weight, while coated apples lost 11–12% of their weight (p < 0.05). The three formulations prevented water loss by producing high relative humidity at the surface of sliced apples. The best coating to prevent water loss was 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan, which allowed a 11.2% loss compared to 15.1% for control apples at day 2 (p < 0.05). 1% pullulan allowed 12.3% weight loss at day 2, and also showed good water loss prevention. 1% pullulan, 0.8% glutathione + 1% chitooligosaccharides, and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan allowed about 11%, 33%, and 29% less weight loss than the control apple slices, respectively (p < 0.05) at day 10 (Fig. 3). Results indicate that pullulan coatings play important role in water loss prevention.
Fig. 4. The microbiological changes of minimally processed Fuji apples during storage at 5 ◦ C ((♦) control; () 1% pullulan; () 0.8% glutathione + 1% chitooligosaccharides; () 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan). Values are means of three replicates. Vertical bars represent standard deviation.
3.4. Microbial growth in apple slices during hypothermia storage The changes in bacterial count of apple slices during storage are shown in Fig. 4. In the control and 1% pullulan, bacterial count increased steadily in 10 d (Fig. 4). However, bacterial count in the apple slices treated with 0.8% glutathione + 1% chitooligosaccharides, and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan decreased continuously, respectively, compared with the control (p < 0.05). This indicated the antimicrobial activity of chitooligosaccharides towards psychrophilic bacteria in apple slices even during hypothermia storage (Fig. 4). 3.5. Initial respiration rate of apple slices during hypothermia storage The physical damage caused by coring, peeling, and cutting increases respiration rate shortly, thus reducing initial respiration rate is essential for extending shelf-life of minimally processed fruits [5]. The effect of pullulan coatings on initial respiration rate is shown in Fig. 5. 1% pullulan, 0.8% glutathione + 1% chitooligosaccharides, and 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan had a significantly greater effect than the control. Oxygen barrier
90
25
85
Respiratory rate (CO2 mg/kg h)
Weight loss (%, w/w)
20
15
10
5
80 75 70 65 60 55 50 45
0
40 0
2
4
6
8
10
Time (d) Fig. 3. Weight loss of minimally processed Fuji apples during storage at 5 ◦ C ((♦) control; () 0.8% glutathione + 1% chitooligosaccharides; () 1% pullulan; () 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan). Values are means of three replicates. Vertical bars represent standard deviation.
0
2
4
6
8
10
Time (d) Fig. 5. The initial respiration rate of minimally processed Fuji apples during storage at 5 ◦ C ((♦) control; () 1% 0.8% glutathione + 1% chitooligosaccharides; () 1% pullulan; () 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan). Values are means of three replicates. Vertical bars represent standard deviation.
S. Wu, J. Chen / International Journal of Biological Macromolecules 55 (2013) 254–257
properties of pullulan coatings and related enzyme activity inhibition by glutathione made them suitable as coating to reduce respiration rate. 4. Conclusions Browning during storage, firmness decreasing, weight loss, microbial growth, and high respiration rate decrease the shelflife of apple slices. Pullulan-based coating treatments effectively inhibited enzymatic browning, retarded tissue softening, inhibited microbial growth, decreased weight loss and respiration rate of the minimally processed apple slices during storage. Therefore, treatment of apple slices with 0.8% glutathione + 1% chitooligosaccharides + 1% pullulan means the extended shelf-life of apple slices. References [1] J.R. Gorny, Fresh Cut 11 (2003) 14–15. [2] M.A. Rojas-Graü, M.S. Tapia, O. Martín-Belloso, LWT – Food Science and Technology 41 (2008) 139–147.
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