Xylan chitosan conjugate - A potential food preservative

Food Chemistry 126 (2011) 520–525

Contents lists available at ScienceDirect

Food Chemistry journal homepage: www.elsevier.com/locate/foodchem

Xylan chitosan conjugate - A potential food preservative Xiaoxia Li, Xiaowen Shi, Miao Wang, Yumin Du ⇑ Department of Environmental Science, College of Resource and Environmental Science, Wuhan University, Wuhan 430079, China

a r t i c l e

i n f o

Article history: Received 19 July 2010 Received in revised form 8 October 2010 Accepted 9 November 2010

Keywords: Xylan Chitosan Maillard reaction Antioxidant Antimicrobial

a b s t r a c t Xylan, a hemicellulose extracted from corn cobs, was co-heated with chitosan to prepare a polysaccharide-based food preservative. UV absorbance, browning and fluorescence changes indicated the presence of Maillard reaction between the two reactants. Antioxidant capacities, including 2,2-diphenyl-1picrylhydrazyl (DPPH) radical-scavenging activity and reducing power, showed that the xylan-chitosan conjugates possessed excellent antioxidant activity depending on the heating time while chitosan or xylan alone did not possess any. The antimicrobial activity of the conjugates against Escherichia coli and Staphylococcus aureus was higher than chitosan. These results indicated that the Maillard reaction conjugate of xylan and chitosan was a promising preservative for various food formulations to enhance microbial safety and extend shelf life. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The current use of environmentally friendly products has resulted in an extensive exploitation of natural materials. Among the sustainable sources, hemicelluloses, representing about 20– 35% of lignocellulosic biomass, have emerged as an immense renewable resource of biopolymers. Xylan-type polysaccharides are the main hemicellulose components of secondary cell walls and account for 50% of the biomass of annual and perennial plants, but their application potential has not yet been exploited commercially due to their source-dependent diversity and high heterogeneity (Ebringerova, Hromadkova, & Heinze, 2005). In recent years, functional modification of xylan has received considerable interest for use in packaging films, food coatings as well as in biomedical areas (Hansen & Plackett, 2008). Chitosan, the deacetylated derivative of chitin, is a nontoxic and biodegradable polymer. Though chitosan has received attention as a potential food preservative because of its film-forming properties (Park & Bae, 2006) and antibacterial activity (Liu, Du, Wang, & Sun, 2004), some studies showed that chitosan was ineffective in preventing oxidative rancidity (Rao, Chander, & Sharma, 2005). The amino groups of chitosan are responsible for several easy and site-selective methods of chemical modification, such as Schiff base, N-acylation and reductive alkylation (Kurita, 2006). Maillard reaction between the amino groups of chitosan

⇑ Corresponding author. Tel.: +86 27 68778501. E-mail address: [email protected] (Y. Du). 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.11.037

and the aldehydes or ketones of reducing sugars has been reported (Chung, Kuo, & Chen, 2005; Kanatt, Chander, & Sharma, 2008; Umemura & Kawai, 2007). Some Maillard reaction products (MRPs) show excellent antioxidative and antimicrobial effects (Morales & Jimenez-Perez, 2001). For example, chitosan–glucose complex exhibited high antioxidative properties without changing the antimicrobial activity of chitosan (Kanatt et al., 2008). It has also been reported that the MRPs of lysozyme–chitosan conjugates improved emulsion properties (Song, Babiker, Usui, Saito, & Kato, 2002), However, most research is limited to the condensation between protein and polysaccharide or amino-based polysaccharide and monosaccharide; there are few reports about Maillard products between two polysaccharides, due to the immiscibility of the polymers. It was found that the partner saccharides in the protein–polysaccharide conjugates, such as dextran, galactomannan, pectin and chitosan, are easily attached to the amino groups of proteins because of the reactive carbonyl group in the reducing end (Kato, 2002). Here we hypothesised that the reaction between chitosan and acid soluble xylan might be viable, based on research related to chitosan and hemicellulose models (Umemura & Kawai, 2008), though they did not consider the reaction between hemicellulose and chitosan or the functional applications of the conjugates. Taking into account the potential exploitation of xylan and the functional properties of chitosan-based MRPs, our aim was to evaluate the use of xylan from corn cobs and prepare xylan–chitosan conjugates by heating the two polymers. The antioxidant and antimicrobial activities accompanied by characteristic changes were investigated, in order to gain more insight into the development of a natural food preservative.

521

X. Li et al. / Food Chemistry 126 (2011) 520–525

2. Materials and methods

2.6. Free radical-scavenging and reducing power

2.1. Chemicals

The radical-scavenging activity was determined by the DPPH assay of Bondet, Brand-Williams, and Berset (1997) with slightly modifications. Five-fold diluted sample (200 ll) was added into 2.0 ml of 0.1 mM DPPH in ethanol. The solution was then mixed vigorously and allowed to stand at room temperature in the dark for 20 min, then absorption was measured at 517 nm. The percentage of DPPH radical-scavenging activity was calculated as follows:

Chitosan was obtained from Zhejiang Yuhuan. Ltd. (China) with a deacetylation degree of 80% and a molecular weight of 210 kDa. Xylan isolated by alkaline extraction of corn cobs was obtained from Shanghai Hanhong Ltd. (China). 2,2-Diphenyl-1-picrylhydrazyl (DPPH) was purchased from Sigma. 2.2. Sugar composition and molecular weight of xylan The xylan sample was hydrolysed in 3 M trifluoroacetic acid solution for 4 h at 100 °C; then the hydrolysate was derivatised with 1-phenyl-3-methyl-5-pyrazolone (PMP) and detected by high performance liquid chromatography (HPLC) according to the method of Lv et al. (2009). Kromasil C18 was used as stationary phase and 15% CH3OH aqueous solution (0.1 M KH2PO4, pH = 6.9) as mobile phase. Flow rate was set at 1 ml/min and UV detection was used at wavelength of 245 nm. An average of three samples was tested. The substitution degree of arabinose in xylan backbone was calculated by the ratio of arabinose in the sum of arabinose and xylose. The molecular weight of xylan was detected by dynamic light scattering (DLS). Average scattering intensity from five different concentrations (in 0.5 M NaOH) was recorded using a modified commercial light scattering spectrometer (ALV/SP-125, ALV, Germany) equipped with an ALV-5000/E multi-s digital time correlator, and an He-Ne laser (at k = 632.8 nm) was used at a scattering angle range of 30–150° at 25.0 °C with dn/dc of 0.146 ml/g (Dervilly-Pinel, Thibault, & Saulnier, 2001). The measurement of the intensity– intensity time correlation function led to the hydrodynamic radius and molecular weight of the polysaccharide. 2.3. Preparation of xylanchitosan conjugates Chitosan was prepared in 1% (v/v) glacial acetic acid at a concentration of 1% (w/v) and filtered to remove insoluble residue. Xylan 4% (w/v) was dissolved in the chitosan solution and refluxed in an oil bath for heating reaction. To monitor the reaction, 8 ml aqueous solution were withdrawn periodically and placed in an ice bath. Chitosan 1% (w/v) and xylan 4% (w/v) alone were heated under the same experimental conditions as controls. 2.4. Structure analysis of conjugates The xylan, chitosan and conjugate solutions were freeze-dried and grounded to powder. FTIR spectra were obtained with an FTIR 170S spectrometer (American Nicolet Company) using the KBr pellet method. The hydrodynamic radius distributions of the aggregates were determined by DLS. Samples of initial chitosan, xylan and the conjugate solutions were made optically clear by filtration through 0.45 lm Millipore filters and detected with DLS using scattering angle h = 90o. 2.5. Spectrophotometric analysis The heated solutions were appropriately diluted for spectrophotometric analysis and antioxidant activity. UV and browning indices were established by measuring the absorbances at 280 nm and 420 nm, respectively. The fluorescence intensity was measured at an excitation wavelength of 343 nm and emission wavelength of 415 nm on a Hitachi-F4500 fluorescence spectrophotometer. An average of six readings was recorded.

Scavenging activityð%Þ ¼ ½1  ðAsample =Ablank Þ  100:

ð1Þ

where Asample was the absorbance of sample, and Ablank was the absorbance of 1% acetic acid (v/v) blank solution under the same conditions as the sample. The percentage of remaining DPPHagainst time was plotted to obtain the reaction time necessary to decrease the initial concentration by 50% (IC50). Butylated hydroxy toluene (BHT) in ethanol was used for comparison. Reducing power of the conjugates during heating was also determined. One millilitre of the solution was mixed with 1.0 ml of 0.2 M sodium phosphate buffer (pH 6.6) and 1.0 ml of 1% potassium ferricyanide, and then incubated at 50 °C for 20 min. Trifluoroacetate (10%, 2.5 ml) was added to the mixture and centrifuged at 10000g for 5 min. The supernatant (2 ml) was mixed with 2 ml of water and 1 ml of 0.1% ferric chloride and the absorbance was measured at 700 nm. There were three replicate samples in each group for the measurements of free radical scavenging and reducing power. 2.7. Antibacterial activity The antibacterial activity of xylan, chitosan and conjugates heated for different time against S. aureus or E. coli was determined using the disk diffusion method. Cell suspension (107cfu/ml; 50 ll) was added onto agar plates before paper disks (diameter of 7.5 mm) containing test solution were placed on plates. Inhibition zones were observed and measured after incubation at 37 °C for 20 h. This experiment was carried out in triplicate and average values with standard errors are reported. A statistical difference at p < 0.05 was considered significant. 3. Results and discussion 3.1. Properties of xylan Composition of the corn cob xylan is given in Table 1. This kind of hemicellulose mainly contains arabinose and xylose as neutral sugars. The substitution degree (0.26) of the xylan backbone with arabinose was much higher than the value of 0.11 reported by others (Garcia et al., 2001). It is reasonable that the water-soluble xylan has more than 15% of the backbone substituted (Ebringerova, Hromadkova, Alfodi, & Hribalova, 1998). The content of uronic acid in this corn cob xylan was only 1.4 ± 0.20%, much lower than that in birchwood-sourced xylan (3%); and the latter could be directly converted into hydrogels with chitosan through polyelectrolyte interactions (Gabrielii & Gatenholm, 1998). The molecular weight of the corn cob xylan was 1.0  104 g/mol, also much lower than that obtained by others (Garcia et al., 2001). Thus, the obtained

Table 1 Sugar compositions of xylan. Sugar composition (mol%) Xylose

Arabinose

Glucose

Galactose

Mannose

70.7 ± 2.21

25.6 ± 1.69

1.3 ± 0.20

0.7 ± 0.14

0.3 ± 0.03

Glucuronic acid 1.4 ± 0.20

X. Li et al. / Food Chemistry 126 (2011) 520–525

Maillard reaction and caramelisation are two kinds of nonenzymatic browning, resulting from condensation between the carbonyl group of reducing sugars and nitrogen-containing compounds and direct heating of carbohydrates, respectively. Fig. 1a and b depict the time course of UV absorbance at 280 nm and browning as observed by absorbance at 420 nm. Xylan did not show any browning when heated alone, indicating that the caramelisation was minimal under our experimental conditions. Also, there was no significant colour change when the aminosugar chitosan was heated alone. In contrast, a colour change from light yellow to dark brown was observed for the xylan and chitosan mixture, suggesting Maillard reaction between the reducing sugar xylan and aminosugar chitosan (Ajandouz & Puigserver, 1999). Generally, Maillard reaction follows a complex mechanism with three major stages (early, advanced and final stage), and the coloured compounds are formed from Amadori intermediate or even melanoidins (Hofmann, Bors, & Stettmaier, 1999). Interestingly, both browning and UV absorbance showed a slight induction period, and the time was longer than that of the reaction between chitosan and monosaccharide or amino acids (Kanatt et al., 2008; Morales & Jimenez-Perez, 2001). It was assumed that the steric hindrance of the long chains of the polymers delayed the formation of intermediate compounds. The fluorescence intensity of xylan was always weak while heating for a long time (Fig. 1c). The intensity value of the heated chitosan was also low overall in spite of minimal increase because of its Maillard reaction. When the two polymers were heated together, the fluorescence intensity increased rapidly to a maximum value at 120 min after an induction of 20 min, revealing fluorophores produced in the Maillard reaction between xylan and chitosan. Although fluorescent molecules have been found to be rather stable when heating sugar/amino acid model systems (Morales & Jimenez-Perez, 2001), the fluorescence intensity decreased during long time heating in our experiment, probably due to the formation of different fluorescent chemical structures. Moreover, the colour of the solution started changing only after the maximum fluorescence arrived, indicating that the fluorogens were the precursors of brown pigments (Morales & van Boekel, 1998).

Absorbance at 280nm

3.2. Browning and fluorescence of the conjugate

(a) 1.6

1.2

0.8

0.4

0.0 0

0.6 Absorbance at 420 nm

xylan was a water-soluble branched polysaccharide with low molecular weight. These features made it miscible with chitosan but unable to form hydrogels due to the weak polyelectrolyte interaction. In this case, it was expected that the reactive carbonyl groups in the soluble xylan were easily attached to the amino groups of chitosan.

50

100 150 200 250 Reaction time (min)

300

350

50

100 150 200 250 Reaction time (min)

300

350

300

350

(b)

0.5 0.4 0.3 0.2 0.1 0.0 0

2500

(c) Relative fluorescence intersity

522

2000

1500

1000

500

0

3.3. Structures of conjugates

0

50

100

150

200

250

Reaction time (min)

FTIR was used for the analysis of polymeric molecules in the reaction. As shown in Fig. 2, the characteristic absorption bands of chitosan observed at around 1659 cm1 and 1597 cm1 are assigned to amide I and primary amino group respectively. For xylan, the absorption at 1641 cm1 is principally associated with adsorbed water, and the prominent band at 1043 cm1 is attributed to the C–O, C–C stretching or C–OH bending; while the sharp band at 897 cm1, corresponding to the C1 group frequency, is the characteristic b-glycosidic linkage between the sugar units (Kacurakova, Belton, Wilson, Hirsch, & Ebringerova, 1998). When the two polymers were heated together, the absorption peaks at 1659 cm1 and 1597 cm1 disappeared, and new bands at 1560 cm1 and 1716 cm1 were observed, suggesting that Schiff base (C@N double bond) formed between the reducing end of xylan and the amino groups of chitosan (Liu, Nishi, Tokura, & Sakairi, 2001; Umemura & Kawai, 2007). When the conjugate was heated

Fig. 1. Spectrophotometric changes of xylan-chitosan conjugates as a function of heating time (a) UV absorption at 280 nm; (b) Browning at 420 nm; (c) fluorescent intensity at emission wavelength of 415 nm with excitation wavelength of 343 nm (xylan:4; chitosan:s; xylan-chitosan conjugate:h).

for 120 min, the spectra did not change much, except that the intensity of 1560 cm1 decreased. The result was consistent with the study of Umemura & Kawai (2008), in which it was believed that the secondary amines changed into tertiary amines without much change of the polymer structures. Though the structure of the conjugate did not change from the analysis of FTIR, the molecular size may change due to the rearrangement of Amadori product and the formation of heterocyclic compounds. As shown in Fig. 3, the hydrodynamic radius of the aggregates for chitosan was 1.26  103 nm, but this value reached

523

X. Li et al. / Food Chemistry 126 (2011) 520–525

60 DPPH scavenging activity (%)

Xylan

Transmittance (%)

Chitosan

0min 15min 30min 60min 120min

(a)

50 40 30 20 10 0 0

4000

3500

3000

2500

2000

1500

1000

50

500

100

150

200

250

300

350

300

350

Reaction time (min)

-1

Wave number (cm )

1.2

Fig. 2. FTIR spctra of xylan, chitosan and xylan-chitosan conjugates heated for different time.

(b)

3.28  103 nm after mixing with xylan. However, when increasing heating time the size of the aggregates decreased; for example, the hydrodynamic radius was only 1.0  103 nm when the solution was heated for 60 min. It is reasonable that heating the mixture of xylan and chitosan resulted in Amadori product via the Schiff base, and the aggregates became larger due to crosslinking of the long chain polymers in the early stage of the Maillard reaction; during intense heating, Amadori compounds may undergo several degradation reactions with cleavage in the sugar unit and formation of heterocyclic compounds in the subsequent advanced and final stages of Maillard reaction (Silvan, van de Lagemaat, Olano, & del Castillo, 2006).

Several MRPs have been shown to contribute greatly to the shelf-life of heat-treated foods, through their antioxidant, antiallergenic, antimicrobial and cytotoxic properties (Manzocco, Calligaris, Mastrocola, Nicoli, & Lerici, 2000). The antioxidant activity and the reducing power of the heated solution were also investigated and are shown in Fig. 4. The heated mixture attained fine antioxidant activity against relatively stable DPPH radical depending on heating time, while xylan and chitosan alone did not have high scavenging potential. The IC50 of scavenging activity was reached at 220 min. It can be described that the 220 min-heated mixture of 2 mg/ml chitosan and 8 mg/ml xylan (the conjugate

1.2 Chitosan

0min

1.0

Intensity

0.8 0.6 0.4 0.2 0.0 100

1000 Radius (nm)

0.8 0.6 0.4 0.2 0.0 0

50

100 150 200 250 Reaction time (min)

Fig. 4. DPPH free radical-scavenging activity and reducing power of xylan-chitosan conjugates as a function of heating time (xylan:4; chitosan:s; conjugate:h).

3.4. DPPHradical-scavenging activity and reducing power

Xylan 120min 60min

Absorbance at 700nm

1.0

10000

Fig. 3. Hydrodynamic radius (Rh) of (a) chitosan; (b) xylan-chitosan conjugate heated for 0 min; (c) conjugate heated for 30 min; (d) conjugate heated for 120 min.

was five-fold diluted and tested) was endowed with good antioxidant activity, equal to the antioxidant ability of 2 mg/ml BHT. The antioxidant activity of the xylan-chitosan conjugate can offer substantial health-promoting activity to the shelf-life of the foods, but the mechanisms may be different from chitosan itself. According to Xie, Xu, and Liu (2001), chitosan derivatives scavenge hydroxyl radicals via hydrogen transmission by OH or NH2 groups in the pyranose rings. The better radical-scavenging activity of the conjugates can be attributed to the advanced Maillard reaction product melanoidins, which showed high antioxidant capacity through a chain-breaking, oxygen-scavenging and metal-chelating mechanism (Silvan et al., 2006). Studies have indicated that the antioxidant effect is related to the development of reductones (Shon, Kim, & Sung, 2003), which are terminators of free radical chain reactions. The reducing activity of the solution is shown in Fig. 4b. The solution of conjugate showed an increasing reducing power when prolonging the heating time, but there was no noteworthy reducing activity of chitosan or xylan alone, consistent with the results of DPPH radical-scavenging activity above. Based on the spectrophotometric studies and radical-scavenging activity analysis, the xylan-chitosan conjugate could be used as a natural antioxidant with controllable capacities. In the process of initial heating, there was no fluorescence or UV absorbance, and the antioxidant activity was very low. When prolonging heating time, the fluorescence intensity increased to maximum in the process of advanced Maillard reaction, while the conjugate possessed increased antioxidant capacity and brown colour. At prolonged heating, some of the MRPs could be involved in the formation of melanoidins with dark colour and significant antioxidant capacity, as the molecular chains of the polymers broke down. In this sense,

524

X. Li et al. / Food Chemistry 126 (2011) 520–525

Antibacterial zone diameter (mm)

13

xylan-chitosan conjugate was endowed with high antioxidant and antibacterial activities by Maillard reaction, and can be used as a promising candidate material for food storage.

E. Coli S. aureus

12 11

Acknowledgments

10

We are grateful to Prof. Lina Zhang and Dr. Sheng Li in assistance of DLS detection. This work was supported by a grant from the Major State Basic Research Development Program of China (973 Program) (No. 2010CB732204).

9 8

References

7 Xylan

CS

0

80

160

240

320

Reaction time (min) Fig. 5. Antimicrobial activities of xylan, chitosan and xylan-chitosan conjugates heated for different times.

UV absorbance, browning and fluorescence changes were good indirect indices to monitor the formation procedures of MRPs with free radical-scavenging activity, as stated by Morales and JimnezPerez (2001). 3.5. Antibacterial evaluation The inhibition of antimicrobial activity of xylan-chitosan conjugates against E. coli and S. aureus is depicted in Fig. 5 by the method of antibacterial zone diameter. There was no antimicrobial effect for xylan, with or without heating (data not shown). For chitosan, the diameter of the inhibition zone was around 9.1 mm vs. control of 7.5 mm against the two microbes, and no visible changes were observed during heating. In contrast, the conjugates exhibited increased zone area both against E. coli and S. aureus with prolongation of heating time, indicating that the antibacterial activities of the MRPs were higher than chitosan. The reason for the inhibitory activity of chitosan towards Gram-negative bacteria is that the polycationic amino groups interact with the predominantly anionic components (LPS) on the bacterial surface and change the membrane permeability (Tsai & Su, 1999), whereas the formation of a film over the surface of the cell membrane preventing the nutrients from entering the cell was postulated to be the main mechanism for the antibacterial activity of chitosan against S. aureus (Liu et al., 2004). With regard to the xylan-chitosan conjugate, it has been reported that the MRPs might destabilise the outer membrane and inhibit the growth of bacterial cells, due to their excellent surfactant properties (Nakamura, Kato, & Kobayashi, 1992). Similarly, the conjugates of chitosan with soy protein, b-lactoglobulin and glucosamine are all reported to enhance bactericidal action (Chung et al., 2005; Miralles, Martinez-Rodriguez, Santiago, van de Lagemaat, & Heras, 2007; Usui et al., 2004) . 4. Conclusion In this work, a novel conjugate based on the polysaccharides xylan and chitosan was developed for food preservation. Browning and fluorescence reaction of the heated solution indicated the presence of Maillard reaction products, which showed excellent antioxidant activity, while chitosan or xylan alone did not have any. During heating, the structure of the polymers did not change much, but the diameter of the aggregates decreased, based on the analysis of dynamic light scattering. The antimicrobial activity of the xylan-chitosan conjugate was higher than chitosan. Thus,

Ajandouz, E. H., & Puigserver, A. (1999). Nonenzymatic browning reaction of essential amino acids: Effect of pH on caramelization and Maillard reaction kinetics. Journal of Agricultural and Food Chemistry, 47(5), 1786–1793. Bondet, V., Brand-Williams, W., & Berset, C. (1997). Kinetics and mechanisms of antioxidant activity using the DPPH free radical method. Food Science and Technology International, 30(6), 609–615. Chung, Y. C., Kuo, C. L., & Chen, C. C. (2005). Preparation and important functional properties of water-soluble chitosan produced through Maillard reaction. Bioresource Technology, 96(13), 1473–1482. Dervilly-Pinel, G., Thibault, J. F., & Saulnier, L. (2001). Experimental evidence for a semi-flexible conformation for arabinoxylans. Carbohydrate Research, 330(3), 365–372. Ebringerova, A., Hromadkova, Z., Alfodi, J., & Hribalova, V. (1998). The immunologically active xylan from ultrasound-treated corn cobs: Extractability, structure and properties. Carbohydrate Polymers, 37(3), 231–239. Ebringerova, A., Hromadkova, Z., & Heinze, T. (2005). Hemicellulose. In: Polysaccharides 1: Structure, Characterization and Use, Vol. 186 (pp. 1-67). Berlin: Springer-Verlag Berlin. Gabrielii, I., & Gatenholm, P. (1998). Preparation and properties of hydrogels based on hemicellulose. Journal of Applied Polymer Science, 69(8), 1661–1667. Garcia, R. B., Nagashima, T., Praxedes, A. K. C., Raffin, F. N., Moura, T., & do Egito, E. S. T. (2001). Preparation of micro and nanoparticles from corn cobs xylan. Polymer Bulletin, 46(5), 371–379. Hansen, N. M. L., & Plackett, D. (2008). Sustainable films and coatings from hemicelluloses: A review. Biomacromolecules, 9(6), 1493–1505. Hofmann, T., Bors, W., & Stettmaier, K. (1999). Studies on radical intermediates in the early stage of the nonenzymatic browning reaction of carbohydrates and amino acids. Journal of Agricultural and Food Chemistry, 47(2), 379–390. Kacurakova, M., Belton, P. S., Wilson, R. H., Hirsch, J., & Ebringerova, A. (1998). Hydration properties of xylan-type structures: An FTIR study of xylooligosaccharides. Journal of the Science of Food and Agriculture, 77(1), 38–44. Kanatt, S. R., Chander, R., & Sharma, A. (2008). Chitosan glucose complex - A novel food preservative. Food Chemistry, 106(2), 521–528. Kato, A. (2002). Industrial applications of Maillard-type protein-polysaccharide conjugates. Food Science and Technology Research, 8(3), 193–199. Kurita, K. (2006). Chitin and chitosan: Functional biopolymers from marine crustaceans. Marine Biotechnology, 8(3), 203–226. Liu, H., Du, Y. M., Wang, X. H., & Sun, L. P. (2004). Chitosan kills bacteria through cell membrane damage. International Journal of Food Microbiology, 95(2), 147– 155. Liu, X. D., Nishi, N., Tokura, S., & Sakairi, N. (2001). Chitosan coated cotton fiber: Preparation and physical properties. Carbohydrate Polymers, 44(3), 233–238. Lv, Y., Yang, X. B., Zhao, Y., Ruan, Y., Yang, Y., & Wang, Z. Z. (2009). Separation and quantification of component monosaccharides of the tea polysaccharides from Gynostemma pentaphyllum by HPLC with indirect UV detection. Food Chemistry, 112(3), 742–746. Manzocco, L., Calligaris, S., Mastrocola, D., Nicoli, M. C., & Lerici, C. R. (2000). Review of non-enzymatic browning and antioxidant capacity in processed foods. Trends in Food Science & Technology, 11(9–10), 340–346. Miralles, B., Martinez-Rodriguez, A., Santiago, A., van de Lagemaat, J., & Heras, A. (2007). The occurrence of a Maillard-type protein-polysaccharide reaction between beta-lactoglobulin and chitosan. Food Chemistry, 100(3), 1071–1075. Morales, F. J., & Jimenez-Perez, S. (2001). Free radical scavenging capacity of Maillard reaction products as related to colour and fluorescence. Food Chemistry, 72(1), 119–125. Morales, F. J., & van Boekel, M. (1998). A study on advanced Maillard reaction in heated casein/sugar solutions: Colour formation. International Dairy Journal, 8(10–11), 907–915. Nakamura, S., Kato, A., & Kobayashi, K. (1992). Bifunctional lysozymegalactomannan conjugate having excellent emulsifying properties and bactericidal effect. Journal of Agricultural and Food Chemistry, 40, 735–739. Park, S. K., & Bae, D. H. (2006). Antimicrobial properties of wheat gluten-chitosan composite film in intermediate-moisture food systems. Food Science and Biotechnology, 15(1), 133–137. Rao, M. S., Chander, R., & Sharma, A. (2005). Development of shelf-stable intermediate-moisture meat products using active edible chitosancoating and irradiation. Journal of Food Science, 70(7), M325–M331.

X. Li et al. / Food Chemistry 126 (2011) 520–525 Shon, M. Y., Kim, T. H., & Sung, N. J. (2003). Antioxidants and free radical scavenging activity of Phellinus baumii (Phellinus of Hymenochaetaceae) extracts. Food Chemistry, 82(4), 593–597. Silvan, J. M., van de Lagemaat, J., Olano, A., & del Castillo, M. D. (2006). Analysis and biological properties of amino acid derivates formed by Maillard reaction in foods. Journal of Pharmaceutical and Biomedical Analysis, 41(5), 1543–1551. Song, Y., Babiker, E. E., Usui, M., Saito, A., & Kato, A. (2002). Emulsifying properties and bactericidal action of chitosan-lysozyme conjugates. Food Research International, 35(5), 459–466. Tsai, G. J., & Su, W. H. (1999). Antibacterial activity of shrimp chitosan against Escherichia coli. Journal of Food Protection, 62(3), 239–243.

525

Umemura, K., & Kawai, S. (2007). Modification of chitosan by the Maillard reaction using cellulose model compounds. Carbohydrate Polymers, 68(2), 242–248. Umemura, K., & Kawai, S. (2008). Preparation and characterization of Maillard reacted chitosan films with hemicellulose model compounds. Journal of Applied Polymer Science, 108(4), 2481–2487. Usui, M., Tamura, H., Nakamura, K., Ogawa, T., Muroshita, M., Azakami, H., et al. (2004). Enhanced bactericidal action and masking of allergen structure of soy protein by attachment of chitosan through Maillard-type proteinpolysaccharide conjugation. Nahrung-Food, 48(1), 69–72. Xie, W. M., Xu, P. X., & Liu, Q. (2001). Antioxidant activity of water-soluble chitosan derivatives. Bioorganic & Medicinal Chemistry Letters, 11(13), 1699–1701.