Preparation and characterization of chitosan film incorporated with thinned young apple polyphenols as an active packaging material.

TL;DR: The results indicated that the addition of YAP resulted in a significant increase in the thickness, density, swelling degree, solubility and opacity of chitosan film, but the water content, water vapor permeability and mechanical properties of the film were decreased.

About: This article is published in Carbohydrate Polymers.The article was published on 2017-05-01 and is currently open access. It has received 334 citations till now.

Summary

1. Introduction

• In recent years, biodegradable and biobased packaging materials have been obtained a widespread interest due to the limited natural resources, food safety and environmental problems caused by the use of petrochemical-based plastics.
• Thinned young apples actually are agricultural resource since the content of total polyphenols is approximately ten times higher than that in ripe apples.
• Hence, the aim of this work is to prepare and characterize the chitosan film incorporated with thinned young apple polyphenols (YAP) and analyze the effect of YAP on the physical, mechanical and bioactive properties of the film to determine if the YAP-chitosan film has a potential as an active packaging material.

2.1 Materials

• Chitosan with the degree of deacetylation of 90% and molecular weight of around 91000 Da was supplied by Shanghai Lanji Technology Development Co. Ltd. .
• All the chemicals in this study were of analytical grade.
• 2 Preparation and determination of YAP YAP was extracted and separated according to their previously reported method (Sun, Guo, Fu, Li, & Li, 2013).
• Finally, the respective YAP-chitosan solutions (70 ml) were cast over the petri dishes (diameter of 15 cm) for 48h at 25°C.
• 4 Physical properties of film 2.4.1 Scanning electron microscopy (SEM) TM3030 scanning electron microscope (Hitachi Co. Ltd., Kyoto, Japan) was used to observe the surface and cross-section morphology of the films with and without YAP.

2.4.2 Thickness and density

• The film thickness was measured using a digital micrometer (Mitutoyo Absolute, Tester Sangyo Co. Ltd., Tokyo, Japan).
• The film density was determined by the film weight and volume.
• Then, they were placed in 100 ml beakers with 50 ml distilled water covered with plastic wraps and stored at 25°C for 24 h.
• Next, the films were dried superficially with filter papers and dried at 105°C to constant weight to obtain the final dry mass (M2).
• The swelling degree was calculated using the following equation: Film swellng degree = 𝑀2−𝑀1 𝑀1 × 100% (2).

2.4.4 Water vapor permeability (WVP) and water content

• WVP was determined according to the method provided by Talja et al. (2008) with some modifications.
• The films were cut into pieces (6 cm×6 cm) before sealed onto a special aluminum cup (internal diameter: 6 cm, external diameter: 9 cm, exposed area: 28.27 cm 2 , depth: 1.3 cm) containing anhydrous calcium chloride and ensured that the gap between the film and anhydrous calcium chloride was less than 6mm.
• Then the cup was placed into a desiccator with distilled water at room temperature.
• The cup was weighed every 2 h using an analytical balance with a precision of 0.1 mg (Mettler-Toledo Instrument (Shanghai) Co. Ltd., China).
• The film pieces (2 cm×2 cm) were prepared and weighed (M1) for water content according to Jimenez et al. (2012) with some modifications.

2.4.5 Film color and opacity

• The film color was determined using a Konica ® colorimeter (CR-300, Japan).
• The absorbance value was measured at 600 nm using a spectrophotometer (UV 2100, Unico Instruments Co. Ltd., Shanghai, China).
• The films were cut into 1.5cm×15cm strips for the determination of mechanical properties.
• Yeast strains were cultured at 28°C and moulds strains were cultured at 25°C.
• The growth diameters were measured after incubation for 144 h. 2.7 Characterization of YAP-chitosan film 2.7.1 FTIR analysis.

2.7.4 Thermal stability analysis

• Differential scanning calorimetry (DSC) analysis was performed using a Q1000 DSC system (TA instruments, New Castle, USA).
• 10mg of the film pieces were sealed in a standard aluminum pan heated at a constant rate of 10°C/min from 0 to 350°C at the nitrogen atmosphere.
• The results were expressed as mean values and standard deviation (SD).
• The experiment data were subjected to variance analysis and the Tukey's test using SPSS 18.0 at 5% significance level (p<0.05).

3.1 Phenolic compositions and contents in YAP

• The individual phenolic compositions in YAP were determined using HPLC method and summarized in Fig.
• There was a small amount of procyanidin B2, epicatechin, caffeic acid, rutin and quercetin-3-galactoside.

3.2 Physical properties of YAP-chitosan film

• The chitosan films incorporated with different concentrations of YAP were photoed and presented in Fig. 2A, where the colour appearance of the films became darker with the YAP concentration increasing.
• The effects of YAP on the thickness, density, water content, swelling degree and solubility of films are shown in Table 1.
• The control films showed the lowest swelling degree and solubility.
• The high values of swelling degree and water solubility of the YAP-chitosan films may be attributed to the hydrophilic groups of polyphenols that can easily interact with water molecules (Mathew, Brahmakumar, & Abraham, 2006).
• To keep foods fresh, the WVP value should be maintained as low as possible.

3.3 Mechanical properties of YAP-chitosan film

• The effects of different concentrations of YAP on the mechanical properties of chitosan film are presented in Fig. 3B.
• The presence of YAP caused significant (p<0.05) differences in the tensile strength and elongation at break (p<0.05).
• Tensile strength represents the maximum strength that a film can withstand, and elongation is the measurement of the ability of a film to stretch.
• Chitosan composition, film network microstructure and intermolecular forces play an important role in the mechanical properties of chitosan film.
• Besides, the mechanical property also strongly depends on the crystallites of film.

3.4.1 DPPH• scavenging activity

• Fig. 3C showed the DPPH scavenging activity of the control and YAP-chitosan films.
• The control films were shown a low DPPH scavenging activity, which was similar to a previous study (Yen, Yang, & Mau, 2008).
• As shown, YAP-chitosan films had the concentration-dependent antimicrobial activities against bacteria and moulds (Fig. 4A-F), but no activity against yeasts (Fig. 4G-I).
• Similarly, due to the interactions between polyphenols and chitosan, including hydrogen bonding and hydrophobic force (Peng, Wu, & Li, 2013), the changes of internal bonds between atoms occurred in certain functional groups, which induced the peaks located at 2941 cm -1 , 2885 cm -1 , 1650 cm -1 becoming less discernible.
• The non-covalent interactions (such as physical binding) between the two compounds do not alter the internal chemical bonds of polysaccharides (Le Bourvellec, Bouchet, Renard, 2005), which normally results in the characteristic chemical shifts of carbohydrates (around 3.0-5.3 ppm) unchanged because of the relatively weaker interactions (Prakash, Iturmendi, Grelard, Moine, & Dufourc, 2016; Khoo, Abas, Abdullah, Tohit, & Hamid, 2014).

3.5.3 X-ray diffraction analysis

• The X-ray spectra of film samples are displayed in Fig. 5C.
• In the control films, the chitosan was in a crystalline state with four main diffraction peaks.
• When the YAP concentration increased to 0.50%, the diffraction peaks still existed but became more flattened and less discernible, suggesting a lower crystallinity in the film.
• As discussed, the mechanical properties of a film strongly depend on the crystallites in its film structure.
• Thus, the amorphous structures in chitosan films that appeared after the addition of YAP leaded to lower mechanical properties (Fig. 3B) of films in this study as described above.

3.5.4 DSC analysis

• DSC analysis was used to measure the enthalpy change (ΔH) and the peak melting temperature (Tm) of the films.
• The thermograms showed that peak 1, an endothermic peak at around 100 °C related to water composition, and that peak 2, a major degradation exothermic peak at around 300°C was associated with the depolymerization and pyrolytic decomposition of the polysaccharide backbone (Martins, Cerqueira, & Vicente, 2012).
• The addition of YAP had negative influence on thermal stability of chitosan films, which can be demonstrated from peak 2.
• The decrease in thermal stability may be attributed to chemical etching caused by bond disrupt, chain scission or chemical degradation of macromolecules (Pankaj et al., 2014).
• Also, the thermal stability of macromolecules is related to the crystalline structure in them, meaning that higher crystal degree corresponds to higher thermal stability, because more energy (heat) is required to break the higher crystalline structure.

4. Conclusion

• Thinned young apple is a rich source of antioxidant compounds, in which chlorogenic acid and phlorizin accounted for a large proportion of total polyphenols.
• Besides, the YAP-chitosan films were shown antimicrobial effects on both bacteria and moulds, but no effect on yeast.
• Both the NMR and FTIR spectra of YAP-chitosan films suggested that the interactions between YAP and chitosan were likely to be non-covalent.
• Interestingly, the changing tendency of crystalline degree kept pace with that of thermal stability for all the tested chitosan films.
• Conclusively, the modified colour, increased bioactivities and enhanced water barrier properties of chitosan film, as well as the slight changes of mechanical properties at low concentration of polyphenols indicate that chitosan film incorporated with YAP can be a promising alternative to synthetic materials, potentially contributing to food shelf-life extension.

Figures

Table 2 Color and opacity of YAP-chitosan films.

Table 1 Physical properties of YAP-chitosan films.

Table 3 DSC thermal parameters (peak temperature (Tm) and peak area (ΔH)) of YAP-chitosan films.

Fig. 1 HPLC chromatograms of 13 phenolic standards (A) and YAP (B). Individual peaks: 1. Gallic acid, 2. Catechin, 3. Procyanidin B2, 4. Chlorogenic acid, 5. 4-Hydroxyl benzoic acid, 6. Epicatechin, 7. Caffeic acid, 8. Rutin, 9. Quercetin-3-galactoside, 10. Ellagic acid, 11. Quercetin-3-rhamnoside, 12. Phlorizin, 13. Quercetin. The contents of individual phenolic compounds in YAP were summarized in the table inset in (B).

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Highlights 1. basic physical properties of chitosan film were increased by the addition of yap 2. water content, vapor permeability and mechanical properties were decreased by yap 3. antioxidant and antimicrobial properties of chitosan film were enhanced by yap 4. the interactions between yap and chitosan were likely to be non-covalent 5. change of crystalline degree kept pace with that of thermal stability for yap films preparation and characterization of chitosan film incorporated with thinned young apple polyphenols as an active packaging material" ?

The objective of this study was to characterize the physical, mechanical and bioactive properties of chitosan film incorporated with thinned young apple polyphenols ( YAP ). Furthermore, the thermal stability of the film was decreased by YAP addition, suggested by DSC. Overall, YAP-chitosan film was shown a potential as a bioactive packaging material to extend food shelf-life. 

Q2. What was the effect of YAP on the diffraction peaks?

When the YAP concentration increased to 0.50%, the diffraction peaks still existed but became more flattened and less discernible, suggesting a lower crystallinity in the film. 

Q3. What is the main function of the preservative films?

One of the main functions of the preservative films is separating foods from surrounding atmosphere vapor to prevent or retard food deterioration. 

Q4. What is the effect of YAP on thermal stability?

The decrease in thermal stability may be attributed tochemical etching caused by bond disrupt, chain scission or chemical degradation of macromolecules (Pankaj et al., 2014). 

Q5. What was the effect of YAP on the thermal stability of chitosan films?

The peak temperature and peak area decreased with the increase of YAP concentration, indicating a declining tendency of thermal stability. 

Q6. What are the main factors that affect the mechanical properties of chitosan film?

Chitosan composition, film network microstructure and intermolecular forces play an important role in the mechanical properties of chitosan film. 

Q7. What is the effect of YAP on the water barrier properties of chitosan?

Polyphenols may establish the interactions with chitosan molecules through the potential hydrogen bonding, which may limit the interactions between hydrophilic groups of chitosan and water molecules due to theABCcompetitive binding effect (Siripatrawan, & Harte, 2010; Sascha, Rawel, & Jürgen, 2004). 

Q8. Why have biobased packaging materials been obtained a widespread interest?

In recent years, biodegradable and biobased packaging materials have been obtained a widespread interest due to the limited natural resources, food safety and environmental problems caused by the use of petrochemical-based plastics. 

Q9. What is the effect of polyphenols on the chitosan film?

It has been reported that the incorporation of polyphenols into chitosan film may interrupt the ordered crystalline structure formation in the chitosan matrix, weakening the intermolecular hydrogen bonding, hindering the polymer-polymer chain interactions and providing the flexible domains within the films, and thus resulted in the decreased mechanical properties (Ahmed, Mulla, & Arfat, 2016; Ying, Creber, Peppley, & Bui, 2003). 

Q10. What is the effect of YAP on chitosan film?

the modified colour, increased bioactivities and enhanced water barrier properties of chitosan film, as well as the slight changes of mechanical properties at low concentration of polyphenols indicate that chitosan film incorporated with YAP can be a promising alternative to synthetic materials, potentially contributing to food shelf-life extension. 

Q11. How much water content of yap-chitosan films decreased after Y?

After the addition of 0.75% YAP and 1.0% YAP, the water content of YAP-chitosan films decreased by 19.45% and 42.73%, respectively. 

Q12. What is the effect of YAP on the chitosan film?

In the YAP-chitosan films, the interactions between YAP and chitosan may hinder the inter- and intramolecular hydrogen bonds formation in chitosan itselfbecause of the competitive effect of hydrogen bonds between chitosan and YAP (this could be shown from FTIR results as well), resulting in a low crystallinity. 

Q13. What is the effect of yap on the thickness and density of chitos?

By this way, polyphenols might act as a bridge, binding with more than one chitosan molecules due to the existence of polyhydroxyl groups in the molecular structures; therefore, the distance between chitosan molecules became shorter, inducing the film structure more compact and thus increased thickness and density. 

Q14. What is the effect of YAP on the thermal stability of chitosan films?

Their results indicated the changing tendency of crystalline degree kept pace with that of thermal stability of YAP-chitosan film with the YAP concentration increasing, supporting this statement. 

Q15. What is the role of polyphenols in food packaging?

Although polyphenol, a class of bioactive antioxidant, has been widely applied in food preservative field, the incorporation of it with chitosan film as a new-type and effective bio-preservative film needs to be further shed light on in terms of its structural characterization and bioactivities. 

Q16. What is the effect of YAP on the tensile strength and elong?

the addition of YAP with the concentration from 0.25% to 1.0% decreased the tensile strength from 23.37 MPa to 16.10 MPa and decreased the elongation at break from 140.09% to 93.13%. 

Q17. What did the change in temperature of chitosan film reflect?

the changing tendency of crystalline degree kept pace with that of thermal stability for all the tested chitosan films. 

Q18. What was the effect of YAP on the thickness of chitosan films?

The chitosan films incorporated with different concentrations of YAP were photoed and presented in Fig. 2A, where the colour appearance of the films became darker with the YAP concentration increasing.