WANG Wenjie ,XUE Changhu, ,and MAO Xiangzhao, ,

1) College of Food Science and Engineering, Ocean University of China,Qingdao 266003,China

2) Laboratory for Marine Drugs and Bioproducts of Qingdao National Laboratory for Marine Science and Technology,Qingdao 266200,China

Abstract Hydrogel has high water content and structural similarity with natural extracellular matrix.So it has been widely studied and applied in the field of biomedicine.In order to further develop multifunctional hydrogels,we prepared mixed gels with antiultraviolet properties.This study found that the addition of polysaccharides and polyphenols was beneficial to the rheological,mechanical properties,and biological activity of the protein.Chitosan (CS) could significantly improve the viscoelasticity,hardness,gel strength,thermal stability and crystallinity of gelatin.Interestingly,the addition of gallic acid (GA) could not only provide significant cross-linking effect,improve gel properties and microstructure,but also improve the UV resistance of the mixed gel.

Key words gallic acid;chitosan;gelatin;gel properties;UV resistance

1 Introduction

Hydrocolloids have been widely used in the food industry as stabilizers,thickeners,and gelling agents to improve the elasticity,mechanical properties and stability of food(Shaoet al.,2018;Wanget al.,2018;Al-Sherbiniet al.,2019).Due to the increasing demand for hydrocolloids with specific functions,it has become very important to find new hydrocolloid sources with potential industrial applications (Huet al.,2017;Chuet al.,2019).Therefore,some natural polymers with strong hydrophilicity,good rheology,gelation or effective biological activity have attracted worldwide attention (Youssefet al.,2018;Dinget al.,2020;El-Sayedet al.,2020).

Gelatin is a natural biopolymer derived from collagen by hydrolysis.It not only has superior film-forming properties,biodegradability,and edibility,but also is cheap and readily available.However,gelatin has poor mechanical properties and viscosity.It was reported that sodium alginate could improve the texture properties and nanostructure of tilapia fish gelatin (Sowet al.,2019).Arabinoxylan ferulate and gelatin could be prepared as fibrous mats as a wound-healing drug delivery platform (Aduba Jr.et al.,2019).Furthermore,psyllium gum and tragacanth gum could improve the film-forming properties and mechanical properties of gelatin (Hedayatniaet al.,2019).Therefore,polysaccharides can enhance the mechanical properties,gel strength and film-forming properties of gelatin.

Chitosan (CS) is a wide range of natural linear alkaline polysaccharides with antibacterial,and antioxidant properties.It is a non-toxic,safe,and inexpensive polymer with great biodegradability and compatibility.CS can be used as a universal carrier matrix for cosmetics,pharmaceuticals,food,and other products.Additionally,CS has high viscosity and can form gels.So it is often used as a cross-linking agent and gel enhancer to improve the gel properties with other polymers.For example,when CS and oxidative hyaluronic acid are mixed together,they can form a more stable hydrogel for medicine (Nairet al.,2011),and CS can replace calcium ion and react with sodium alginate to form mixed gel (Kimet al.,2008).CS can also enhance the physical and biological properties of silk protein scaffold (Wanget al.,2020).Therefore,the composite of CS and biopolymer can improve its gel and biological properties,and can effectively expand the application of biopolymer.As mentioned above,CS plays an important role on improving gel properties.However,the effects of CS on gelatinization,mechanical properties,and viscoelasticity have not been thoroughly studied.

The gel formed by polysaccharides and proteins is a complex coacervate under different conditions and has poor stability.It has been reported that chemical crosslinking can stabilize polysaccharide-protein gels.For example,formaldehyde and glutaraldehyde have been used successfully by linking hydroxyl residues on polysaccharides or amine residues on proteins (Farriset al.,2011;Anvariet al.,2016).However,the safety of chemical crosslinkers has been controversial.In recent years,polyphenols which are rich in plants have attracted attention as natural crosslinking agents (Bertoloet al.,2020).Natural phenols not only can improve the mechanical properties of polysaccharides and proteins,but also have different degrees of absorption of ultraviolet rays.At the same time,they can remove peroxides and hydroxyl radicals (Azeredo and Waldron,2016;Shaoet al.,2017,2019;Fenget al.,2019).Therefore,phenols can also help improve the functional properties of polysaccharides and proteins.Gallic acid (GA) is a natural phenolic antioxidant,which is extracted from plants and widely used in food,medicine,and cosmetics to prevent lipid peroxidation.GA can effectively scavenge free radicals and have strong antioxidant properties.The antioxidant activity of CS-GA is significantly enhanced compared with CS alone (Ruiet al.,2017).Moreover,GA can enhance the elasticity of the zein film and has the effect of a plasticizer,thereby improving the film-forming property of the zein film (Sunet al.,2019).More interesting is that gallate is closely related to UV resistance (Wuet al.,2016).So GA has the potential to improve the gelation properties and UV resistance of CS-gelatin gel.

However,as far as we know,very limited work has been done on the gelation properties and anti-ultraviolet activity of GA-CS-gelatin gel.Therefore,the purpose of this study was to investigate the effects of CS and GA on the gelation properties of gelatin,including viscoelasticity,mechanical properties,thermal stability,crystallinity,and microstructure.Furthermore,GA-CS-gelatin system was evaluated for its UV resistance.Based on the results,the applicability of the GA-CS-gelatin mixed gel as an ultraviolet shielding material was evaluated.

2 Materials and Methods

2.1 Materials and Reagents

CS (average molecular weight 300 kDa,degree of deacetylation ≥ 95%,water-soluble) was bought from Macklin Biochemical Co.,Ltd.(Shanghai,China).GA and gelatin with analytical grade were purchased from Sigma Chemical Co.(St.Louis,MO,USA).Water used in the experiment was purified by Milli-Q water purification system(Millipore,Bedford,MA,USA).

2.2 Preparation of GA-CS-Gelatin Mixed Gels and Films

The CS was dissolved in a 0.5% acetic acid solution,and stirred at 25℃ using a magnetic stirrer (500 r min-1) for 60 min to prepare a 3% CS (w/v) stock solution.The CS stock solution was stored overnight for complete hydration (Jinet al.,2009).Then,a 15% gelatin stock solution was prepared in a water bath at 50℃,and 0.6% GA was dissolved in ultrapure water to obtain a gallic acid stock solution.

CS,gelatin,and GA stock solution were mixed in a ratio of 1:1:1 (v/v) at 45℃,and stirred for 30 min with a magnetic stirrer (300 r min-1) to obtain a solution including 0.2%GA,1% CS and 5% gelatin.The mixture was heated at 90℃ for 20 min,then was quickly cooled to 25℃,and stored in a refrigerator at 4℃ overnight to form a gel.The concentration of CS gel was 3%,which was the maximum solubility of CS in 0.5% acetic acid.For the consistency of the mixed gel system,the concentration of gelatin was selected to be 6%.The mass of 0.2% GA was much smaller than that of the gel system,which can be ignored.

Following the above method,the mixture of CS,gelatin and GA stock solution were heated at 90℃ for 20 min,and quickly cooled to 45℃.Then the mixture was poured into a 9 cm diameter acrylic plates (10 g solution/plate) to prepare a GA-CS-gelatin gel film (Wanget al.,2019a).

2.3 Rheological Measurement

The rheological properties of GA-CS-gelatin gels were evaluated using ARES-G2 rheometer (TA Instruments,New Castle,USA) equipped with parallel plate with 50 mm diameter and 1 mm gap.Preparation of mixed gel was completed and allowed to stand at 25℃ for 12 h.A solvent trap cover was applied to avoid evaporation and mitigate interference.Before each measurement,samples were equilibrated for 2 min (Wanget al.,2018).

The frequency-dependent storage modulus (G'),loss modulus (G") and loss angle (tan δ=G"/G') of samples were measured at a constant temperature of 25℃ and a fixed strain of 1% followed by frequency sweep (0.1 change to 10 Hz) tests.

In the creep recovery test,all gel samples were subjected to a continuous stress of 100 Pa,and the corresponding strain of the samples under the applied stress were recorded.At the end,the applied stress was instantly cancelled,that was,the external force was zero,and the strain of the sample was also recorded.The strain curve obtained twice is the creep curve (Huanget al.,2016).

Stress relaxation experiment was performed at a constant strain (20%) at set temperature to demonstrate changes in the structure of the mixed gel network.When the normal force was close to zero,the stress relaxation test was started and the relaxation time was 280 s (Wanget al.,2019b).

2.4 Texture Properties of GA-CS-Gelatin Mixed Gels

The analysis of the gel texture was performed according to the method described by Wanget al.(2018) with minor modifications.Briefly,the gel was prepared into a block of 40 mm × 40 mm × 4 mm,and equilibrated at 25℃for 30 min.Then a texture instrument (TMS-Pro,Ensoul,UAS) was applied for measurement.In order to explore the hardness,adhesiveness,springiness,and cohesiveness of the gel,the TPA measurement was first performed.The parameters were set as follows:the pre-test speed was 40 mm min-1;the test speed was 40 mm min-1;the up-test speed was 40 mm min-1;test distance was 10.0 mm;and trigger force was 1.5 N.Then the gel strength was measured and the parameters were set as follows:test speed was 40 mm min-1;initial force was 1.5 N;sample height was 4 mm;and piercing distance was 3 mm.Finally,the stress relaxation of the sample was measured.The parameters were set as follows:test speed was 40 mm min-1,initial force was 1.5 N,deformation was 50% (Wanget al.,2018).

2.5 Thermal Properties

Thermal gravimetric analyzer (TGA 4000,PE Instruments Inc.,USA) was used to assess the thermal properties of GA-CS-gelatin mixed gels.Briefly,the sample (5–10 mg) was weighed.Then it was placed in the equipment alumina crucible,while the reference was an empty aluminum pan.The measured temperature was from 30 to 700℃ under N2atmosphere,and the heating rate was 10℃ min-1(Liet al.,2019).

2.6 Measurement of X-Ray Diffraction (XRD)

XRD was performed using a D8-Advance Diffractometer (Bruker AXS Inc.,Germany).The mixed gel was measured at 40 kV and 50 mA,which explored its molecular arrangement,molecular amorphous and crystalline properties (Peng and Yao,2018).

2.7 Scanning Electron Microscope (SEM) Analysis

Microstructure of gel was observed according to the method of Xionget al.(2018) with some modifications.Samples were sprayed with gold,then was fixed on the sample stage and observed by a scanning electronic microscope (JSM6701F,Japan).Acceleration voltage was 5.0 kV,and the XT Microscope Control software was used to collect the images.

2.8 Anti-Ultraviolet of GA-CS-Gelatin Mixed Gel Films

The UV resistance of the composite film was evaluated using UV1100 UV-visible spectrophotometer (Tech-comp,China).The mixed gel films were prepared into a 1 cm × 3 cm rectangle,placed in a cuvette,and scanned at a wavelength range of 200– 800 nm to obtain the light transmittance of the film (Wanget al.,2019a).

2.9 Statistical Analysis

All studies were performed in parallel three times and the data were expressed as mean ± standard deviation (SD).Significant difference (P<0.05) between the data was determined by the analysis of variance (ANOVA) using Duncan’s test with SPSS software version 21 (IBM software,NY,USA).

3 Results and Discussion

3.1 Rheology Analysis

3.1.1 Dynamic rheology analysis

G'andG''are representative parameters in dynamic rheology and can be used to evaluate the viscoelastic behavior of the samples (Nadzharyanet al.,2018).Fig.1 shows theG',G''and tan δ of the gel sample.It could be observed that theG'andG''values of CS increased with the increasing frequency.It was interesting that the increased rate ofG'was greater thanG'',that is,G''was higher thanG'at low frequencies,G'was higher thanG''as the frequency increasing.It was shown that CS changes from liquid viscosity to solid elasticity,exhibiting the characteristics of weak gel.TheG'andG''values of the other samples increased with increasing frequency,andG'was always higher thanG'',indicating that gelatin and mixed gels both showed typical gel behavior.In addition,G'andG''of the sample at the same frequency were both GA-CS-gelatin mixed gel >CS-gelatin mixed gel >gelatin,showing that the CS could increase the viscoelasticity of gelatin,and GA could effectively enhance the viscoelasticity of GA-CSgelatin mixed gel.

Fig.1 Variation of G',G" (A) and tan δ (B) of 3% CS,6%gelatin,CS-gelatin (1% CS,5% gelatin) and GA-CS-gelatin (0.2% GA,1% CS,5% gelatin) mixed gels with frequency from 0.1 to 10 Hz at 25℃.

The viscoelastic behavior of gel samples can be observed by the tan δ,which is an important value to evaluate the viscoelastic behavior (Razmkhahet al.,2017;Wanget al.,2019).If tan δ >1,the sample appears viscous.When tan δ <1,the sample is elastic.The tan δ of the gel sample is shown in Fig.1B.With the increasing frequency,the tan δ of CS gradually decreased from 1.61 to 0.71,the tan δ of gelatin was less than 0.1,and the tan δ of CS-gelatin and GA-CS-gelatin mixed gel were less than 1 and greater than 0.1.The change of the tan δ of samples more intuitively indicated that CS mainly exhibited viscous behavior,gelatin showed significant solid elastic behavior,and the mixed gels had stronger viscoelasticity than CS and gelatin.The reason for this phenomenon may be that the hydrogen bonding interaction between the carboxyl and hydroxyl groups on gelatin and the hydroxyl and amino groups on CS promoted CS-gelatin to form a more stable gel (Cuiet al.,2015;Afshar and Ghaee,2016).Furthermore,GA introduced more hydroxyl and carboxyl groups,and the enhanced interaction made the three-dimensional structure of the gel more stable (Quanet al.,2019;Sunet al.,2019).Therefore,the viscoelasticity of the mixed gel increased.

3.1.2 Creep and recovery analysis

The effects of CS and GA on the creep recovery characteristics of the gel sample are shown in Fig.2.It could be observed that gelatin,CS-gelatin mixed gel and GACS-gelatin mixed gels had typical viscoelastic characteristics.The strain rapidly rose to a value over time,and then the rate of rise gradually slowed down to a stable value.Furthermore,the total deformation of the gel gradually decreased with the addition of CS and GA.The total deformation could be used as a measure of the strength of the gel sample (Patelet al.,2015;Petcharat and Benjakul,2018).Therefore,it was shown that the gel network structure becomes stronger with the addition of CS and GA.In addition,CS exhibited the typical behavior of liquid unstructured systems,which was similar to the CS creep recovery results explored by Diez-Saleset al.(2007).The deformation of CS increased continuously and almost completely deformed when the stress disappeared (Diez-Saleset al.,2007).Therefore,the sticky behavior of CS was dominated.Gelatin,CS-gelatin and GA-CS-gelatin mixed gels had solid elastic characteristics.The hardness of the gel samples increased with the addition of CS and GA.

Fig.2 Creep and recovery of 3% CS,6% gelatin,CS-gelatin (1% CS,5% gelatin) and GA-CS-gelatin (0.2% GA,1% CS,5% gelatin) mixed gels with time from 0 to 260 s.

3.1.3 Relaxation modulus analysis

The relaxation modulus refers to the process of gradually reducing the stress of the material to maintain its original shape with the increase of time when a deformation is applied.The relaxation modulus is closely related to the network structure of the gel (Wanget al.,2019b;Liet al.,2020).Therefore,the relaxation network can be used to reflect the gel network structure.The effects of CS and GA on the relaxation modulus of gelatin are shown in Fig.3.The results showed that the addition of CS and GA could increase the relaxation modulus.CS could interact with gelatin and promote the relaxation modulus of GA-CSgelatin mixed gel.In addition,GA promoted the crosslinking between CS and gelatin molecules,thereby enhanced the gel strength of CS-gelatin.Therefore,the network structure of the gel was more compact and stable.The addition of polysaccharides and phenols could enhance the rheological properties of gelatin,and enhance the intermolecular bonding,so the gelatin exhibited stronger viscoelasticity(Muhozaet al.,2019).In addition,the results of the relaxation modulus further showed that the addition of CS and GA effectively increased the viscoelasticity of gelatin,enhanced the intermolecular interaction,and stabilized the gel network.

Fig.3 The relaxation modulus of 3% CS,6% gelatin,CSgelatin (1% CS,5% gelatin) and GA-CS-gelatin (0.2% GA,1% CS,5% gelatin) mixed gels.

3.2 Texture Analysis

3.2.1 TPA analysis

Texture properties are important indicators for evaluating the properties of gels and can be used to assess the physical properties and mechanical strength of gels (Moonet al.,2017;Chenet al.,2019).Table 1 shows the effects of CS and GA on the texture properties of gelatin.CS could significantly increase the hardness,adhesiveness,cohesiveness and springiness of gelatin.The addition of GA and CS promoted the mixed gels to have higher gel hardness,adhesiveness,cohesiveness,and springiness.The results of the texture properties of the gel samples were consistentwith the rheological results,indicating that CS and GA were active and effective in the gelatin formation process.

Table 1 Gel strength of 3% CS,6% gelatin,CS-gelatin (1% CS,5% gelatin) and GA-CS-gelatin (0.2% GA,1% CS,5% gelatin) mixed gels

3.2.2 Gel strength analysis

The strength of the mixed gel is another important indicator for evaluating the texture of the gel.The gel strength can directly reflect the mechanical properties of the gel sample (Petcharat and Benjakul,2018;Wanget al.,2018).Fig.4 shows the gel strength of the gel samples.CS could significantly enhance the gel strength of gelatin.Polysaccharides could improve gelatin properties as gelling agents.Charged gelatin and polysaccharide macromolecule can generate electrostatic effects to form polyelectrolyte complexes.In addition,CS has a large number of hydroxyl and amino groups,which can form hydrogen bonding with the carboxyl and hydroxyl groups of gelatins.These interactions promoted the gel to exhibit stronger gel strength and improved the mechanical properties of gelatin.Furthermore,the addition of GA could significantly improve the gel strength of CS-gelatin.GA could effectively cross-link CS and gelatin,which promoted the gel network of CS-gelatin to be more stable.Therefore,the strength of the gel was stronger.Previous study has also proved that the gel strength of gelatin can be increased with the addition of polysaccharides and polyphenols (Bertoloet al.,2020).

Fig.4 The gel strength of 6% gelatin,CS-gelatin (1% CS,5% gelatin) and GA-CS-gelatin (0.2% GA,1% CS,5% gelatin) mixed gels.Each value represents the mean ± standard deviation (n=3).Values with different letters in the same column differ significantly (P <0.05).

3.2.3 Stress relaxation analysis

In order to further evaluate the relationship between experimental time and structural rearrangement,the stress relaxation characteristics of these gels have been determined(Fig.5).The relaxation characteristics of gel are not only related to the network structure of the gel,the uniformity of network formation,but also closely related to the texture characteristics of the gel,such as the gel strength,hardness,and viscoelasticity (de Jonget al.,2015).When the deformation of the gel sample reached 50%,its relaxation increased with the addition of CS and GA.In addition,the equilibrium stress of GA-CS-gelatin was greater than that of gelatin or CS-gelatin,indicating that the addition of GA and CS increased the gel strength,hardness,and viscoelasticity of gelatin.Moreover,the beginning time of relaxation is the result of the interaction of internal elastic and viscous behavior of gel.Therefore,an increase of relaxation time indicates that the internal binding force of gel becomes stronger and is difficult to compress and deform.The relaxation times of gelatin,CS-gelatin and GA-CSgelatin were 2.34 s,2.68 s,and 3.64 s,respectively.The addition of CS and GA significantly increased the relaxation time of gelatin,which showed that CS and GA could improve the internal binding force of gelatin,make the gel mechanical properties and viscoelastic stronger.Interestingly,these analysis results were consistent with the rheological measurement results shown in Figs.2– 3.

Fig.5 The stress relaxation of 3% CS,6% gelatin,CS-gelatin (1% CS,5% gelatin) and GA-CS-gelatin (0.2% GA,1% CS,5% gelatin) mixed gels.

3.3 Thermal Gravimetric Analyzer (TGA)

According to the theory of Muhozaet al.(2019),the thermal decomposition process of the gel is divided into three stages.The first stage is the loss of free water and bound water.The second stage is the preliminary thermal decomposition process,including gel long-chain dehydration reaction,depolymerization,and fracture.In the third stage,the gel molecular chain is completely cracked and carbonized.As shown in Fig.6,the first stage of the thermal decomposition of GA-CS-gelatin and gelatin were 30– 170℃,and the mass loss amounts were 3.4% and 6.3%,respectively.The result showed that the loss of water dispersion of the mixed gel was affected by CS and GA.The more water was lost meaning that the gels possessed poor water retention capacity.Therefore,CS and GA increased the water retention capacity of the gelatin gel.This phenomenon could be confirmed by SEM analysis.Gelatin underwent second-stage thermal decomposition near 90–220℃,as the temperature increased.The degree of thermal decomposition of the molecule increased,and the carbon chain and hydrogen bonding were broken to generate H2O and CO2(Liu and Zhen,2018;Di Donatoet al.,2020).In addition,the second-stage thermal decomposition of GACS-gelatin occurred at 160– 350℃ that was significantly higher than that of gelatin,which indicated that CS and GA increased the thermal stability of gelatin.This might be due to the stronger interaction between gelatin and CSGA.Therefore,higher temperatures were required for fracture.Gelatin and GA-CS-gelatin had been completely decomposed and carbonized when the temperature increased to 800℃,and the final remaining masses were 26.1% and 30.1%,respectively.Therefore,thermal stability was improved,and carbonization required higher temperatures.

Fig.6 Thermal properties of 3% CS,6% gelatin,CS-gelatin (1% CS,5% gelatin) and GA-CS-gelatin (0.2% GA,1% CS,5% gelatin) mixed gels.A,TGA;B,DTG.

3.4 X-Ray Diffraction (XRD) Analyses

X-ray diffraction patterns can determine the amorphous or crystalline nature of the molecule (Wanget al.,2018).The XRD patterns of gelatin,CS-gelatin and CA-CS-gelatin gels are shown in Fig.7.CS showed two diffraction peaks at 2θ=10.7? and 20.8?,which was very similar to the previous work of Wanget al.(2019).The peak near 10.7? was a crystalline peak formed by the movement of the CS chain due to the formation of intramolecular hydrogen bonding between NH3+and -OH.The peak near 20.8? was crystalline peak resulting from the dissolution of CS in acetic acid solution (Wanget al.,2019a).In addition,a sharp peak appeared near 44.2?,which might be caused by the formation of a weak gel by CS.Gelatin formed a diffraction peak around 19.4?,which indicated that gelatin was crystalline.The diffraction peak of CS-gelatin moved to around 18.9? with the addition of CS,and the peak width and height increased.This was due to the interaction between gelatin and the CS chain,and the enhanced intermolecular attraction,which promoted the shift of the diffraction peak and the increased crystallinity.The diffraction peak of GA-CSgelatin gel was near 19.9?,indicating that CS,GA,and gelatin interacted to form a crystalline complex.Furthermore,the GA-CS-gelatin gel had small partially crystalline bands near 31.1? and 41.6?.The appearance of crystalline bands further indicated that GA affected the interaction between CS and gelatin.

Fig.7 X-ray diffraction spectra of freeze dried 3% CS,6%gelatin,CS-gelatin (1% CS,5% gelatin) and GA-CS-gelatin (0.2% GA,1% CS,5% gelatin) mixed gels.

3.5 Microstructure Analysis

As an effective tool for analyzing the surface morphology,SEM can be used to analyze the microstructure of gel(Zhuanget al.,2016).The results are shown in Fig.8.CS formed a gel with disordered pores.The surface of gelatin was rough and uneven,and there were holes with different sizes.With the addition of CS,the pores of the microstructure disappeared,and the surface was wrinkled.It was smoother compared with CS and GA.GA-CS-gelatin showed reduction in wrinkles and smoother appearance due to the effective cross-linking effect of GA between gelatin and CS.Although gelatin and CS alone could form gel,the gel structure was unstable and the water retention was poor.Therefore,the holes remained after the water had sublimed.The interaction between gelatin and CS enhanced the gel network and made it more stable,which improved the water retention performance.Therefore,the sublimation of water did not cause holes in the CS-gelatin.However,the gel formed by gelatin and CS was not stable due to the absence of cross-linking agent,which led CS-gelatin gel to possess more wrinkles.These results were corresponded to the changes in the TGA value of the gel shown in Fig.6.

Fig.8 The microstructure of gels analyzed with scanning electron microscope.A,3% CS gel;B,6% gelatin gel;C,CSgelatin mixed gel (1% CS,5% gelatin);D,GA-CS-gelatin mixed gel (0.2% GA,1% CS,5% gelatin).

Based on the above investigation,we proposed the general mechanism of the formation of GA-CS-gelatin.Gelatin can be denatured by heating and the structure is destroyed.When chitosan is dissolved in dilute acid solution,the amino group of chitosan will be protonated.When gelatin and chitosan are mixed,the protonated amino group and the carboxyl group on the gelatin can combine together,and the electric charge can lead to the system an electrostatic interaction.In addition,the gelatin chain and the chitosan chain also contain a large amount of -OH,which provide the possibility for hydrogen bonding interaction.Under these interactions,chitosan and gelatin form a new system.The introduction of gallic acid can enhance these interactions.Therefore,mixing chitosan,gelatin and gallic acid into one system can produce a stable three-dimensional gel network structure.

3.6 UV-Visible Spectrum

Ultraviolet radiation can be divided into UVA (320– 380 nm),UVB (280– 320 nm) and UVC (210– 280 nm) according to the wavelength (Qianet al.,2017).Due to the destruction of the ozone layer,the ultraviolet rays reaching the earth’s surface is increasing (Wanget al.,2016).As a result,the intensity of ultraviolet radiation in human skin is also increasing.Ultraviolet light is an important external factor for human skin aging and disease (Naidooet al.,2018).Ultraviolet radiation with shorter wavelengths has higher energy and is more likely to cause skin damage.Exposure to UVR may cause skin damage,such as immune dysfunction,inflammation,skin aging,skin cancer,etc.(Tobin,2017;Linet al.,2018).Therefore,there is an urgent need for UV-resistant materials to resist UV radiation.The anti-ultraviolet radiation performances of the samples are shown in Fig.9.CS,gelatin and CS-gelatin films all had certain resistance to UVC,but had little effect on UVB and UVA.0.2% GA solution showed better resistance to UVB.This could be attributed to the absorption of ultraviolet light (280– 320 nm) by the hydroxybenzene structures (Sadeghifaret al.,2016).The GA-CS-gelatin film could not only resist UVB effectively,but also resist some UVA.This was because the mixed gel system introduced hydroxybenzene.In addition,GA,CS,and gelatin could form hydrogen bonding.The existence of hydrogen bonding might be an important reason for promoting the gel system to expand the ultraviolet resistance range (Zhonget al.,2002).Interestingly,the GA-CS-gelatin gel film was more transparent and had no significant effect on the visible light transmittance compared with gelatin and CS-gelatin.This provided a good basis for UV protection with GA-CS-gelatin gel film.

Fig.9 Transmittance of gels between 200 and 800 nm (3%CS gel,6% gelatin gel,CS-gelatin mixed gel (1% CS,5%gelatin),GA-CS-gelatin mixed gel (0.2% GA,1% CS,5%gelatin),0.2% GA).

4 Conclusions

The focus of this research is to explore the effects of CS and GA on gelatin gels,with the aim of improving the viscoelasticity and mechanical strength of gelatin,and developing new functional properties of the mixed gel systems In order to evaluate whether the addition of CS and GA did significantly change the functional properties of gelatin,the rheology,texture,thermal stability,and UV resistance of the mixed gel were studied.The results indicated that the structure of 6% gelatin gel was unstable,with poor mechanical properties and viscoelasticity.CS could significantly improve the viscoelasticity,hardness,strength,thermal stability,crystallinity,and microstructure of gelatin gel when keeping the total concentration unchanged (1% CS+5%gelatin).At the same time,the addition of GA further enhanced the improvement effect of CS,making the CS-gelatin mixed gel stronger.In addition,the UV transmittance results showed that GA-CS-gelatin gel film was more effective in blocking UVC,UVB,and UVA.Accordingly,the results of this study demonstrated that GA could improve the functional properties of CS-gelatin,thereby enhanced its potential application performance.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No.31922072),the China Agriculture Research System (No.CARS-48),the Fundamental Research Funds for the Central Universities (No.2019 41002),the Taishan Scholar Project of Shandong Province(No.tsqn201812020).