Physical and antimicrobial properties of chitosan film containing cinnamaldehyde

This research aims to produce antimicrobial film by incorporating cinnamaldehyde into chitosan film. The films were prepared by dissolving chitosan powder (95% degree of deacetylation) into 1 %v/v acetic or lactic acid solution and blending with glycerol or sorbitol at different concentration (20, 40 and 60 %w/w of chitosan powder) as plasticizer. Physical properties including thickness, tensile strength (TS), elongation at break (EAB), water vapor permeability (WVP) surface color and transparency were examined. Films from acetic acid and lactic acid solution showed similar visual appearance. However, chitosan film-forming solution from lactic acid was difficult to form into film, especially when the plasticizers were added, and thus physical property measurements were not possible. The results showed that an increase in amount of the plasticizers resulted in significant (p 0.05) increase in thickness, decrease in mechanical resistance (decrease in TS) and increase in extensibility (increase in EAB). TS decreased but EAB and WVP increased when the concentration of sorbitol and glycerol increased from 20 to 60 %w/w of chitosan powder. Moreover, film plasticized with sorbitol had lower WVP than those with glycerol at each concentration. The results suggested that 40 %w/w sorbitol was the optimum concentration of plasticizer for forming the chitosan film and thus used for further studies. In further studies, cinnamaldehyde was incorporated into chitosan film-forming solution (plasticized with sorbitol 40 % w/w) at different concentrations (50, 100 and 150 µl/g of chitosan powder). Addition of cinamaldehyde led to a significant increase in both TS and EAB, but a significant decrease in WVP of the films. Increasing of TS and EAB could be due to the interaction between fuctional group of chitosan and cinnamaldehyde. This could be proved by FT-IR spectra results. Antimicrobial property of chitosan film containing cinnamaldyhyde was tested against target microorganisms (S. aureus, B. lichenformis, B. subtilis, E. coli, P. aeruginosa and S. putrefaciens). The film was found to have small inhibition zone against S. aureus while inhibited other target microorganisms only on the contact surface. Addition of cinnamaldehyde could improve physical properties of chitosan film in terms of TS, EAB and WVP. However, only small amount can release from chitosan matrix due to the interaction between functional groups of chitosan and cinnamaldehyde.

本研究旨在将肉桂醛（cinnamaldehyde）掺入壳聚糖（chitosan）膜中，制备抗菌复合膜。具体制备流程为：将脱乙酰度95%的壳聚糖粉末溶解于体积分数1%的乙酸或乳酸溶液中，随后分别添加不同浓度（以壳聚糖粉末质量计为20%、40%、60%）的甘油（glycerol）或山梨醇（sorbitol）作为增塑剂（plasticizer），制得壳聚糖基膜。对所制备膜的厚度、拉伸强度（tensile strength, TS）、断裂伸长率（elongation at break, EAB）、水蒸气透过率（water vapor permeability, WVP）、表面色泽及透明度等物理性能进行了表征测试。
以乙酸溶液制备的成膜液与乳酸溶液制备的成膜液所成膜外观相似，但以乳酸溶液制备的壳聚糖成膜液难以成膜，添加增塑剂后该问题尤为突出，因此无法对其进行物理性能测试。结果表明，增塑剂添加量提升会显著（p<0.05）增加膜厚度、降低力学强度（拉伸强度TS下降）并提升延展性（断裂伸长率EAB升高）。当甘油或山梨醇的添加浓度从20%提升至60%（以壳聚糖粉末质量计）时，膜的拉伸强度TS均出现下降，而断裂伸长率EAB与水蒸气透过率WVP则均有所上升。此外，在相同增塑剂添加浓度下，以山梨醇为增塑剂的膜的水蒸气透过率WVP低于以甘油为增塑剂的膜。
上述结果表明，以40%（以壳聚糖粉末质量计）的山梨醇作为增塑剂时，可制备出性能最优的壳聚糖膜，因此该配方被用于后续实验。后续实验中，我们将不同浓度（50、100、150 μl/g壳聚糖粉末）的肉桂醛掺入以40%山梨醇增塑的壳聚糖成膜液中。添加肉桂醛可显著提升膜的拉伸强度TS与断裂伸长率EAB，并显著降低其水蒸气透过率WVP。拉伸强度TS与断裂伸长率EAB的提升，可能源于壳聚糖与肉桂醛之间的官能团相互作用，傅里叶变换红外光谱（Fourier Transform Infrared Spectroscopy, FT-IR）测试结果可验证该推测。
将含肉桂醛的壳聚糖膜针对以下目标微生物进行抗菌性能测试：金黄色葡萄球菌（Staphylococcus aureus, S. aureus）、地衣芽孢杆菌（Bacillus lichenformis, B. lichenformis）、枯草芽孢杆菌（Bacillus subtilis, B. subtilis）、大肠杆菌（Escherichia coli, E. coli）、铜绿假单胞菌（Pseudomonas aeruginosa, P. aeruginosa）及腐败希瓦氏菌（Shewanella putrefaciens, S. putrefaciens）。测试结果显示，该膜对金黄色葡萄球菌仅能产生较小的抑菌圈，而对其余目标微生物仅在接触表面产生抑制作用。添加肉桂醛可从拉伸强度TS、断裂伸长率EAB及水蒸气透过率WVP三方面改善壳聚糖膜的物理性能，但由于壳聚糖与肉桂醛之间的官能团相互作用，仅有少量肉桂醛可从壳聚糖基质中释放出来。