Abstract:

Objective A lot of caffeine and tea polyphenols are contained in the waste tea residue, which have excellent antibacterial effect and are ideal raw materials for the preparation of food preservation films. However, the molecular chain of tea residues is short, and the film formation rate is low. Herein, a phase conversion casting film strategy is proposed, where cellulose and tea residue are dissolved in ion liquids. The caffeine and tea polyphenols are well extracted and dispersed into cellulose, and hydrogen bonds drive cellulose to build films in the ion liquids.

Method The cellulose/tea residue composite film was prepared by casting film method. Cellulose and tea residue were dissolved with ionic liquid as solvent, and the mixed solution was obtained by adding glycerol as plasticizer. The composite film precursor was obtained by centrifuging the mixed solution. The precursor was prepared into wet film by phase change method, and then transferred to the mold and air-dried at room temperature to prepare cellulose/tea residue composite film. The mechanical properties and antioxidant properties of the composite films were measured by tensile testing machine and ultraviolet visible spectrophotometer. The food preservation performance of the composite films was determined by plate counting method. The water content, water solubility and mass ratio of the composite film were obtained by measuring the weight change of the composite films in different solution media.

Results The final dissolution temperature of tea residue in ionic liquid ([EMIm]Ac) was 90 ℃, which was lower than the [AMIm]Cl. The DPPH free radical scavenging properties of the cellulose/tea residue composite films was 45.9%, which was increased about 32 times than the pure cellulose film. There was a strong positive relationship between tea residue and moisture content. The moisture content of the composite film was 4.8% higher than that of the pure cellulose film with 80% tea residue, and water solubility decreased to 11%. Moreover, the gas permeability value was also positively correlated with the content of tea residue. The gas permeability value of the composite film with 80% tea residue revised to 7.2 mol·m/(m²·s·Pa), which was about 67% higher than pure cellulose film. The food preservation performance of the cellulose/tea residue composite films was between the pure cellulose film and commercial PE film. When the tea residue content reached 80%, the water loss rate of the fresh beef embedded with the composite film was only 28.9%, which was significantly lower than pure cellulose film (57%). The prepared composite film demonstrated excellent gas permeability. Therefore, the film was able to prolong the storage time of beef for 3-4 d compared to PE film. Tea residue was effectively capable of inhibiting the microbial colonies, which further improved the food preservation performance of the composite films to 12 d.

Conclusion The [EMIm]Ac had the lowest dissolution time and temperature for tea residue, which was the optimal solvent for tea residue. The DPPH free radical scavenging rate of the prepared films was 32 times higher than pure cellulose film, and the antioxidant activity was also significantly enhanced. These were attributed to the large amount of antioxidant active component contained in the tea residue. With the increase of tea residue content, the water solubility of the composite films gradually decreased, and the moisture content was increased. These were mainly attributed to high hydrophobicity of the tea residue, and the network of the prepared films contained massive pores. Compared to commercial PE films, the cellulose/tea residue composite film had an excellent preservation performance, and the preservation time was able to be prolonged by 3-6 d. Therefore, the research opened a new avenue for the utilization of tea residue waste.

Key words: composite film, preservation material, tea residue, cellulose, ionic liquid, caffeine, tea polyphenol