To explore the synergistic effects of different catechins on antioxidant and bacteriostasis compared with single catechins, in order to achieve the application of reducing dose and increasing efficiency. The scavenging ability of four single catechins (EGCG, ECG, EGC, EC) on DPPH free radicals was compared under the concentration gradient of 0.025~0.2 mg/mL. The optimal combination of different catechins was investigated based on orthogonal design with antioxidant properties (DPPH free radical scavenging ability) as the index. In the experimental concentration gradient, the best combination was set as three different concentrations of high, middle and low groups, and the inhibitory effects of different complex catechins on Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) were compared. The antioxidant and antibacterial effects of complex catechins were better than those of single catechins. The best antioxidant effects of catechins were 0.1 mg/mL EGCG, 0.025 mg/mL EC, 0.1 mg/mL ECG, 0.05 mg/mL EGC (volume ratio 1:1:1:1). The scavenging rate of DPPH free radical reached 95.03%, which was significantly better than that of EGCG at 0.1 mg/mL concentration (p <0.05), and the antibacterial effect of high, medium and low concentrations of complex catechins on S. aureus and E. coli was better than that of the four single catechins. Compared with single catechin, the optimized complex catechins showed better DPPH free radical scavenging ability and inhibition of S. aureus and E. coli, it shows that they have synergistic effect.

Tea polyphenols, catechins and theaflavins are active substances in tea, which have obvious antibacterial activities in vivo and in vitro against various pathogenic microorganisms. As an important part of human microbiota, the relationship between oral microbiota and human health has become the focus of research in various fields. The experimental results have shown that phenolic compounds of tea have a definite inhibitory effect on oral opportunistic pathogens such as Streptococcus mutans and Porphyromonas gingivalis, etc., and can be used as potential oral flora regulators, which are widely used in dental caries, periodontal diseases and other oral diseases. This paper summarized the inhibitory activities of representative phenolic compounds in tea against oral pathogens and their effects on oral health, in order to provide reference for related basic research and clinical techniques.

In order to explore the dynamic changes of microorganisms during the drinking process of freshly-made commercial black milk tea, this paper took iced and hot black milk tea of 3 different brands on the market as the research object. They were placed at room temperature (25 ℃) and natural humidity for 6 h to determine the total contents of mold, yeast, coliform and bacterial colonies in the freshly-made black milk tea according to the national standard method. The results showed that compared with the 0 h, the contents of yeast, coliform and total colonies of iced black milk tea placed at 25 ℃ for 6 h significantly increased (p < 0.05). The contents ranged from 1.2×10²~2.8×10³ CFU/mL, 3.0×10²~1.4×10⁵ CFU/mL, 6.4×10⁴~2.7×10⁵ CFU/mL, respectively. No significant changes were found in the contents of mold (p > 0.05), and the contents ranged from 0 to 15 CFU/mL. When the hot black milk tea was at 25 ℃ for 6 h, no mold and yeast were detected, and the total contents of coliforms and colonies ranged from 0~9.9×10⁴ CFU/mL and 0~1.6×10⁵ CFU/mL, respectively. In conclusion, the total number of bacterial colonies and coliform groups increased after the black milk tea was unsealed. Coliform groups, bacteria and yeasts were more likely to breed after opening iced black milk tea than hot. This study provides a theoretical basis for the hygienic quality control of freshly-made milk tea, and provides guidance for consumers to drink freshly-made milk tea scientifically.