Impact of cold atmospheric plasma on microbial safety, total phenolic and flavonoid contents, antioxidant activity, volatile compounds, surface morphology, and sensory quality of green tea powder

Abstract

The effects of cold atmospheric plasma (CAP) at different voltages of 20, 22, and 25 kV for 2, 4, 6, and 8 min were investigated on microbial safety, total phenolic content (TPC), total flavonoid content (TFC), antioxidant activity (AA), volatile compounds profile, microstructure and sensory properties of green tea powder. CAP treatments effectively reduced the total microbial, mold and yeast, total coliform, Escherichia coli, and Entrococcus faecalis counts of green tea. E. coli and E. faecalis were completely deactivated in the sample exposed to plasma treatment at 25 kV for 8 min. E. coli showed higher minimum inhibitory and minimum bactericidal concentrations compared to Staphylococcus aureus. TPC, TFC and AA of green tea decreased with increases in CAP voltage and exposure time. GC–mass spectrometry identified 48 different volatile components in green tea. CAP treatment profoundly influenced the amounts of volatile compounds but the trends of changes did not follow the same pattern as the time and/or voltage of the process increased. SEM analysis showed that CAP at 20 kV for 2 or 4 min did not change the surface morphology of green tea powder while higher exposure times and voltages caused substantial ruptures in microstructure of the samples. Sensory acceptability of green tea decreased noticeably after plasma treatment at 25 kV for longer than 4 min. In conclusion, CAP treatment at 22 kV for 4 or 6 min considerably diminished microbial load of green tea powder while relatively satisfactorily preserved its TPC, TFC, AA, volatile components and sensory desirability.