Quality enhancement and microbial reduction of mung bean (Vigna radiata) sprouts by non-thermal plasma pretreatment of seeds

Mung bean (Vigna radiata) sprouts are widely consumed worldwide due to their high nutritional value. However, the low yield and microbial contamination of mung bean sprouts seriously reduces their economic value. This study investigates the effects of non-thermal plasma on the quality and microbial reduction of mung bean sprouts by pretreatment of seeds in water for different times (0, 1, 3 and 6 min). The quality results showed that short-time plasma treatment (1 and 3 min) promoted seed germination and seedling growth, whereas long-time plasma treatment (6 min) had inhibitory effects. Plasma also had a similar dose effects on the total flavonoid and phenolic contents of mung bean sprouts. The microbiological results showed that plasma treatment achieved a reduction of native microorganisms ranging from 0.54 to 7.09 log for fungi and 0.29 to 6.80 log for bacteria at 96 h incubation. Meanwhile, plasma treatment could also efficiently inactivate artificially inoculated Salmonella typhimurium (1.83–6.22 log) and yeast (0.53–3.19 log) on mung bean seeds. The results of seed coat permeability tests and scanning electron microscopy showed that plasma could damage the seed coat structure, consequently increasing the electrical conductivity of mung bean seeds. The physicochemical analysis of plasma-treated water showed that plasma generated various long- and short-lived active species nitric oxide radicals (NO·), hydroxyl radicals (·OH), singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), nitrate (NO₃^-), and nitrite (NO₂^-) in water, thus the oxidizability, acidity and conductivity of plasma-treated water were all increased in a treatment time-dependent manner. The result for mimicked chemical mixtures confirmed the synergistic effect of activity of H₂O₂, NO₃^- and NO₂^- on bacterial inactivation and plant growth promotion. Taken together, these results imply that plasma pretreatment of mung bean seeds in water with moderate oxidizability and acidity is an effective method to improve the yield of mung bean sprouts and reduce microbial contamination.