Abstract: The effects of temperature, culture time and plant extract, an aqueous extract obtained from mixed Lycium barbarum and Eleutherococcus senticosus at a ratio of 1:1 (m/m) and concentrated by rotary evaporation until a concentration of 1. 0 g/mL (total material weight/concentrated extract solution volume ratio) on the production of superoxide dismutase (SOD) by liquid shake-flask fermentation of Streptococcus thermophilus were studied using completely randomized design. Experimental data processing was carried out using analysis of variance (ANOVA) and regression analysis. The ANOVA results showed that: 1). The effect of culture temperature on SOD production was highly significant (P＜0.01); when the culture temperature was 41 ℃, the highest SOD yield was 901.667 U/g wet cells. 2). The effect of culture time on SOD production was also highly significant (P＜0.01); when the culture time was 48 h, the highest yield of SOD was 932.6667 U/g wet cells. 3). The effect of plant extracts on SOD production was significant (P＜0.01); when plant extract addition was at 8%, the highest SOD yield was 1347.0000 U/g wet cells. Therefore, the optimal culture conditions that can maximize SOD yield and result in a higher input-to-output ratio were culture temperature at 41 ℃, fermentation duration of 24 h, and 8% plant extract. The regression analysis showed no significant correlation between SOD production and culture temperature (P＞0.05). SOD production (Y) showed a highly significant linear relationship with plant extract amount or culture time, and the corresponding regression equations were Y = 865.033 + 52.383X (P＜0.01) and Y = 238.757 + 17.346X (P＜0.01), respectively.

Key words: Streptococcus thermophilu, superoxide dismutase, NBT reduction, culture conditions