Effects of copper on the post acidification of fermented milk by St. thermophilus.

UNLABELLED: As an initial investigation of using Cu²⁺ to control post acidification, the fermentation time, pH and the viable counts of bacteria of fermented milk during fermentation and storage were studied.Streptococcus thermophilus was used as the starter culture to ferment the whole milk. Cu²⁺ was added to whole milk at a concentration ranging from 1.25 to 20 mg/kg before fermentation. It was observed that increasing the concentration of Cu²⁺, the fermentation time of yoghurt was prolonged, while the post acidification of fermented milk was decreased during storage. The results showed that Cu²⁺ at the concentration of 1.25 mg/kg could reduce the post acidification of fermented milk effectively and showed no significant effect on neither the fermentation time nor the viable counts of S. thermophilus (P > 0.05) compared with the control sample. These findings indicated that Cu²⁺ could be used as a potential additive to inhibit the post acidification of yoghurt. PRACTICAL APPLICATION: Adding copper to milk has an effect on fermented milk products such as yoghurt. Increasing the concentration of Cu2⁺ decreased the post acidification of fermented milk. The fermentation time is prolonged with the increase of Cu2⁺ concentration. Cu2⁺ at 1.25 mg/kg exhibited inhibition on post acidification and had no significant effect on fermentation time and the viable counts of fermented milk.