Response surface methodology based on Box-Behnken design was employed to optimize the fermentation conditions of juice of “Milpa Alta” cactus stems (Opuntia ficus-indica Mill.) added with skimmed milk powder for producing fermented milk beverage. The optimal fermentation parameters were determined as follows: the concentrations of cactus stem juice, sucrose and starter culture (Lactobacillus bulgaricus:Streptococcus thermophilus = 1:1) were 15.3%, 9.4% and 0.16%, respectively; the fermentation was proceeded at 42 ℃ for 6.8 h. The acidity value of fermented beverage produced under these conditions was 93.2 °T, and it had a distinctive flavor and abundant nutrients.

The impact of mixed starters consisting of Streptococcus thermophilus PZST4 and Lactobacillus bulgaricus PZLB5 in different proportions on the fermentation period of fermented milk and its acidity, texture, syneresis and flavor during storage was investigated. The results showed that viable cell ratios between Streptococcus thermophilus PZST4 and Lactobacillus bulgaricus PZLB5 had a striking effect on the quality of fermented milk, and a ratio of 50000:1 was the optimal combination based on the quality parameters tested.