Optimization of fermentation process of blueberry and Chinese yam flavored fermented milk by fuzzy mathematics comprehensive evaluation and response surface method

Using blueberries, Chinese yam, and milk as raw materials, blueberry and Chinese yam flavored fermented milk was prepared. Using sensory evaluation score as the evaluation index, the fermentation process was optimized by single factor experiments, fuzzy mathematical comprehensive evaluation, and response surface methodology. The physiochemical indicators, microbial indicators, and antioxidant activity of the fermented milk were detected and analyzed. The results showed that the optimal fermentation conditions for blueberry and Chinese yam flavored fermented milk were as follows: milk 77.4%, blueberry juice 10.5%, Chinese yam pulp 5.9%, sugar 6%, bacterial starter addition 0.2%, and fermentation time 6.1 h. Under these optimized conditions, the sensory score of blueberry Chinese yam flavored fermented milk reached 93.28 points, and its acidity, protein content, soluble solids content were 93 °T, 3.67 g/100 g and 15.1%, respectively. The total number of bacterial colony was 1.73×10⁵ CFU/ml, and the lactic acid bacteria counts was 1.82×10⁶ CFU/ml. Escherichia coli and Staphylococcus aureus were not detected. All indicators met the requirements of relevant national standards. The antioxidant activity results showed that the scavenging rates of 1,1-diphenyl-2-phenylhydrazyl (DPPH) free radicals, hydroxyl free radicals and the reduction/antioxidant capacity of Fe³⁺ were 79.0%, 72.6% and 1.396, respectively, indicating that the fermented milk had certain antioxidant capacity.