In this paper, the culture conditions for higher antioxidant activity in the culture supernatant of Bifidobacterium animalis subsp. lactis strain B013 were optimized by response surface methodology. The results showed that an inoculum size (defined by OD600 nm) of 0.66, a culture time of 17.84 h and a glucose concentration of 3.04% were determined as the optimal conditions. Treatment with different enzymes and at different temperatures conformed that the antioxidant activity of the culture supernatant was due to the presence of bioactive peptides. A total of 8 peptides were separated and purified by solid phase extraction and ultrafiltration, and they were analyzed by liquid chromatography tandem mass spectrometry. One (QYPLGPK) of the peptides was screened for possible antioxidant activity by bioinformatics and computer simulation. It was verified that the total antioxidant capacity of QYPLGPK at a concentration of 10 mg/mL was (0.249 ± 0.011) mmol Trolox/L.

Lactobacillus paraplantarum L-ZS9, isolated from fermented meat foods, as a probiotic strain and a fermentation starter has a great application potential in the dairy industry. In order to achieve high-density culture of strain L-ZS9, the modified MRS medium and culture conditions were optimized. The optimal carbon and nitrogen sources were selected by one-factor-at-a-time method. The Plackett-Burman experimental design was used to recognize the most significant medium components and culture conditions. The steepest ascent method was used to determine the levels for these variables. A central composite rotary design (CCRD) was used to fit cell density as a function of the independent variables and response surface methodology was employed to determine optimal culture conditions. The results showed that sucrose and yeast powder were used as carbon and nitrogen sources, respectively. The optimized medium formulation consisted of sucrose 3%, yeast extract 7.26%, sodium acetate 0.5%, dipotassium phosphate 0.2%, magnesium sulfate 0.02%, triammonium citrate 0.2%, and Tween-80 0.1%; the optimized culture conditions were as follows: the initial pH value of the medium was 9.53, the volume of inoculum was 1.74%, and the culture temperature was 37 ℃. It was verified that after 14 hours of cultivation under the optimized conditions, the optical density at wavelength of 600 nm of the broth was 1.214, and the viable count was 4.53 × 109 CFU/mL at this time, which was 6 times higher than that before optimization.

Fermented okara with good processing suitability containing 13.4% soluble dietary fiber (SDF) was homogenized with a blender. After addition of other ingredients, the homogenate was formulated to produce a healthy high SDF milk beverage that was uniform and tasted good. Using one-factor-at-a-time method and response surface methodology, the optimum homogenization conditions were determined as follows: blender treatment time 15 min, blender rotation rate 10 000 r/min, stabilizer mixture 0.2%, temperature 45 ℃, pressure 30 MPa, and homogenization time 120 s, and the optimum combination of ingredients was fermented okara 7% (particles sieved through 60 mesh), white sugar 6%, and milk powder 9%. The milk beverage prepared under the optimized conditions had a good particle size distribution. Its soluble solids content was 11%, stability coefficient 0.8 and centrifugal sedimentation rate 2.3%. The product was milky white, and it was a uniform system with good taste and good stability.