Investigation of lactic acid production in optimized dairy wastewater culture medium

Dairy wastewater is one of the main and inexpensive wastes of the dairy industry that contains nutrients necessary for the growth of lactic acid bacteria (LAB) in order to produce lactic acid. The fermentation variables including dairy wastewater concentration, glucose concentration, and fermentation time were optimized using the response surface methodology (RSM) with central composite design (CCD) in order to maximize the biomass production of Lactobacillus delbrueckii subsp. lactis PTCC 1743. The culture medium containing 18.16 g/L of dairy wastewater, 57.07 g/L of glucose, and a reaction time of 41.41 h were identified as the optimal conditions. This strain produced 14.2 g/L of lactic acid, with a yield of 0.78 g/g sugars and a productivity of 0.34 g/L.h. In the kinetics of pH, lactic acid and biomass production and reducing sugar concentration were analyzed under these conditions. Based on the results, it was observed that over time, the pH of fermentation and the amount of reducing sugars decreased. The pH ranges 4-6 was the best condition for the production of lactic acid. Also, lactic acid production and LAB survival increased up to 41 h of fermentation, and after 41 h, these parameters had a decreasing trend. The antioxidant activity of the sample containing bacterial cells (78%) was higher than the microbial supernatant sample (19%). This study emphasizes the efficacy of dairy wastewater as potential alternative feedstock to produce lactic acid through fermentation. Because in addition to being cheap, it also contains the nutrients needed to produce lactic acid.