Optimized biomass and bioethanol production from dairy industry side streams by Saccharomyces cerevisiae ATCC 13007 under aerobic and microaerophilic conditions

Dairy industry by-products possess high nutritional value, making them suitable substrates for microbial biomass, enzyme, and biofuel production. This study evaluated the potential of sweet whey (SW) and acid whey (AW) for yeast biomass and bioethanol production using the ale-brewing super-attenuated strain Saccharomyces cerevisiae ATCC 13007. To enhance production yields, SW and AW were pretreated with an archaeal thermostable β-glucosidase (EC 3.2.1.21) to hydrolyze lactose into fermentable sugars. Experiments were conducted under aerobic and microaerophilic conditions to assess the effects of oxygen availability on yeast growth and fermentation efficiency. The highest biomass (OD 600 = 2.20) and specific growth rate (0.40 ± 0.01 h -1 ) were obtained in 2-fold diluted AW under aerobic conditions, representing a 30–50% increase compared to microaerophilic conditions. Both SW and AW exhibited significant reductions in pH (∼1.00 ± 0.03) and oxidation–reduction potential (150 ± 10 mV), correlating with yeast metabolic activity. Volatile solids reached 33 ± 0.9 g L -1 in 2-fold diluted AW at 48 h, indicating strong fermentative activity. Protein content approximately doubled under aerobic growth in both substrates but declined in microaerophilic conditions, highlighting the importance of oxygen in biomass and metabolite synthesis. The maximal protein yield (∼52% of total organic carbon) was achieved in non-diluted SW after 120 h of aerobic cultivation, while the highest ethanol concentration (4.5 ± 0.1 g L -1 ) was observed in 2-fold diluted SW after 24 h under microaerophilic conditions. Aerobic conditions resulted in a 10-fold increase in alcohol dehydrogenase activity compared to microaerophilic cultures. Overall, oxygen availability significantly influenced yeast biomass and bioethanol production, demonstrating the feasibility of dairy by-products as cost-effective substrates for single-cell protein and bioethanol generation.