Optimization of lactic acid production from dairy whey using indigenous lactic acid bacteria

Sweet whey from cheese production represents a major environmental challenge due to its high organic load, yet it remains an underutilized nutrient-rich substrate. This study valorized sweet whey from three dairy processors in Northern Rwanda (Burera Dairy Limited, Mukamira Dairy Ltd and Fromagerie La Lumière) into lactic acid through microbial fermentation using indigenous lactic acid bacteria (LAB). Whey samples were physicochemically and microbiologically characterized. Indigenous LAB were isolated from local yoghurt and strain LAB-4 (Lactobacillus sp.) was selected for superior acidification (pH reduction to 3.8 within 72 h). Fermentation parameters (temperature, initial pH and inoculum size) were optimized using Central Composite Design under Response Surface Methodology. Whey contained 4.68–5.15% lactose, 0.72–0.92% protein and pH 6.08–6.35. The optimized conditions (37°C, pH 6.0, 10% v/v inoculum) yielded 43.2 ± 1.2 g/L lactic acid after 48 h, with a substrate-to-product yield of 0.873 g/g and fermentation efficiency of 75.4%. A 67.3% COD reduction was achieved. Downstream recovery by calcium lactate precipitation gave 75.0% overall extraction yield and 82.5% purity. The unit production cost was estimated at USD 2.10 per kg at 500 L/day scale. The process simultaneously valorizes whey waste and produces a high-value platform chemical suitable for food and industrial applications.