The study was conducted to evaluate the potential of mixed culture of Kluyveromyces marxianus and Candida krusei to enhance COD removal efficiency, minimize contamination at extreme conditions (high temperature 40 degrees C and low pH 3.5) during batch and continuous aerobic fermentation and to obtain improved quality single-cell protein (SCP) using whey as substrate. The batch fermentation of mono-culture and mixed culture result showed that the mixed culture resulted in 8.8% higher COD removal efficacy with 19% higher biomass yield and 33% increased productivity. The maximum COD removal 80.2% (including residual protein) was obtained at 24 h HRT with biomass productivity of 0.17 g/L/h; however, maximum biomass productivity of 0.38 g/L/h and 34% COD removal were obtained at 6 h HRT. The results showed that the mixed culture of acid resistance and thermo-tolerant yeasts was a potential way to produce SCP (animal feed) and simultaneous COD removal under extreme operating conditions. (C) 2014 Elsevier Ltd. All rights reserved.