Kluyveromyces marxianus and Bacillus coagulans co-cultivation: Efficient co-production of bioethanol and lactic acid from pomegranate peels

Lignocellulosic biomass utilization is challenging due to the presence of several carbon sources. Thus, microorganisms with different sugar preferences can be used in co-cultures to overcome this hurdle. This work addressed the simultaneous production of lactic acid (LA) from C5 sugars (i.e., xylose) and bioethanol from C6 (i. e., glucose/fructose) with Bacillus coagulans and Kluyveromyces marxianus, respectively. Sequential inoculation and co-inoculation of microorganisms were also compared. At pH 6, co-inoculation in synthetic media resulted in higher bioethanol (0.51 g/g) and LA (0.98 g/g) yields than sequential inoculation. Furthermore, when using lignocellulosic hydrolysates obtained after enzymatic hydrolysis of 20 % w/w pomegranate peels (PP), 92 % and 98 % of the theoretical maximum bioethanol and LA, respectively, were obtained. This study demonstrated the efficient bioethanol/LA co-generation despite the different optimum fermentation conditions of microorganisms and will pave the way to consider co-cultures for improving process efficiency in lignocellulosic biorefineries.