Bioconversion of lignocellulosic wastes to zwitterionic biosurfactants by an alkaliphilic bacterium: Process development and product characterization

Abstract

The bioconversion of lignocellulosic wastes to value-added biosurfactants was investigated using a single flask to carry out both substrate pretreatment and biosurfactant production. Lignocellulosic wastes were added to an alkaline medium and autoclaved, which led to a combined alkaline and hydrothermal pretreatment. For biosurfactant production, alkaliphilic Brevibacterium casei NK8 was added to the medium without pH neutralization. Among several lignocellulosic wastes, the media with corn husk, coconut oil cake, and defatted rice bran gave the highest biosurfactant concentrations (1.14–1.32 g L−1). These wastes provided a relatively high concentration of soluble substrates in the pretreated liquid, while solid residues swelled. The bacterium could use the pretreated liquid and solid residues for growth and biosurfactant production. The produced biosurfactants in cell-free broth could be applied directly or after lignin removal. The residual lignin decreased biosurfactant performance in terms of surface tension, contact angle, and oil displacement; conversely, lignin enhanced the biosurfactant emulsification capacity. Cell-free broths with and without lignin were appropriate for different tasks. Biosurfactants synthesized from different lignocellulosic wastes were similar and were identified as a mixture of zwitterionic lipopeptides and phospholipids. The surface tension of the crude biosurfactant solution was extremely low, ranging between 25.9 and 27.6 mN m−1 over a wide range of pH values (1–11), salinities (2–10% NaCl), and temperatures (4–121 °C). In addition, biosurfactants as crude solution and cell-free broth with lignin showed lower phytotoxicity than the commercial surfactant Tween 80. These findings suggest a convenient method for manufacturing biosurfactants from lignocellulosic wastes.