Influence of solid-state fermentation with Streptomyces on the ability of wheat and barley straws to thicken castor oil for lubricating purposes

This work reports the influence of solid-state fermentation (SSF) of two agricultural residues, wheat and barley straws, by using Streptomyces MDG301 strain, on their capacity to thicken and structure castor oil via crosslinking with hexamethylene diisocyanate in order to obtain gel-like systems. The analytical composition of the lignocellulosic residues and the enzymatic profile of the strain on both substrates were screened in order to establish the best conditions to achieve optimal oleogel properties. A different rheological response was found for oleogels obtained from each residue. This result could be explained in terms of the different enzymatic profile observed when the microorganism grow on these substrates. Thus, a notable difference in the production levels of CMCase and xylanase activities were detected. The assessment of the resulting oleogels as potential lubricating grease formulations was carried out by means of rheological characterization at different temperatures and tribological and mechanical stability tests. Overall, these oleogels demonstrate to possess similar rheological and tribological characteristics compared to other commercial lubricating greases.