Anaerobic biotransformation process of lignocellulosic waste with Lactobacillus casei and Bacillus subtilis and its effect on the physicochemical parameters case of study: craft brewers’ spent grains

In the present study, the anaerobic biotransformation process of craft brewers’ spent grains (CBSG) was evaluated using two strains of lactic acid bacteria (LAB), Lactobacillus casei and Bacillus subtilis, as well as their effect on rheological properties at the end of the process. Three doses of inoculum were applied (5, 10, and 15% v v⁻¹) with three replications at constant temperature and pH (37 ± 0.5 °C, 6.5); the process was monitored every 4 h for 72 h to determine carbohydrate consumption and cell growth. At the beginning and end of the process, the substrate was rheologically characterized to observe physical changes during the process of biotransformation, especially in terms of viscosity, shear stress, and rheological parameters ( \(k\) $k$ , \(n\) $n$ ). The results showed that the substrate exhibited dilating behavior (n > 1) prior to fermentation. At the end of the process, the same flow behavior was observed in the substrate; however, there was a thinning of the fluid due to the action of probiotic bacteria when degrading lignocellulosic compounds and in turn by cell growth, which resulted in the production of probiotic biomass, of which the species B. subtilis produced the highest content of total nitrogen (TN) and proteins with 4.10% and 25.62% respectively (p < 0.05). This represents an increase of 126% of TN and proteins with respect to their initial contents (1.80% and 11.29%, respectively). In this way, it was possible to evaluate the nutrient content of CBSG and to correlate the rheological parameters during the development of the process as an efficient control strategy, establishing regions of change in the final values of the variables, such as viscosity, shear stress, consistency index, and flow index, at the end of the process.