A new kinetic model for growth based on simultaneous substrate and biomass limitation in solid-state fermentation using agar spheres as the model substrate

Substrate limitation is a major concern in solid-state fermentation, especially for bacteria, as they are not capable of penetrating into the substrate particles. The use of a model substrate, such as agar spheres, makes it possible to study the intra-particle transfer phenomena in solid-state fermentation, reduce the system complexity, and determine the cell dry weight (CDW). A semi-mechanistic model was used to predict the growth of Pseudomonas aeruginosa on agar spheres that contain starch as the only carbon source. The model was developed by introducing a new simultaneous substrate- and biomass-limited growth kinetic equation. The parameters of the growth kinetic were estimated by fitting the experimental data of both the CDW and cumulative CO2 production on agar spheres containing glucose. Statistical analysis (F-test) showed that the new growth kinetic equation was significantly better than the conventional substrate-limited growth kinetics. The model predictions were validated by the experimental data of growth, starch concentration and cumulative CO2 production (CCP) of Pseudomonas aeruginosa growing on agar spheres containing starch in aerated small packed-bed bioreactors. The model confirmed that at high initial starch concentration, free glucose and O2 were not the limiting factors, and bacterial growth is limited by other factors that are related to the amount of biomass in the biofilm.