Lactic acid production from corn cobs by simultaneous saccharification and fermentation: a mathematical interpretation

Milled corn cob samples were treated with water (autohydrolysis treatment) under optimised operational conditions to cause the selective depolymerisation of hemicelluloses, and the cellulose-enriched residue was used as a substrate for lactic acid production by simultaneous saccharification and fermentation (SSF) in media containing cheap nutrients (spent yeast cells and corn steep liquor (CSL)), cellulolytic enzymes and Lactobacillus rhamnosus CECT-288 cells. SSF experiments were carried out at 45 °C under a variety of operational conditions (enzyme to substrate ratios in the range 5–55 FPU/g substrate, liquor to solid ratios in the range 6–10 kg/kg and spent yeast cells concentrations in the range 5–20 g/L). In the initial SSF stages, cellobiose and glucose accumulate in the reaction media, and then decreased. The cellulose conversion into lactic acid obtained at the end of experiments was modelled by means of an empirical model. For process times longer than 25 h, cellulose hydrolysis became the rate limiting step of the SSF process, and the lactic acid concentration profiles followed a typical hyperbolic behaviour. Based on this finding, a simple model able to reproduce the time course of lactic acid was developed.