After the screening of fifty LA-producing bacteria, a G. stearothermophilus strain was selected for its capability of growing on potato starch as sole carbon source, and of converting it to LA with high selectivity for the L- optical isomer. Another characteristic of this strain is the optimum of growth and activity at temperatures between 55 and 60°C.
In this study a thermophilic process for direct use of starch with G. stearothermophilus is compared to a traditional LA process based on glucose operated at 37°C with L. casei. While in the latter case glucose is converted with yields around 90%, in the thermophilic process yields on total starch are never higher than 60%, but productivities are 4-fold increased and no sterile handling is required. In 3 L lab-scale batch fermentations with pH control L. casei IFA 32 produced 108 g/L LA in 66 hours in MRS medium enriched of glucose with a yield of 88%. G. stearothermophilus IFA 301 produced instead 32 g/L LA in 24 hours (MRS medium without glucose) and 46 g/L LA in 13 hours (NB/YE medium) with yields of 58 and 50% respectively.
Research on this topic is still in progress. The next steps include the scale-up to pilot scale and the establishment of a continuous process.