Optimization of d-lactic acid production by terrilactibacillus laevilacticus SK5-6 and fermentation process scale up in 30 litre-fermentor

Abstract

PLA, which is one of biodegradable plastic stereocomplex is made by block polymerization of optically pure L- and D-lactic acid. With stereoblock structure, heat and mechanical properties of PLA products can be improved. Thus, the demand of D-lactic acid has been increased. However, the market supply is still limited. In-house work reported that Terrilactibacillus laevilacticus SK5-6, the novel bacterium has been isolated from soil and later identified as the potent D-lactic acid producer. Moreover, it could produce high D-lactic acid from glucose with high optical purity in flask scale. To improve the fermentation performance, further process optimization was conducted in a 5 L stirred fermentor. The effects of inoculum size, seed age and volume of seed on D-lactic acid production were determined for establishing the platform. This work shows that the novel strain could produce high concentration of D-lactic acid with high yield and optical purity as well as the flask scale production. It was observed that 88.0 g/L of D-lactic acid with yield of 0.87 g/g glucose and productivity of 1.83 g/L/h were obtained with the mid log phase of 1% inoculum size and 10% seed transfer. Furthermore, the high optical purity of 99.3% could also be obtained from this isolate in the 5 L fermentor operation with above conditions. To develop the fermentative process scale up platform for D-lactic acid in the 30 L stirred fermentor. The seed train was developed for fermentation by used the conditions that had optimized in 5 L fermentor. In addition, the engineering parameters were tested as scale up factors by assuming constant mass transfer pattern in both scales. The results of testing scale up in 30 L fermentor show that T. laevilacticus SK5-6 could produce 81.5 g/L of D-lactic acid and productivity of 2.09 g/L/h, which was higher than 5 L scale slightly. They were obtained with 10% preculture seed transfer (seed in 250 mL flask) and using power input per volume (P/V) for agitation criteria. In addition, the high optical purity of 99.3% could also be obtained in the 30 L fermentor as well as production in 5 L scale. Therefore, the novel D-lactic acid producer, T. laevilacticus SK5-6 should be suggested as the potential strain for D-lactic acid production in the industrial scale.