Production of lactic acid from red algal galactan and its recovery by membrane separation

Abstract

A process has been developed for lactic acid production from red algal galactan. This process consists of four steps: 1) saccharification of red algal galactan; 2) detoxification of the acid hydrolysate; 3) production of lactic acid from the detoxified hydrolysate by fermentation; and 4) lactic acid recovery from the fermentation broth by membrane separation. First, for the saccharification of agarose, the main component of red algal galactan, hydrochloric acid (HCl) was used. Agarose was readily hydrolyzed to galactose, 5-hydroxymethylfurfural (5-HMF), and levulinic acid by HCl. To overcome disadvantages of liquid acid catalysis such as excessive acid consumption with no reuse and waste generation during the subsequent neutralization step, HCl was replaced by a solid acid catalyst, Amberlyst 36. In order to identify optimal reaction conditions, the effects of reaction time, temperature and catalyst concentration on galactose yield and production rate were investigated. In the subsequent step, a nanofiltration (NF) process with TFC-SR3 membrane was employed for the removal of 5-HMF and levulinic acid from the acid hydrolysate. 5-HMF and levulinic acid are inhibitory to fermentation, but at the same time they themselves are useful compounds with many applications. The effects of various operating parameters such as pH, temperature and feed composition were investigated. Galactose rejection remained constant at a very high level over 0.99 with no respect to the pressure, while the rejections of 5-HMF and levulinic acid ranged from 0.03 to 0.13 and from 0.17 to 0.46 respectively, indicating that the NF process was very effective for the removal of 5-HMF and levulinic acid while retaining galactose. For the complete removal of 5-HMF and levulinic acid, NF in a diafiltration mode was carried out. In the subsequent step of electrodialysis (ED), 5-HMF and levulinic acid could be effectively separated from each other. The feasibility of lactic acid production from gal...