Lactic acid production enhancement using metagenome-derived bifunctional enzyme immobilized on chitosan-alginate/nanocellulose hydrogel

Abstract

Lactic acid (LA) production from renewable and abundant lignocellulosic resources has received great interest due to its various applications in food, cosmetic, pharmaceutical, and textile industries. Herein, the efficient immobilized bi-functional enzyme on an innovative hydrogel (nano)carrier with boosted stability/activity were prepared and utilized for hydrolyzing lignocellulosic biomass to enhance the fermentative production of LA. First, different samples of sodium alginate (SA)-chitosan (CH) hydrogel grafted to acrylic acid and N-isopropylacrylamide and reinforced with nanocellulose (NCs) were synthesized. Selected hydrogels of pristine SA-CH hydrogel and reinforced SA-CH/NCs hydrogel were utilized for the immobilization of a cellulase/xylanase enzyme (PersiCelXyn1). While free PersiCelXyn1 had the optimum temperature of 50 °C, the immobilized enzyme on SA-CH and SA-CH/NCs hydrogels showed optimum temperatures at 70, and 80 °C, respectively. Furthermore, the immobilization of PersiCelXyn1 onto both hydrogels shifted the optimum pH to alkaline pHs, and increased the enzyme specific activity. The enzyme immobilized on nanocarrier (PersiCelXyn1@SA-CH/NCs) showed higher reusability than PersiCelXyn1@SA-CH, after 14 cycles (70 % activity vs. 52 % activity, respectively), and less enzyme leaching. PersiCelXyn1@SA-CH/NCs showed higher efficiency in the hydrolysis of sugar beet pulp to obtain the hydrolysate containing fermentable sugar as a substrate for lactic acid (LA) production, than PersiCelXyn1@SA-CH hydrogel. After 96 h, the concentration of LA produced with the PersiCelXyn1@SA-CH/NCs was 33.1 g/L, while it was 26 g/L and 21.6 g/L, in the fermentation of hydrolysate with PersiCelXyn1@SA-CH hydrogel, and free PersiCelXyn1, respectively.