Aspergillus niger production of pectinase and α-galactosidase for enzymatic soy processing

Abstract

Soybean is a most promising sustainable protein source for feed and food to help meet the protein demand of the rapidly rising global population. To enrich soy protein, the environment-friendly enzymatic processing requires multiple carbohydrases including cellulase, xylanase, pectinase, α-galactosidase and sucrase. Besides enriched protein, the processing adds value by generating monosaccharides that are ready feedstock for biofuel/bioproducts. Aspergillus could produce the required carbohydrases, but with deficient pectinase and α-galactosidase. Here we address this critical technological gap by focused evaluation of the suboptimal productivity of pectinase and α-galactosidase. A carbohydrases-productive strain A. niger (NRRL 322) was used with soybean hull as inducing substrate. Temperatures at 20 °C, 25 °C and 30 °C were found to affect cell growth on sucrose with an Arrhenius-law activation energy of 28.7 kcal/mol. The 30 °C promoted the fastest cell growth (doubling time = 2.1 h) and earliest enzyme production, but it gave lower final enzyme yield due to earlier carbon-source exhaustion. The 25 °C gave the highest enzyme yield. pH conditions also strongly affected enzyme production. Fermentations made with initial pH of 6 or 7 were most productive, e.g., giving 1.9- to 2.3-fold higher pectinase and 2.2- to 2.3-fold higher α-galactosidase after 72 h, compared to the fermentation with a constant pH 4. Further, pH must be kept above 2.6 to avoid limitation in pectinase production and, in the later substrate-limiting stage, kept below 5.5 to avoid pectinase degradation. α-Galactosidase production always followed the pectinase production with a 16-24 h lag; presumably, the former relied on pectin hydrolysis for inducers generation. Optimal enzyme production requires controlling the transient availability of inducers.