Enhancement of water soluble wheat bran polyphenolic compounds using different steviol glucosides

Abstract

Background: Production of wheat bran (WB) for human consumption is estimated to be about 90 million tons per year. WB contains abundant source of dietary fiber, minerals, vitamins, and bioactive compounds. WB is a by-product of milling and contains abundant source carbohydrate (60%), protein (12%), fat (0.5%), minerals (2%), and bioactive compounds such as phenolic acids, arabinoxylans, flavonoids, caroteinoids alkylresorcinol and phytosterols. These are known for health promoting properties such as controlling glycemic index, reducing plasma cholesterol level, and reducing the human colon cancer cell growth antioxidant, antimicrobial, anti-inflammatory, and anticarcinogenic activities. Several terpene glycosides such as mogroside V, paenoiflorin, geniposide, rubusoside (Ru), stevioside (Ste), rebaudioside A (RebA), steviol monoside, and stevioside glucoside have been discovered to enhance the solubility of a number of pharmaceutically and medically important compounds that normally show poor solubility in water.

Context and purpose of this study: In this study, to increase soluble extraction of polyphenol compounds of WB using Ru, the expression of β-galactosidase from Thermus thermophilus was optimized with different E. coli hosts and with different concentration of lactose inducer instead of isopropyl-1-thio-β-D-galactopyranoside (IPTG) for industrial production. Furthermore, the effect of different steviol glucosides (Ru, Ste, RebA, and SG) on the enhancement of polyphenol compounds extraction from wheat bran was studied. 

Results: β-galactosidase from Thermus thermophilus was used for the specific conversion of stevioside (Ste) to rubusoside (Ru) with 92% productivity. The enzyme was optimized to be expressed in E. coli. With 7 mM lactose, the β-galactosidase activity expressed was 34.3, 14.2, or 34.4 ± 0.5 U/mL in E. coli BL21(DE3)plysS, Rosetta(DE3)plysS, or BL21(DE3) at 37°C, and 9.8 ± 0.2, 7.0 ± 0.5, or 7.4 ± 0.2 U/mL at 28°C, respectively. The expression of β-galactosidase was dependent on the lactose concentration and the highest activity was obtained with the conditions of 5 mM lactose in E. coli Rosetta(DE3)plysS, 53.3 ± 1.5 U/mL. 78% of the mesophilic proteins was eliminated by heating at 70°C for 15 min with 89% β-galactosidase activity recovery. The total polyphenol content of WB extracted by water, Ru, Ste, rebaudioside A (RebA), and steviol glucosides (SG) were 533.8 ± 9.6 µg/mL, 633.3 ± 1.25 µg/mL, 604.4 ± 10.1 µg/mL, 654.8 ± 26.5 µg/mL, and 601.2 ± 33.4 µg/mL, respectively. The DPPH radical scavenging activity prepared by water, Ru, Ste, RebA, and SG extraction were 8.76 ± 0.3 mg/mL, 4.87 ± 0.3 mg/mL, 5.34 ± 0.22 mg/mL, 7.27 ± 0.1 mg/mL, and 7.82 ± 0.02 mg/mL, respectively.

Conclusions: To increase soluble extraction of polyphenol compounds of WB using Ru, the expression of β-galactosidase from Thermus thermophilus was optimized with different E. coli hosts and with different concentration of lactose inducer instead of isopropyl-1-thio-β-D-galactopyranoside (IPTG) for industrial production of the enzyme. The highest antioxidant activity was shown in WB extracted by Ru. The number of glucosyl units attached to steviol can possibly affect the efficiency of antioxidant activity of WB extracted by steviol glucosides. 

Keywords: Rubusoside, ß-galactosidase, lactose induction, immobilized enzyme, wheat bran, steviol glucosides