Preparation and Antioxidant Activity In Vitro of Fermented Tremella fuciformis Extracellular Polysaccharides

Abstract

This study was aimed at increasing the capacity of fermented Tremella fuciformis extracellular polysaccharides (TEPS) for possible functional food applications. Thus, strain varieties, fermentation parameters and purification conditions, and the in vitro antioxidant activities of purified EPS fractions were investigated. An EPS high-yield strain Tf526 was selected, and the effects of seven independent fermentation factors (time, temperature, initial pH, inoculum size, shaking speed, carbon, and nitrogen source) on the EPS yield were evaluated. By single factor optimization test, yeast extract and glucose were chosen as nitrogen sources and carbon sources, respectively, and with initial pH of 6.0, inoculum size of 8%, shaking speed of 150 rpm, and culture at 25 °C for 72 h, the optimal yield of TEPS reached 0.76 ± 0.03 mg/mL. Additionally, A-722MP resin showed the most efficient decoloration ratio compared to six other tested resins. Furthermore, optimal decoloration parameters of A-722MP resin were obtained as follows: decoloration time of 2 h, resins dosage of 2 g, and temperature of 30 °C. Decoloration ratio, deproteinization ratio, and polysaccharide retention ratio were 62.14 ± 2.3%, 81.21 ± 2.13%, and 73.42 ± 1.96%, respectively. Furthermore, the crude TEPS was extracted and four polysaccharide fractions were isolated and purified as Tf1-a, Tf1-b, Tf2, and Tf3 by the DEAE-Sepharose FF column and the Sephasryl S100 column. In general, the antioxidant activities of the Lf1-a and Lf1-b were lower compared with Vc at the concentration of 0.1 to 3 mg/mL, but the FRAP assay, DPPH scavenging activity, and hydroxyl radical scavenging activity analysis still revealed that Tf1-a and Tf1-b possess significant antioxidant activities in vitro. At the concentration of 3 mg/mL, the reducing power of Lf1-a and Lf1-b reached 0.86 and 0.70, the maximum DPPH radical were 54.23 ± 1.68% and 61.62 ± 2.73%, and the maximum hydroxyl radicals scavenging rates were 58.76 ± 2.58% and 45.81 ± 1.79%, respectively. Moreover, there were significant correlations (r > 0.8) among the selected concentrations and antioxidant activities of TEPS major fractions Tf1-a and Tf1-b. Therefore, it is expected that Tf1-a and Tf1-b polysaccharide fractions from fermented TEPS may serve as active ingredients in functional foods.