Synthesis and Characterization of a Novel Extracellular Polysaccharide by Rhodotorula glutinis

Abstract

The aim of this work was to characterize an exopolysaccharide by Rhodotorula glutinis KCTC 7989 and to investigate the effect of the culture conditions on the production of this polymer. The extracellular polysaccharide (EPS) produced from this strain was a novel acidic heteropolysaccharide composed of neutral sugars (85%) and uronic acid (15%). The neutral sugar composition was identified by gas chromatography as mannose, fucose, glucose, and galactose in a 6.7:0.2:0.1:0.1 ratio. The molecular weight of purified EPS was estimated to be 1.0-3.8 x 10(5) Dalton, and the distribution of the molecular weight was very homogeneous (polydispersity index = 1.32). The EPS solution showed a characteristic of pseudoplastic non-Newtonian fluid at a concentration >2.0% in distilled water. The maximum EPS production was obtained when the strain was grown on glucose (30 g/L). Ammonium sulfate was the best suitable nitrogen source for EPS production. The highest yield of EPS was obtained at a carbon to nitrogen ratio of 15. The EPS synthesis was activated at the acidic range of pH 3.0-5.0 and increased when the pH of the culture broth decreased naturally to <2.0 during the fermentation. When the yeast was grown on glucose (30 g/L) and ammonium sulfate (2 g/L) at 22 degrees C at an initial pH of 4.0, EPS production was maximized (4.0 g/L), and the glucose-based production yield coefficient and carbon-based production yield coefficient were 0.30 g of EPS/g of glucose and 0.34 g (carbon of EPS)/g (carbon of glucose), respectively.

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“…Peptone, yeast extract, as well as inorganic sources in the form of ammonium salts or nitrates were used as nitrogen source. The largest amounts of exopolysaccharides were obtained in media containing 0.2% to 0.25% of ammonium sulphate (Cho et al, 2001). The use of other ammonium salts, e.g., NH 4 Cl or NH 4 NO 3 , resulted in a greater increase of biomass, but the reduction in polymers biosynthesis effi ciency was observed at the same time (Grigorova et al, 1999).…”

“…Therefore, the physical parameters such as, for example, rheological traits of aqueous EPS solutions, stability in water-oil emulsions, and the ability to bind water, are essential for their application. Aqueous solutions of EPS from Rhodotorula yeasts showed the features of pseudoclassical non-Newtonian fl uids (Cho et al, 2001). Glucomannan produced by S. salmonicolor AL 1 increases the effi ciency of emulsions formation and their stability in the tanning lotions (Kuncheva et al, 2007).…”

“…The factor determining the effi ciency of EPS biosynthesis, except carbon and nitrogen source, is their molar ratio. It was found in the study conducted by Cho et al (2001) that Rhodotorula glutinis yeasts were able to produce the highest amount of polymers when C:N ratio was 15:1. Higher C:N ratio in the medium caused an activation of the mechanism of other reserve substances accumulation, including fats.…”

“…The inoculum culture of EPS forming strains is carried out in conditions ensuring optimal growth of biomass for 42-48 hours (Cho et al, 2001;Pavlova et al, 2009;Poli et al, 2010), usually in YPD medium. The size of inoculum introduced to the production medium applied in the cited publications was from 1% to 10% v/v.…”

“…In order to obtain EPS, Cryptococcus laurentii AL 62 strain was cultured at 22°C. Sporobolomyces salmonicolor AL 1 (Pavlova et al, 2004) and Rhodotorula glutinis (Cho et al, 2001) were cultured at the same temperature. Cryptococcus fl avus A 51 was incubated at 24°C (Pavlova et al, 2009), while Rhodotorula acheniorum MC under conditions of the bioreactor at a temperature of 26°C (Pavlova et al, 2005).…”