Structures and properties of gellan polymers produced by sphingomonas paucimobilis ATCC 31461 from lactose compared with those produced from glucose and from cheese whey

Abstract

The dairy industry produces large quantities of whey as a by-product of cheese production and is increasingly looking for new ways to utilize this waste product. Gellan gum is reliably produced by Sphingomonas paucimobilis in growth media containing lactose, a significant component of cheese whey, as a carbon source. We studied and compared polysaccharide biosynthesis by S. paucimobilis ATCC 31461 in media containing glucose, lactose (5 to 30 g/liter), and sweet cheese whey. We found that altering the growth medium can markedly affect the polysaccharide yield, acyl substitution level, polymer rheological properties, and susceptibility to degradation. Depression of gellan production from lactose compared with gellan production from glucose (approximately 30%) did not appear to occur at the level of synthesis of sugar nucleotides, which are the donors of monomers used for biosynthesis of the repetitive tetrasaccharide unit of gellan. The lactose-derived biopolymer had the highest total acyl content; the glucose- and whey-derived gellans had similar total acyl contents but differed markedly in their acetate and glycerate levels. Rheological studies revealed how the functionality of a gellan polysaccharide is affected by changes in the acyl substitution.

FIG. 1

Gellan production (expressed as the concentration of the ethanol precipitate isolated from culture broth), culture OD₆₄₀, residual carbon source concentration, and broth viscosity (shear rate, 24 s⁻¹) after 48 h of S. paucimobilis ATCC 31461 batch growth at 30°C and 250 rpm in basal S medium containing glucose (○) or lactose (■) at concentrations ranging from 5 to 30 g/liter.

FIG. 2

Gellan production (expressed as the concentration of the ethanol precipitate isolated from culture broth) (●), residual carbon source concentration (○), broth viscosity (shear rate, 24 s⁻¹) (▴), and OD₆₄₀ (■) for batch cultures of S. paucimobilis ATCC 31461 grown at 30°C and 250 rpm with 20 g of glucose per liter (A) or 20 g of lactose per liter (B) as the carbon source.

FIG. 3

Specific activities of PGI, PGM, UGP, UGD, TGP, and TRS in cell extracts prepared from cells of S. paucimobilis ATCC 31461 harvested after 48 h of growth in glucose-containing or lactose-containing media. The bars indicate standard deviations based on at least three enzyme assays for each extract prepared from cells resulting from at least two independent growth experiments.

FIG. 4

Gellan production (expressed as the concentration of the ethanol precipitate isolated from culture broth) and broth viscosity (shear rate, 0.6 s⁻¹) during S. paucimobilis ATCC 31461 growth at 30°C and 250 rpm in cheese whey diluted 1:4 (○), 1:5 (□), 1:6 (●), 1:8 (■), or 1:10 (▵).

FIG. 5

400-MHz ¹H NMR spectra (90°C) of S. paucimobilis gellan polysaccharides. S(G), cells grown in glucose-containing medium; S(L), cells grown in lactose-containing medium; S(W), cells grown in cheese whey-containing medium. Signal assignments: a, Rha H-1 (no glycerate); b, Rha H-1 (glycerate on 1,3-Glc); c, acetate CH₃; d, Rha CH₃.

FIG. 6

Viscosities in water of 0.2% TMA gellan samples from three different growth media, lactose-containing broth (●), glucose-containing broth (■), and cheese whey-containing broth (▴).

FIG. 7

Frequency dependence of the storage moduli of 0.4% gellan gels as determined with 0.03 M KCl (1% deformation). Symbols: ●, lactose-containing broth; ■, glucose-containing broth; ▴, cheese whey-containing broth.