Viscoelastic properties of microbial alginate gels by oscillatory dynamic tests

The jellifying capacity of some alginate gels excreted by Azotobacter vinelandii DSM 576 was studied by oscillatory tests at alginate concentrations (c) of 1.0% and 1.5% w/w. Frequency dependence of the complex shear modulus (G*) permitted these gels to be discriminated in terms of their different orders of the relaxation function (alpha) and "gel stiffness" (A(alpha)) values using Friedrich and Heymann theory [Journal of Rheology, 32 (1988) 235]. Of the samples tested that characterised by a guluronate content of 25% and a number-average molecular mass (M-n) of 263.4 kDa exhibited a gel stiffness at c = 1% w/w of the same order of magnitude of that pertaining to alginates extracted from Macrocystis pyrifera or Lessonia nigrescens with greater guluronate contents (35-40%) but smaller number-average molecular masses (70-110 kDa) with about half of alpha. Whereas alpha was found to depend exponentially on the ratio of guluronate to mannuronate monad frequencies (N-G/(M)) only, A(x) was expressed as a power function of c, M-n N-G/M, and average G-block length (N-G). The limited data available, as well as incomplete understanding of the real controlling factors for ionotropic gelation of alginates, made such empirical correlations just useful for a preliminary screening of alginates independently of their weed or microbial origin. (C) 2003 Elsevier Ltd. All rights reserved.