Abstract
Apparent yield stress is a very useful rheological property which is important in determining the extent of mixing in xanthan fermentations as well as the suspending ability of the gum solutions. Using a previously developed stress relaxation technique, three commercial products, one home-made product and one dehydrated-reconstituted fermentation broth (DRFB) were characterized in terms of the apparent yield stressτ y of solutions containing different polymer concentrations and when mono and divalent salts or sucrose were added and also when exposing the samples to heat treatment in the case of DRFB. Sodium, potassium, calcium, magnesium, acetate, pyruvate, the mean molecular weight (MMW) and the molecular weight distribution (MWD) were determined. Although the four products showed differences in their chemical characteristics, the pyruvic acid content and the mean molecular weight played the main role in determining the yield stress patterns. With no salt added, the slope of theτ y vs gum concentration plot was a strong and inverse function of the pyruvate content. With NaCl 0.5%, the mentioned slope correlated linearly with the MMW. The three commercial xanthans showed very similar MWD. Xanthan broths showed an exponential, rather than a linear (as it was the case for end-products) function ofτ y in terms of xanthan concentration. No significative differences were found in τy depending on the type of salt. Sucrose decreasedτ y and this effect was more pronounced in DRFB than in a solution of a commercial xanthan.
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This work was supported in part by CONACyT (grant 1020-N9111). We are grateful to S. Rodríguez for the atomic absorption determinations. We thank G. Araiza for someτ y measurements and N. Zambrano for helping us with the pyruvate and acetate determinations. We especially thank L. Güereca for his valuable assistance and advise in developing the GPC technique and Prof. A. W. Nienow (University of Birmingham, U.K.) for kindly allowing us to use the Contraves 30 rheometer for the MMW estimation.
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Torres, L.G., Flores, F. & Galindo, E. Apparent yield stress of xanthan solutions and broths. Bioprocess Engineering 12, 41–46 (1995). https://doi.org/10.1007/BF01112992
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DOI: https://doi.org/10.1007/BF01112992