Thermal denaturation and renaturation of a fermentation broth of xanthan: rheological consequences

The rheological properties of an unpasteurised and concentrated xanthan fermentation broth (c = 30 g/l 0.02 M in salt) were studied before heat treatment and after a thermal heating/cooling cycle performed at various polymer concentration conditions (10-30 g/l). At concentrations below 10 g/l heat denaturation occurs with dissociation of the native double-stranded structure into two single strands. At higher concentration, no complete dissociation happens. Changes in both viscoelastic properties and molecular weight are observed after heating above the melting order-disorder temperature (Tm). They are related to the order disorder conformational transition of the xanthan molecules. Xanthan renatured in concentrated conditions (above 10 g/l) has a higher viscosity than that of the native sample and displays more gel-like properties. The inhibition of the dissociation in two single strands in the high concentration range is attributed to the presence of nematic phases observed by viscoelastic measurements and apolar microdomains evidenced by the addition of a neutral detergent.