Rheological characterization of cellulosic microfibril suspensions. Role of polymeric additives

We have investigated the theological behaviour of cellulosic microfibril suspensions which may be considered as thickening systems for food applications. We have demonstrated the existence of a critical microfibril concentration, C* approximate to 3 g/l, above which the viscosity increases rapidly and a gel-like behaviour is obtained. No decrease in the storage (G(I)) and loss (G(II)) moduli was found with an increase of temperature, whereas the addition of NaCl to the suspensions seems to enhance the gel-like behaviour, due to the reinforcement of the microfibril entanglements and interactions. To allow the redispersion of a dried sample, water-soluble polymers need to be added to the suspensions before their drying. The influence of some water-soluble polymers on the physicochemical properties of suspensions was studied. For the initial aqueous suspension, the general trend is a decrease of G(I) and G(II) with an increase of the additive amount, whatever the polyelectrolyte. For dried samples, the ability to redisperse and the behaviour of the redispersion strongly depend on the charge parameter, chemical structure and sign of the charge of the polyelectrolyte used. This theological dependence was related with streaming potential measurements. A positive effect is observed with anionic polymers. The proposed mechanism is the formation of an anionic layer around the cellulosic fibres during drying preventing cellulose-cellulose binding. This water-soluble polymeric layer is able to swell during re-hydration allowing the redispersion of the fibres. (C) 2001 Elsevier Science Ltd. All rights reserved.