Effect of salts on the rheology of hydrocolloids from mulberry (Morus alba L.) leaves in concentrated domain

Abstract

Rheological properties of 2-8% mulberry leaf hydrocolloids extracted by hot water (Hw) or sodium bicarbonate (Alk or Alk-v) were evaluated. Steady shear theological results of mulberry leaf hydrocolloids indicated the structural behavior with a Newtonian viscosity at low shear rate and shear-thinning behavior over intermediate to high shear rate regime. As simulated by Carreau model, the characteristic time constant of mulberry leaf hydrocolloids increased, but the power-law behavior index decreased with increasing concentration. Results from the dynamic theological experiments revealed that the behavior of 2% mulberry leaf hydrocolloid solutions were characteristic concentrated polymer solution. However, as the concentration was raised up to 4-8%, a gel-like behavior may occur depending on the extraction methods, possibly owing to the formation of aggregates at high polymer concentration. Furthermore, theological properties of Alk and Alk-v were generally higher than those of Hw. Though mulberry leaf hydrocolloids were ionic polymers, the theological properties in concentrated domain were not influenced pronouncedly by the addition of 0-200 mM of NaCl, KCl or MgCl(2). In contrast, under sufficiently high CaCl(2) concentration (>= 68 mM or 200 mM for alkaline extracted and water extracted mulberry leaf hydrocolloids, respectively), formation of a three-dimensional gel network was observed, possibly due to the cross-linking of Ca(2+) with carboxyl groups of mulberry leaf hydrocolloids in addition to charge screening effects. (C) 2009 Elsevier Ltd. All rights reserved.