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

Rheological properties of 2–8% mulberry leaf hydrocolloids extracted by hot water (Hw) or sodium bicarbonate (Alk or Alk-v) were evaluated. Steady shear rheological 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 rheological 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, rheological properties of Alk and Alk-v were generally higher than those of Hw. Though mulberry leaf hydrocolloids were ionic polymers, the rheological properties in concentrated domain were not influenced pronouncedly by the addition of 0–200 mM of NaCl, KCl or MgCl2. In contrast, under sufficiently high CaCl2 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 Ca2+ with carboxyl groups of mulberry leaf hydrocolloids in addition to charge screening effects.