Hydrocolloid Gums – Their Role and Interactions in Foods - Practical Food Rheology - page 65

High pH
Low salt concentration

Low pH
High salt concentration

Fig. 4.3 Molecular conformation of charged hydrocolloids in response to pH and salt
concentration, which affects rheological response.

should be paid in the application of charged hydrocolloids, since molec-
ular conformation can be drastically altered as indicated in Fig. 4.3.
With increasing concentration, macromolecules begin to interact as
sketched in Fig. 4.4; the critical concentration for the onset of interac-
tions is denoted as the coil-overlap concentration, c * . The nature and
extent of interaction is affected by the solvent conditions and the solu-
tion behaviour of the individual molecule. At c * , a change in the slope
of the specific viscosity,

η sp , defined as:

η sp = η − η s
η

(4.3)

s

where η is the viscosity of the solution and η s is the viscosity of the

solvent, can be observed (note that η sp is dimensionless).

The behaviour is linear with the slope for c

<
c * with characteristic values for different hydrocolloids (see Table 4.1).
Plotting the dimensionless product c [

> c * larger than for c

], also referred to as the coil-
overlap parameter, on the abscissa collapses the

η

sp curves for a large
number of hydrocolloids (Morris et al ., 1981). Values reported for the
slope are

η

∼ 1.4 and ∼ 3.3 for the dilute regime and the concentrated

η sp

c

c*

c < c*
(dilute regime)

c = c*
Onset of overlap

c > c*
(concentrated regime)

Fig. 4.4 η sp ) versus concentration

(c) relationships for random coil polymers, where c * is the coil-overlap concentration.

Macromolecular conformation and specific viscosity (