Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
measured and the dielectric permeability, ε′, giving the polarization of the sample,
and the dielectric loss, ε″, a measure of the energy loss, as a function of frequency
and temperature are calculated. Dipoles and ions try to follow the direction of the
applied alternating current. At low frequencies, the charges move freely and ε′
remains constant and ε″ is zero. At high frequencies, the charges cannot move
quickly enough with the field and are fixed in a position that does not have to be
parallel to the field. During a frequency scan, a transition takes place depending on
the characteristic relaxation time, τ, of the orientation process (Figure 23.3). In
analogy, at low temperatures the charges are frozen in and at increased temperature
they become mobile.
For polymers, several transitions are found. At temperatures below the T g, peaks
are found because certain chain segments are more mobile than other parts of the
polymer. A β-transition can be seen for the mobilization of chain segments, and at
even lower frequencies or temperatures γ- and δ-transitions are found for smaller
segments such as the rotation of large side groups. The relaxation times of these
transitions follow Arrhenius law, τ = τ 0 exp(E a/RT), where τ 0 is the preexponent
factor at infinite T and E a the thermal activity energy of the dipole rotation The αtransitions
at high frequencies and temperatures are due to mobility changes of whole
chains (the T g). The α-transition behaves according to Vogel–Fulcher–Tammann
(VFT) law. VFT accounts for the dipole rotations, which are determined by polymer
free volume changes during the T g, τ = τ 0 exp [E v/R(T – T v)], where E v is VFT
activation energy and T v the VFT temperature.
For dry solid polysaccharides, four relaxation processes are observed below the
T g: the local main chain motion or the β-relaxation, the side group motion in the
repeating unit or the γ-relaxation, the δ-relaxation in the low frequency side for welldried
samples, and finally a β-relaxation found in wet samples around room temperature,
of which the origin is not entirely clear. At higher temperatures, the αrelaxation
can be measured, which is often associated with the hopping motion of
ions in the disordered structure of an ionic biopolymeric material. 8,59
Although DRS was used for synthetic polymers, only recently DRS became
more popular for carbohydrates. 8,59,60 A recent review on DRS for analysis of
polysaccharides was given by Einfeldt et al. 59 DRS is used to study physical ageing
phenomena of polysaccharides. 61 Several studies concern complex polysaccharide
systems (wood, bread, cereal). 8,62,63 Local or side-group motions in modified celluloses
and starches were evaluated with the Havriliak–Negami relaxation model. 8,64
The electrical and dielectric properties of Ba 2+ and Ca 2+ cross-linked alginate hydrogels
were studied by single-particle electrorotation using microelectrodes. 65 Some
studies were related to the effect of water on the chain mobility in polysaccharides. 8,64
Another area is the gelation, coil-helix, and sol-gel transitions occurring in gelforming
ionic polysaccharides. 66,67 The problem of the origin of the dielectric secondary
relaxations of pure polysaccharides was described not long ago. 68 The activation
energy of the lower temperature transition for amylose films, obtained from
the frequency dependence of the relaxation, was typical of a primary α-relaxation
or T g. This indicated that although glycerol is an effective plasticizer of amylose,
amylose–glycerol mixtures are only partially miscible. 69 DRS is important for studies