145
study involved subjecting the solutions of native pectin mixture (1.15% apple pectin, 41.40% sucrose, pH 1.50) having high viscosity at time zero to stress at different
gelation times. It was observed that with the increase in gelation time, the magnitude of the stress response to the oscillatory sweeps increased display in the elastic
nature of the solution. Also the storage modulus (G’), having dependency on frequency became less evident specifying the change from viscous to rubbery nature.
This behaviour can be explained on the basis of network formation. In the beginning
the pectin behaves as single and dispersed molecule. During shearing, owing to the
high mobility of the pectin dispersion, there is quick relaxation of pectin molecules.
This keeps the building up of excessive stress at low oscillation frequencies. With
the passage of time, as result of hydrogen bonding and hydrophobic interactions
association of pectin molecules begins. This forms the 3Dnetwork which blocks the
molecules to interact with the superimposed flow. As a matter of fact there is a shift
from viscous region to rubber plateau.
Da Silva et al. (1995) reported that gelation of HMP is highly temperature dependent. At 5 °C gelation of HMP was performed and it resulted in a weak gel due to
slow gelation. The reason for this being the lack of hydrophobic interaction at low
temperature. At temperatures up to 30 °C Pseudo-equilibrium G increased and then
decreased. It was also observed that at an intermediate temperature range, hydrogen
bonds and hydrophobic interactions can result in formation of the network having
higher elasticity. On heating the structure of HMP-sugar gels, they tend to be irreversible. When the gel was heated and cooled an unusual ‘thermal annealing’
behaviour was found (Evageliou et al. (2000). This showed its dependency on
pH. In presence of 65 wt% sucrose and at pH 4.7, a HMP (of DM = 70%, 0.5%) gel
was fully reversible with no detectable thermal hysteresis on heating. Upon heating,
there was slight reduction in G’ and G” (pH 3.0 and 3.5) Despite having pH difference, stability remained virtually constant at 30 °C.
Low Methoxy Pectin (LMP)
The stabilization of LMP gels is brought by ionic cross-linkages via calcium
bridges between two carboxylates from two different chains. There is seldom any
hydrogen bonding or any hydrophobic interaction. The structure hence formed is
given the term “egg-box” model (Gidley et al. 1979; Grant et al. 1973).The egg
box model consists of junction zones that are formed by the sequential, side by
side units of GalA, in which the ionic and electrostatic ionic bonding of carboxyl
groups results in interlocking of GalA monomers in adjacent chains. This forms an
egg-box structure and was first explained for alginate. Various workers have studied the egg-box models for alginate and pectin (Braccini and Pérez 2001). The
“egg-box” model is best suited to explain the gelation mechanism of alginate but
in case of LMP, a model termed as ashifted “egg-box” can be used to understand
the gelation (Low et al. 2015). It is termed as “ashifted egg-box” as one of the
chains is shifted with respect to the other. The workers stated that there is two-step
Pectin
study involved subjecting the solutions of native pectin mixture (1.15% apple pectin, 41.40% sucrose, pH 1.50) having high viscosity at time zero to stress at different
gelation times. It was observed that with the increase in gelation time, the magnitude of the stress response to the oscillatory sweeps increased display in the elastic
nature of the solution. Also the storage modulus (G’), having dependency on frequency became less evident specifying the change from viscous to rubbery nature.
This behaviour can be explained on the basis of network formation. In the beginning
the pectin behaves as single and dispersed molecule. During shearing, owing to the
high mobility of the pectin dispersion, there is quick relaxation of pectin molecules.
This keeps the building up of excessive stress at low oscillation frequencies. With
the passage of time, as result of hydrogen bonding and hydrophobic interactions
association of pectin molecules begins. This forms the 3Dnetwork which blocks the
molecules to interact with the superimposed flow. As a matter of fact there is a shift
from viscous region to rubber plateau.
Da Silva et al. (1995) reported that gelation of HMP is highly temperature dependent. At 5 °C gelation of HMP was performed and it resulted in a weak gel due to
slow gelation. The reason for this being the lack of hydrophobic interaction at low
temperature. At temperatures up to 30 °C Pseudo-equilibrium G increased and then
decreased. It was also observed that at an intermediate temperature range, hydrogen
bonds and hydrophobic interactions can result in formation of the network having
higher elasticity. On heating the structure of HMP-sugar gels, they tend to be irreversible. When the gel was heated and cooled an unusual ‘thermal annealing’
behaviour was found (Evageliou et al. (2000). This showed its dependency on
pH. In presence of 65 wt% sucrose and at pH 4.7, a HMP (of DM = 70%, 0.5%) gel
was fully reversible with no detectable thermal hysteresis on heating. Upon heating,
there was slight reduction in G’ and G” (pH 3.0 and 3.5) Despite having pH difference, stability remained virtually constant at 30 °C.
Low Methoxy Pectin (LMP)
The stabilization of LMP gels is brought by ionic cross-linkages via calcium
bridges between two carboxylates from two different chains. There is seldom any
hydrogen bonding or any hydrophobic interaction. The structure hence formed is
given the term “egg-box” model (Gidley et al. 1979; Grant et al. 1973).The egg
box model consists of junction zones that are formed by the sequential, side by
side units of GalA, in which the ionic and electrostatic ionic bonding of carboxyl
groups results in interlocking of GalA monomers in adjacent chains. This forms an
egg-box structure and was first explained for alginate. Various workers have studied the egg-box models for alginate and pectin (Braccini and Pérez 2001). The
“egg-box” model is best suited to explain the gelation mechanism of alginate but
in case of LMP, a model termed as ashifted “egg-box” can be used to understand
the gelation (Low et al. 2015). It is termed as “ashifted egg-box” as one of the
chains is shifted with respect to the other. The workers stated that there is two-step
Pectin