Rheological study of the interactions between agar (gracilaria chang/i) and specific hydrocolloids
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Date
2004-07
Authors
Soo Lee, Foo
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Abstract
The rheological properties of mixtures containing agar Gracilaria changii and different
types of hydrocolloid (with a total polymer concentration of 1.5 % (w/v)) were studied
as a function of mixing ratio (100/0, 80/20, 60/40, 40/60 and 20/80) and pH (3.5, 4.5
and 5.5). The incorporated hydrocolloids are K-carrageenan, xanthan gum, high
methoxyl (HM) pectin and low methoxyl (LM) pectin. Studies were conducted using
dynamic oscillatory measurements, creep-compliance tests and large deformation
uniaxial compressive measurements. Temperature dependence of dynamic moduli (G'
and G") of agar/hydrocolloid systems studied, excluding agar/xanthan gum mixtures,
closely resembled that of agar during cooling. However, the gelation temperatures (Tgel)
decreased as the proportion of incorporated hydrocolloids increased. On heating, the
melting profiles as well as the melting temperatures (Tmelt) showed considerable
variability. The observed melting variability was attributed to the specific mode of
gelation of the incorporated hydrocolloids. Generally, incorporation of the selected
· hydrocolloids resulted in a significant decrease in gel rigidity and elasticity, as observed
in a sharp drop in G' and loss tangent (tan 8), respectively, determined by mechanical
spectra. Reduction in gel rigidity has also been demonstrated by a marked increase in
instantaneous compliance (Jo) evaluated by creep-compliance tests. Zero shear viscosity
( 1Jo) determined by creep-compliance tests decreased on the addition of the
hydrocolloids. Large defo~ation studies revealed that agar/K-carrageenan and
agar/xanthan gum mixed gels were less brittle, with a higher failure strain (cr), but had
lower gel strength and firmness indicated by a marked decrease in Young's modulus (E)
and failure stress (crr), compared to agar gel. For agar/HM pectin and agar/LM pectin
mixtures, the resulting mixed gels generally showed a decrease in the values of E, crr
and ~::r. These results suggest that the gelation of agar-hydrocolloid systems studied
appeared to have occurred through a segregative phase-separation. A decrease in pH
gave rise to a reduced tendency to gel formation in agar he-carrageenan mixtures and led
to a substantial loss in viscoelastic (G', tan 8, J07 and E) and fracture properties ( crr and
~::r), attributed to a decrease in polymer chain lengths induced by acid hydrolysis.
. .
Reduction in pH did not seem to affect the viscoelastic and fracture properties of
agar/xanthan gum mixtures. The opposite effect was observed for Tgel and Tmelt·
Viscoelastic properties of agar/HM pectin mixtures show pH independence whereas the
fracture properties were highly pH-dependent. Additionally, the Tgel and Tmelt decreased
with decreasing pH. This would indicate that the effects of pH on rheological properties
were found to vary depending on the degree of acid hydrolysis. For agar/LM pectin
mixtures, the Tgel and Tmelt were highly pH-dependent. At high LM pectin content of 60
- 80 %, decrease in pH led to a significant increase in gel rigidity with a corresponding
. decrease in elasticity, as a result of precipitation_ On the other hand, fracture properties
decreased with decreasing pH. Acid hydrolysis was the dominant factor in explaining
these behaviours. Poor correlation between viscoelastic and fracture properties was
observed for agar/LM pectin mixtures.
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Keywords
Interactions between agar , Agar (gracilaria changii) , Specific hydrocolloids