Rheological study of the interactions between agar (gracilaria chang/i) and specific hydrocolloids
| dc.contributor.author | Soo Lee, Foo | |
| dc.date.accessioned | 2015-09-02T08:22:56Z | |
| dc.date.available | 2015-09-02T08:22:56Z | |
| dc.date.issued | 2004-07 | |
| dc.description.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. | en_US |
| dc.identifier.uri | http://hdl.handle.net/123456789/1141 | |
| dc.language.iso | en | en_US |
| dc.subject | Interactions between agar | en_US |
| dc.subject | Agar (gracilaria changii) | en_US |
| dc.subject | Specific hydrocolloids | en_US |
| dc.title | Rheological study of the interactions between agar (gracilaria chang/i) and specific hydrocolloids | en_US |
| dc.type | Thesis | en_US |
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