Investigations of the behaviour of pectin in casein micelle systems and their analogues : thesis presented for the degree of Doctor of Philosophy in Physics
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Date
2009
DOI
Open Access Location
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Massey University
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Abstract
Firstly, the effect of pectin on acid milk gels in concentrated, quiescent systems was
investigated by passive microrheology using two complementary techniques: diffusive
wave spectroscopy (DWS) and multiple particle tracking (MPT). DWS, by allowing
probing the mechanical properties of the network at high frequency, gave information
on its microstructure. The addition of high methoxyl pectins was shown to change the
network structure which has been explained by bridging of the casein micelles by the
polymer as the system was undergoing acidification. On the other hand, the presence of
low methoxyl pectin in the acid milk gel was shown to have no effect on the
microstructure of the network at low concentration of polymer (0.1%w/w) which has
been attributed to the sensitivity of this low DM pectin to calcium: LM pectin are
trapped by calcium and not able to interact with casein micelles anymore. Multiple
particle tracking was used to probe the effect of pectin on the heterogeneity of the
system by following the distribution of the displacements of added micro beads at a
given time lag during the gelation using the Van Hove distribution. Furthermore, the
surface chemistry of the probes was modified in an attempt to control their location in
the system. Finally, the mean square displacements of the casein micelles obtained by
DWS and, of k-casein coated particles obtained by MPT were shown to give good
agreement for the same acid milk system.
Having established that the interaction between casein micelles and low methoxyl
pectin is prevented by the pectin sensitivity to calcium, the effect of the pectin fine
structure was investigated on the interaction between k-casein and pectin by surface
plasmon resonance (SPR). The amount of pectin binding on a k-casein coated gold
surface was shown to be strongly dependant on the pectin fine structure. It was
concluded that small negative patches on the pectin backbone, likely to comprise of
around two consecutive unmethylesterified galacturonic acid, are the most effective for
pectin binding to k-casein. The effect of the direct interaction between pectin and k-
casein on ‘calcium-free casein micelle mimics’ in pectin solution was then investigated
using coated latex beads. A pectin structure with a limited number of negative patches
on its backbone was also shown to limit the potential for destabilization via bridging.
Description
Keywords
Acid milk gels, Pectin-casein interaction