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Abstract
Pectin, a polysaccharide derived from plant cells of fruit, is commonly used as stabilizer
in acidified milk drinks. To gain a better understanding of the way that pectin stabilizes
these drinks, we studied the adsorption and layer thickness of pectin on casein micelles
in skim milk dispersions. Dynamic light scattering was used to measure the layer thickness
of adsorbed pectin onto casein micelles in situ during acidification. The results
indicate that the adsorption of pectin onto casein micelles is multilayered and takes
place at and below pH 5.0. Renneting, i.e., cleaving-off kappa-casein from the casein
micelles, did not alter the adsorption pH. It did, however, show that pectin arrests
the rennet-induced flocculation of casein micelles below pH 5.0. From the findings
we concluded the attachment of pectin onto casein micelles is driven by electrosorption.
Adsorption measurements confirmed the multilayered nature of the adsorption of pectin
onto casein micelles. Both the adsorbed amount and the layer thickness increased with
decreasing pH in the relevant range 3.5-5.0. The phase behavior of a casein micelles/pectin
mixture was determined and could be explained in terms of thermodynamic incompatibility
being relevant above pH 5.0 and adsorption, leading to either stabilization and bridging,
being relevant below pH 5.0. The results confirm that electrosorption is the driving
force for the adsorption of pectin onto casein micelles.