Casein Interactions: Does the Chemistry Really Matter? - Food Colloids: Self-Assembly and Material Science - page 160

unlike the usual a s1 -casein + Ca 21 system behaviour, where the CT and the
slope of the linear growth phase are inversely related, the slope in the presence
of phosphate first increases and then decreases as the included phosphate
concentration is increased.
Before considering possible explanations for the above behaviour, we must
explore the possibility that the precipitation of calcium phosphate alone
(independent of a s1 -casein) was causing the increase in turbidity. Experiments
carried out in the absence of a s1 -casein showed measurable precipitation of
calcium phosphate at higher concentrations of phosphate (P i ) and calcium ions.
Taking the slope of the linear growth in turbidity of these solutions as a
measure of the rate of aggregation, the results are conveniently plotted as log
(rate) versus the ion product [Ca 21 ][P i ], as this representation shows linear
behaviour (Figure 4). Inclusion of a s1 -casein produces precipitation at low ion-
product values, lying well away from this linear behaviour and at rates
approximately two orders of magnitude greater than predicted by extrapolation
of the line into the same low ion-product range (Figure 4). The lowering of CT
cannot be due to some independent precipitation of calcium phosphate because
the inclusion of phosphate in the a s1 -casein + Ca 21 mixture speeds up the
precipitation of this metastable system. There is nothing to suggest that the
phosphate will precipitate the protein on its own. The mixture of a s1 -casein +
P i remains perfectly clear in the absence of ionic calcium. The three component
mixture, a s1 -casein + Ca 21 +P i , is destabilized by the inclusion of both
protein and phosphate. Remove either protein or phosphate in this limited

-1

-1.5

-2

With α S1-casein

-2.5

-3

No α S1-casein

-3.5

-4

-4.5

-5

0 5 10 15 20 25 30

Ion product [Ca][P]

Figure 4 Effect of inclusion of a s1 -casein on the rate of precipitation of calcium phosphate
mixtures. Semi-log plot of dependence of rate of reaction (given by rate of
increase of solution turbidity with time) on the value of the input ion product
[Ca][P i ] (mM 2 ). Data points ( E ) obtained in the absence of protein, showing
a linear dependence, appear to the right. Data points ( ' ) obtained in the
presence of protein appear to the left (well above the extrapolated line)