Casein Interactions: Does the Chemistry Really Matter? - Food Colloids: Self-Assembly and Material Science - page 158

3.5

3

2.5

2

1.5

1

0.5

0

0 5 10 15 20 25 30 35

Q-squared

Figure 1 Logarithm of the coagulation time CT as a function of the square of the net
protein charge Q for chemically modified a s1 -caseins. Closed symbols distin-
guish the different modifications: ' , lysine residues reacted with fluorescamine;
E , lysine residues reacted with dansyl chloride; and m , tyrosine residues
converted to di-iodotyrosine. In all cases coagulation times were varied by
combining various calcium solutions with proteins having differing extents of
modification. Open symbols refer to native protein data at varying Ca 21
concentrations. Dashed lines are linear regression fits through all points in each
data set. For clarity the data sets are shifted along the log (CT) axis (+1 for
dansyl, and +2 for di-iodotyrosine)

which is another calcium sequestrant, but one for which the complex is fully
soluble in water.
The results for citrate addition are shown in Figure 2. The reaction was
monitored by following the increase in solution turbidity with time. When Ca 21
was added to an a s1 -casein + citrate mixture, similar behaviour to that in the
absence of citrate was observed - a lag phase followed by a linear increase in
turbidity with time - except that the lag period increased and the slope of the
linear phase decreased with increasing concentrations of citrate added. The
sequestering action of citrate at pH ¼ 7.0 is almost stoichiometric with one Ca 21
bound for every citrate anion introduced. Taking this reduction in available free
calcium into account in the calculation of casein-bound calcium, we were able to
recover the linear log (rate) versus Q 2 behaviour seen in the absence of citrate. 7
So it appears that citrate acts simply in a passive sequestrant role.
Inclusion of phosphate in the a s1 -casein solution produces a different effect
when calcium is added. Unlike citrate, the coagulation time (CT) for the
aggregation reaction is shortened (Figure 3). Moreover, in the presence of
phosphate, precipitation is induced at Ca 21 levels where such mixtures do not
normally precipitate under the same conditions of pH and temperature. Also,