Chemistry Reference
In-Depth Information
Added Ca
2+
Added P
i
Decreasing salt
balance ratio
Increasing salt
balance ratio
pK
5.8
6
6.2
6.4
6.6
6.8
pH
Figure 9.5.
Stability phase diagram of milk as a function of pH or other treatments; schematic,
meant to indicate trends for both ethanol and heat stability behaviour of milk (adapted from
Horne, 2003).
competing effects on the micellar system, destabilization through loss of the
hairy layer and shifts in the Ca phosphate equilibria, first noted by Pierre
(1985). If the ethanol promotes the precipitation of Ca phosphate external to
the micelle, it would first reduce the concentration of free Ca, reduce the level
of caseinate-bound Ca and disrupt the binding through Ca phosphate
nanoclusters. Moderate losses would increase the negative charge on the
caseins and increase the thickness of the steric stabilizing layer. The higher
the alcohol concentration, the faster and more extensive would be the pre-
cipitation of Ca phosphate. The ensuing adjustment in protein charge and
conformation, although relatively rapid, still requires a finite response time.
Countering these changes are the effects of ethanol as a non-solvent for the
proteins, promoting crosslinking and collapse of the hairy layer. When the
coagulation reaction occurs faster than the adjustment of charge and con-
formation resulting from shifts in Ca phosphate equilibria or the extent of the
latter is limited by insufficient ethanol, the aggregation reaction dominates
and precipitation of micelles follows.
The origin of the sigmoidal ethanol stability/pH profile (Figure 9.5) can
also be explained through the effect of pH on Ca phosphate precipitation.
Increasing pH brings about increased Ca phosphate precipitation, possibly
further enhanced by the ethanol, which means that more ethanol is required
to precipitate the protein, i.e. to overcome the increased energy barrier being
erected following the transfer of Ca phosphate from the nanoclusters. Con-
versely, decreasing pH acts to diminish the influence of ethanol-induced
