Chemistry Reference
In-Depth Information
1974; Pierre et al., 1983; van Hooydonk et al., 1986; Dalgleish and Law, 1989;
Mariette et al., 1993). The pH at which CCP is completely solubilized pre-
sumably varies with the conditions (e.g. rate and temperature) of acidifica-
tion. At pH values
<
5 milk is unsaturated with respect to most types of
calcium phosphate (Lyster, 1979). At high pH values (
<
6), concentrated
milk products (e.g. processed cheeses) have an increased likelihood of pre-
cipitation of some types of Ca phosphate. Increasing the pH of milk results in
the formation of additional CCP. McGann and Pyne (1960) described a
method for increasing the CCP content of milk (by up to 200%). Milk pH
was increased by the addition of NaOH at about 08C followed by exhaustive
dialysis against a large excess of the original milk.
Lucey et al. (1996) studied the impact of (cold) acidification and neu-
tralization of milk on the properties of casein micelles. Acidification of milk
to pH 5.0 or 4.6, followed by neutralization to pH 6.6, resulted in a reduction
in the buffering maximum of milk at pH
5.1; this buffering peak is caused by
the solubilization of CCP. The reduced buffering in reformed milk suggests
that little reformation of CCP occurs on neutralization; reformed milks also
had an elevated Ca
2+
activity. Acidification of milk to pH
>
5.5, followed by
neutralization to pH 6.6, hardly reduced buffering (at pH
5.1), suggesting
that either little CCP dissolved on acidification in that pH range or reforma-
tion of CCP occurred on neutralization. Canabady-Rochelle et al. (2007) also
reported that milk had a higher soluble Ca level after acidification and
neutralization.
Gevaudan et al. (1996) used high-pressure CO
2
to acidify milk reversi-
bly (pH was restored to the original value after depressurization). Acidifica-
tion to pH
5 with high-pressure CO
2
resulted in a reduction in the buffering
peak at pH
5.1 but this peak increased during chilled storage of this milk
(Raouche et al., 2007).
Heat treatment has little impact on the pH-dependent release of Ca and
phosphate from micelles during acidification (Law, 1996; Singh et al., 1996).
9.4.3.
Concentration of Milk
Concentrating milk by evaporation results in a decrease in milk pH, e.g.
a decrease of
0.3 and 0.5 pH units for 2:1 and 3:1 concentrations, respec-
tively (Walstra and Jenness, 1984). The [Ca
2+
] increases with concentration
but less than the concentration factor (Walstra and Jenness, 1984). Presum-
ably, the smaller increase in Ca
2+
is at least partly due to the formation of
additional CCP (even though the pH decreases in evaporated milk). Mem-
brane filtration of milk using either ultrafiltration or microfiltration results in
retentates where CCP is a greater proportion of the total Ca content as some
soluble Ca is lost in the permeate during processing (Lelievre and Lawrence
