Chemistry Reference
In-Depth Information
Table 15.1.
Principal buffering groups in milk
Approximate
concentration (mM)
a, b
Expected pK
a
c,d
pK
a
(in milk)
a
Group
Salts
Inorganic phosphate
Citrate
Organic phosphate esters
Carbonate
Lactic acid
Formic acid
Acetic acid
Various amines
21.0
e
9.0-9.2
2.5-3.5
2.0
<
0.4
0.2-1.8
0.05-0.8
1.5
3, 5.8, 6.6
f
3, 4.1, 4.8
f
1.7, 5.9
f
6.4, 10.1
3.9
3.6
4.8
7.6
2.1, 7.2, 12.3
3.1, 4.7, 5.4
1.4, 6.6
6.4, 10.1
3.9
3.6
4.7
7.6
Titratable group
c
Expected pK
a
c,d
pK
a
(in milk)
a
Protein-bound residues
-COOH
g-COOH
Imidazole
Phenol
"
-NH
3
+
Phosphate
COOH
-COOH
-NH3+
Aspartic acid
Glutamic acid
Histidine
Tyrosine
Lysine
Phosphoserine
N-acetylneuraminic acid
Terminal carboxyl
Terminal amino
4.6
4.6
6.1
9.7
10.4
1.5, 6.5
2.6
3.6
7.9
4.1
4.6
6.5
-
10.5
2, 6
5.0
3.7
7.9
a
Data from Walstra and Jenness (1984).
b
Data from Jenness (1988).
c
Data from Tanford (1962).
d
Data from Edsall and Wyman (1958).
e
About 10 mM colloidal phosphate, 11 mM in solution (at pH 6.6).
f
pK
a
values from titration with Ca(OH)
2
.
There are many reports on the buffering properties of milk (Buchanan
and Peterson, 1927; Whittier, 1933; Wiley, 1935a, b; Jenness and Patton,
1959; Kirchmeier, 1979, 1980; Unnikrishnan and Doss, 1982; Walstra and
Jenness, 1984; Miyagawa and Namba, 1988; Sherbon, 1988; Srilaorkul et al.,
1989; Lucey, 1992; Lucey et al., 1993) calculated from titration curves. How-
ever, the titration curve obtained depends on the methodology used, e.g., the
forward and back-titration curves of milk do not coincide (Lucey, 1992;
Lucey et al., 1993). For instance, when milk was acidified, maximum buffer-
ing occurred at approximately pH 5.1 but when acidified milk was back-
titrated with base, there was low buffering at pH 5.1 and maximum buffering
occurred at pH 6.3 (Lucey et al., 1993). The removal of CCP by acidification
to pH 4.9 at 28C and dialysis against an excess of normal milk (Pyne and
McGann, 1960) resulted in the disappearance of the buffering maximum at
