Food Biochemistry and Food Processing (2 edition)

BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come

460 Part 4: Milk

Table 24.3.Distribution of Organic and Inorganic Ions Between Soluble and Colloidal

Phases of Bovine Milk

Soluble

Species

Concentration

(mg/L) % Form Colloidal (%)

Sodium 500 92 Ionised 8

Potassium 1450 92 Ionised 8

Chloride 1200 100 Ionised _

Sulphate 100 100 Ionised _

Phosphate 750 43 10% bound to Ca+^2 and Mg+^257

51% H 2 PO4−

39% HPO 42 −

Citrate 1750 94 84% bound to Ca and Mg 6

14% Citr^3 −

1% HCitr^2 −

Calcium 1200 34 35% Ca^2 + 66

55% bound to citrate

10% bound to phosphate

Magnesium 130 67 Probably similar to calcium 33

Source: Modified from Fox and McSweeney 1998.

casein micelles; these salts are referred to as CCP, although some

magnesium, citrate and traces of other elements are also present

in the micelles. The typical distribution of the principal organic

and inorganic ions between the soluble and colloidal phases

is summarised in Table 24.3. The actual form of the principal

species can be determined or calculated after making certain

assumptions; typical values are shown in Table 24.3. Many of

the micro-elements are bound by proteins, including enzymes,

some of which are present in the MFGM.

The precise nature and structure of CCP are uncertain. It is

associated with the caseins, probablyviathe casein phosphate

residues; it probably exists as nanocrystals that include PO 4

residues of casein. The simplest stoichiometry is Ca 3 (PO 4 ) 2 but

spectroscopic data suggest that CaHPO 4 is the most likely form.

CCP plays a major integrating role in the casein micelles, as

discussed earlier.

The solubility and ionisation status of many of the principal

ionic species are interrelated, especially H+,Ca^2 +,PO 43 −and

citrate^3 −. These relationships have major effects on the stabil-

ity of the caseinate system and consequently on the processing

properties of milk. The status of various species in milk can be

modified by adding certain salts to milk, for example [Ca^2 +]by

PO 43 −or citrate^3 −; addition of CaCl 2 to milk affects the distri-

bution and ionisation status of calcium and phosphate and the

pH of milk.

The distribution of species between the soluble and colloidal

phases is strongly affected by pH and temperature. As the pH

is reduced, CCP dissolves and is completely soluble<ca.pH

4.9; the reverse occurs when the pH is increased. These pH-

dependent shifts mean that acid-precipitated products, for ex-

ample acid casein and acid-coagulated cheeses, have a very low

concentration of Ca.

The solubility of calcium phosphate decreases as the temper-

ature is increased. Consequently, soluble calcium phosphate is

transferred to the colloidal phase, with the release of H+and a

decrease in pH:

CaHPO 4 /Ca(H 2 PO 4 ) 2 ↔Ca 3 (PO 4 ) 2 +3H+

These changes are quite substantial, but are at least partially

reversible on cooling.

Since milk is supersaturated with calcium phosphate, concen-

tration of milk by evaporation of water increases the degree of

super-saturation and the transfer of soluble calcium phosphate to

the colloidal state, with the concomitant release of H+. Dilution

has the opposite effect. The concentration of salts in milk may

be modified by electrodialysis, ion exchangers or addition of

chelators or selected salts; such operations are practised widely,

for example for sterilised milks, processed cheese or nutritional

products, such as infant formulae.

Milk salts equilibria are also shifted on freezing; as pure water

freezes, the concentrations of solutes in unfrozen liquid are in-

creased. Soluble calcium phosphate precipitates as Ca 3 (PO 4 ) 2 ,

releasing H+(the pH may decrease to 5.8). The crystallisation

of lactose as a monohydrate aggravates the situation by reducing

the amount of solvent water.

There are substantial changes in the concentrations of the

macroelements in milk during lactation, especially at the begin-

ning and end of lactation and during mastitic infection (see White

and Davies 1958, Keogh et al. 1982, O’Keeffe 1984, O’Brien

et al. 1999a). Changes in the concentration of some of the salts

in milk, especially calcium phosphate and citrate, have major

effects on the physico-chemical properties of the casein system

and on the processability of milk, especially rennet coagulability

and related properties and heat stability.