Chemistry of Milk

Lesson 31. PHYSICAL EQUILIBRIUM AMONG MILK SALTS

Module 7. Salt composition of milk

Lesson 31
PHYSICAL EQUILIBRIUM AMONG MILK SALTS

31.1 Introduction

Milk contains several elements but all of them are not entirely in a soluble state. Some minerals exist in colloidal and ionic state at the normal pH of milk. The compounds which are in soluble condition play an important role in keeping various milk constituents in stable condition. A balance exists between the components which are in soluble state and those which are in colloidal state.

31.2 Salts Solution in Milk

The dissolved salts of milk are phosphate, citrate, chloride, sulphate, bicarbonate, sodium,potassium, magnesium calcium. Sodium and potassium in milk are present entirely in soluble state. It is necessary have physical equilibria among the milk salts for stability of milk, especially during heat processing. Colloidal particles of casein contain predominantly calcium, magnesium, phosphate and citrate. The chloride and sulfate are entirely present as the free ions Cl ^- and SO₄^{2 -} at the pH of milk The salts of the various weak acids (phosphates, citrates, and carbonates) are distributed among various ionic forms. Calcium and magnesium form soluble ions with citrate, phosphate and bicarbonates.

• H©^-2 → © ^-2 + H ⁺

• © ^-2 + Ca⁺² → Ca© ^-

• Ca © ^- +H ⁺ → CaH©

• 2 Ca © ^- + Ca⁺² → Ca₃©₂

Where © =Citrate

A considerable portion of the calcium is held in the form of complex soluble ions such as Ca©^- Similar complex ions are also formed with phosphate CaPO₄^- and with bicarbonate CaHCO₃⁺.The formation of such complex ions has the effect of reducing the concentration of calcium and magnesium ions in the solution. Only calcium salts are included but a similar scheme would apply to the magnesium salts.

31.3 Salt Balance

Salts in milk exist in colloidal and soluble form. A large amount of salt is present in casein micelles structure and is in equilibrium with salts in the soluble phase of milk. During acidification/fermentation of milk some salts gets solubilized from the colloidal state and come in to the soluble state. This destabilizes the casein micelle structure and tends to precipitate out as happens during curd setting of milk and preparation of acid casein. It also happens that the casein micelle stability is lost during heating of milk at high temperature such as in UHT processing, sterilization and boiling of milk. This is due to the in solubilization of colloidal calcium phosphate and final loss in the stability of milk, this in turn affects the quality of dairy products. Such changes in the salt balance also take place during manufacture of condensed milk,evaporated milk and milk powder.

The salt balance in milk is defined by the following equation

It has been observed that the balance between certain salts such as Ca⁺² and Mg⁺² with citrate and phosphate in the colloidal and soluble phase plays apivotal role in the stability of casein micelles structure. It appears adelicate balance cations and anions in the colloidal and soluble state is important in determining the stability of milk. The monovalent cations such as Na⁺ have a dispersing effect, while divalent and trivalent cations such as Ca⁺² and Mg⁺² have an aggregating effect. So during heat processing of cow milk and buffalo milk the heat stability would be governed by the concentration of above ions in addition to other factors. Therefore,addition of sodium citrates and phosphate improves the heat stability of milk.

Colloidal salts remain in equilibrium with the dissolved salt (diffusate form). Various treatments of milk may cause transfer of salts from one phase to the other. Various methods have been used to study the partition of salts between dissolved and colloidal phases. Two thirds of the calcium and one third of the magnesium and on half of the phosphorus are colloidal in normal milk. It is presumed that all the colloidal phosphorus is present in the micellar calcium phosphate and it appears that all the colloidal citrate is similarly located where as colloidal calcium and magnesium are partly in corporate in milk colloidalphosphate and partly bound in a more direct manner to casein. A small portion of colloidal calcium is bound to α lactalbumin. Β lactoglobulin can also bind calcium and magnesium and there is other minor calcium binding protein. Nearly all the sodium, potassium and chlorine are diffusible.

The principle dissolved salt constituents or milk consists of phosphate, citrate, chloride,sulphate, bicarbonate, sodium, potassium, magnesium and calcium. At pH 6.6 of milk sodium and potassium are not found in any combination with other constituents and are present as the cations Na⁺ and K⁺ while the chloride and sulfate being anions of strong acids are present as freeion Cl^- and SO₄^-. However the salts of the weak acids phosphate citrates and carbonates are distributed among various ionic forms. Calcium and magnesium form soluble complex ions with citrate, phosphate, and bicarbonates. Figure 31.3 shows the changes in salt balance due to various treatments of milk. Various factors effect salt equilibria. These are

1. Temperature
2. Variation in acidity
3. Variation in carbon dioxide content
4. Concentration

Sequestering agents and ion exchangers

Fig. 31.1 Equilibrium among milk salts
(Source: Jenness and Patton Principles of Dairy Chemistry)

31.3.1 Temperature

The temperature will shifts the balance among the various forms as milk is subjected to various cooling and heating treatments after it is drawn from the cow at 37⁰C. With the rise in the temperature the dissolved will be transferred to colloidal phase. Similarly lowering the temperature below that at which the milk is drawn would cause a transfer of calcium and phosphate from the colloidal particles to the dissolved state.

31.3.2 Acidity

There will be pronounced shift in the salt equilibria with the addition of acid to milk wither directly or indirectly by bacterial action. Decrease in the pH withdraws the calcium and phosphate from colloidal particles until at about pH5.2 all the calcium and phosphate is in the dissolved state.

31.3.3 Carbon dioxide content

Milk as secreted by the cow contains about 20 mg of CO₂ per 100ml or about 10% by volume. This gas is rapidly lost from milk owing to the low content in the air. The loss is irreversible under ordinary conditions of handling. The loss of CO₂ from milk is accelerated by heating and agitation. Removal of CO₂ would affect the balance in the rest of the system. It would be expected that the removal of CO₂ and the consequent rise in pH would be reflected in a shift in calcium phosphate from the dissolved to the colloidal state and probably also in a shift of calcium ions activity.

31.3.4 Concentration

As the milk is concentrated there is a tendency for calcium phosphate and calcium citrate to accumulate in the colloidal particles because the solubility is exceeded. As these materials are insolubilized , hydrogen ions are liberated, lowering the pH. The net result is an increase in concentration of citrate and phosphate in both the dissolved and colloidal state.

31.3.5 Sequestering agents and ion exchangers

In order to stabilize the milk or even to improve the utility of milk for a particular purpose it is desirable to treat milk so as to alter its ionic balance. The best known example for this is the addition of phosphate or citrate to stabilize milk against subsequent heat coagulation. This practice is common during the manufacture of evaporated milk, Addition of phosphate or citrate result in the binding of more of the calcium in the form of soluble complexes and decreasing activity of the calcium ions.

Soluble reagents which thus tie up a particular ions are called sequestering agents and are said to sequester that ion. Ethylenediamine tetra acetic acid is on such sequestering reagent. It is very good reagent for divalent and polyvalent cations. Treatment of milk with ion exchange column will remove both anions and cations from milk by a process called “monobed resin” treatments. It is mixture of cation exchange resine in the hydrogen form and an anion exchange resin in the hydroxyl form. When a salt containing solution is passed through such a mixture of resins both anions and cations are absorbed by the resin and hydrogen and hydroxyl ions are released which immediately combine to form water. By such treatments milk can be deionized to any extent desirable in a single pass through a resin bed or by batch wise treatment with a mixture of the two resins.