CHEMISTRY OF MILK

Module 7. Salt composition in milk

Lesson 30
SALTS IN MILK

30.1 Introduction

The salts of the milk include those constituents that are present as ions or in equilibrium with ions, except hydrogen and hydroxyl ions. Salts are important from nutritional point of view. They also largely determine the physicochemical state of the casein micelles, which in turn influences heat stability of the milk. Some metals acts catalysts in oxidation of milk fat leading to oxidative rancidity. Certain metals like Ca, P, Mg, Mo,Co, Fe, etc. are components of metallo proteins such as lactoferrin, lactoperoxidase, xanthine oxidase and parts of phospholipids and certain vitamins.

The salt composition in milk cannot be expressed either as an equivalent to inorganic or organic substance because of the fact that some salts exist as organic substances also. It is also not possible to express them as ionic substances as a considerable part of these minerals are present in non ionized form. The ash content of the milk also does not represent the salt composition since during ashing of milk, some of the salts are lost due to volatilization. It is also possible that some non salt substances get included in ash. The minerals in human, cow and buffalo milk plays an important role in the availability of these minerals for the young ones. They also play an important role in the digestibility of milk proteins.

30.2 Overall Composition of Minerals

The most important salts of bovine milk are given in a Table 30.1.

Table 30.1 Overall composition of minerals in bovine milk

(Source: Fundamentals of Dairy Chemistry, Wong et. al. 1988)

Not all of the salt constituents are found in the dissolved state in milk. Calcium, magnesium, phosphate, and citrate are partitioned between the solution phase and the colloidal casein micelles. For analytical purposes, partition of the salt constituents can be achieved by equilibrium dialysis or by pressure ultra filtration.

Table 30.1 presented above shows the proportion of the several constituents found in the dissolved and diffusible state. Actually,phosphate is present in five classes of compounds: inorganic dissolved,inorganic colloidal, water soluble, esters, ester bound in caseins, and lipid. These can be determined by making the following analyses:

-Total P in the dry or wet ashed sample-I

-Lipid Pin digested Rose-Gottlieb extract-II

-Dissolved P in digested ultrafiltrate-III

-In organic dissolved P in undigested ultrafiltrate-IV

-Acid soluble P in undigested 12.5% trichloroacetic acid filtrate-V

Then:

-Inorganic dissolved P = IV

-Inorganic colloidal P = V - IV

-Water soluble ester P = III- IV

-Casein P = I - (II + V)

-Lipid P = II

The total inorganic phosphate (Pi) is, of course, V. The salt constituents in the dissolved state (ultra filterable or diffusible) interact with each other to form various complexes. The concentrations of each of these constituents can be calculated (with suitable computer programs) from a knowledge of their several interaction or association constants. Na, K, andC1 are primarily present as free ions but Ca, Mg, phosphate, and citrate are distributed throughout many complexes; those in the highest concentration are CaCit^-, Mg Cit^-, H₂PO₄^-HPO₄^- and CaHPO₄. The calculation yields Ca²⁺and Mg²⁺ concentrations of 2.0 and 0.8 mm respectively.

30.3 Trace Elements

A large number of trace elements are found in milk.Although these elements are very less quantitatively they have significant influence in the various properties of milk and also play a significant role inhuman nutrition. In addition to the major salt constituents discussed up to this point, the elements listed in the Table 30.2 are given here under. These elements have been detected in normal bovine milk by spectroscopic and chemical analyses. They include a large number of metals, the metalloids such as, B, and Si, and the halogens F, Br, and I. The reported concentrations of the trace elements exhibit large variation. (e.g., I, Mo, Zn), The concentration of elements like I, Mo, and Zn in the milk depends markedly on the diet consumed by the cow. The concentrations of some of them are increased by contamination withutensils and equipment to which milk is exposed while handling and processing.

Table 30.2 concentration of trace minerals in bovine milk

(Source: Fundamentals of Dairy Chemistry, Wong et. al. 1988)

Some high values for the concentrations of certain trace elements may have resulted from contamination during laboratory analysis. Molybdenum appears to be found exclusively in xanthine oxidase and Co in vitamin B₁₂. Iron is an essential component of xanthine oxidase,lactoperoxidase, and catalase. About half of the total Fe and 10% of the Cu are in the fat globule membrane. Copper has been studied extensively in relation to oxidation of milk lipids. The trace metal present in highest concentration in milk is Zn; its concentration of 3.5 mg/liter is about 3% of that of Mg, the major salt constituent present in lowest concentration. About 85% of the Zn is associated with casein micelles. Alkaline phosphatase, a Zn-containing enzyme, is located primarily in the fat globule membrane but accounts for only a small fraction of the total Zn. Manganese is required for fermentation of citrate by certain lactic acid bacteria, and with some milks the bacterial formation of diacetyl in cultures is inhibited by lack of sufficient Mn. Apparently, iodine is present in milk solely in the form of iodide ion; its concentration depends markedly on the amount consumed by the cow.

The casein micelles contain undissolved calcium phosphate and a little citrate; it is often called "colloidal calcium phosphate". Some cations, notably Ca²⁺and Mg²⁺, are associated with the negatively charged proteins. Small quantities of other ions (e.g. Cl^-) also may be bound to the proteins. Almost all of the salt is in the serum or in the casein micelles, and very little is bound to the fat globules. The distribution of salt between various casein micelles in serum is presented in the Table 30.2. The salt in the casein micelles can be dissolved by lowering the pH to about 4.6 or lower,and also, though slowly and incompletely, by dialysis against water or a calcium free solution.

30.4 Distribution of Some Salts Between Casein Micelles and Serum

Milk contains phosphorus in many forms. It is present as orthophos­phate, but part of it is bound to organic components like serine and threonine residues of casein, molecules of hexoses and glycerol,phospholipids, etc. Phosphorus is also found in the hydroxyapatite structure of the colloidal calcium phosphate in the micellar structure of casein. Table 30.3gives particulars of the distribution of salts between casein micelles and serum. The sulfur content of milk is about 0.36 g per kg, but most of it is in the amino acid residues methionine and cysteine of the proteins. About 10% is present as inorganic sulfate. There is, of course, a significant variation between lots of milk.

Table 30.3 Distribution of salt between casein micelles and serum

(Source: Davies and white, J. Dairy Res., 1960)

The electrical conductivity is largely determined by the salt solution. The pH of milk dialysate is a function of its salt composition and tem­perature. The presence of the proteins in milk does affect this pH, of course, as it affects the salt composition. The titratable acidity of milk depends both on its salt composition (largely phosphate content) and its protein content. The freezing point of milk follows from the total concentration of dissolved substances especially chloride and lactose.