Ions in milk

The ash content of milk remains relatively constant at 0.7-0.8%, but the relative concentrations of the various ions can vary considerably. Table 5.1 shows the average concentration of the principal ions in milk, the usual range and the extreme values encountered. The latter undoubtedly include abnormal milks, e.g. colostrum, very late lactation milk or milk from cows with mastitic infection. 

In milk, the critical dissociation constants are pX, for citric acid, pK for phosphoric acid and pX for carbonic acid. Bearing in mind the limitations and assumptions of the above data, the following calculations can be made for the distribution of the various ions in milk at pH 6.6. 

Holt, C. (1981) Some principles determining salt composition and partitioning of ions in milk. J. Dairy Set., 64, 1958-64. 

Smeets, W.J.G.M. (1955) The determination of the concentration of calcium ions in milk ultrafiltrate. Neth. Milk Dairy J., 9, 249-60. 

Oilier examples of this type of reaction are file conversion of the antibiotic streptomycin sulfate to its corresponding chloride hy means of anion exchange, the exchange of Na ions in milk for the K ion. and the conversion of NaCKdj to H CiOi by cation exchange The latter process is used extensively in the plaling industry to concentrate ICCrO, Iron rinse waters, with subsequent reuse of a toxic chemical ami reuse of ihe rinse water in what might be termed a closed system. 

S. S. Ali, G. H. Kazi, T. G. Kazi, Use of comparative sample preparation methods for the extraction of metal ions in milk as determined by atomic absorption spectroscopy, ACGC Chem. Res. Commun., 6 (1997), 9-12. 

In the absence of calcium ions but in the presence of other casein components the normally insoluble K-casein is apparently stabilized by the calcium-sensitive caseins (157). Thus calcium ions are required to coagulate whole casein after treatment with rennin. Therefore in the native milk system the micelle-stabilizing power of K-casein is specifically destroyed by rennin, and in the presence of calcium ions in milk a coagulum is formed (2). This offers a dramatic example of how the functionality of an entire protein system can be altered by specific proteolytic action on a component of that system. 

In vitro studies suggest that estramustine combines with calcium ions in milk and food to form a poorly-soluble complex that is not as well absorbed as the parent compound. ... 

The opportunity of use of a ternary complex of ions Eu(III) with oxatetracycline (OxTC) and citrat-ions (Cit) for luminescent detection of OxTC in milk after chromatographic isolation is shown. 

Helmerson 2881, and more recently Bartels 2891, determined chloride in serum by adding excess silver and then measuring the excess silver ion in the filtrate. Ezell 29°) used a similar procedure to determine chloride in plant liquors and Gutsche etal.29 determined chlorine in milk by measuring the excess silver by flame photometry. 

Insoluble complex with ions like Ca, Fe, Zn in milk, meat, vegetables. Decreased absorption and bioavailability... 

Korfmacher WA, Rushing LG, Siitonen PH, et al. 1987. Confirmation of heptachlor epoxide and octachlor epoxide in milk via fused silica gas chromatography/negative ion chemical ionization mass spectrometry. J High Resolut Chromatogr Commun 10 332-336. 

Iodide ion-selective electrode The iodide electrode has broad application both in the direct determination of iodide ions present in various media as well as for the determination of iodide in various compounds. It is, for example, important in the determination of iodide in milk [44,64,218, 382, 442], This electrode responds to Hg ions [150, 306, 439] and can be used for the indirect determination of oxidizing agents that react with iodide, such as 10 [305], lOi [158], Pd(II) [117, 347,405] and for the determination of the overall oxidant content, for example in the atmosphere [393], It can also be used to monitor the iodide concentration formed during the reactions of iodide with hydrogen peroxide or perborate, catalyzed by molybdenum, tungsten or vanadium ions, permitting determination of traces of these metals [12,192,193, 194, 195]. The permeability of bilayer lipid membranes for iodide can be measured using an I"... 

A number of methods have been described for determination of tetracycline (chlortetracycline, tetracycline, and oxytetracycline) residues in tissues of food-producing animals (53-62), fish (63), eggs (64), and honey (65,66). Most of these methods use reversed-phase HPLC for determination. However, one uses TLC with UV densitometry ( ) and one uses GLC ( ), and one uses a direct mass spectrometric method CAD MIKE spectrometry (collisionally activated decomposition mass-analyzed ion kinetic spectrometry) for oxytetracycline in milk and meat (62). Several use solid-phase extraction in the cleanup procedure using XAD-2 resin (56,58) or Cj g cartridges... 

The most remarkable observation is that all 22 minerals considered to be essential to the human diet are present in milk. Some of these are sodium (Na), potassium (K), and chloride (Cl). The electroneutrality of milk is maintained by free ions (negatively charged to lactose). 

Van Staden reported a rapid, reliable automated method for direct measurement of the chloride content in milk based on the principles of flow injection analysis and the use of a dialyser to remove interferents. Dialysed chloride was measured by means of a coated tubular chloride ion-selective electrode. Potential changes arising from the interference of casein were thus avoided and baseline stability ensured. The results obtained for chloride in milk compared well with those provided by standard recommended methods. The linear range for chloride was 250-5000 pg/mL for 30 pL of sample, and the coefficient of variation was better than 0.5%. The throughput was ca. 120 samples/h [132],... 

Calcium and magnesium. Some calcium and magnesium in milk exist as complex undissociated ions with citrate, phosphate and bicarboante, e.g. Ca Citr-, CaP04, Ca HCOj. Calculations by Smeets (1955) suggest the following distribution for the various ionic forms in the soluble phase ... 

It should be possible to determine experimentally the concentrations of anions such as HPO - and Citrate3- in milk using ion-exchange resins or by nuclear magnetic resonance spectroscopy, but no such experimental work has been reported and available data are by calculation only. 

Cation-exchange resins. Using ion-exchange resins, Ca2+ and Mg2+ are adsorbed on to a cation-exchange resin added to milk the resin is removed and the Ca2+ and Mg2+ desorbed. It is assumed that the treatment does not alter the ionic equilibrium in milk. 

Holt, C., Dalgleish, D.G. and Jenness, R. (1981) Calculation of the ion equilibria in milk diffusate and comparison with experiment. Anal. Biochem., 113, 154-63. 

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