240 DAIRY CHEMISTRY AND BIOCHEMISTRY each ion in a mixture is frequently complicated by interfering ions. The major elements/ions in foods, including milk, may be determined by the following specific method Inorganic phosphate reacts with molybdate to form phosphomolybdate which may be reduced to a blue compound that can be quantified spectrophotometrically at 640 nm Calcium and magnesium may be determined by titration with EDTA or by atomic absorption spectroscopy on TCA filtrates or on wet-or dry-ashed sample Citrate forms a yellow complex with pyridine(which is carcinogenic)in the presence of acetic anhydride; the complex may be quantified spectro- photometrically. Alternatively, citrate can be determined by an enzymatic Ionized calcium may be determined spectrophotometrically after reaction with murexide or using a Ca2*-specific Sodium and potassium may be quantified by flame photometry, atomic absorption spectroscopy or ion specific electrodes e Chloride can be titrated with AgNO, using potentiometric or indicator end-point detection FeCl2, or by an enzymatic assay(using lactate dehydrogenase which can quantify both D-and L-isomers)or by HPLC References to these and other methods can be found in Jenness (1988) Detailed analytical procedures are published in the Official Methods of Analysis of the Association of Official Analytical Chemists(Arlington, VA SA)or in Standard Methods of the International Dairy Federation(Brus els, Belgium) 5.3 Composition of milk salts 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 The ash content of human milk is only about 0. 2%; the concentration of all principal and several minor ions is higher in bovine than in human milk (Table 5.2). Consumption of unmodified bovine milk by human babies causes increased renal load and hence demineralized bovine milk or whey should be used for infant formulae240 DAIRY CHEMISTRY AND BIOCHEMISTRY each ion in a mixture is frequently complicated by interfering ions. The major elements/ions in foods, including milk, may be determined by the following specific methods: 0 Inorganic phosphate reacts with molybdate to form phosphomolybdate which may be reduced to a blue compound that can be quantified spectrophotometrically at 640 nm. 0 Calcium and magnesium may be determined by titration with EDTA or by atomic absorption spectroscopy on TCA filtrates or on wet- or dry-ashed samples. 0 Citrate forms a yellow complex with pyridine (which is carcinogenic) in the presence of acetic anhydride; the complex may be quantified spectro￾photometrically. Alternatively, citrate can be determined by an enzymatic assay. 0 Ionized calcium may be determined spectrophotometrically after reaction with murexide or using a Ca*'-specific electrode. Sodium and potassium may be quantified by flame photometry, atomic absorption spectroscopy or ion specific electrodes. 0 Chloride can be titrated with AgNO, using potentiometric or indicator end-point detection. 0 Sulphate is precipitated by BaCl, and quantified gravimetrically. 0 Lactate may be quantified spectrophotometrically after reaction with FeCl,, or by an enzymatic assay (using lactate dehydrogenase which can quantify both D- and L-isomers) or by HPLC. References to these and other methods can be found in Jenness (1988). Detailed analytical procedures are published in the Oflcial Methods of Analysis of the Association of Oficial Analytical Chemists (Arlington, VA, USA) or in Standard Methods of the International Dairy Federation (Brus￾sels, Belgium). 5.3 Composition of milk salts 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. The ash content of human milk is only about 0.2%; the concentration of all principal and several minor ions is higher in bovine than in human milk (Table 5.2). Consumption of unmodified bovine milk by human babies causes increased renal load and hence demineralized bovine milk or whey should be used for infant formulae