5 Salts of milk 5.1 Introduction The salts of milk are mainly the phosphates, citrates, chlorides, sulphates, carbonates and bicarbonates of sodium, potassium, calcium and mag- nesium. Approximately 20 other elements are found in milk in trace amounts, including copper, iron, silicon, zinc and iodine. Strictly speaking, the proteins of milk should be included as part of the salt system since they carry positively and negatively charged groups and can form salts with counter-ions; however, they are not normally treated as such. There is no lactate in freshly drawn milk but it may be present in stored milk and in milk products. The major elements are of importance in nutrition, in the reparation, processing and storage of milk products due to their marked influence on the conformation and stability of milk proteins, especially caseins, and to a lesser extent the stability of lipids and the activity of some indigenous enzymes 5.2 Method of analysis he mineral content of foods is usually determined from the ash prepared by heating a sample at 500-600 C in a muffle furnace for about 4 h to oxidize organic matter. The ash does not represent the salts as present in he food because 1. the ash is a mixture, not of the original salts, but of the carbonates and oxides of the elements present in the food; 2. phosphorus and sulphur from proteins and lipids are t in the ash while organic ions, such as citrate, are lost during incineration; and 3. the temperature usually employed in ashing may vaporize certain volatile elements, e.g. sodium and potassium Therefore, it is difficult or impossible to relate the ash obtained from a food with its salts system, and low values are obtained for certain mineral elements by analysis of the ash compared to direct analysis of the intact food. Titrimetric, colorimetric, polarographic, flame photometric and atomic absorption spectrophotometric techniques are frequently used to analyse for the various mineral constituents; however, the quantitative estimation of
5 Salts of milk 5.1 Introduction The salts of milk are mainly the phosphates, citrates, chlorides, sulphates, carbonates and bicarbonates of sodium, potassium, calcium and magnesium. Approximately 20 other elements are found in milk in trace amounts, including copper, iron, silicon, zinc and iodine. Strictly speaking, the proteins of milk should be included as part of the salt system since they carry positively and negatively charged groups and can form salts with counter-ions; however, they are not normally treated as such. There is no lactate in freshly drawn milk but it may be present in stored milk and in milk products. The major elements are of importance in nutrition, in the preparation, processing and storage of milk products due to their marked influence on the conformation and stability of milk proteins, especially caseins, and to a lesser extent the stability of lipids and the activity of some indigenous enzymes. 5.2 Method of analysis The mineral content of foods is usually determined from the ash prepared by heating a sample at 500-600°C in a muffle furnace for about 4h to oxidize organic matter. The ash does not represent the salts as present in the food because: 1. the ash is a mixture, not of the original salts, but of the carbonates and 2. phosphorus and sulphur from proteins and lipids are present in the ash, 3. the temperature usually employed in ashing may vaporize certain volatile oxides of the elements present in the food; while organic ions, such as citrate, are lost during incineration; and elements, e.g. sodium and potassium. Therefore, it is difficult or impossible to relate the ash obtained from a food with its salts system, and low values are obtained for certain mineral elements by analysis of the ash compared to direct analysis of the intact food. Titrimetric, colorimetric, polarographic, flame photometric and atomic absorption spectrophotometric techniques are frequently used to analyse for the various mineral constituents; however, the quantitative estimation of
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 formulae
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 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 spectrophotometrically. 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 (Brussels, 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
SALTS OF MILK Table 5.1 C ation of milk salt constituents(mg litre milk(from various sources) Usual range Extremes reported 500 350-60 110-1150 1350-1550 1150-2000 Calcium 1000-1400 650-2650 100150 750-1100 800-1400 540-2420 Sulphate Carbonate (as cO 1750 Total phosphorus incle idal inorganic phosphate, casein(organic) phosphate, soluble sphate and phospholipid bPhosphorus (inorganic)includes colloidal inorganic phosphate and soluble inorganic phos- Table 5. 2 Mineral composition (mg or ug-)of mature human or bovine milks (from Flynn and Power. 1985) Mature human milk Cows'milk Constituent Mean Range odium(mg) Potassium (mg) 350-550 Calcium (mg) 350 320-360 1200 1100-1300 Magnesium(mg) 26-30 Iron (ug) Zinc (ug) 2060 Copper (g lodine (ug) 70 20-120 Fluoride (ug) 21-155 30-220 Cobalt(ug) 05-1.3 6-100 Nickel (ug) 0-50 150-1200 750-7000 Vanadium (ag) Tin (ug) 0-500 45 Tr. trace
SALTS OF MILK 24 1 Table 5.1 Concentration of milk salt constituents (mg litre-' milk (from various sources) Constituent Average content Usual range Extremes reported Sodium 500 350-600 1 10- 1 150 Potassium 1450 1350-1550 11 50-2000 Calcium 1200 1000-1400 650-2650 Magnesium 130 100-150 20-230 Phosphorus (total)" 950 750-1100 470- 1440 Chloride 1000 800-1400 540-2420 Phosphorus (inorganic)b 750 Sulphate 100 Carbonate (as CO,) 200 Citrate (as citric acid) 1750 "Total phosphorus includes colloidal inorganic phosphate, casein (organic) phosphate, soluble inorganic phosphate, ester phosphate and phospholipids. bPhosphorus (inorganic) includes colloidal inorganic phosphate and soluble inorganic phosphate. Table 5.2 Mineral composition (mg or pgl-') of mature human or bovine milks (from Flynn and Power, 1985) Mature human milk Cows' milk Constituent Mean Range Mean Range Sodium (mg) Potassium (mg) Chloride (mg) Calcium (mg) Magnesium (mg) Phosphorus (mg) Iron (pg) Zinc (pg) Copper (pg) Manganese (pg) Iodine (pg) Fluoride (pg) Selenium (pg) Cobalt (pg) Chromium (pg) Molybdenum (pg) Nickel (pg) Silicon (pg) Vanadium (pg) Arsenic (pg) Tin (Peg) 150 600 430 350 28 145 760 2950 390 12 70 77 14 12 40 8 25 700 7 50 - 110-200 570-620 350-550 320-360 26-30 140-150 620-930 2600-3300 370-430 7-15 20- 120 21-155 8-19 1-27 6-100 4-16 8-85 150- 1200 Tr-15 - - 500 1500 950 1200 120 950 500 3500 200 30 260 - - 1 10 73 25 2600 170 45 - 350-900 1100-1700 900- 1100 1100- 13OO 90-140 900-1000 300-600 2000-6000 100-600 20-50 - 30-220 5-67 0.5-1.3 8-13 18-120 0-50 750-7000 Tr-310 40-500 20-60 Tr. Trace
242 AIRY CHEMISTRY AND BIOCHEMISTRY 5.4 Secretion of milk salts The secretion of milk salts, which is not well understood has been reviewed and summarized by Holt (1985). Despite the importance of milk salts in determining the processing characteristics of milk, relatively little interest has been shown in the nutritional manipulation of milk salts composition. Three factors must be considered when discussing the milk salts system the need to maintain electrical neutrality, 2. the need to maintain milk isotonic with blood as a result of this, a set of correlations exist between the concentrations of lactose, Na, K- and Cl 3. the need to form casein micelles which puts constraints on the pH and [Ca2+] and requires the complexation of calcium phosphate with casein Skim milk can be considered as a two-phase system consisting of casein-colloidal calcium phosphate micelles in quasi-equilibrium with an aqueous solution of salts and proteins; the phase boundary is ill-defined because of the intimate association between the calcium phosphate and the caseins(phosphoproteins) Oo A fat-free primary secretion is formed within vesicles formed by blebbing- of the Golgi dicytosomes; the vesicles pass through the cytoplasm o the apical membrane where exocytosis occurs. The vesicles contain casein(synthesized in the rough endoplasmic reticulum toward the base of the mammocyte): fully-formed casein micelles have been demonstrated within the Golgi vesicles. The vesicles also contain lactose synthet (UDP: galactosyl transferase and a-lactalbumin)and there is good evidence showing that lactose synthesis occurs within the vesicles from glucose and UDP-galactose transported from the cytosol The intracellular concentrations of sodium and potassium are established by a Na/K-activated ATPase and Na and K can permeate across the vesicle membranes. Calcium is probably necessary to activate the UDP: galactosyl transferase and is transported by a Ca"/Mg-AtPase which concentrates Ca2+against an electrical potential gradient from uM concentrations in the cytosol to mM concentrations in the vesicles. Inor nic P(P) can be formed intravesicularly from UDP formed during the synthesis of lactose from UDP-galactose and glucose. UDP, which cannot cross the membrane, is hydrolysed to UMP and Pi, both of which can e-enter the cytosol (to avoid product inhibition) however, some of the P is complexed by Ca-t. Ca are also chelated by citrate to form largely oluble undissociated complexes and by casein to form large colloidal casein micelles Water movement across the vesicle membranes is controlled by osmotic pressure considerations. Since lactose is a major contributor to the osmotic pressure of milk, the concentrations of both soluble and colloidal salts in
242 DAIRY CHEMISTRY AND BIOCHEMISTRY 5.4 Secretion of milk salts The secretion of milk salts, which is not well understood, has been reviewed and summarized by Holt (1985). Despite the importance of milk salts in determining the processing characteristics of milk, relatively little interest has been shown in the nutritional manipulation of milk salts composition. Three factors must be considered when discussing the milk salts system: 1. the need to maintain electrical neutrality; 2. the need to maintain milk isotonic with blood; as a result of this, a set of correlations exist between the concentrations of lactose, Na', K+ and c1-; 3. the need to form casein micelles which puts constraints on the pH and [Ca"] and requires the complexation of calcium phosphate with casein. Skim milk can be considered as a two-phase system consisting of casein-colloidal calcium phosphate micelles in quasi-equilibrium with an aqueous solution of salts and proteins; the phase boundary is ill-defined because of the intimate association between the calcium phosphate and the caseins (phosphoproteins). A fat-free primary secretion is formed within vesicles formed by blebbingoff of the Golgi dicytosomes; the vesicles pass through the cytoplasm to the apical membrane where exocytosis occurs. The vesicles contain casein (synthesized in the rough endoplasmic reticulum toward the base of the mammocyte); fully-formed casein micelles have been demonstrated within the Golgi vesicles. The vesicles also contain lactose synthetase (UDP : galactosyl transferase and sr-lactalbumin) and there is good evidence showing that lactose synthesis occurs within the vesicles from glucose and UDP-galactose transported from the cytosol. The intracellular concentrations of sodium and potassium are established by a Na+/K+-activated ATPase and Na+ and K+ can permeate across the vesicle membranes. Calcium is probably necessary to activate the UDP : galactosyl transferase and is transported by a CaZ +/Mg2 +-ATPase which concentrates Ca2 + against an electrical potential gradient from pM concentrations in the cytosol to mM concentrations in the vesicles. Inorganic P (Pi) can be formed intravesicularly from UDP formed during the synthesis of lactose from UDP-galactose and glucose. UDP, which cannot cross the membrane, is hydrolysed to UMP and Pi, both of which can re-enter the cytosol (to avoid product inhibition); however, some of the Pi is complexed by Ca2+. Caz+ are also chelated by citrate to form largely soluble, undissociated complexes and by casein to form large colloidal casein micelles. Water movement across the vesicle membranes is controlled by osmotic pressure considerations. Since lactose is a major contributor to the osmotic pressure of milk, the concentrations of both soluble and colloidal salts in
SALTS OF MILK 243 act。se UDP-Ga GOLGI VESICLE Ca-ATPase Ca CYTOSOL Figure 5. 1 Summary of some transport mechanisms for calcium, phosphate and citrate from the cytosol of the secretory cell to the inside of Golgi vesicles(from Holt, 1981). milk are strongly influenced by lactose concentration and the mechanism by which it is synthesized Inter-relationships in the biosynthesis of the principal milk salts are summarized in Figure 5. 1. Transport of several ionic species via the junctions between cells(paracellular)occurs during early and late lactation and during mastitic infection when the junctions between cells are more 5.5 Factors influencing variation in salt composition The composition of milk salts is influenced by a number of factors, including breed, individuality of the cow, stage of lactation, feed, mastitic infection and season of the year. The more important factors are discussed below 5.5.1 Breed of cow Milk from Jersey cows usually contains more calcium and phosphorus than milk from other breeds, including Holstein, but the concentrations of odium and chloride are usually lower
SALTS OF MILK 243 / GO1OLGI VESICLE Casein u D P-G a1 ac t ose ,GI VESICLE Casein -----=2 CanPO, \ Cll D ’ CYTOSOL ci I Figure 5.1 Summary of some transport mechanisms for calcium, phosphate and citrate from the cytosol of the secretory cell to the inside of Golgi vesicles (from Holt, 1981). milk are strongly influenced by lactose concentration and the mechanism by which it is synthesized. Inter-relationships in the biosynthesis of the principal milk salts are summarized in Figure 5.1. Transport of several ionic species via the junctions between cells (paracellular) occurs during early and late lactation and during mastitic infection when the junctions between cells are more open. 5.5 Factors influencing variation in salt composition The composition of milk salts is influenced by a number of factors, including breed, individuality of the cow, stage of lactation, feed, mastitic infection and season of the year. The more important factors are discussed below. 5.5.1 Breed of cow Milk from Jersey cows usually contains more calcium and phosphorus than milk from other breeds, including Holstein, but the concentrations of sodium and chloride are usually lower
DAIRY CHEMISTRY AND BIOCHEMISTRY 0.16 0.14 0.12 0.04 0.02 Weeks of lactation Figure 5.2 Changes in the concentrations of calcium(----)and phosphorus (-)in bovine milk during lactation. 5.5.2 Stage of lactation a. an bv tation of total calcium is generally high both in early and late The concen station but in the intervening period no relation with stage of lactation is evident(Figure 5.2), Phosphorus shows a general tendency to increase as lactation advances(Figure 5. 2). The concentrations of colloidal calcium and inorganic phosphorus are at a minimum in early and at a maximum in late lactation milk. The concentrations of sodium and chloride( Figure 5.3)are high at the beginning of lactation, followed by a rapid decrease, then increase gradually until near the end of lactation when rapid increases occur The concentration of potassium decreases gradually throughout lactation The concentration of citrate which has a marked influence on the distribu tion of calcium, shows a strong seasonal variation(Figure 5.4), influenced more by feed than the stage of lactation. The ph of milk shows a strong
244 DAIRY CHEMISTRY AND BIOCHEMISTRY 0.18 - 0.16- 0.14 - 0.12 - 0.1- a 6 & G 0.08- 0.06 - p ...d ...*.. , .o,. ... . .. * .... ....*O"' 9.. o"..*....* ,... * ... *... , , . , , .*. .... I.1.0 .I..*.. = = - ~ - Weeks of lactation Figure 5.2 Changes in the concentrations of calcium (----) and phosphorus (-) in bovine milk during lactation. 5.5.2 Stage of lactation The concentration of total calcium is generally high both in early and late lactation but in the intervening period no relation with stage of lactation is evident (Figure 5.2). Phosphorus shows a general tendency to increase as lactation advances (Figure 5.2). The concentrations of colloidal calcium and inorganic phosphorus are at a minimum in early and at a maximum in late lactation milk. The concentrations of sodium and chloride (Figure 5.3) are high at the beginning of lactation, followed by a rapid decrease, then increase gradually until near the end of lactation when rapid increases occur. The concentration of potassium decreases gradually throughout lactation. The concentration of citrate, which has a marked influence on the distribution of calcium, shows a strong seasonal variation (Figure 5.4), influenced more by feed than the stage of lactation. The pH of milk shows a strong
SALTS OF MILK 245 Percent of lactation Figure 5.3 Changes in the concentration of chloride in bovine milk during lactation Figure 5. 4 Seasonality of the concentration of citric acid in bo
SALTS OF MILK 245 0.25 0.2 0.15 3 2 u ct 0.1 0.05 Percent of lactation Figure 5.3 Changes in the concentration of chloride in bovine milk during lactation. Monih Figure 5.4 Seasonality of the concentration of citric acid in bovine milk
DAIRY CHEMISTRY AND BIOCHEMISTRY seasonal trend: the ph of colostrum is about 6 but increases to the normal value of about 6 6-6. 7 shortly after parturition and changes little until late lactation, when the ph raises to as high as 7. 2, i.e. approaches that of blood (pH 7. 4)due to degeneration of the mammary cell membrane. The pH of milk also increases during mastitic infection(e.g 6.8-6.9), due to the influx of constituents from blood Figure 5.5 Correlations between the concentration of sodium and potassium(a) and sodium and chloride(b)in bovine milk
246 DAIRY CHEMISTRY AND BIOCHEMISTRY seasonal trend; the pH of colostrum is about 6 but increases to the normal value of about 6.6-6.7 shortly after parturition and changes little until late lactation, when the pH raises to as high as 7.2, i.e. approaches that of blood (pH 7.4) due to degeneration of the mammary cell membrane. The pH of milk also increases during mastitic infection (e.g. 6.8-6.9), due to the influx of constituents from blood. X Figure 5.5 Correlations between the concentration of sodium and potassium (a) and sodium and chloride (b) in bovine milk
SALTS OF MILK 247 5.5.3 Infection of the udder Milk from cows with mastitic infections contains a low level of total solids, especially lactose, and high levels of sodium and chloride, the concentration of which are directly related(Figure 5.5). The sodium and chloride ions come from the blood to compensate osmotically for the depressed lactose synthesis or vice versa These are related by the Koestler number Koestler number= 100×%Cl which is normally 1.5-3.0 but increases on mastitic infection and has been used as an index of such(better methods are now available, e.g. somatic cell count, activity of certain enzymes, especially catalase and N-acetyl glucosamidase). The pH of milk increases to approach that of blood during mastitic infection 5.5.4 Feed Feed has relatively little effect on the concentration of most elements in milk because the skeleton acts as a reservoir of minerals. the level of citrate in milk decreases on diets very deficient in roughage and results in the ' Utrecht phenomenon, i.e. milk of very low heat stability. Relatively small changes in the concentrations of milk salts, especially of Ca, Pi and citrate, can have very significant effects on the processing characteristics of milk and hence these can be altered by the level and type of feed, but definitive studies or his are lacking 5.6 Interrelations of milk salt constituents Various milk salts are interrelated and the interrelationships are affected by H Table 5.3). Those constituents, the concentrations of which are related to ph in the same way, are also directly related to each other(e.g. the concentrations of total soluble calcium and ionized calcium), while those related to pH in opposite ways are inversely related(e. g. the concentrations of potassium and sodium Relationships between some of the more important ions/molecules are shown in Figure 5.6. Three correlations are noteworthy: The concentration of lactose is inversely related to the concentration of soluble salts expressed as osmolarity. This results from the requirement that milk be isotonic with blood
SALTS OF MILK 247 5.5.3 Infection of the udder Milk from cows with mastitic infections contains a low level of total solids, especially lactose, and high levels of sodium and chloride, the concentration of which are directly related (Figure 5.5). The sodium and chloride ions come from the blood to compensate osmotically for the depressed lactose synthesis or vice versa. These are related by the Koestler number: 100 x %C1 %lactose Koestler number = which is normally 1.5-3.0 but increases on mastitic infection and has been used as an index of such (better methods are now available, e.g. somatic cell count, activity of certain enzymes, especially catalase and N-acetylglucosamidase). The pH of milk increases to approach that of blood during mastitic infection. 5.5.4 Feed Feed has relatively little effect on the concentration of most elements in milk because the skeleton acts as a reservoir of minerals. The level of citrate in milk decreases on diets very deficient in roughage and results in the ‘Utrecht phenomenon’, i.e. milk of very low heat stability. Relatively small changes in the concentrations of milk salts, especially of Ca, Pi and citrate, can have very significant effects on the processing characteristics of milk and hence these can be altered by the level and type of feed, but definitive studies on this are lacking. 5.6 Interrelations of milk salt constituents Various milk salts are interrelated and the interrelationships are affected by pH (Table 5.3). Those constituents, the concentrations of which are related to pH in the same way, are also directly related to each other (e.g. the concentrations of total soluble calcium and ionized calcium), while those related to pH in opposite ways are inversely related (e.g. the concentrations of potassium and sodium). Relationships between some of the more important ions/molecules are shown in Figure 5.6. Three correlations are noteworthy: 1. The concentration of lactose is inversely related to the concentration of soluble salts expressed as osmolarity. This results from the requirement that milk be isotonic with blood
248 DAIRY CHEMISTRY AND BIOCHEMISTRY Relationships between the pH of milk and the concent ons of certain milk salt constitue Inversely related to pH Directly related to pH Total phospho Ester phosphor Pota 云67 (c Figure 5.6 Interrelationship nd soluble salts(osmolarity)and between
248 7- 68 6.7 6.6 - 6.S - 6.4 DAIRY CHEMISTRY AND BIOCHEMISTRY 3.2 - 69:/ ;l\ 29 - 2.n - IIIII1 27 I I I I 29 3 3.1 3.2 3.3 Table 5.3 Relationships between the pH of milk and the concentrations of certain milk salt constituents Inversely related to pH Titratable acidity Colloidal inorganic calcium Total soluble calcium Caseinate calcium Soluble unionized calcium Ionized calcium Colloidal calcium phosphate Soluble magnesium Sodium Soluble citrate Soluble inorganic phosphorus Total phosphorus Ester phosphorus Potassium Directly related to pH Colloidal inorganic phosphorus Chloride 140 - 130- 120- 110- loo - RO DO 100 110 120 130 140 Salt osmoluity (mM) Figure 5.6 Interrelationships between lactose and soluble salts (osmolarity) and between some soluble salts in bovine milk
