A01

Figure 5-1 Examples of Metal Chelates. Only the relevant portions of the molecules are shown. The chelate formers are: (A) thiocarbamate, (B) phosphate, (C) thioacid, (D) diamine, (E) o-phenantrolin, (F) a-aminoacid, (G) o-diphenol, (H) oxalic acid. Source: From K. Pfeilsticker, Food Components as Metal Chelates, Food Sci. Technol., Vol. 3, pp. 45-51, 1970.

absorption of calcium by causing formation of fiber-bound calcium in the intestines.

Iron bioavailability may be increased in the presence of meat (Politz and Clydesdale 1988). This is the so-called meat factor. The exact mechanism of this effect is not known, but it has been suggested that amino acids or polypeptides that result from digestion are able to chelate nonheme iron. These complexes would facilitate the absorption of iron. In nitrite-cured meats some factors promote iron bioavailability (the meat factor), particularly heme iron and ascorbic acid or erythor-bic acid. Negative factors may in-clude nitrite and nitrosated heme (Lee and Greger 1983).

Minerals in Milk

The normal levels of the major mineral constituents of cow's milk are listed in Table 5-1. These are average values; there is a considerable natural variation in the levels of these constituents. A number of factors influence the variations in salt composition, such as feed, season, breed and individuality of the cow, stage of lactation, and udder infections. In all but the last case, the variations in individual mineral constituents do not affect the milk's osmotic pressure. The ash content of milk is relatively constant at about 0.7 percent. An important difference between milk and blood plasma is the rela-

Table 5-1 Average Values for Major Mineral Content of Cow's Milk (Skim Milk)

Constituent

Normal Level (mg/100 mL)

Sodium

50

Potassium

145

Calcium

120

Magnesium

13

Phosphorus (total)

95

Phosphorus (inorganic)

75

Chloride

100

Sulfate

10

Carbonate (as C02)

20

Citrate (as citric acid)

175

tive levels of sodium and potassium. Blood plasma contains 330 mg/100 mL of sodium and only 20 mg/100 mL of potassium. In contrast, the potassium level in milk is about three times as high as that of sodium. Some of the mineral salts of milk are present at levels exceeding their solubility and therefore occur in the colloidal form. Colloidal particles in milk contain calcium, magnesium, phosphate, and citrate. These colloidal particles precipitate with the curd when milk is coagulated with rennin. Dialysis and ultrafiltration are other methods used to obtain a serum free from these colloidal particles. In milk the salts of the weak acids (phosphates, citrates, and carbonates) are distributed among the various possible ionic forms. As indicated by Jenness and Patton (1959), the ratios of the ionic species can be calculated by using the Henderson-Hasselbach equation,

The values for the dissociation constants of the three acids are listed in Table 5-2. When these values are substituted in the Henderson-Hasselbach equation for a sample of milk at pH 6.6, the following ratios will be obtained:

Citrate Citric acid

Citrate" Citrate"

Citrate

From these ratios we can conclude that in milk at pH 6.6 no appreciable free citric acid or monocitrate ion is present and that trici-trate and dicitrate are the predominant ions, present in a ratio of about 16 to 1. For phosphates, the following ratios are obtained:

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