Food Biochemistry and Food Processing (2 edition)

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516 Part 4: Milk

Stage of lactation (wk)

0 2 4 6 8 10 12 14 16 18

Salt concentration (mg/mL)

0

200

400

600

800

1000

1200

1400

1600

Figure 26.5.Influence of the stage of lactation on the concentration

of calcium (•), magnesium (◦), phosphate (
), potassium (∇)or

sodium (
) in equine milk. (Data from Schryver et al. 1986.)

by the stage of lactation (Fig. 26.5), which shows a progressive

decrease in the concentrations of calcium, magnesium, phos-

phorus, sodium and potassium from the end of the first week of

lactation. The concentration of calcium in equine milk increases

during the first week of lactation, before decreasing steadily

thereafter (Ullrey et al. 1966). As in the milk from all mammals

which have been studied in sufficient detail, the concentrations

of calcium and phosphate in equine milk far exceeds the solubil-

ity of calcium phosphate in milk. As a result, part of the calcium

and phosphate exist in a non-ultrafiltrable, micellar, form, that

is MCP. Holt and Jenness (1984) reported that approximately

60%, 20% or 40% of total calcium, magnesium or inorganic

phosphorus, respectively, in equine milk are non-ultrafiltrable,

compared to approximately 70%, 35% or 45% of total calcium,

magnesium and inorganic phosphorus in bovine milk. On the

basis of measurements of the distribution of salts between the

ultrafiltrable and non-ultrafiltrable phase of milk, Holt and Jen-

ness (1984) estimated the concentrations of ionic calcium and

magnesium in equine milk at pH 7.0 are 2.5 and 0.6 mmol L−^1 ,

respectively, compared to 2.0 or 0.8 mmol L−^1 , respectively, in

bovine milk at pH 6.7. For human infant nutrition, a Ca:P ratio

of approximately 2:1 is considered optimal; for bovine milk, the

ratio is approximately 1:1, but in equine milk it is about 2:1, and

is very close to that in human milk.

Trace Elements

Data on concentrations of trace elements, that is, those elements

present at concentrations less than 30 mg L−^1 , in equine milk

have been sporadic. The concentrations of trace elements in

equine, bovine and human milk are compared in Table 26.15.

Compared to bovine milk, equine milk contains markedly higher

levels of aluminum, copper, iron and titanium but lower lev-

els of boron, barium, lithium, molybdenum, manganese, silicon

Table 26.15.Concentrations of Trace Elements

(μg.L−^1 ) in Equine, Bovine and Human Milk

Species

Element Equine Bovine Human

Aluminum 123 98 125

Boron 97 333 273

Barium 76 188 149

Copper 155 52 314

Iron 224 194 260

Lithium 15 24 7

Molybdenum 16 22 17

Manganese 14 21 7

Silicon 161 434 472

Strontium 442 417 60

Titanium 145 111 25

Zinc 1835 3960 2150

Source: Modified from Anderson 1992.

and zinc. Human milk contains more boron, barium, copper,

iron, silicon and zinc, but less lithium, manganese, strontium

and titanium than equine milk (Table 26.15). Concentrations

of zinc, iron and copper in equine milk decrease progressively

with advancing lactation, whereas the concentration of man-

ganese increases during the first 5 days of lactation, after which

it decreases progressively (Csapo-Kiss et al. 1995, Ullrey et al. ́

1974).

PHYSICAL PROPERTIES OF EQUID MILK

The physical properties of the milk of some species are compared

in Table 26.16.

Density

The density (kg m−^3 ) of equine colostrum is higher than that of

equine milk (Waelchli et al. 1990, Ullrey et al. 1966, Mariani

et al. 2001), due primarily to its considerably higher protein

content. Values for colostrum can reach up to approximately

1080 kg m−^3 (Ullrey et al. 1966) and show a significant linear

correlation with the IgG content of colostrum (Waelchli et al.

1990, LeBlanc et al. 1986). Density is highest immediatelypost-

partumand decreases rapidly during the first 12 h (Ullrey et al.

1966); considerably smaller decreases in density are observed

during the rest of lactation (Ullrey et al. 1966, Mariani et al.

2001). Density values reported for mature equine milk range

from approximately 1028 to 1035 kg m−^3. The density of whole

mature bovine milk normally ranges from 1027 to 1033 kg m−^3

(Singh et al. 1997).

Refractive Index

The refractive index for equine colostrum ranges from 1.340

to 1.354, whereas that of mature equine milk is approximately