Food Biochemistry and Food Processing (2 edition)

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26 Equid Milk: Chemistry, Biochemistry and Processing 517

Table 26.16.Physical Properties of the Milk of Equid Species, with Comparative Data for Bovine and Human Milk

Property Equine Milk Equine Colostrum Asinine Milk Bovine Milk Human milk

Freezing point (◦C) −0.525 –

−0.554

• −0.55 –
  −0.49

−0.512 –

−0.55

pH (25◦C) 7.1–7.3 – 7.01–7.35 6.5–6.7 6.8

Density (kg.m−^3 ), (20◦C) 1032 1080 1029–1037 1027–1033 1031

Refractive index, nD^20 1.3394 1.340–1.354 – 1.344–1.349 –

Viscosity (mPa s) 1.5031 – – 1.6314 –

Zeta potential (mV) −10.3 – – −20.0 –

Colour L∗a∗b∗ 86.52,−2.34,−0.15 – 80.88

−2.27

−3.53

79.12,−7.46

−2.31

Source: Modified from Uniacke and Fox 2011.

1.339 (Waelchli et al. 1990). The higher refractive index of

colostrum than of mature milk is probably related to its higher

total solids content, since the refractive index increases with

increasing mass fraction of each solute. The refractive index of

bovine milk is in the range 1.344–1.349 (Singh et al. 1997).

pH

There is considerable variation among reported values for the

pH of equine milk. Mariani et al. (2001) reported that its pH

4 dayspost-partumis approximately 6.6 and increases to ap-

proximately 6.9 after 20 days and to approximately 7.1 at 180

days post-partum. A value of approximately 7.0 for the pH for

mature equine milk was reported by K ̈uc ̈ukcetin et al. (2003),

but Pagliarini et al. (1993) reported an average value of ap-

proximately 7.2. The pH of bovine milk is generally between

6.5 and 6.7 (Singh et al. 1997) and increases during lactation

(Tsioulpas et al. 2007). These differences are presumably re-

lated to differences in protein and salt composition of the milks.

Freezing Point

The freezing point of milk is directly related to the concen-

trations of water-soluble compounds therein. Fat globules and

proteins have a negligible influence on freezing point, with the

main effect arising from lactose and minerals. A freezing point

of−0.554◦C (Pagliarini et al. 1993) or−0.548◦C (Neseni et al.

1958) has been reported for equine milk, whereas the vast ma-

jority of bovine milk samples have a freezing point in the range

−0.512 to−0.550◦C (Singh et al. 1997). The lower freezing

point of equine milk is probably related to its higher lactose

content.

Viscosity

Equid milks are less viscous than bovine milk due to a lower

total solids content.

Colour

Equid milk may be expected to be less white than bovine milk

due to its low protein content and large casein micelles but this

is not the case and equid milk is considerably whiter due to the

absence ofβ-carotene that confers a yellow colour to bovine

milk.

PROCESSING OF EQUID MILK

Because of its unique physico-chemical properties outlined ear-

lier, the processing of equine and asinine milk into traditional

dairy products is not possible; cheese is not produced from these

milks as a firm curd is not formed on renneting. Equid milk will

form a weak coagulum under acidic conditions and this is ex-

ploited in the production of yoghurt-type products with reputed

probiotic and therapeutic properties. Traditionally, and to date,

the only significant product from equine milk is the fermented

product, koumiss, the production and properties of which are de-

scribed below in section ‘Koumiss’. Interest in koumiss produc-

tion has grown recently which may be attributed to the fact that,

worldwide, the overall consumption of fermented milk products

has grown faster than the consumption of fresh milk (IDF 2009).

Fermentation is one of the oldest methods for preserving

milk and probably dates back approximately 10,000 years to

the Middle East where the first organised agriculture occurred.

Traditional fermented milk products have been developed inde-

pendently worldwide and were, and continue to be, especially

important in areas where transportation, pasteurisation and re-

frigeration facilities are inadequate. Worldwide, milk from eight

species of domesticated mammals (cow, buffalo, sheep, goat,

camel, horse, reindeer and yak) have been used to produce tradi-

tional fermented milk products. There are three categories of fer-

mented milks, those resulting from lactic fermentations, yeast-

lactic fermentations and mould (Geotrichum candidum)-lactic

fermentations. Koumiss and kefir belong to the yeast-lactic fer-

mentation group where alcoholic fermentation by yeasts is used