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26 Equid Milk: Chemistry, Biochemistry and Processing 503120014001600600800100002004006.36.46.56.66.76.86.9 7 7.17.27.37.47.57.67.77.8
pHHeat coagulation time (s)Figure 26.4.Heat coagulation time-pH profile of raw unconcentrated skimmed equine milk at 140◦C(-----), preheated and unconcentrated
milk ( ) and concentrated milk at 120◦C (---).microbes (Malacarne et al. 2002). Together, Ig A, G, M, Lf
and Lyz provide the neonate with immune and non-immune
protection against infection (Baldi et al. 2005).β-Lactoglobulinβ-Lg is the major whey protein in the milk of most ruminants and
is also present in the milk of some monogastrics and marsupials,
but is absent from the milk of humans, camels, lagomorphs and
rodents.β-Lg is synthesised in the secretory epithelial cells of
the mammary gland under the control of prolactin. Although sev-
eral biological roles forβ-Lg have been proposed, for example
facilitator of vitamin A (retinol) uptake and an inhibitor, modifier
or promoter of enzyme activity, conclusive evidence for a spe-
cific biological function ofβ-Lg is not available (Sawyer 2003,
Creamer and Sawyer 2011).β-Lg of all species studied binds
retinol;β-Lg of many species, but not equine or porcine, binds
fatty acids also (Perez et al. 1993). During digestion, milk lipids ́
are hydrolysed by pre-gastric and pancreatic lipases, greatly in-
creasing the amount of free fatty acids that could potentially bind
toβ-Lg, displacing any bound retinol, and implying that fatty
acid metabolism, rather than retinol transport, is the more impor-
tant function ofβ-Lg (P ́erez and Calvo 1995). Bovineβ-Lg is
very resistant to peptic digestion and can cause allergenic reac-
tions on consumption. Resistance to digestion is not consistent
among species, with ovineβ-Lg being far more digestible than
bovineβ-Lg (El-Zahar et al. 2005). The digestibility of equine
β-Lg, which has, to our knowledge, not been studied, warrants
research, particularly considering the potential applications of
equine milk as a hypo-allergenic dairy product.Two isoforms of equineβ-Lg have been isolated,β-Lg I
and II, which contain 162 and 163 amino acids, respectively.
The extra amino acid in equineβ-Lg II is a glycine residue
inserted after position 116 ofβ-Lg I (Halliday et al. 1991).
Asinine milk also has two forms ofβ-Lg, I and II (MW. 18.5
and 18.2 kDa, respectively); two variants ofβ-Lg I, that is, A
and B, are known and four variants ofβ-Lg II, A, B, C and
D (Cunsolo et al. 2007). Godovac-Zimmermann et al. (1985,
1988a,b) reported thatβ-Lg I from asinine milk has 162 amino
acids, similar to equineβ-Lg I (from which it differs by 5 amino
acids).β-Lg II in both asinine and equine milks has 163 amino
acids and shows substantial differences between both milks, with
only six clusters of amino acid residues conserved (Godovac-
Zimmermann et al. 1990). Criscione et al. (2009) reported the
absence ofβ-Lg II from more than 23% of Ragusana donkeys in
one study.
Bovineβ-Lg occurs mainly as two genetic variants, A and
B, both of which contain 162 amino acids and differ only at
positions 63 (Asp in variant A, Gly in variant B) and 117
(valine (Val) in variant A, alanine (Ala) in variant B); a fur-
ther 11, less common, genetic variants of bovineβ-Lg have
also been reported (Sawyer 2003). On the basis of its amino
acid sequence, unmodified equineβ-Lg I has a molecular mass
of 18,500 Da and an isoelectric pH of 4.85, whereas equine
β-Lg II, despite having one more amino acid, has a molecu-
lar mass of 18,262 Da (ExPASy ProtParam Tool 2009), and
an isoelectric pH of 4.71 (Table 26.5). Bovineβ-Lg A and
B have a molecular mass of 18,367 and 18,281 Da, respec-
tively, and an isoelectric pH of 4.76 and 4.83, respectively
(Table 26.5). Using the hydropathy scale proposed by Kyte and
Doolittle (1982), equineβ-Lg I and II have a GRAVY score of