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26 Equid Milk: Chemistry, Biochemistry and Processing 509widely between well-nourished and poorly nourished mothers,
the amount of Lyz is conserved.
Lyz found in egg white, tears and saliva do not generally bind
calcium but equine and canine milk Lyz do and this is believed
to enhance the stability and activity of the enzyme (Nitta et al.
1987). The binding of a Ca^2 +by Lyz is considered to be an
evolutionary linkage between non-Ca^2 +-binding Lyzs andα-La
(Tada et al. 2002, Chowdhury et al. 2004). The conformation
of the calcium-binding loop of equine Lyz is similar to that of
α-La (Tsuge et al. 1992, Tada et al. 2002) and both equine Lyz
andα-La form stable, partially folded, ‘molten globules’ under
various denaturing conditions (Koshiba et al. 2001,) with that
of equine Lyz being considerably more stable thanα-La (Lyster
1992, Morozova-Roche 2007). The molten state of canine Lyz is
significantly more stable than that of equine Lyz (Koshiba et al.
2000, Spencer et al. 1999). Equine milk Lyz is very resistant to
acid (Jauregui-Adell 1975) and proteolysis (Kuroki et al. 1989),
and may reach the gut relatively intact.
Asinine Lyz contains 129 amino acids, is a C-type Lyz,
binds calcium strongly and has 51% homology to human Lyz
(Godovac-Zimmermann et al. 1988b). Two genetic variants of
Lyz, A and B, have been reported in asinine milk (Herrouin et al.
2000) but only one is found in equine milk. Asinine Lyz is re-
markably heat stable and requires 121◦C for 10 minutes for inac-
tivation. The Lyz content of equid milks is one of the main attrac-
tions for use of these milks in cosmetology as it is reputed to have
a smoothing effect on the skin and may reduce scalp inflamma-
tion when incorporated into shampoo. Equid milk has very good
antibacterial activity, presumably due to its high level of Lyz.Other Indigenous Enzymes in Equine MilkLactoperoxidase, catalase, amylase, proteinase (plasmin), li-
pase, lactate dehydrogenase and malate dehydrogenase have
been reported in equine milk. Bovine milk is a rich source of
xanthine oxidoreductase (XOR) but the milk of other species for
which data are available have much lower XOR activity, because
in non-bovine species, most (up to 98% in human milk) of the
enzyme molecules lack Mo and are inactive. XOR has not been
reported in equine milk, which is unusual considering the role of
XOR in the excretion of fat globules from the secretory cells and
also considering that equine milk contains quite a high level of
molybdenum (Mo), which presumably is present exclusively in
XOR. Chilliard and Doreau (1985) characterised the lipoprotein
lipase activity of equine milk and reported that the milk has high
lipolytic activity, comparable to that in bovine milk and higher
that in caprine milk. There are no reports on hydrolytic rancidity
in equine milk, which is potentially a serious problem in equine
milk products and warrants investigation.
Plasmin, a serine proteinases, is one of a number of proteolytic
enzymes in milk. Visser et al. (1982) and Egito et al. (2002) re-
portedγ-caseins in equine milk and, it is therefore assumed, that
equine milk contains plasmin. Humbert et al. (2005) reported
that equine milk contains five times more plasmin activity than
bovine milk and 90% of total potential plasmin activity was plas-
min, with 10% as plasminogen; the plasmin:plasminogen ratio
in bovine and human milk is 18:82 and 28:72, respectively.Alkaline phosphatase (ALP) is regarded as the most important
indigenous enzyme of bovine milk because ALP activity is used
as the index of the efficiency of high-temperature short-time
pasteurisation. About 40% of ALP activity in bovine milk is
associated with the MFGM. Equine milk has 35–350 times less
ALP activity than bovine milk and there are no reports on ALP
in the equine MFGM. Because of the low level of ALP in equine
milk it has been suggested that it is not suitable as an indicator of
pasteurisation efficiency of equine milk (Marchand et al. 2009)
although one would expect that once the exact initial concentra-
tion of ALP is known, the use of a larger sample size or a longer
incubation period would overcome the low level of enzyme.CARBOHYDRATES
Lactose and GlucoseThe chemistry, properties and applications of lactose are de-
scribed in Chapter 24 and have been reviewed extensively
elsewhere for example Fox (1985, 1997) and McSweeney and
Fox (2009) and will not be considered here. The concentration
of lactose in asinine milk is high (51–72.5 g.kg−^1 ), probably
marginally higher than equine milk (approximately 64 g.kg−^1 )
(Table 26.2), which is similar to the level in human milk and sig-
nificantly higher than that in bovine milk. As an energy source,
lactose is far less metabolically complicated than lipids but the
latter provides significantly more energy per unit mass. As well
as being a major energy source for the neonate, lactose affects
bone mineralisation during the first few monthspost-partumas
it stimulates the intestinal absorption of calcium (Schaafsma
2003). Equine milk contains a significant concentration of glu-
cose, approximately 50 mg/L in colostrum which increases to
approximately 150 mg/L 10 dayspost-partumand then de-
creases gradually to approximately 120 mg/L (Enbergs et al.
1999). Although the lactose content of equid milks is high, the
physico-chemical properties of lactose that cause problems in
the processing of bovine milk are of no consequence for equid
milks, which are consumed either fresh or fermented. In Mongo-
lia, where approximately 88% of the population is lactose intol-
erant (Yongfa et al. 1984), lactose intolerance is not a problem
with fermented equine milk, koumiss, as approximately 30% of
lactose is converted to lactic acid, ethanol and carbon dioxide
during fermentation.OligosaccharidesThe milk of all species examined contains OSs but the concen-
tration varies markedly (see Urashima et al. 2009). The OSs
in milk contain 3 to 10 monosaccharides and may be linear or
branched; they contain lactose at the reducing end and also con-
tain fucose, galactosamine andN-acetylneuraminic acid. The
highest levels are in the milk of monotremes, marsupials, ma-
rine mammals, humans, elephants and bears. OSs are the third
most abundant constituent of human milk that has an exception-
ally high content (approximately 20 g.L−^1 in colostrum, which
decreases to 5–10 g.L−^1 in milk) and structural diversity of
OSs (>200 molecular species), which have a range of functions,