BLBS102-c26 BLBS102-Simpson March 21, 2012 13:51 Trim: 276mm X 219mm Printer Name: Yet to Come
506 Part 4: MilkWhey Protein DenaturationWhey proteins are susceptible to heat-induced denaturation. The
thermal stability of equine Lf and BSA is comparable to that of
the bovine proteins but equineβ-Lg andα-La are more heat
stable than the bovine proteins (Bonomi et al. 1994). Equine
β-Lg is more thermally stable than equineα-La, which is dif-
ferent from bovine milk, whereα-La is the more thermally
stable (Civardi et al. 2007). The high thermal stability of equine
β-Lg may be related to its lack of a sulphydryl group. Thermal
denaturation of bovineβ-Lg is a two-stage process, unfolding
of the polypeptide chain and exposure of the sulphydryl group,
followed by self-association or interaction with other proteins
viasulphydryl–disulphide interchange (Sawyer 2003). Owing to
the lack of a sulphydryl group, equineβ-Lg cannot undergo the
second denaturation step and therefore its structure may refold
on cooling. Denaturation ofα-La is commonly a result of com-
plex formation withβ-Lgviasulphydryl–disulphide interchange
and its higher thermal stability may therefore be a result of dif-
ferences in its environment, rather than its molecular structure.
Recent research suggests that equineα-La andβ-Lg are also
less susceptible to denaturation than their bovine counterparts
under high pressure.DIGESTIBILITY OF EQUID MILK
Milk is highly digestible and, because it is liquid, the gastroin-
testinal tract of mammals has mechanisms for delaying its pas-
sage; coagulation of milk in the stomach delays the degradation
of proteins and improves their assimilation by the body. Caseins
are precipitated by gastric acid and enzymes, forming a clot
in the stomach that entraps fat. The hardness of this clot de-
pends on the casein content of the milk; high casein-containing
milks will produce firm clots. Generally, species that nurse their
young at frequent intervals, for example equids and humans,
tend to produce dilute milk in which<60% of total protein is
casein and which form a soft clot, whereas those that nurse infre-
quently, for example cattle and sheep, produce milk that is high
in fat and casein and has much longer gastric retention (Jenness
1986). Degradation of casein in the gastrointestinal tract is slow
but extensive and whileβ-Lg is relatively resistant to gastric
proteolysis,α-La is readily hydrolysed when the gastric pH is
approximately 3.5 (Savalle et al. 1988).
The physico-chemical differences between human and bovine
caseins result in the formation of different types of curd in the
stomach (Hambræus 1982) and because the protein profile of
equine milk is quite similar to that of human milk, equine milk
may be more suitable in human nutrition than bovine milk.
Turner (1945) compared the digestibility of equine, human and
bovine milk based on the average percentage conversion of
acid-insoluble protein to acid-soluble protein during digestion.
Equine and human milk have a much lower buffering capacity
than bovine milk and, while equine milk is very digestible, it is
slightly less than human milk but significantly better than bovine
milk. Turner (1945) concluded that both equine and human milk
form soft curds in the stomach that pass through the digestive
tract more quickly than bovine milk curd. Kalliala et al. (1951)also reported that the overall digestibility of equine and human
milk (byin vitroexperiments) appeared to be quite similar and
both were easier to digest than bovine milk. Human milk forms
fine, soft flocs in the stomach with an evacuation time of 2–2.5
hours, whereas bovine milk forms compact hard curds with a
digestion time of 3–5 hours.TOTAL AMINO ACIDS
Guo et al. (2007) investigated the total amino acid composition
of asinine milk and expressed the results both in grams of in-
dividual amino acids per 100 grams of milk and per 100 grams
of protein and compared values to those for equine, bovine and
human milk (Table 26.6). Results expressed per 100 g of milk
demonstrated differences related, most likely, to differences in
the total protein content between the milks, but when expressed
as g/100 g protein, the differences were not so apparent. It has
been reported that glycine is exceptionally high in equine casein
(Lauer and Baker 1977) and other studies have reported that the
mean values of peptide-bound amino acids in equid milks are
generally higher than those in camel and buffalo milks and may
indicate that equid milks are more suitable for human consump-
tion than other milks studied to date. Asinine milk has noticeably
higher levels of serine, glutamate, arginine and valine and much
less cystine and the percentage of seven of the eight essential
amino acids (isoleucine, leucine, lysine, methionine, phenylala-
nine, threonine, tyrosine, valine) is also higher than those of
equine and bovine milk (Guo et al. 2007). In equine, bovine
and human milk cystine, glycine, serine, threonine and alanine
decrease as lactation progresses while glutamate, methionine,
isoleucine and lysine tend to increase (Davis et al. 1994).NON-PROTEIN NITROGEN
The non-protein nitrogen (NPN) of milk consists primarily of
urea, peptides, amino acids and ammonia. NPN constitutes
10–15% of the total nitrogen in mature equine milk which is
intermediate between the values for human milk and ruminant
milk, 25% and 5%, respectively (Hambræus 1984, Oftedal et al.
1983, Atkinson et al. 1989, Walstra et al. 2006b). In equine
milk, NPN increases from<2% of total nitrogen at parturi-
tion to>10% after 2 weeks (Zicker and Lonnerdal 1994). The ̈
components of the NPN in human and bovine milk have been
characterised (see Atkinson et al. 1989, Atkinson and Lonnerdal ̈
1995, Rudloff and Kunz 1997, Carratu et al. 2003), but the NPN`
of equine milk has not been studied in detail.
Up to 50% of the NPN of human milk is urea and free amino
acids, the exact function of which is, as yet, unknown. Asinine
milk has a significantly higher level of NPN than equine milk
and is close to that of human milk (Table 26.2). Equine and
asinine milk have similar urea levels.Free Amino AcidsThe free amino acid content of equine, bovine and human milk
are 1960, 578 and 3019μmol.L−^1 , respectively (Rassin et al.
1978, Agostini et al. 2000) (Table 26.7). Glutamine, glutamate,