BLBS102-c26 BLBS102-Simpson March 21, 2012 13:51 Trim: 276mm X 219mm Printer Name: Yet to Come
502 Part 4: Milk1.000010.0000100.00000.00010.00100.01000.1000Log G' (Pa)0 12 24 36 45 56 68 80 92 104 116
Time (min)Figure 26.3.Coagulation of equine milk (----), asinine milk (---) and bovine milk ( ) acidified with 3% glucono-δ-lactone at 30◦C.review, see Lucey and Singh 2003). Acid-induced flocculation
of bovine casein micelles is believed to result from a reduction
in the solvency of theκ-casein brush on the micellar surface
due to protonation of the negatively charged carboxylic acid
groups of Glu and aspartic acid (Asp). Equine casein micelles
are considerably less susceptible to acid-induced flocculation. Di
Cagno et al. (2004) reported that equine milk acidified at pH 4.2,
the point of minimum solubility of equine caseins (Egito et al.
2001), had an apparent viscosity only approximately seven times
higher than that of equine milk at its natural pH (Waelchli et al.
1990) and is probably indicative of micellar flocculation rather
than gelation. By comparison, the viscosity of acidified bovine
milk is approximately 100 times higher than that of bovine milk
at natural pH. Differences in acid-induced flocculation between
equine and bovine casein micelles may be related to differences
in the mechanism by which they are sterically stabilised. Figure
26.3 shows the effect of acidification of bovine, equine and
asinine milk at 30◦C and 3% glucono-δ-lactone (GDL). Asinine
milk appears to form a weak gel when treated with GDL, unlike
equine milk that shows little or no gel formation. Elucidation of
the mechanism of steric stabilisation of equine casein micelles
is likely to shed further light on this subject.Heat-induced Coagulation of Equine MilkAlthough milk, compared to most other foods, is extremely heat-
stable, coagulation does occur when heated for a sufficiently long
time at> 120 ◦C. Unconcentrated bovine milk, usually assayed
at 140◦C, displays a typical profile, with a heat coagulation
time (HCT) maximum (∼20 minutes) at pH approximately 6.7
and a minimum at pH approximately 6.9 (O’Connell and Fox
2003). In contrast, the HCT of unconcentrated equine milk at
140 ◦C increases with pH, that is, it has an almost sigmoidal
pH-HCT profile (Fig. 26.4), from<2 minutes at pH 6.3–6.9 to
>20 minutes at pH 6.9–7.1; a slight maximum is observed at pH
7.2. Pre-heating unconcentrated milk shifts the pH-HCT profile
and reduces the HCT in the pH region around the maximum,
similar to the effect reported for bovine milk (O’Connell and
Fox 2003). The HCT of concentrated equine milk at 120◦Cin-
creases up to pH 7.1 but decreases progressively at higher pH
values. While the profile for concentrated bovine milk is some-what similar, the maximum HCT occurs at a considerably lower
pH, that is, approximately 6.6. Differences in heat stability be-
tween equine and bovine milk may be related to differences
in steric stabilisation of the micelles and, while heat-induced
complexation ofβ-Lg withκ-casein greatly affects the heat sta-
bility of bovine milk (O’Connell and Fox 2003), it is unlikely
to do so in equine milk due to lack of a sulphydryl group in
equineβ-Lg. The lower protein, particularly casein, concentra-
tion in equine milk is also likely to contribute to its higher heat
stability.
The colloidal stability of equine casein micelles differs con-
siderably from that of bovine casein micelles, which may have
significant implications for the conversion of equine milk into
dairy products. On the basis of the evidence outlined, manufac-
ture of cheese and yoghurt from equine milk is unlikely to be
successful using conventional manufacturing protocols.Stability of Equine Milk to EthanolThe ethanol stability of bovine milk (for review, see Horne
2003), defined as the minimum concentration of added aque-
ous ethanol that causes it to coagulate at its natural pH (∼6.7),
is 70–75% (added 1:1 to milk), whereas the ethanol stability
of equine milk (pH∼7.2) is 40–45% (Uniacke-Lowe, 2011).
The high concentration of ionic calcium and low level of
κ-casein in equine milk probably contribute to its low ethanol
stability.Whey ProteinsSimilar to bovine milk, the major whey proteins in equine and
asinine milk areβ-Lg,α-La, Igs, blood serum albumin (BSA), Lf
and Lyz (Bell et al. 1981a, Salimei et al. 2004, Guo et al. 2007).
Except forβ-Lg, all these proteins are also present in human
milk. However, the relative amounts of the whey proteins differ
considerably between these milks (Table 26.2). Compared to
bovine milk, equine milk contains lessβ-Lg but moreα-La
and Igs. The principal anti-microbial agent in equine milk is
Lyz and to a lesser extent Lf (which predominates in human
milk (Table 26.2). Both Lf and Lyz are present at low levels
in bovine milk, in which Igs form the main defense against