Food Biochemistry and Food Processing (2 edition)

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26 Equid Milk: Chemistry, Biochemistry and Processing 497

Table 26.2.Concentration of Caseins and Whey Proteins (g. kg−^1 ) in Equine, Asinine,

Human and Bovine Milk

Equine Asinine Human Bovine

Total casein 13.56 7.8 2.4 26

αs1-casein 2.4 Identified 0.77 10.7

αs2-casein 0.20 Unknown – 2.8

β-casein 10.66 Identified 3.87 8.6

κ-casein 0.24 Unknown 0.14 3.1

γ-casein Identified Unknown – 0.8

Total whey protein 8.3 5.8 6.2 6.3

β-lactoglobulin 2.55 3.3 – 3.2

α-lactalbumin 2.37 1.9 2.5 1.2

Serum albumin 0.37 0.4 0.48 0.4

Proteose peptone – – – 0.8

Immunoglobulins 1.63 1.30 0.96 0.80

IgG1,2 0.38 0.03 0.65

IgA 0.47 0.96 0.14

IgM 0.03 0.02 0.05

Lactoferrin 0.58 0.37 1.65 0.10

Lysozyme 0.87 1.00 0.34 126 × 10 −^6

NPN (mg.L−^1 ) 375 455 454 266

Casein micelle size (nm) 255 ∼100–200 64 182

Source: Modified from Uniacke et al. 2010, with asinine data from Guo et al. 2007 and Salimei et al. 2004.

NPN, non-protein nitrogen.

αs2-,β- andκ-caseins, of which the first three are calcium sen-

sitive. All caseins lack secondary structure, which led Holt and

Sawyer (1993) to consider them as rheomorphic proteins. The

lack of secondary structure may be attributed, at least partially,

to the relatively high level of proline residues in casein. As a

result, caseins do not denature or associate on heating (Paulson

and Dejmek 1990). The biological function of the caseins lies

in their ability to form macromolecular structures, casein mi-

celles, which transfer large amounts of calcium to the neonate

with a minimal risk of pathological calcification of the mam-

mary gland. The individual caseins will be discussed separately

in the following sections with focus on their interactions to form

casein micelles and the colloidal stability thereof.

Fractionation and characterisation of individual equine ca-

seins has been poorly researched to date in comparison to those

of bovine milk and it had been reported that equine, and presum-

ably asinine, caseins exhibit greater heterogeneity and a higher

level of post-translational modifications than those of bovine

milk (Miranda et al. 2004). Table 26.3 shows the biochemical

properties of individual casein proteins that are discussed later.

Table 26.3.Properties of Equine, Bovine and Humanαs1-,β- andκ-Caseins

Protein Species

Primary

Accession

Numbera

Amino

Acid

Residues

Molecular

Weight (Da) pH GRAVYb

Cysteine

Residues

αs1-casein Equine Q8SPR1 205 24,614.4 5.47 −1.127 0

Bovine P02662 199 22,974.8 4.99 −0.704 0

Human P47710 170 20,089.4 5.17 −1.013 3

β-casein Equine Q9GKK3 226 25,511.4 5.78 −0.415 0

Bovine P02666 209 23,583.2 5.13 −0.355 0

Human P05814 211 23,857.8 5.33 −0.289 0

κ-casein Equine P82187 165 18,844.7 8.03 −0.313 2

Bovine P02668 169 18,974.4 5.93 −0.557 2

Human P07498 162 18,162.6 8.68 −0.528 1

Source: Modified from Uniacke et al. 2010.

aPrimary accession number for the protein in SWISS-PROT database.

bGrand average hydropathy (GRAVY) score using the scale of Kyte and Doolittle (1982).