400 Chapter 16
α (^) s1 - and α (^) S2 - fractions and β - casein are
located in the interior of the micelles, with
κ - casein predominantly wrapped around the
surface of the micelle. Casein fractions in the
interior of the micelle are sensitive to calcium
and become insoluble in its presence.
However, κ - casein is not sensitive to calcium
and thereby keeps the micelles containing
calcium - sensitive caseins intact and sus-
pended in the aqueous phase. κ - casein is a
protein with hydrophilic carbohydrate moiety
(sialic acid) that extends into the aqueous
phase, further stabilizing the micelle. Casein
micelles are stable under most heating,
homogenization, and other dairy processing
conditions.
Caseins have a certain distinctive amino
acid makeup that impacts their processing
and functional properties. They are rich in
apolar and hydrophobic amino acids, namely
valine, leucine, isoleucine, phenylalanine,
tyrosine, and proline. The apolar amino acids
normally are insoluble in water, but their
nature is balanced by phosphate groups so
that caseins exhibit some solubility.
Methionine and cysteine, the sulfur -
containing amino acids, are relatively low in
Table 16.11. Comparative nutritional measures of the quality of various food proteins.
Protein PER AAS BV PD PDCAAS NPU
Milk protein 3.1 1.27 91 95 1.21 86.45
Casein 2.5 1.24 77 100 1.23 76
Whey protein 3.2 1.16 104 100 1.15 92
Whole egg 3.9 1.21 100 98 1.18 94
Soy protein conc. 1.7 0.96 — 95 0.91 —
Wheat fl our 0.6 0.38 61 91 0.42 56
Rice, polished 2.2 0.66 64 — — 59
PER(protein effi ciency ratio): Gain in body weight divided by weight of protein consumed by growing rats fed 10%
(w/w) of test or reference protein
AAS (amino acid score): Content of the fi rst limiting essential amino acid in test protein compared with the content
of that essential amino acid in a reference pattern of essential amino acids
BV (biological value): Proportion of absorbed protein that is retained for body maintenance and growth
PD (protein digestibility): Proportion of food protein absorbed
PDCAAS (protein digestibility corrected amino acid score): Ratio of mg of limiting amino acid in 1 g of test protein
and mg of the same amino acid in the reference requirement pattern multiplied with true digestibility
True Digestibility: I (F - f)/I, where I = nitrogen intake, F = total nitrogen excretion, and f = fecal nitrogen excretion
on a protein - free diet
NPU (net protein utilization): Proportion of protein intake that is retained, calculated as BV × P D
Adapted from Schaafssma and Steijns (2000) , Southward, (2002) , Miller, et al, (2007). Source: Chandan and Kilara
(2008)
Table 16.12. Major proteins, their fractions and
polypeptides of cow ’ s milk.
Casein fractions Concentration
(g/100 mL)
α - s1 - casein 1.2 – 1.5
α - s2 - casein 0.3 – 0.4
β - casein 0.9 – 1.1
κ - casein 0.3 – 0.4
Casein fragments
(including γ - casein)
0.2 – 0.35
Total casein 2.4 – 2.8
Whey protein fractions
β - lactoglobulin 0.2 – 0.4
α - lactalbumin 0.1 – 0.17
Immunoglobulins 0.05 – 0.18
Bovine serum albumin 0.02 – 0.04
Total whey protein 0.5 – 0.7
Adapted from Schaafsma and Steijns (2000) , Chandan
(2007).
Source: Chandan and Kilara (2008)
κ - casein exists in as many as nine glycosyl-
ated forms. It contains two cysteine
molecules/molecule. As a result of disulfi de
bond formation, it can exist as polymers of
two to eight units. Similarly, α (^) S - 2 – casein also
contains two cysteines and exists in a dimeric
form.
Casein micelles contain α (^) s1 - , α (^) S2 - - , β - , and
κ - casein in the ratio of 3 : 1 : 3 : 1. Most of the