Milk characteristics can be specifically associated with the environment and spe- cies. For example, there is a positive linear relationship between the logarithm of milk protein quantity and the body weight of the mother, which indicates that some mammalian species may produce insufficient milk for the newborn (Fig. 4.1).
Nowadays 4027 species are classified as mammals, but the milk composition of only 200 species, 60 of these with fewer than ten samples, has been studied (Jenness, 1974, 1980). High intraspecific variations in protein percentage are commonly observed, probably due to differences in genotype and/or in environ- mental conditions. A high number of scientific studies have demonstrated that milk protein content of 200 mammalian species can vary from 1% up to 20%.
Milk is a colloidal heterogeneous system where mineral salts, lactose and some whey proteins dissolved in water represent the continuous phase, while casein micelles represent the dispersed phase. Fat is present as tiny globules in this
mouse
Body weight
guinea pig monkey woman
horse cow
goat
dog rabbit
rat
Log of produced proteins (g/day)
Fig. 4.1. Relationship between body weight and produced proteins in mammals (logarithmic scale).
emulsion, the membranes of which are constituted mainly by a phospholipid–
protein complex. The biochemical composition and characteristics of milk vary from species to species, reflecting the newborn’s nutritional needs (Jenness and Sloan, 1970; Jenness, 1979; Davis and Collier, 1983; Targowski, 1983).
For a long time, the use of milk in the diet of adults and, in particular, of infants has been studied. In comparison to the milk of other species, goat’s milk has been considered a good source of proteins and amino acids (Tables 4.1–4.4).
In recent decades, milk has been considered as a message carrier to nursing infants (Teschemacheret al., 1977; Teschemacher and Brantl, 1994; Teschemacher, 1995) and adults as well. In the past, very few studies indicated the presence of neurotransmitters or hormonal substances in milk (Blanc, 1982). On the other hand, in the last few years, hundreds of works have shown the presence of prolactin, melatonin, oxytocin, growth hormone, luteinizing hormone-releasing hormone, thyroid-stimulating hormone, vasoactive intestinal peptide, calcitonin, neurotensin and cholecystochinin in milk, indicating that milk can be considered an exogenous endocrine system for the newborn (Mackle and Bauman, 1998).
Proteins Lipids Lactose
Primates
Human 9–15 38–41 70–72
Simian 16 40 70
Rodents
Rat 81 88 38
Ruminants
Goat 29–31 35–45 41–44
Cow 32–34 37–39 48
Cetaceans
Dolphin 68 330 11
Whale 109 423 13
Table 4.1. Milk composition of different species (g/l).
Human milk Cow’s milk Goat’s milk
Water (%) 87.6 87.3 87.5
Dry residue (g/l) 11.7 12.5 13.6
Dust (g/l) 2.0 8.0 8.9
Total proteins (g/l) 10 34 33
Carbohydrates (g/l) 70 48 51
Total lipids (g/l) 38 37 29
Non-protein N (g/l) 3.2 2.5 3.2
Ca (mg/l) 33 125 124
P (mg/l) 15 96 105
Table 4.2. Composition of human, cow’s and goat’s milk.
Human milk Cow’s milk Goat’s milk
Total proteins 9–15 32–34 28–32
Caseins 2.0–2.5 26–37 22–28
aS1-Casein – 11–15 10
aS2-Casein – 3–4 3
b-Casein 1.5 9–11 11
k-Casein 0.5 2–4 4
Whey proteins 6.3 5.8–6.5 5.5–6.5
a-Lactoalbumin 1.9–2.6 0.6–1.5 1.2
b-Lactoglobulin – 3–4 3.1
Serum albumin 0.4 0.4 0.5
Immunoglobulin 1.1 1.0 1.0
Lactoferrin 1.7–2 0.1 0.02–0.2
Lysozyme 0.04–0.2 – –
Table 4.3. Composition of the protein fraction of human, cow’s and goat’s milk (g/l).
Human milk Cow’s milk Goat’s milk
Total proteins (g/l) 9–15 32–34 28–32
Essential amino acids (mg/g protein)
Cysteine 20 9 9
Phenylalanine 37 52 47
Isoleucine 53 64 48
Histidine 23 28 26
Leucine 104 100 96
Lysine 71 83 80
Methionine 16 27 25
Tyrosine 46 53 38
Threonine 44 51 49
Tryptophan 17 14 –
Valine 51 68 61
Non-essential amino acids (mg/g protein)
Aspartate 86 79 75
Glutamate 190 208 209
Alanine 40 35 34
Arginine 36 37 29
Glycine 22 21 18
Proline 95 101 106
Serine 61 56 49
Table 4.4. Protein amino acid composition of human, cow’s and goat’s milk.
These hormonal substances should be considered as passengers that use milk to reach their target organ (Choick, 1998).
Knowledge of milk proteins has also increased enormously. For example, the functional meaning of some caseins and whey proteins has been identified and evidence for the importance of biopeptides (i.e. amino acid sequences in alimentary proteins) has been found. These peptides are inactive when they are present in their native protein, but they can be released in their active form after proteolytic digestion (i.e. during gastric digestion or simply during any tech- nological treatment). Since the 1980s, many authors have described active biopeptides derived from milk proteins which, in contrast with endogenous biopeptides, have many biological functions (Yoshikawa et al., 1988). Since then, the nutritional importance of milk, such as in the diet of athletes, has increased. Athletes need to consume proteins with high biological value, in order to: (i) maintain a positive N balance during physical activities; (ii) allow an increase of muscular mass; and, occasionally, (iii) repair muscular lesions after training (Fig. 4.2). In these cases, since protein needs increase from twofold to threefold with respect to normal needs, kidneys may suffer due to high protein intake. The consequences from intense physical activity can be: (i) nutritional deficiencies; (ii) formation of free radicals that could also lead to immunological deficiencies; and (iii) a lack of Fe, during aerobic stress, with consequent limited performance. Iron integration into the diet may have undesirable effects. Therefore, in all of these cases, utilization of goat’s milk could be helpful (Domeniconi and Balzola, 1980).
Milk has two main protein fractions: caseins, which are predominant, and whey proteins (Law and Brown, 1994). The qualitative and quantitative content of each protein fraction depends on many factors: (i) physiological (lactation stage, lactation order); (ii) environmental (climate, hygiene); (iii) genetic (breed, gene- alogy); and (iv) nutritional (Polidoriet al., 1991). More precisely, six main pro- teins are present in milk: four caseins (aS1,aS2,bandk) and two whey proteins (a-lactoalbumin andb-lactoglobulin) (Perez and Calvo, 1995). The protein frac- tion of goat’s milk, like that of other domestic ruminants, is constituted mainly by caseins, which account for 80% of total proteins (Ambrosoliet al., 1988).
CARBOHYDRATES 200–260 g/day PROTEINS
40–110 g/day
AMINO ACIDS GLUCOSE Acetyl-CoA
LIPIDS 65–95 g/day
FATTY ACIDS
TRIGLYCERIDES 13 kg Energy
GLYCOGEN 0–0.5 kg PROTEINS
13 kg
Fig. 4.2. Energy production and protein metabolism in the athlete.
It has been recently demonstrated that mRNA levels for the four caseins produced in the mammary epithelial cells during lactation are almost equally abundant (~25% each), while mature protein levels in milk differ greatly (Bevilacqua et al., 2006). In particular, translational efficiency for aS1- and b-casein is about four times higher than that foraS2- andk-casein, thus demon- strating that regulation of casein biosynthesis is also controlled at translational level.
The most abundant milk proteins are the three caseins (aS1, aS2 and b), which are sensitive to Ca. Casein biosynthesis is performed at cellular level under the influence of external agents such as hormones (Figs 4.3 and 4.4). Casein is a globular protein, present in milk as a colloidal suspension, having the aspect of micelles. It is characterized by a hydrophobic core and a charged polar hydro- philic part. The anionic regions of the polar part are responsible for sensitivity to Ca2+ and for some physicochemical properties of this protein (Jaubert et al., 1999). Moreover, caseins are relatively hydrophobic, and under ionic condi- tions, such as those present in milk, they tend to associate tightly into their typical colloidal form (micelles). Micelles are constituted by a protein fraction and by a mineral component (Ca and phosphate). Caseins are a group of spe- cific phosphoproteins in milk which precipitate: (i) when milk is acidified to a pH
Progesterone
Glucocorticoids Prolactin Insulin
P P
P (CHO)n Ca2+
Golgi Endoplasmic reticulum
DNA mRNA
Fig. 4.3. Casein biosynthesis. Following external signalling (i.e. hormones) and signal transduction inside the nucleus, casein genes are transcribed into mRNA molecules, which undergo post-transcriptional processing once inside the cytosol.
Mature mRNA molecules are translated into single casein molecules which are transferred inside the endoplasmic reticulum for post-translational modification.
Casein micelle formation begins in the cis-Golgi with condensation of casein molecules; addition of Ca2+, possibly in the secretory vesicle, leads to maturation of the casein micelles into particles sufficiently dense to be seen in the electron microscope.
of 4.6–4.7 at 20°C; (ii) by enzymatic treatment (chymosin); or (iii) by ultracentri- fugation. Caseins are rich in proline, glutamate and glutamine, and poor in glycine and aspartate. A high amount of Ca and phosphate is required to make casein soluble in milk in a stable form, so that it becomes a nutritional source of amino acids.
A very important milk feature, especially in goat’s milk, is related to its con- tent of complex oligosaccharides, either in free form or conjugated to proteins. In fact, beneficial anti-inflammatory effects of such oligosaccharides, especially on inflammatory bowel disease, have been demonstrated (Daddaouaet al., 2006).