The lipid fraction of goat’s milk is organized in fat globules made up of glycerides (97–99%), mainly present within the globule core, and of phospholipids, glyco- lipids and sterols (1–3%), as an integral part of the globule membrane. This dis- tribution remains almost unchanged even when milk fat content changes (Cerbuliset al., 1982), and is very similar to that described for cow’s milk. The fat globule core is made up of apolar molecules, such as triglycerides (96.8%), diglycerides (2.3%) and monoglycerides (0.9%), whereas the membranes contain both polar lipids, such as glycolipids (8.5%) and phospholipids (44.7%), and apolar lipids, such as triglycerides (26.5%), diglycerides and monoglycerides (4.7%) and cholesterol (15.6%). The polar lipids are the determinants of the main
phospholipid fractions contained in the milk fat globule membrane of goat’s milk (Table 3.1). Even though the distribution of phospholipids in the membrane of milk fat globules varies according to individual characteristics and stage of lacta- tion, the main phospholipid classes remain phosphatidyl ethanolamine, phos- phatidyl choline and sphyngomyelin.
Milk samples collected from five different goat flocks of Granadina breed were analysed (Fontechaet al., 2000) by capillary gas chromatography and silver ion adsorption–liquid chromatography, to determine the composition of triglyc- erides of goat’s milk by classifying them according to the number of carbon atoms (CN). It was observed that 55% of glycerides were composed of SFAs, 29% of MUFAs and 16% of PUFAs. Analysis of the same samples by silver ion adsorption–thin layer chromatography allowed them to be classified into four groups or elution bands, named A, B, C and D (Table 3.2). Fraction A, which represented 38.5% of total triglycerides, was composed mainly of SFAs (95.3%) and traces of MUFAs (0.9%). Fraction A contained 73%, 80% and 74% of total milk content of 8:0, 10:0 and 12:0, respectively. By contrast, the level of 4:0 was low in fraction A, being concentrated in fraction B (94%). The latter fraction accounted for 16.5% of triglycerides, of which 94.3% were SFAs and 1.5%
MUFAs. Fraction C, containing 28.9% of total triglycerides, was mainly made up of palmitic acid, MUFAs and LA, in the form of isolated and conjugated diene.
The last band, fraction D, was constituted by 16% triglycerides whose main FAs were MUFAs and PUFAs.
The main classes of triglycerides, subdivided by CN, were sorted out by gas chromatography. Triglycerides were mostly represented by CN40and CN36in fractions A and B, by CN38and CN44in fraction C, and by CN46and CN54in fraction D. As the degree of unsaturation increased, the number of carbon atoms of the apolar lipids increased. In goat’s milk the main saturated glycerides pres- ent are (10:0; 14:0; 16:0), (10:0; 16:0; 16:0) and (10:0; 16:0; 18:1), whereas the main polyunsaturated triglyceride is (16:0; 18:1; 18:1), similar to bovine and human milk.
Conjugated linoleic acids
In goats, as well as other ruminant species, the lipid fraction of milk contains an appreciable amount of CLA. The most represented isomer in milk lipids of Phospholipids
(g/100 g phospholipids)
Cerbuliset al.
(1982)
Pattonet al.
(1977)
Meleet al.
(unpublished data)
Phosphatidylethanolamine 35.4 25.5 46.3
Phosphatidylcholine 28.2 27.6 20.7
Phosphatidylserine 3.2 9.6 14.8
Phosphatidylinositol 4.0 1.4 6.1
Sphyngomyelin 29.2 35.9 12.2
Table 3.1. Phospholipids of the milk fat globule membrane of goat’s milk.
ruminants is RA, which originates from two metabolic pathways: (i) the bio- hydrogenation of LA in the rumen (Fig. 3.2); and (ii) the desaturation of VA, formed in the rumen, within the mammary gland (Fig. 3.5). The first pathway involves the already described lipolysis and successive reductions of dietary UFAs in the rumen. For many years, B. fibrisolvens was the only bacterium thought capable of performing biohydrogenation (Kepleret al., 1967). Succes- sively, other microorganisms capable of reducing the double bond of FAs were identified:Eubacterium lentum,Propionibacterium freudenreichi,Lactobacillus acidophilus, Lactobacillus reuteri, Megasphaera elsdeni and Bifidobacterium breve(Fukudaet al., 2005).
Studies on pure cellular cultures demonstrated that the whole biohydro- genation process is not performed by a single microorganism. Instead, it is coor- dinated by a pool of rumen bacteria, each controlling the various reaction steps, which can be divided into two groups: (i) group A hydrogenates LA and LNA to
Fraction
Fatty acid % of total A B C D
4:0 5.09 0.56 19.26 4.43 3.01
6:0 4.42 3.83 6.87 3.94 2.50
8:0 4.15 6.70 5.81 4.57 3.98
10:0 12.91 18.57 10.79 11.74 7.75
10:1 0.36 0.60 1.84
12:0 5.62 8.72 6.96 5.21 3.80
12:1 0.21 0.05 0.41 0.40
14:0 9.86 13.54 10.01 7.95 5.15
14:1 + 15:0 0.39 0.85 0.73 0.69 0.86
ai-15:0 + 15:0 0.83 2.18 1.67 1.06 1.08
15:1 0.09 0.37 0.53
i-16:0 + 16:0 25.38 30.54 24.26 19.64 12.30
16:1 1.41 2.79 3.98
i-17:0 + ai-17:0 + 17:0 1.26 0.88 1.05 0.66 0.98
17:1 0.34 0.60 0.73
18:0 7.17 9.57 7.17 6.58 4.47
18:1 15.46 0.90 1.47 24.24 27.35
18:2 2.83 1.19 13.80
18:3 0.35 1.44
CLA 0.57 0.06 0.12 0.76 0.76
20:0 0.11 0.18 0.42 0.11 0.28
20:1 0.05 0.03 0.18
Other acids 1.14 2.92 3.37 2.43 2.83
i, iso; ai, anteiso; CLA, conjugated linoleic acid.
Table 3.2. Percentage composition of fatty acids in the triglycerides of goat’s milk, subdivided into four elution groups after silver ion adsorption–thin layer chromatography. (From Fontechaet al., 2000.)
VA; and (ii) group B concludes the sequence of biohydrogenation by reducing VA to 18:0 (Harfoot and Hazlewood, 1988). The synthesis of CLA in the rumen is illustrated in Fig. 3.2. The initial step is the isomerization ofcis-9,cis-12 18:2 (LA) to cis-9,trans-11 18:2 (RA) (Harfoot, 1978). This passage is catalysed by the enzyme linoleic isomerase, which does not need the aid of cofactors and acts on double bonds located in the middle of the carbon chain and far from activating functional groups. The enzyme adheres to the bacterial membrane and is very selective, because it recognizes only dienes of thecis-9,cis-12 type present along the carbon chain of FAs with the free carboxyl function. The second step is the reduction of RA totrans-11 18:1 (VA), which is a fast reaction as demonstrated by in vitro studies incubating marked LA with rumen liquor. By contrast, the reduction of VA to 18:0 is much slower (rate-determining step), thus permitting the accumulation of VA in the rumen and its passage to plasma, via intestinal absorption. Recently, new strains of B. fibrisolvens that have high ability to isomerize LA to CLA or to hydrogenate LA to VA have been isolated, aiming to enhance the rumen production of VA and RA (Fukudaet al., 2005, 2006). Once VA is absorbed into plasma and arrives at the mammary gland, it may be reconverted into RA by the action of∆9-desaturase (Fig. 3.5). As demonstrated in dairy cows, this second pathway of RA production provides more than 80% of the RA present in milk fat (Corlet al., 1998). The pathway of LNA reduction is very similar to that of LA biohydrogenation. The most prevalent 18:3 isomer in feeds, i.e.a-LNA (cis-9,cis-12,cis-15 18:3), is hydrogenated totrans-11 18:1, the precursor of CLA synthesized by the mammary tissue (Fig. 3.2).g-Linolenic acid (g-LNA;cis-6,cis-9,cis-12 18:3), less common in feeds, is fermented in a similar way (Griinari and Bauman, 1999). Because of both processes, the most repre- sented CLA isomer and trans 18:1 FA in ruminant milk fat are RA and VA, respectively, except under particular feeding conditions that markedly change
12 9 Rumen
Linoleic acid
cis-9,cis-12 18:2 isomerase 9 11 Rumenic acid (CLA)
cis-9,trans-11 18:2
saturase
Vaccenic acid trans-11 18:1
11
saturase
Stearic acid 18:0
Vaccenic acid 11 trans-11 18:1
Mammary gland
∆9-desaturase
Rumenic acid (CLA) cis-9,trans-11 18:2
11 9 OH
O
OH
OH OH
OH OH
O
O
O O
O
Fig. 3.5. Synthesis of rumenic acid, a conjugated linoleic acid (CLA), in the rumen and mammary gland.
the composition of the rumen bacterial population. In these cases, the biohydro- genation process shifts towardstrans-10,cis-12 CLA andtrans-10 18:1, instead of RA and VA. These first two FAs seem to be responsible for the MFD in dairy cows (Griinari et al., 1998), but these aspects have not been confirmed yet in goats.
In addition to the transformation of VA to RA,∆9-desaturase is also responsi- ble for the production of another CLA isomer (trans-7,cis-9) and a non-conjugated 18:2 (cis-9,trans-13) (Ulberth and Henninger, 1994; Yuraweczet al., 1998). The mammary gland appears to be the tissue of greatest activity of∆9-desaturase in lactating ruminants (Kinsella, 1972). This enzyme, also named stearoyl-CoA desaturase (SCD), is expressed by the homonymousScdgene located on chro- mosome 26 in cattle and goats and on chromosome 22 in sheep (Wardet al., 1998; Bernardet al., 2001; Taniguchiet al., 2003). Among ruminants, the cod- ing sequence of this gene is completely known for cattle and goats only. How- ever, based on the information currently available, the structure of theScdgene appears to be very well preserved in the ruminant species. The expression of the Scdgene and the activity of the related enzyme are sensitive to nutritional fac- tors, such as the presence of PUFAs in feeds, and to endogenous factors, such as physiological stage (e.g. lactation phase) and hormonal balance (Ntambi, 1995).
In a few cases, species-related differences in CLA content of milk have been reported for cows, goats and sheep grazing on the same pasture (Jahreiset al., 1999; Nuddaet al., 2003). In these cases, variations were ascribed to differences among species in feeding habits and feed passage rate in the digestive system.
On the other hand, recent studies have highlighted that factors other than diet may be associated with variations in milk FA composition, including VA and CLA, in both goats and sheep (Chilliardet al., 2006; Meleet al., 2007a). In par- ticular, Chilliardet al. (2006) found that two groups of dairy goats with low or highaS1-casein milk content had different proportions of at least 17 FAs in milk fat, particularly of saturated medium-chain FAs and stearic, oleic, linoleic and rumenic acids.
Branched fatty acids of goat’s milk
Caproic acid (6:0), caprylic acid (8:0) and capric acid (10:0) are among the most characteristic FAs of goat’s milk and are named after the species name.
They are characterized by a sharp and persistent odour, typical of goats. In fact, a widely used parameter to detect the authenticity of goat’s milk is the estimate of the ratio 12:0 to 10:0, as proposed by Ramos and Juarez (1986). The BCFAs are very important, because they are responsible for the typical aroma of goat’s milk and cheese. There is an increasing interest in OBCFAs as potential diagnostic tools of rumen function (e.g. rumen fermentation pattern and bacte- rial nitrogen). Other reasons for the interest in OBCFAs are their anticarcino- genic effects on cancer cells, their influence on milk fat melting point and their potential as indicators of dairy product intake by humans (Vlaeminck et al., 2006b). The iso and anteiso forms of 15:0, the iso 16:0 and the iso and anteiso forms of 17:0, which were the first ones to be identified, are the main BCFAs in
milk of goats and cows (Massart-Leen et al., 1981). Monomethyl BCFAs with chain length shorter than ten carbon atoms were identified, successively, only in goat’s milk (Ha and Lindsay, 1993). Another 31 BCFAs, present at very low concentrations, were also identified: 25 of them are monomethyl branched, two are dimethyl and four are diethyl branched (Alonsoet al., 1999).
Among the ethyl ones, 4-ethyloctanoate, together with 4-methyloctanoate, give characteristic goat-like or mutton-like flavours to dairy products. Even though 3-methylbutanoate, 4-methylpentanoate and 8-methylnonanoate have also been identified in goat’s milk, they are not typical because they can be found in cow’s milk as well.