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* Unité de Recherches sur les Herbivores, Equipe Tissu Adipeux et Lipides du Lait, INRA, Theix, 63122 St-Genès-Champanelle, France
Unité de Recherches Laitières et Génétique Appliquée, INRA, 78352 Jouy-en-Josas, France
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMILK FAT SECRETION AND...LIPASE AND LIPOLYSISCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Peculiarities of goat milk FA composition and lipolytic system play an important role in the development of either goat flavor (release of branched, medium-chain FA) or rancidity (excessive release of butyric acid). The lipoprotein lipase (LPL) activity, although lower in goat than in cow milk, is more bound to the fat globules and better correlated to spontaneous lipolysis in goat milk. The regulation of spontaneous lipolysis differs widely between goats and cows. Goat milk lipolysis and LPL activity vary considerably and in parallel across goat breeds or genotypes, and are low during early and late lactation, as well as when animals are underfed or receive a diet supplemented with protected or unprotected vegetable oils. This could contribute to decreases in the specific flavor of goat dairy products with diets rich in fat.
Key Words: goat milk fat • fatty acids • lipoprotein lipase • lipolysis
Abbreviation key: CLA = conjugated linoleic acids, DHA = C22:6 n-3, EPA = C20:5 n-3, FA = fatty acids, LPL = lipoprotein lipase, RA = rumenic acid (cis-9, trans-11 CLA
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMILK FAT SECRETION AND...LIPASE AND LIPOLYSISCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Fat supplementation of diets could improve goat milk composition for greater control of cheese processing and satisfaction of consumer demand. Dietary lipid supplementation may indeed change milk fat FA composition and result in positive or adverse changes in the physical characteristics and the nutritional or dietetic properties of goat dairy products, and/or modify the lipolytic system (Chilliard, 1982) and hence the flavor of these products. Furthermore, the expected positive effects of fat supplementation on goat milk fat content (Chilliard and Bocquier, 1993) could be useful in solving the technological problems of the goat cheese industry, which are linked to a low milk fat content, especially when fat content falls below protein content (the so-called "inversion of percentages syndrome") (Morand-Fehr et al., 2000b). Although fat supplementation in dairy cows and ewes often decreases the milk protein content and the associated coagulation properties, this negative effect could not exist in goats (Chilliard and Bocquier, 1993).
The aim of this paper is to review the main effects of physiological and nutritional factors, and more particularly recent studies on fat supplementation, on goat milk fat and protein contents, fatty acid composition, lipase activity and lipolysis.
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MILK FAT SECRETION AND COMPOSITION |
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TOPABSTRACTINTRODUCTIONMILK FAT SECRETION AND...LIPASE AND LIPOLYSISCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Milk unsaturated FA may contain one or several trans double bonds. About 5 to 15% of total C18:1 are of trans configuration in goat (Bickerstaffe et al., 1972; Calderon et al., 1984; Alonso et al., 1999), cow (Storry and Rook, 1965; Selner and Schultz, 1980) and human species (Jensen, 1989; Guesnet et al., 1993). However, the proportion of different trans isomers varies between species: the main FA (35 to 40%) is trans-vaccenic acid (C18:1, n-7 or 
11) in goat and cow milk (Bicherstaffe et al., 1972; Alonso et al., 1999; LeDoux et al., 2002; Figure 1A and B
), whereas human milk fat trans C18:1 contains larger percentages of FA with the double bond located on carbons 6 to 14 (Figure 1C). The profile of human milk fat is probably related to the consumption of a mixture of ruminant milk fat and of margarines, the latter being richer in 6 to 14 - trans C18:1, especially 6 to 10 (Figure 1 D). Quantitatively, the trans C16:1 isomers represent less than 0.2% of total FA, or 5% of all trans C16:1 and C18:1 isomers in ruminant milk fat. The distribution patterns of cis and trans C16:1 isomers are very similar for goat, cow, and ewe cheese fat (Destaillats et al., 2000).
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). Conjugated linoleic acid (CLA) is a precursor of trans-vaccenic acid in the rumen and a product of the delta-9 desaturation of this FA in the mammary gland (Figure 2). The major isomer (more than 90%) of bovine milk CLA (cis-9, trans-11 C18:2, or rumenic acid, RA) originates mainly from the latter pathway (Griinari and Bauman, 1999). It is interesting to emphasize that milk fat from monogastric farm animals such as mare or sow (that do not consume ruminant milk fat or margarines) is almost devoid of trans-vaccenic acid and RA, whereas human milk fat is of an intermediate composition (Jahreis et al., 1999). In that study, the trans-vaccenic and RA contents of goat milk fat were lower than those of milk fat from cow or ewe receiving similar diets. However, the mean milk RA values from three other goat studies were in the range 0.4 to 0.9% of total FA (Alonso et al., 1999; Gulati et al., 2000; Chilliard et al., 2002), i.e., similar to observations in dairy cows receiving diets without added lipids (Griinari and Bauman, 1999; Chilliard et al., 2000).
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).
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). Correlations between the percentages of individual FA in these milks show three main families: C18-FA, C10 to C16-FA (negatively correlated to the C18-FA family), and C4 to C8-FA (not highly correlated to the two other families, with the exception of the negative correlation to C16:0) (Sauvant et al., 1973, and Figure 4). These correlations result mainly from the negative effect of long-chain (C18) FA that are mobilized from adipose tissue on de novo synthesis of medium-chain FA (C10 to C16; Barber et al., 1997), and from the fact that short-chain FA arise in part from metabolic pathways that do not involve malonyl-CoA and acetyl-CoA carboxylase activity (Bauman and Davis, 1974; Palmquist and Jenkins, 1980).
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).
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), lipid supplementation tended to increase milk yield (+0.1 to +0.4 kg/d or more when the control diet was very low in fat) and fat content (+2 to +7 g/kg). Effects on protein content were highly variable. The calculated energy balance increased or decreased according to respective effects on intake of DM and energy, and milk fat secretion. In three other early-lactation trials, milk yield and protein content remained unchanged, and fat content increased, with either protected sunflower seeds (15% of concentrate; Morand-Fehr et al., 1984a), extruded soybeans (160 g/d; Morand-Fehr et al., 1984b) or calcium soaps of palm oil (100 g/d; Martin et al., 1999). There were no clear trends concerning effects of fat supplementation on goat BW changes or body fat mobilization during early lactation. Results from early-lactation experiments are limited by their lack of precision, linked to possible differences in milk potential of animals and limited use of covariates, when lipid supplementation begins before or immediately after parturition.
Contrary to what was observed in dairy cows (Chilliard et al., 2001), feeding fat supplements to mid- or late-lactation goats did not increase milk yield, whereas milk fat content always increased sharply (+5.7 g/kg in 23 supplemented groups in Table 2). The ranges of observed responses were similar with different types of fat supplements: saturated free FA, calcium salts or triglycerides; animal fat; vegetable oils (C18:1-, C18:2-, or C18:3-rich oils; free oils, encapsulated oils); oilseed (whole, crushed, extruded, or formaldehyde-treated oil seeds) (Table 2, and Schmidely and Sauvant, 2001). Remarkably, goat milk fat content did not decrease even when vegetable oils (rich in polyunsaturated FA) were added to a low-forage diet (e.g., footnote 8 in Table 2), contrary to what was very clearly observed in dairy cows (Bauman and Griinari, 2001). As previously observed for early-lactation goats, response of milk protein content was highly variable in midlactation goats (Table 2). Body weight gain was either higher (Baldi et al., 1992), lower (Gelaye and Amoah, 1988), or unchanged (footnote 8 in Table 2) in fat supplemented vs. control goats.
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The response of milk fat secretion to fat supplementation could be lower during midlactation than during early lactation (Figure 5). This could be related to the fact that goat adipose tissue anabolic enzymes involved in de novo lipogenesis, and lipoprotein lipase (the enzyme involved in the uptake of blood lipoproteins carrying dietary FA absorbed from the intestine), are more active after the lactation peak than before it (Chilliard et al., 1977, 1979a), because they are positively related to energy balance (Chilliard et al., 1987). Body lipid mobilization dominates during early lactation, and this would favor the partitioning of dietary FA towards the mammary gland (Chilliard, 1993). It results from these events that a greater part of the exogenous FA are taken up by the adipose tissue after the lactation peak (Chilliard et al., 1991). However, contrary to milk fat secretion, the highest milk fat content responses were observed in late-lactating or low-yielding goats (Table 2), probably because the dietary FA were less diluted in the milk of these animals.
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The reasons for these differences in dairy performance response to fat supplementation between ruminant species are not easy to identify, as fewer trials and less information are available for ewes and goats than for dairy cows. The differences may be linked to complex digestive and metabolic interactions (as observed in dairy cows) between the basal diet (nature and proportion of forages and concentrates), fat supplementation (nature and technological treatment, dose and/or duration) and animal characteristics (species, breed, lactation stage, milk potential, etc.; see reviews from Chilliard et al., 2000; Bauman and Griinari, 2001). It has been suggested that the rate of passage of digesta is higher in goats than cows (Hart, 2000). This could decrease in goats the effects of dietary FA on the yield of some ruminal factors that reduce mammary lipogenesis in cows.
Effects on mammary metabolism of the balance and availability of glucogenic, lipogenic, and aminogenic nutrients (including related endocrine changes) are not completely understood, and may vary between species, thus changing their relative responses in lactose, fat, and protein secretions. Furthermore, the caprine 
-s1 casein locus is remarkable for its high level of polymorphism and for the fact that important differences exist in milk protein content and, in some cases, in fat content between alleles or groups of alleles (Grosclaude et al., 1994; Tables 3 and 4). It is not known whether the response to dietary fat supplementation would be different between these genotypes.
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). Furthermore, the clear positive effects of almost all types of fat supplementation on milk fat content could be useful to solve the technological problems of the goat cheese industry which are linked to the so-called "inversion of percentages syndrome" during the spring and summer period (Bocquier et al., 2000; Morand-Fehr et al., 2000b). This effect could be related to the combined effects of lactation stage (see above and Table 4), day length (dilution effect, due to stimulation of milk yield; Linzell, 1973; Chilliard and Bocquier, 2000), and/or nutritional factors. Indeed, high-concentrate diets used in high-yielding goats (Sauvant and Morand-Fehr, 2000) are more frequently subject to this syndrome than corn silage-based diets (Rouel et al., 2000). The inversion syndrome occurred whatever the -s1 casein genotype of the goats (Table 4). Changes in milk fat FA composition are another very important consequence of dietary lipid supplementation that may result in positive or adverse changes in the flavor, the physical characteristics and the nutritional or dietetic properties of dairy products.
Effects of Lipid Supplementation on Milk FA Composition
Palmitic or stearic acid.
Feeding of palmitic acid increased goat milk C16:0 percentage considerably (and C16:1 to a lesser extent) at the expense of C10:0 to C14:0 and of C18:1. When stearic acid was fed instead, milk C18:0 and C18:1 percentages increased considerably, at the expense of C10:0 to C16:1 (Table 5). These results illustrate the important role of mammary delta-9 desaturase in regulating the monounsaturated:saturated FA ratio, especially for C18 FA.
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). Another goat trial also reported an increase in milk C16:0 percentage (Sleiman et al., 1998). These results are comparable to those observed in dairy cows, showing increases in both palmitic acid and C18:1 (Table 6), although the responses were more marked in goats.
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), unless these lipid supplements are efficiently protected by encapsulation in a formaldehyde-treated protein coat (McDonald and Scott, 1977). This is illustrated by data in Table 5 showing that feeding protected canola seeds to goats increased milk C18:1, C18:2, and C18:3 proportionally to the respective percentages of these FA in canola oil. On the other hand, feeding unprotected oil increased mainly C18:0 and C18:1, the latter increase probably being due, to a large extent, to unidentified trans isomers of C18:1 (cf. Griinari and Bauman, 1999; Chilliard et al., 2000, 2001, for data in cows). This illustrates that both total and partial hydrogenation of unsaturated FA take place in the rumen (Figure 2).
Feeding protected soybean oil sharply increased milk C18:2 percentage (Table 5). The increase in C18:0 (and probably part of the C18:1) could be because some soybean oil escaped protection and was hydrogenated to C18:0 and trans-C18:1. In this trial, the increase in milk C18-FA was compensated for by a sharp decrease in the C16:0 percentage, although there was a trend for increasing short- and medium-chain FA percentages.
The effect of feeding protected cottonseeds differed from that of other vegetable oils, resulting in a large increase in the milk C18:2 percentage and in the C18:0 to C18:1 ratio, despite the fact that this oil is poor in C18:0 and contains only 16% of C18:1 (Table 5). This response is related to the fact that cottonseeds are rich in cyclopropenoic FA, which are strong inhibitors of mammary delta-9 desaturase activity. With unprotected cottonseeds, the increase in milk C18:2 did not occur, whereas the increase in milk C18:1 (Table 5) was probably due in part to unidentified trans isomers of C18:1 arising from C18:2 hydrogenation in the rumen.
Responses were different when linseeds were added to the diet. Milk C18:3 (n-3) percentage was significantly increased by 159 or 320% when goats received a diet with 4% added fat from either crushed linseeds or formaldehyde-treated crushed linseeds, respectively (Y. Chilliard, P. Capitan, J. Rouel, A. Ferlay, unpublished results). These results show that C18:3 was partially protected in crushed linseeds, and that formaldehyde treatment approximately doubled (P < 0.01) its degree of protection.
Unprotected oils or seeds.
The response to feeding different kinds of unprotected lipid supplements consisted mainly of an increase in the percentages of milk C18:0 and C18:1, at the expense of mainly C8 to C14 (and C16:0 in most trials; Astrup et al., 1985; De Maria Ghionna et al., 1987; Bartocci et al., 1988; Baldi et al., 1992; Mir et al., 1999; Schmidely and Sauvant, 2001; Tables 7 and 8). This was probably due to the ruminal hydrogenation of polyunsaturated FA into C18:0 and trans-C18:1, which are inhibitors of the de novo FA synthesis, mainly C8 to C16. The final response of milk C16:0 percentage depended on the level of dietary intake, that is, the C16:0 percentage in the lipid supplement which was studied. The decreases of C12:0 to C16:0 resulted in a sharp decrease in the atherogenicity index of the milk fat (Tables 7 and 8). The responses to fat supplementation of C4 to C8 were less marked, or even opposite to those of C10 to C14 (Tables 5, 7, and 8), and this peculiarity is probably related to their origin from nonmalonyl CoA pathways (see above).
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We recently compared (Table 7) the effects of dietary supplementation with either free oil or oilseeds, from either linseed (rich in C18:3) or sunflower (rich in C18:2). These four treatments sharply increased goat milk fat content (+3 to 6 g/kg), but had very different effects on milk FA composition. Linseed oil had a greater effect on increasing the percentages of milk linolenic acid (+325%) and cis 9, trans 13 isomer of C18:2 (+350%), whereas sunflower oil had more effect on milk linoleic (+55%), trans-vaccenic (+290%) and rumenic (+283%) acids, probably undergoing the mechanisms indicated in Figure 2. trans-Vaccenic acid and RA and, surprisingly, polyunsaturated FA were more significantly increased by free oil than by oilseeds, whereas stearic and oleic acids were less affected. This suggests that biohydrogenation was less efficient when oil was added free than as part of the seeds, and that a low concentration of free oil (3 to 4% of diet DM) was sufficient to disturb rumen metabolism in a way that inhibited the biohydrogenation of its own FA, thus increasing the transfer of polyunsaturated and trans FA to milk.
The comparison of these two whole seeds with lupine seeds and whole soybeans showed that all four seeds sharply increased milk stearic and oleic acids. However, the highest oleic:stearic ratio was observed with lupine seeds, probably because they did not increase the secretion of polyunsaturated FA, contrary to the other seeds, which increased either C18:3 (linseeds) or C18:2 (sunflower and soya beans). Thus the mammary delta-9 desaturase activity was probably higher with lupine seed, because desaturase activity is inhibited by polyunsatured FA, and/or because lupine seeds tended to decrease trans-vaccenic acid (Table 7). This later decrease could also explain why lupine seeds decreased milk RA. Among the four seeds, only sunflower seeds increased trans-vaccenic acid and RA (Table 7). This suggests that when biohydrogenation occurred on the polyunsaturated FA released from linseeds or soya beans it occurred slowly but almost completely. These results on effects of soybeans are in agreement with observations in dairy cows by Morales et al. (2000) who suggested that polyunsaturated FA from roasted whole soybeans, compared to those from tallow, were, in part, protected against biohydrogenation and that the other part was slowly but completely hydrogenated, since both C18:2 and C18:0, but not trans-C18:1, increased in milk fat.
Lupine seeds FA were totally and completely hydrogenated and these seeds could even bring special (nitrogenous?) compounds, which could increase the biohydrogenation of polyunsaturated FA from the basal diet. Thus these three seeds (linseeds, soybeans, lupine seeds) would need to be physically treated (ground, heated, extruded,...) in order to specifically increase trans FA and RA in goat milk. On the contrary, oil from whole untreated sunflower seeds was probably more rapidly released and interacted with rumen microflora, in a way that significantly increased trans-vaccenic. However, the increase in RA was much lower than when free sunflower oil was used. Further research is needed to understand the mechanisms that could explain these differences.
The trial presented in Table 7 thus shows that the four whole seeds studied are not efficient when used to increase goat milk RA, in contrast to the use of free linseed or sunflower oils. Feeding free canola oil also increased (+204%) goat milk total CLA content (Mir et al., 1999).
Interaction between forages and oil supplements.
The RA content of goat milk fat was lower during winter than during summer, when animals received fresh grass (Figure 6). The effect of grass feeding on milk RA is probably due to its high content in linolenic acid (cf. Figure 2). Furthermore, it is likely that different types of winter diets (hay vs. corn silage) are not equivalent for goat milk RA content, and that they could also interact differently with dietary fat supplements. We tested this possibility recently, comparing the effects of either linseed oil or high oleic sunflower oil, added to either hay or corn silage-based diets (Table 8).
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, and results not shown). Compared with control diets, linseed oil addition increased milk yield, and fat and lactose contents, C4:0, C18:0, trans-10 C18:1, trans-vaccenic, rumenic and linolenic acid percentages, and lowered C10:0 to C16:1, branched-chain and odd- numbered FA percentages, and the atherogenicity index. Effects of linseed oil addition on C4:0, C6:0 and trans-10 C18:1 were higher when combined with the corn silage diet, while effects on milk fat content and C14:0 to linolenic acid and RA (except trans-10 C18:1, oleic and linoleic acids) were higher with the hay diet. The effect on RA (+853% with hay diet) was spectacular. Compared with control diets, high oleic sunflower oil addition increased milk fat, protein and lactose contents, C4:0, C18:0, oleic, trans-vaccenic and RA percentages, and lowered C8:0 to C16:1, branched-chain and odd-numbered FA, linoleic and linolenic acid percentages, and the atherogenicity index. Effects of high oleic sunflower oil addition on C4:0 and trans-10 C18:1 were greater with the corn silage diet, while effects on C16:0, C16:1, oleic acid, branched-chain and odd-numbered FA and RA were higher with the hay diet. The trans-10 cis-12 CLA isomer was never present in significant amounts, whatever the diet used in Table 8.
Thus the differences between hay and corn silage diets were small. High oleic sunflower oil addition sharply increased stearic and oleic acid percentages. Linseed oil addition sharply increased trans-vaccenic and RA percentages, and the effects were higher with hay diet. Important interactions between forage and oil effects were observed. Furthermore, any increase in RA was systematically (for 38 different diets) accompanied by a 2.5-fold higher increase in trans-vaccenic acid (Figure 7), as in dairy cows (Griinari and Bauman, 1999). This implies that the potential effects on human health of both CLA and trans-C18:1 isomers have to be evaluated together with care (e.g., Jensen, 2002) in order to predict the putative effects of lipid supplementation on goat milk fat nutritional quality. A sharp decrease in the atherogenicity index of goat milk fat was observed for the two vegetable oils, whatever the forage. These results are an illustration of how dietary factors can broadly modify goat milk FA composition and have potential effects on its quality for human nutrition.
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Conclusion.
Despite differences in the quantitative responses (milk yield and milk fat content), the changes in milk FA composition after lipid supplementation are very similar in goats (see above) and cows (reviews by Palmquist et al., 1993; Griinari and Bauman, 1999; Chilliard et al., 1993, 2001). It is likely that specific changes in minor FA that play an important role in the inhibition of mammary lipogenesis (such as trans-10 C18:1 or trans-10 cis-12 CLA in cows, Bauman and Griinari, 2001), and that are not well-known in goats, could partially explain between-species differences. However, the sharp increase in trans-10 C18:1 due to corn silage-vegetable oil interactions (Table 8) did not result in a decrease in goat milk fat content, contrary to observations in cows. Nevertheless, the trans-10 C18:1 could be involved in the lack of positive effect of oil supplementation on goat milk fat content when corn silage was used, whereas oil supplementation increased sharply milk fat content when hay was used (Table 8). Changing diet composition can allow rapid and efficient changes in goat milk FA composition, but potential effects on other aspects of the quality of caprine dairy products, such as taste, flavor, nutritive and health value for consumers, warrant further investigations.
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LIPASE AND LIPOLYSIS |
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TOPABSTRACTINTRODUCTIONMILK FAT SECRETION AND...LIPASE AND LIPOLYSISCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Milk fat lipolysis and lipase (or LPL) activity are two distinct phenomena, although both result in the hydrolysis of triglycerides. Milk fat lipolysis generally takes place at 4°C, at natural milk pH, in the presence of biochemical factors naturally occuring in milk. It is generally lower than 1.0 mmol released FA/24 h/L of goat milk (Chilliard, 1982). Milk LPL activity is measured as maximal potential activity on an artificial lipid emulsion, at 37 or 39°C, at alkaline pH, in the presence of apoprotein activators from blood serum. It is generally higher than 20 µmol/h/ml of goat milk (Chilliard, 1982). Thus potential LPL activity is more than 500 times higher than spontaneous lipolysis in goat milk.
The lipolytic system differs considerably between goat (Bjorke and Castberg, 1976; Chilliard et al., 1984; Azzara and Dimick, 1989), cow (Cartier and Chilliard, 1990, 1994) and human (Castberg and Hernell, 1975; Neville et al., 1991) milks. Spontaneous lipolysis is not correlated to LPL activity in bovine milk, and lipolysis remains generally low despite the very high LPL activity of this milk. This could be due to the presence of inhibitors in bovine milk (Cartier et al., 1990), as well as to the fact that bovine milk LPL is largely bound to casein micelles, thus decreasing enzyme-fat substrate interactions. On the contrary, large proportions of human and goat milk LPL are bound to cream, which could explain that milk lipolysis is well correlated to milk LPL activity in these two species. This can be related to the lack of effect of heparin addition on goat milk lipolysis, contrary to its strong positive effect on cow milk lipolysis due to the release of casein-bound LPL by heparin (Chilliard et al., 1984). Furthermore, blood serum (a specific activator of LPL) increased lipolysis more in goat than in cow milk, except when goat milk total LPL activity was extremely low and became limiting (Chilliard et al., 1984). Goat milk serum proteose peptone fraction inhibited spontaneous (Chilliard et al., 1984) and induced (Arora and Joshi, 1994) lipolysis of goat milk, as was similarly observed for cow milk (Anderson, 1981; Cartier et al., 1990).
The development of goat flavor in cold, stored fresh milk is due to free FA, especially free C6:0 to C9:0 and more specifically volatile branched-chain C9 and C10 as 4-methyl- and 4-ethyl-C8, which are more abundant in small ruminant than in bovine milk fat (Ha and Lindsay, 1993; Lamberet et al., 2001). The high concentrations of total branched-chain FA in the milk fat of ruminant species results mainly from microbial metabolism of branched-chain amino acids in the rumen, since leucine and isoleucine give rise to iso-valeric and 2-methyl butyric acids; the corresponding acyl-CoA could be used as a primer in the elongating process to form the iso and anteiso series up to C17. Furthermore, goat milk contains minor volatile branched-chain FA with one methyl or ethyl group on carbon 4 (Ha and Lindsay, 1990a; Lamberet et al., 1996; Alonso et al., 1999). These FA probably arise from tissue metabolism of propionate and butyrate absorbed from the rumen, such metabolism probably differing between bovine and caprine species. Interestingly, two of these minor FA (4-methyloctanoic acid which was first found with 4-methylnonanoic acid in mutton meat (Wong et al., 1975), and 4-ethyloctanoic acid) are involved in goat flavor. Ha and Lindsay (1990b) proposed pathways for their formation in ruminant fat, by analogy with descriptions of fat from uropygial glands in waterfowl (e.g., Rainwater and Kolattukudy, 1982). But nothing is known of the way in which such a synthesis is regulated within the different tissues in ruminants. For example, according to Sugiyama et al. (1986), a homologous series of 4-ethyl branched-chain FA, in free and bound forms, is the main fat component from neck sebaceous glands of adult buck. 4-Ethyloctanoic acid, which can be considered as a pheromone, represents less than 5% of this series while 4-ethyldodecanoic acid accounts for about half the total amount. The presence of such a series leads to suppose that addition of ethylmalonyl-CoA at different stages of the FA elongation process takes place in the sebaceous gland cells. In goat milk, only the presence of 4-ethyloctanoic acid was reported, and this FA has the lowest flavor threshold value determined until now for FA (Brennand et al., 1989; Karl et al., 1994).
Thus, the combination of goat milk FA composition, triglyceride structure (i.e., high proportion of C6-C10 FA esterified on carbon 3) and LPL characteristics could explain the link between LPL, lipolysis and goat flavor in caprine milk (Figure 8). Furthermore, it should be pointed out that goat flavor, which is regarded sometimes as a positive, sometimes as a negative feature in cheeses or milk, appears at lipolysis levels much lower than those responsible for the rancid-butyric flavor (Lamberet et al., 1996, 2001 and unpublished results).
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These breed or genetic effects could be related in part to the casein -s1 genotype. Indeed, milks from goats with different alleles of the casein -s1 gene present different protein, casein, and fat contents (Grosclaude et al., 1994; Table 3). Interestingly, goats of FF genotype secrete a milk with lower fat content and higher levels of LPL activity, spontaneous lipolysis and goat flavor than goats of AA genotype (Table 3; Delacroix-Buchet and Lamberet, 2000). Thus, selecting goats for milk flavor is probably the reason for which the frequency of null alleles is very high in Norway (Vegarud et al., 1999). Negative intergoat correlations were found consistently between milk fat content and goat flavor (review by Delacroix-Buchet and Lamberet, 2000). Furthermore, milk fat C16:0 percentage was higher in FF genotypes (Delacroix-Buchet et al., 1996; Lamberet et al., 1996) in agreement with a positive interanimal correlation between C16:0 percentage and goat flavor (Bakke et al., 1977; Astrup et al., 1985).
The creaming ability is also related to the casein -s1 genotype. Indeed, an 11% decrease in milk fat content was followed by a 49% decrease in the fat content of the cream from FF compared with AA goats (Table 3). This could be related either to differences in the size or composition of milk fat globules and in the amount of cytoplasmic fragments or to other factors linked to differences in secretory mechanisms between AA and FF genotypes, that could explain in part the occurrence of apocrine milk secretion in the goat (Neveu et al., 2002).
Physiological Factors
Goat milk flavor (reviews by Skjevdal, 1979; Delacroix-Buchet and Lamberet, 2000), lipolysis and LPL activity (Table 4 and Figure 9) were at their highest after the lactation peak, and were low before wk 4 and after wk 30 of lactation. This confirms that the latter two parameters are well correlated in goats, unlike in cows in which milk LPL activity decreased as lipolysis increased during late lactation (Chazal and Chilliard, 1986). This effect of lactation stage in cows was in fact an effect of pregnancy stage (Chazal and Chilliard, 1986) linked to estrogens (Cartier and Chilliard, 1994) and/or progesterone (Chilliard et al., 1997). One reason for the lack of increase in milk lipolysis during late lactation in goats could be that fecundation occurs later, due to the shorter pregnancy in this species. The involvement of sex steroid hormones in the maturation of the milk lipolytic system is also suggested by the fact that the normal increase of milk LPL activity during early lactation was delayed in goats hormonally induced into lactation without being previously pregnant (Figure 10). In other respects, there is an interaction between lactation stage and -s1 casein genotype: the increase of lipolysis as lactation advances was more pronounced and significant in goats with E, F, or O alleles (Table 4).
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) was increased. Furthermore, hourly milking compared with twice daily milking increased the percentage of milk LPL distributed in the serum fraction (Azzara and Dimick, 1989), thus suggesting that this milk fraction could contain mainly LPL from blood origin in goats (Figure 11). A similar effect of milking frequency was observed recently in dairy cows in which once daily milking decreased milk LPL activity compared with twice daily milking (Rémond et al., 2002).
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). This result was recently confirmed with unprotected C18:1-, C18:2-, and C18:3-rich oils: milk LPL activity and spontaneous lipolysis decreased sharply in goats fed hay- or corn silage-based diets when fat was added (5 to 6% of DMI from either regular or high-oleic sunflower oil or linseed oil, Figure 13). This matches with the negative correlation observed between goat milk flavor and polyunsaturated FA content (Skjevdal, 1979), and with the decrease in goat flavor in milk and cheeses from animals of Table 8 receiving lipid supplements (Gaborit et al., 2002). It can be observed in Figure 13 that the effect of lipid supplementation was less important on milk spontaneous lipolysis than on milk LPL activity, and it can be predicted that milk lipolysis would only increase when milk LPL activity is higher than 15 to 18 µmol/h per milliliter. It could be hypothesized that milk LPL activity decreased when supplemental lipids were fed because mammary LPL was directed towards the basal membrane of secretory cells, where it is needed to allow the uptake of blood triglycerides (Figure 11). Conversely, the higher LPL activity in milk of animals with the FF genotype (Table 3) could be related to the lower flow of fat secretion, thus decreasing the need for LPL at the basal membrane and increasing the availability of LPL for secretion into milk for a given level (constitutive of the lactating status) of mammary LPL synthesis.
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). Milk spontaneous lipolysis did not change markedly and was poorly correlated to milk LPL activity during fasting-refeeding, whereas the free FA content of freshly drawn milk increased during fasting (Figure 14), probably due to a direct passage of blood NEFA to the milk when their concentration was increased by body fat mobilization. As the mammary gland LPL activity did not significantly change during fasting-refeeding (Figure 15), it may be thought either that mammary LPL was partitioned differently between basal membrane and milk when the nutritional status changed, or that blood LPL arising from adipose tissues contributed less to milk LPL secretion because it was decreased in adipose tissues during fasting (Figures 11 and 15). These observations can be related to the negative effect of fasting and to the positive effect of insulin on human milk LPL activity (Neville et al., 1991).
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). This relationship was mainly due to intra-goat and between-lactation stage correlation. Indeed, adipose tissue LPL and other lipogenic activities were low in high-yielding goats at the beginning of lactation, and then increased when animals returned to a positive energy balance, after 3 or 4 mo of lactation (Figure 16). Simultaneously, milk LPL activity followed a similar pattern (Figures 9 and 10), whereas mammary LPL activity did not change significantly (Chilliard et al., 1986). In other respects, the strongest correlation (r = -0.70) was between milk LPL activity and milk fat C16:0 percentage (Table 9). It can be hypothesized that there is a mechanistic link between the physiological or nutritional regulation of the chain-length of synthesized FA and the synthesis and secretion of mammary LPL. This observation is also in agreement with the fact that feeding C16:0 to goats tended to decrease milk lipolysis and goat flavor, whereas there was surprisingly a positive interindividual correlation between C16:0 and goat flavor (Astrup et al., 1985).
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). On the contrary, lipolysis in bovine milk was not correlated to LPL activity, and responded differently to physiological factors compared with observations in goat milk (Table 10). Thus, important differences exist between goat and cow lipolytic systems as well as in the physiological regulation of milk lipolysis, even though the regulation of milk LPL activity was similar in the two species.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMILK FAT SECRETION AND...LIPASE AND LIPOLYSISCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Quantitative and qualitative aspects of milk quality cannot always be increased simultaneously. For example, lipid supplementation could improve the efficiency of goat cheese yield and its FA profile but decrease its sensorial quality. In other respects, the polymorphism of the casein -s1 gene in goats presents an interesting model to study the mechanistic links between mammary protein and fat secretions, as well as the opposite regulations of goat milk fat and LPL secretion and their relationships with the development of goat flavor. The present and future knowledge on genetic, physiological, and nutritional factors regulating goat milk FA composition and lipolytic system will be helpful in the management procedures of goat husbandry to optimize the quantitative and qualitative (nutritional, sensorial, technological, etc.) aspects of goat dairy products.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMILK FAT SECRETION AND...LIPASE AND LIPOLYSISCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Corresponding author: Y. Chilliard; e-mail:
Yves.Chilliard{at}clermont.inra.fr.
Received for publication May 17, 2002. Accepted for publication October 24, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMILK FAT SECRETION AND...LIPASE AND LIPOLYSISCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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s1 caprine sur le rendement fromager et les caractéristiques des fromages. Lait 76:217–241.
-s1 caprine, ses effets, son évolution. INRA Prod. Anim. 7:3–19.
s1 caprine sur la flaveur chèvre: Fabrications fromagères avec échange de protéines et de matières grasses. Lait 76:349–361.
s1-Cn locus? Reprod. Nutr. Dev. 42:163–172.
9, trans 11 linoleic acid (CLA) as well as total fat composition of German human milk lipids. Nahrung 43:233–244.[Medline]
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= lactation after normal parturition (10 goats); 
= lactation after abortion (4 goats); 
= hormonally induced lactation in nonpregnant goats (5 goats). The following correlations were observed: milk C18:0 + C18:1 (%) versus energy balance, r = -0.77; milk fat content versus energy balance, r = -0.58; milk fat content versus plasma NEFA content, r = +0.46; milk fat content versus milk C18:1 (%), r = +0.47.
) (from 
, ewe; 
