Major trends in research into domestic ducks and recent results concerning meat quality

Major trends in research into domestic ducks and recent results concerning meat quality E. BAEZA Station de Recherches Avicoles, INRA Tours, 37380 Nouzilly, France. Email: baeza@tours.inra.fr The introduction presents the main countries producing duck meat and the different species and rearing systems used in production. The review is then divided into two parts. The first part concerns meat quality. The evolution of duck meat post-mortem is described. The effects of high muscle development on technological and sensorial quality are also presented. Finally, the influences of the main factors (genotype, age and nutrition) involved in the variations in intramuscular lipid levels on technological and sensorial quality of duck meat are detailed. The second part reviews the latest trends in research into domestic ducks, particularly nutrition, reproduction, behaviour, welfare and genetics. Keywords: duck; meat; quality; lipids; nutrition Introduction From 1993 to 2003, the world duck production almost doubled, and meat production rose from 1.72 to 3.31 million tons (Evans, 2004). Asia is the leading production region and this is the area where the industry is expanding most rapidly. Asia accounted for 76 and 82% of duck meat output in 1993 and 2003, respectively. While duck meat production in South America has halved, all other regions have recorded growth. In particular, annual production in Europe grew from 262,736 to 448,105 tons, France being the leading producer with 250,000 tons in 2003. Next came Ukraine (60,000 tons) followed by Germany with just over 50,000 tons, Hungary (45,000 tons) and the UK (43,000 tons). Production in North and Central America is dominated by the USA (51,000 tons) followed by Mexico (20,000 tons) and Canada (8,900 tons). In Africa, the industry is primarily centred on Egypt and Madagascar, producing 37,300 and 13,000 tons of duck meat, respectively. In Asia, China s contribution (2.2 million of tons) represents 81% of the total region. The next largest producers are India (160,000 tons), Vietnam (83,000 tons), Thailand (63,000 tons), Malaysia (51,000 tons) and the Republic of Korea (48,000 tons). The main importers of duck meat in Asia and Europe, are Japan and Germany, respectively (Pingel, 2004). In 2002, the main duck exporters to Germany were France (27%), the Netherlands (10%), the UK (13%), Hungary (33%), China (3.3%) and Thailand (2.9%). The main species used to produce duck meat are Pekin, Muscovy (mostly in France) and mule ducks (mostly in France for the production of fatty liver and Taiwan). Other duck species such as Jinding and Shao ducks in China, Tsaiya ducks in Taiwan, Khaki Campbell, Indian runner and Desi ducks in India, Vietnam, Cambodia and Indonesia are used mainly for egg production and meat is a by-product (Pingel, 2004). Different production systems are used in the world. In developed countries, ducks are mainly reared intensively in total-confinement housing or with access to pasture (free-range, organic or label rouge systems, mule ducks reared for fatty liver production) and sometimes to water for swimming. In Asia, extensive systems such as fish-duck farming are largely used, ducks being housed above fishponds (Prinsloo et al., 1999; Syed, 2002). To decrease feed costs, ducks can also be reared in rice fields (Men et al., 2002a; 2002b). As for other poultry species, the industry developed cut (48% of slaughtered ducks in France in 2004, OFIVAL, 2005) and processed duck products. The meat yield of ducks has therefore increased and carcass fattiness has decreased owing to genetic selection and to improvements in bird management, particularly in nutrition. At the same time, interest in the technological quality of duck 1

meat has increased. In the first part of this review, the evolution of duck meat post-mortem will be described and the effects of high muscle development on technological and sensorial quality of duck meat will be presented. Duck meat is red and its oxidative energy metabolism is higher than in chicken or turkey meat. Lipids are therefore an important component of duck meat. The effects of the main factors (genotype, age and nutrition) involved in the variability of intramuscular lipid levels (quantity and quality) and on technological and sensorial quality of duck meat will be detailed. With the development of duck production, interest in scientific research in this species has increased. The second part of this review will briefly summarize the latest trends in different areas such as nutrition, reproduction, behaviour, welfare and genetics. Research into diseases and health aspects is also very important, but this will not be tackled in the present review. I. Duck meat quality 1) Evolution of duck meat post-mortem The evolution of duck meat post-mortem depends mainly on decrease in ph and on protein degradation under the action of proteases. The average ultimate ph is 5.8 and 6.2 in breast and thigh muscles of duck, respectively (for more details see review by Baéza, 1995). In breast muscle, ph decreases from 6.25 (15 min after slaughter) to 5.66 (24 h post-mortem). The ultimate ph in duck breast is similar to that measured in chicken breast but the decrease is more rapid in duck meat. However, the decrease in duck meat is slower and the ultimate ph higher when the fasting period before slaughter is longer. Exposure to high temperatures before slaughter has the same effects (Baéza, 1995). Finally, the activity of glycolytic metabolism in muscle increases with age, inducing a decrease in ultimate ph value (Baéza et al., 1998). The post-mortem action of proteases in chicken meat has been widely studied (Schreurs, 1997). However, there are few reports concerning ducks. Only Chou et al. (1996; 1997) and Cha et al. (2002) described the post-mortem degradation of myofibrillar proteins during storage at 5 C and the possibility of accelerating this process by marinating the meat in lactic acid or white wine. Their studies demonstrated that the post-mortem increase in myofibril fragmentation index of duck breast muscles was consistent with the kinetics obtained in other meats. SDS-PAGE analysis of duck breast myofibrils showed that the amount of troponin-t band decreased from day 0 to day 1, became barely detectable by day 3, and disappeared by day 7. Simultaneously 30 and 32 kda components began to appear by day 3; amounts increasing by days 7 and 14. After 7 days of storage, α-actinin was degraded and a 98 kda component appeared. Titin 1 and nebulin also disappeared after storage for three days post-mortem. It will be interesting to complement these results by measuring the concomitant evolution of meat tenderness and water holding capacity. 2) Effects of high muscle development on meat quality The Muscovy duck is characterized by a high sexual dimorphism for body weight, resulting in greater muscle development in males. The latter is due to greater fibre numbers and length of muscles in males but not to a difference in fibre area (Baéza et al., 1999). The difference in breast muscle weight between females and males is 29 and 60% at 10 and 12 weeks of age, respectively. This characteristic of males does not directly influence meat quality as the main effects of sex on growth, physico-chemical properties, technological and sensorial quality of breast muscle are related to the earlier body development of the females in this species (Baéza et al., 1998), confirming the benefit of slaughtering females two weeks before males (at 10 and 12 weeks of age, respectively). Between 8 and 15 weeks of age, grilled fillets of females are therefore judged less tender, less juicy and less mellow but more stringy and presenting a more pronounced flavour than grilled fillets of males when compared at the same age. In mule ducks, for whom sexual dimorphism for body weight is slight, there is no effect of sex on meat quality; males and females can therefore be slaughtered at the same age, i.e. 10 weeks (Baéza et al., 2000). Selection has considerably increased meat yield, particularly in Muscovy ducks (Baéza et al., 1997; 2002). At 12 weeks of age, the difference in breast muscle weight between control and selected ducks 2

was 39%. Selection increased the area and length of muscle fibres but their typology and their energy metabolism were not modified. However, in selected ducks, the rate and extent of decrease in ph postmortem were lower, and the meat was paler and less red, related to lower content of haeminic pigments, than in control birds. The chemical composition and juice loss after storage at 4 C were slightly modified by selection or unchanged. The technological quality of duck breast meat was therefore not altered by selection on higher muscle development. Age has a major effect on muscle development. Between 8 and 15 weeks of age, the weight of breast muscle is 3.3-fold in males and 2.3 in females (Baéza et al., 1997). Fibre area and length strongly increase with age (Baéza et al., 1999). As the collagen content in breast muscle decreases during the same period while collagen solubility remains unchanged, the decrease in meat mellowness and tenderness, and the increase in stringiness, may be related to fibre size (Baéza et al., 1997; 2002). Muscle levels of haeminic pigments increase with age, and the meat is therefore darker and redder (Baéza et al., 2002). Finally, the Muscovy duck has a higher muscle weight than the Pekin duck. At 14 weeks of age the difference is 84%, resulting from greater area and length of muscle fibres in Muscovy ducks (Chartrin et al., 2005). The muscle collagen content is also higher in this species, and the solubility of collagen is lower (Larzul et al., 2002). These differences may explain why Muscovy breast meat is judged less tender and stringier than Pekin breast meat (Chartrin et al., 2006a). By comparison with Pekin duck, Muscovy duck has late body development. When compared at the same age, the haeminic pigment concentration is therefore probably lower in muscles of Muscovy ducks than in muscles of Pekin ducks, explaining why the meat of Muscovy ducks is paler and less red than meat of Pekin ducks (Chartrin et al., 2006a). 3) Effects of genotype, age and nutrition on intramuscular lipids and meat quality Comparing Pekin and Muscovy ducks and their crossbred mule and hinny ducks, Chartrin et al. (2006b) showed that genotype had a significant effect on lipid levels in muscles. The Pekin duck had the highest lipid, phospholipid and triglyceride levels in breast and thigh muscles and the Muscovy duck had the lowest levels. The proportion of monounsaturated fatty acids (neo-synthesised by the liver) was also highest in muscles of Pekin ducks while the proportions of saturated and polyunsaturated were lowest (Chartrin et al., 2006b). The Muscovy duck exhibited the reverse. In muscles, lipids mainly accumulate in adipocytes, the relative area measured on cross-sections of breast and thigh muscles being highest in Pekin ducks (Chartrin et al., 2005). Lipids also accumulate in muscle fibres, particularly those with oxydo-glycolytic energy metabolism (type IIa), the triglyceride content of which is highest in breast muscles of Pekin ducks (Chartrin et al., 2005). Overfeeding-stimulated hepatic lipogenesis also induces an accumulation of lipids in muscles containing mainly triglycerides rich in monounsaturated fatty acids. The relative surface occupied by adipocytes and the triglyceride content of muscle fibres are significantly increased (Chartrin et al., 2005; 2006b). When considering the slaughter period, age also results in increasing muscle lipid content (Baéza et al., 2000; 2002) and modified lipid composition, as previously described, with preferential storage in intramuscular adipocytes (Chartrin et al., 2006c). Finally, genotype, overfeeding and age have similar effects on the variations in lipid content, lipid composition and lipid localisation. In breast muscle, the lipid content is 2-fold higher in Pekin ducks than in Muscovy ducks, 1.6 higher in overfed ducks than in ad libitum fed ducks and 1.4-fold higher in 98-day-old mule ducks than in 75-day-old mule ducks fed ad libitum (Chartrin et al., 2006b; 2006c). When lipid content in muscles is increased the activity of glycolytic energy metabolism is decreased and that of oxidative energy metabolism is stimulated (Baéza et al., 2005a). By combining genotypes (Muscovy and Pekin ducks and their crossbred hinny and mule ducks) and feeding levels (overfeeding and ad libitum feeding), Chartrin et al. (2006a) were able to obtain a wide range of lipid content in breast meat (from 1.72 to 8.35%). Sensorial analysis showed that increase in muscle lipid content increases lightness, yellowness, juice loss after cooking, tenderness and flavour of meat, with significant correlation coefficients (0.49, 0.47, 0.54, 0.43 and 0.28, respectively). The increase in lipid content of breast muscle with age may be involved in the increase in meat flavour (Baéza et al., 1997; 2000; 2002). 3

Not only the content but also the composition of intramuscular lipids can be modified according the fatty acid composition of the diet. It is possible to enrich duck meat in ω-3 fatty acids by including fish meal or fish oil in the diet, but the sensory acceptance of such meat is lower than meat from ducks fed with diet containing soybean meal (El-Deek et al., 1997; Schiavone et al., 2004). Duck meat has higher lipid content than chicken and turkey meat. As the meat of other poultry species, it contains high levels of unsaturated fatty acids (around 60% of total fatty acids) and also high levels of haeminic pigments (haemoglobin and myoglobin, Baéza et al., 2002) rich in iron which is a good catalyst of oxidation reactions. The susceptibility of duck meat to oxidation is thus higher than chicken or turkey meat. There are many studies concerning the supplementation of diet with high levels of vitamin E to limit the oxidation of turkey and chicken meat (see the review of Buckley et al., 1995) but there are few studies on duck meat. Only Romboli et al. (1997) and Salichon et al. (1998) have demonstrated the value of a diet supplemented with vitamin E to prevent duck meat oxidation during cold storage. II. Major trends in research into domestic ducks 1) Nutrition of growing ducks Recent regulations have restricted the production and use of manure, and the feed industry has proposed using enzymes such as xylanase and phytase to improve feed efficiency and reduce production pollutants. As in chickens, turkeys and hens, these enzymes are also efficient in ducks. It has been demonstrated in ducks that exogenous dietary xylanase and β-glucanase have a positive effect on the viscosity of digesta (Timmler and Rodehutscord, 2001; Adeola and Bedford, 2004), growth, feed conversion ratio and the digestibility of nitrogen and phosphorus, mainly during the starting and growing periods (Hong et al., 2002; Baéza et al., 2005b). Supplementation of diets with exogenous phytase improves growth performance and leg bone development of ducks, avoids the use of large amounts of phosphorus from inorganic sources and reduces the phosphorus content in manure (Shinckel et al., 2005; Bernadet et al., 2004; 2005; Wendt et al., 2005). The need for phosphorus has been precisely defined by Bernadet et al. (2002a; 2002b) during the different rearing periods of mule ducks, indicating a level of 0.31 and 0.14% of available phosphorus for the starting and growing periods, respectively. The availability of inorganic phosphorus from different sources for growing ducks was also investigated, showing for example that it was 100% for monosodium phosphate and 77% for calcium sodium phosphate and depended on the dietary calcium/phosphorus ratio (Wendt and Rodehutscord, 2004; Rodehutscord and Dieckmann, 2005). 2) Reproduction In France, only male mule ducks are reared for fatty liver production. Females are therefore generally sacrificed at the hatchery. The sex-ratio for mule ducks is unbalanced at hatching, with a preponderance of males (around 60% of total hatched ducklings) due to higher late mortality in female mule embryos occurring between egg transfer and hatching (Batellier et al., 2004). Current research is focused on sex determination and sex differentiation in chickens to control the sex-ratio of birds and to avoid sacrificing one sex at hatching (Shimada, 2002; Yamamoto et al., 2003). There are many different duck genotypes. Current conservation of poultry genetic resources is by living flocks, which is costly. The cryopreservation of spermatozoa could play an important role in poultry breeding and conservation of genetic resources. Various research teams are working on characterizing duck semen and on improving the cryopreservation technology (Surai et al., 2000; Han et al., 2005). Duck spermatozoa contain high levels of n-3 and n-6 polyunsaturated fatty acids (55% of total fatty acids) but they exhibit high levels of dimustase superoxide, glutathione peroxidase and antioxidant activity, which may compensate for their low level of vitamin E (4-fold lower than in chicken semen) and help to protect duck spermatozoa from oxidative stress which may occur during storage and cause loss of fertility (Surai et al., 2000). Polyunsaturated fatty acid content in spermatozoa is positively correlated with sperm motility and fertility (Cerolini et al., 2003). It is 4

possible to increase the proportion by supplementing feed with fish oil or linseed oil. The positive effects of diets enriched in polyunsaturated fatty acids have been examined in chickens (Cerolini et al., 2003) but this has not been extended to ducks. 3) Behaviour and welfare European regulations in the past ten years have tried to take into account the welfare of domestic animals, and various points are being or have been discussed concerning ducks, including overfeeding with the use of individual cages restraining duck movement during this period, stocking density and group size, the use of slatted-floors in relation to leg problems, practices that reduce feather pecking such as reduced light intensity and beak trimming, and possible access to an outdoor run and to open water for drinking, bathing and swimming (Rodenburg et al., 2005). Guémené et al. (2001) found no evidence of acute or chronic stress when measuring physiological responses to manipulation, intubation and overfeeding. Faure et al. (2001) showed that mule ducks exhibited less fear towards the caretaker than to an unknown person during the overfeeding period, suggesting that ducks do not learn to treat their regular feeder as an aversive stimulus. Pekin and mule ducks are more reactive to stressful reactions and more often express fear reactions than Muscovy ducks (Faure et al., 2003). Selection breeders aim to adapt their genotypes to production systems where birds are reared in large size groups. Therefore, current research is being focused on the genetic determination of susceptibility to fear and stress in order to determine new selection criteria related to duck behaviour and duck adaptability to new environmental conditions. 4) Genetics Research in genetics constantly improves performance traits in duck production (growing, reproduction, laying and overfeeding performance, slaughter weight and meat yield, see the review of Cheng et al., 2003) and new traits are now taken into account such as duck behaviour, ability to adapt to new environmental conditions and disease resistance. Research is currently being focused on gene and molecular genetic markers such as microsatellites (Stai and Hughes, 2003; Huang et al., 2005), quantitative trait locus detection, genome analysis and identification of candidate genes for important traits which could help to select directly on individuals and not indirectly on collaterals. Conclusion Duck meat is highly appreciated as it combines the characteristics of a red meat (containing for example high levels of phospholipids, precursors of aromas) and the dietetic characteristics of poultry meat (containing for example high levels of unsaturated fatty acids, representing around 60% of total fatty acids). By combining genotype, age and nutrition it is possible to achieve modulation within a wide range of intramuscular lipid levels. The duck is therefore a good model to study the metabolic mechanisms responsible for these variations and their consequences on sensorial (particularly the determination of cooked meat flavour) and technological (particularly the storage ability of crude and processed meat) quality. The duck is very appropriate for extensive production systems as its growth rate is lower than that of the chicken broiler. Moreover, its hardiness allows this species to adapt to different and difficult environmental conditions. Further research is needed to improve production systems, taking into account the hygien quality of products, sustainability of activity and respect for the environment and animal welfare. The development of the biotechnology of reproduction and genetics should help to preserve the biodiversity of duck species and to select birds better adapted to new production systems. 5

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