L58 Meat quality of fast-growing broilers: problems and solutions

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1 L58 Meat quality of fastgrowing broilers: problems and solutions M. Petracci *, F. Soglia, L. Canonico, C. Cavani Department of Agricultural and Food Sciences, Alma Mater Studiorum University of Bologna, Piazza Goidanich 60, Cesena, Italy *Corresponding author: Keywords: chicken meat, quality, abnormalities, appearance, tenderness Summary Nowadays most of the world s chicken meat production is merely based on intensive farming of few fast growing hybrids reaching the slaughter weight in a very short time and having high meat yields. The shift from the sale in whole carcass to readytoeat and readytocook products has increased importance of quality traits of both fresh meat and raw meat material used for the manufacture of products. This evolution has led to an extreme modification of the genetic background of modern hybrids which are currently used worldwide for the production of chicken meat. However, during the last decades, these evolutions have certainly favoured the occurrence of a high number of abnormalities that are increasing the meat downgrading rates for fresh market retailing and sometimes decreasing the nutritional, sensory and technology proprieties of raw meat materials used for further processing. Among these, the poultry industry faced the occurrence of visual defects such bruises or haemorrhages and muscle exhibiting abnormal colour (i.e. PSE and DFDlike conditions) as well as toughening due to early deboning. Nowadays, however, the greater concern is towards occurrence of abnormalities characterized by a myodegeneration affecting breast fillets (whitestriping, woodybreast and spaghettimeat) which seems directly or indirectly induced by high growth rate and hypertrophy characterizing modern fast growing broilers. Introduction The development in industrialization and specialization of broiler meat production chains that took place starting from the end of World War II, has led to a worldwide remarkable increase in both the efficiency and the chicken meat production. In the recent years, the lifestyle changes have also dramatically modified the way in which the poultry meat is marketed and consumed and therefore food technologies have become part of the poultry industry, and today much of the production is marketed in the form of cutup and processed products (Table 1). Table 1 Trend of world chicken meat production (FAO, 2016), progress of broiler performance and evolution of market segments and forms of chicken meat in the US (adapted from NCC, 2016). Year Production (.000 tons) 9,365 16,326 27,293 46,352 70,259 96,338 market age (d) market weight (kg) Broiler performance average daily gain (g/d) feed to meat gain (g/g) mortal ity Market segments retail grocery foodservice whole Market forms cutup parts further processed As a result, nowadays, most of the world s production is merely based on intensive farming of few fastgrowing hybrids rapidly reaching the slaughter weight and having high meat yields. In addition, because of the consumers preference for breast meat and as a consequence of the developing market of 271

2 cutup and processed products, broilers are slaughtered at increased weights. Within this context, as a result of the shift in market form from whole carcass to readytoeat and readytocook products, the importance of quality traits of both fresh meat and meat used as raw materials for processed products manufacture has remarkably increased. This evolution has led to extreme modifications in the modern hybrids which are currently selected and used worldwide to produce chicken meat. Notwithstanding, the differences existing in meat quality among the most popular hybrids (i.e. Ross, Cobb and Hubbard) are very limited if compared to the ones observed among and within the mediumand slowgrowing genotypes. Thus, the changes in meat quality traits existing in different fastgrowing hybrids mainly arise by farming factors and, especially in recent years, by the preslaughter and slaughtering phases. In this regard, it is also well known that some features observed in fastgrowing hybrids (i. e., muscle hypertrophy, accentuation of glycolytic metabolism of the muscles, poor thermoregulatory capacity, skeletal and vascular fragility, insufficient vascularisation), might directly or indirectly be induced by selection predisposing to the occurrence of meat abnormalities with increased incidence within the last 30 years (Petracci et al., 2015; Velleman, 2015). This review is therefore intended to make a summary of the most important qualitative issues affecting the chicken meat of fast growing broilers reared in industrial farming and slaughtering conditions. Main meat quality problems Green muscle disease (GMD) Deep pectoral disease, also known as Oregon disease was observed for the first time in fastgrowing birds about 30 years ago (Siller, 1985). Nowadays, the occurrence of GMD abnormality is still a relevant quality issue in poultry since its incidence may vary between 0.02% and 1.9% (Kijowski et al., 2014). In detail, its incidence is higher in broiler chickens with a higher growth rate and of a great extent in roosters rather than in hens (Lien et al., 2011). GMD exhibit haemorrhagic appearance with a swollen reddishbrown lesion, typical of a early developing stage, that gradually becomes green in the old one. Thus, the presence of GMD may result in significant commercial complaints, if the whole carcass is sold to cutup as well as processors units or butcher shops and economic losses for the poultry industry as a consequence of the trimming procedures resulting in breast meat downgrading (Petracci et al., 2015). The occurrence of GMD certainly appears to be related to the tremendous development of the pectoral muscles observed in modern commercial hybrids coupled with the relatively low activity during growing period. Being confined within an inelastic compartment, the Pectoralis minor muscles are insufficiently enlarged if poorly exercised (Siller, 1985). In addition, according to their behaviour under organic farming conditions, this lack of movement observed in fastgrowing chickens is mainly due to a genetic predisposition rather than to an intensive farming environment (Castellini and Mugnai, 2002). Recently, Lien et al. (2012) found that GMD appears to begin at approximately 26 and 36 days of age in males and female broilers, respectively. Within this context, an increased bird activity (flock nervousness, flightiness, struggle, and wing flapping) induced by several factors such as feed or water outages, lighting intensity and programs, human activity and excessive noises in and around chicken houses should be considered a trigger for the development of GMD in broilers. Lien et al. (2012) suggest to the use of creatine kinase levels as tools in selection programmes to mark the susceptibility of muscle for GMD. Colour abnormalities Among all the quality attributes, the appearance is one of the most critical for the selection of many food commodities, including poultry meat. One of the main components contributing to meat appearance is colour, which is the most important parameters for the selection of both deboned and skinless raw meat and exerts a relevant role in the final evaluation of many cooked products. Indeed, pink or red appearance in cooked poultry meat is generally associated with undercooking and is therefore highly undesirable. In addition, being associated with freezing prior to cooking, bone darkening is also considered a defect in fully cooked products (Fletcher, 2002). Other visual defects are mainly associated with bruises and haemorrhages. Modern broilers are very prone to develop bruises and haemorrhages as a result of the grater body size and the increased blood vessel and skeleton fragilities. In detail, 9095% of the bruises found in broilers occur during the last

3 hours prior to processing with breast, wings and legs being the most affected as a result of the manual catching of broilers (by the legs) through which operatives may carry up to five birds, each held by one leg, in each hand. In this condition, the potential for trauma is therefore considerable (Petracci et al., 2011). In addition to the ante mortem steps involving manual handling of the broilers, the stunning phase is of significant importance. Although electrical stunning represents the most widely used system for poultry, under certain conditions, can result in meat defects such as broken keels and wings, engorged wing veins, and broken capillaries within the breast muscle as a consequence of hypercontractions and subsequent haemorrhages caused by blood vessels rupture and damaged muscle fibres (Savenije et al., 2002). Animal welfare and product quality issues arising as a result of to the use of high current electrical stunning have led in Europe (Council Regulation n. 1099/2009) to the development of alternative techniques such as gas stunning. During gas stunning, the birds are exposed to gases which may induce either anaesthesia (carbon dioxide) or anoxia (argon and nitrogen). Gas stunning has a high potential for humane stunning or stun to kill, it requires sophisticated and expensive technical equipments with the main advantage being the moderate handling stress for the birds since they can remain in transport crates from the time they are carried from the farm to the slaughterhouse reducing both animal distress and subsequent carcass defects (broken bones, haemorrhages, etc.) (Petracci et al., 2011). Raw meat colour differing from the expected pale tan to pink results in consumer rejection of the product itself. The main factors affecting poultry meat colour are: i) its myoglobin content; ii) the chemical state of the haem structure; and iii) the ph. The myoglobin content in chicken meat basically depends on genotype, muscle type and age at slaughter. However, since similar muscle characteristics and age at slaughter are found between the modern fast growing hybrids, the differences were found only when white (i.e. breast) and dark (i.e. leg, wing) meats are compared. In detail, considering breast meat, the postmortem muscle acidification is the major factor responsible for fresh meat colour variations in broilers. Muscle ph has been shown to be primarily related to the biochemical state of the muscle at slaughter and following rigor mortis development. In addition, besides a higher ultimate ph is associated with darker meat, lower ph values are associated with lighter meat. In the extremes, high and low ph meats are often characterized by being dark, firm, and dry (DFD like) and pale, soft, and exudative (PSElike), respectively. Both of them associated with poor functional properties or, at the very last, considered major contributing factors to product variation (Fletcher, 2002). Especially PSElike meat has been the major source of breast meat downgrading at least before the appearance of myodegeneration related abnormalities. The occurrence of PSE like condition in chickens can be associated with both the rapid glycolytic process taking place during postmortem time (ph lower than 6.0 just after few minutes after the death) and the extensive acidification leading to achievement of extremely low ultimate ph values (< ) (Petracci et al., 2015). The genetic selection for increased growthrate and breast yield has resulted in an altered intercompartmental regulation of cations within the muscle cells of modern chicken hybrids, reflecting an adaptive response to an augmented metabolic demand (Sandercock et al., 2003). In addition, an increased stress level of broilers may exacerbate these issues and underlie additional quality defects such as PSE meat. Indeed, it has been generally recognized that stressors during preslaughter phases (i.e. catching, transportation and lairage) exert detrimental effects on meat quality and exposure to high temperatures may exacerbate such negative effects. If compared to their genetic predecessors, fastgrowing and heavier birds have been found to be more susceptible to heat stress because of their reduced thermoregulatory capacity. Moreover, acute heat stress has been demonstrated to increase superoxide free radical production in chicken skeletal muscle. Thus, this mechanism may be responsible for both the heat stressinduced muscle damage and the changes in muscle and meat quality observed in broilers (Petracci et al., 2015). On the other hand, genetic studies suggested that breast yield is highly positively correlated with ultimate ph. The ultimate ph of breast muscle is mainly determined by the glycolytic potential which is associated with glycogen stores at slaughter (Berri et al., 2007). Le BihanDuval et al. (2008) found a strong genetic correlation (very close to 1) between breast muscle glycolytic potential and phu. Therefore, since these studies evidenced that it is possible to select broilers with a higher phu without any adverse effect on growth rate and breast yield, this strategy can be effective in reducing the occurrence of PSE like condition in broiler breast meat (Petracci et al., 2015). On the other hand, the occurrence of DFDlike meat is favoured by depletion of glycogen stores during the antemortem period as a result of prolonged feed withdrawal and transportation. Glycogen deple 273

4 tion determines an increase in muscle ultimate ph leading to a reduced protein denaturation that results in meat with higher water holding capacity and darker colour. Although not all these effects are detrimental for meat, they could undoubtedly contribute to a higher variability in quality. Moreover, according to Allen et al. (1997) darker broiler breast meat with higher phu values exhibits a faster microbiological spoilage than the paler and low ph ones. Furthermore, Delezie et al. (2007) also evidenced that stocking density in transport crates often overruled the effects of feed withdrawal and transport. Tenderness Texture is probably the single most critical quality parameter associated with consumers ultimate satisfaction with poultry meat products. The two major factors contributing to poultry meat tenderness are the maturity of the connective tissue and the contractile state of myofibrillar proteins. As the modern broiler industry developed and began to dominate the chicken meat market, the issue of an agerelated toughness (resulting from connective tissue cross linking) has virtually disappeared (Fletcher, 2002). However, the market shift towards further processed products has pushed processors involved in the production of boneless breast meat towards not only quality and uniformity, but also efficiency. If the carcass is cutup and the breast removed from the carcass prior to the completion of rigor mortis, muscles fibers will contract and shorten unimpeded by the normal skeleton restrains resulting in less tender meat. It was extensively argued that a minimum of 4 to 6 h ageing is required to allow the breast muscles to complete rigor development before cutting it up without any excessive toughening. Thus, since carcasses usually complete the waterimmersed or air chilling at about 1.5 h and 23 h postmortem, respectively, additional 2.5 to 4.5 h of refrigerated storage are needed before deboning (Petracci et al., 2011). Undoubtedly, because ageing is an expensive process with the majority of the costs being associated with the loss in meat yield and energy extensive research has been done to develop slaughter methods which allow to accelerate the onset and resolution of rigor mortis (Fletcher, 2002). Among them, electrical stimulation of carcasses immediately after death reducing the aging time and accelerating the energy depletion in rigor mortis development seems the most promising technique underling a reduced shorteningrelated toughness of early deboned meats. In addition, some of the electrical stimulation systems have the additional effect of inducing physical disruption within the muscle tissue, reducing therefore its integrity (Petracci et al., 2011). Emerging abnormalities as related to myodegeneration In recent years, a new group of muscle abnormalities characterized by myodegeneration has appeared. This group includes manifestation of white striations parallel to muscle fibres mainly occurring on the ventral surface of breast fillets (white striping, Kuttappan et al., 2009), myodegeneration of anterior latissimus dorsi (Zimmerman et al., 2012), woody breast condition (often associated with white striping) where muscles are visually hard, out bulging and pale (Sihvo et al., 2014) and poor cohesion of meat or spaghetti meat abnormality (tendency toward separation of muscle fiber bundles) (Sirri et al., 2016). Histopathological evaluations of skeletal muscles affected by these abnormalities exhibited similar features including the presence of degenerative and atrophic fibres associated with loss in cross striations, variability in fibre size, floccular/vacuolar degeneration and lysis of fibres, mild mineralization, occasional regeneration (nuclear rowing and multinucleated cells), mononuclear cell infiltration, lipidosis, and interstitial inflammation and fibrosis. However, it has been currently shown that the incidence of degenerated fibers is very high also in breast muscle without any macroscopic lesions such as white striped and woody breast (Mazzoni et al., 2015; Soglia et al., 2016). In addition, it is interesting to note that the presence of similar histological lesions (i.e. hypercontraction, mononuclear cell infiltration, alterations in cell membrane integrity and the loss of myocellular constituents) in fastgrowing chickens has been reported in several works since the 90s (Mahon, 1999). Therefore, the existence of a common myodegenerative process leading to the separate or combined emergence of all the muscle abnormalities may be hypothesised. Nowadays, the occurrence of abnormalities affecting breast fillets (whitestriping, woodybreast and spaghetti meat) which is the most valuable part of broiler carcass, causes increased downgrading of meat because of the reduced visual appearance, sensory and technological quality (when used for further processing) of meat, especially for woody breast abnormality. Overall, the incidence rates of these abnor 274

5 malities is alarmingly and seems no longer sustainable for the poultry industry. These abnormalities appear only in fastgrowing broiler hybrids, so it is therefore clear that direct and indirect promoting causes can be found in pectoral muscle hypertrophy and high growth rate. Until now, several studies have been conducted in order to identify the possible factors involved especially in the occurrence of white striping (Table 2). It is clear that the incidence is higher in flocks belonging to highbreast yield hybrids and increases with increasing growth rate and weight at slaughter. On the other hand, males seem more prone to develop especially woody breast abnormality (Trocino et al., 2015). Otherwise, since an overall reduction in the occurrence of muscle abnormalities was observed only as an indirect consequence of reduced growth rate and/or slaughter weight, the dietary and prophylactic treatments seem not to be directly associated with the incidence of emerging abnormalities. As a consequence, no effective management and dietary solutions are available to mitigate the occurrence of these abnormalities without negatively affecting live production traits even if it was recently claimed that there is a strong nongenetic component for all the breast muscle myopathies (Bailey et al., 2015). Factor Genotype Table 2 Factors affecting occurrence of emerging abnormalities as related to myodegeneration. Body weight at slaughter Growth rate Gender Crude protein level Vitamin E Selenium Lysine Larginine Early dietary restriction Full dietary restriction Coccidiosis control Occurrence High > Standard breastyield High > Low Fast > Slow Males > Females Males = Females High > Low 1 High > Low 1 Ad libitum > Feed restricted 1 Anticoccidial Vaccination Anticoccidial > Vaccination 1 References Petracci et al. (2013), Lorenzi et al. (2014), Trocino et al. (2016) Lorenzi et al. (2014), Kindlein et al. (2015), Ferreira et al. (2016) Kuttapan et al. (2012a, 2013) Lorenzi et al. (2014) Trocino et al. (2015) Kuttapan et al. (2012a) Kuttapan et al. (2012b) Ferreira et al. (2016) Cemin et al. (2016) Christensen et al. (2015) Trocino et al. (2015) Livingston et al. (2016) Unpublished data, Dalle Zotte et al. (2015) 1 changes related to decrease of growth rate and/or body weight at slaughter. Conclusions The evolution of the chicken meat market has increased the relative importance of quality parameters that affect the appearance (in terms of absence of visual defects and abnormal colours) and the sensory profile with special emphasis to tenderness. However, during the last decades, the genetic characteristics of current fastgrowing hybrids have certainly favoured the occurrence of a high number of abnormalities that are increasing the meat downgrading rates for fresh market retailing and decreasing the nutritional, sensory and technology proprieties of raw meat materials used as raw material for further processing. Therefore, it seems that poultry industry cannot longer postpone a closer view of problems related to the proper muscle growth and then of the quality of resulting meat when selecting genotypes for broiler production. References ALLEN, C.D., RUSSELL, S.M. and FLETCHER, D.L. (1997) The relationship of broiler breast meat colour and ph to shelflife and odour development. Poultry Science 76:

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