Ciencia animal

 

Effect of sex and fat depot location on fat composition of Rasa Aragonesa lambs

 

Efecto del sexo y del depósito graso en la composición de la grasa de los corderos de raza Rasa Aragonesa

 

Alberto Horcada–Ibáñez^1*, María J. Beriain–Apesteguía², Guadalupe Lizaso–Tirapu² , Kizkitza Insausti–Barrenetxea² , Antonio Purroy–Unanua²

 

¹ Escuela Universitaria de Ingeniería Técnica Agrícola, Universidad de Sevilla, Carretera Utrera Km. 1.41013. Sevilla, Spain. *Autor responsable: (albertohi@us.es).

² Escuela Técnica Superior de Ingenieros Agrónomos, Universidad Pública de Navarra, Campus Arrosadía. 31006. Pamplona, Spain.

 

Received: August, 2008.
Approved: May, 2009.

 

ABSTRACT

The quality of lamb fat is important in sheep carcass markets because it affects the degree of acceptance by consumers. In Spain, consumption of beef and lamb is important and young lambs are highly valued. In addition, the precocity of the female determines slaughter at younger ages than males, to avoid excessive fat. Therefore, the effect of sex and fat depot location (omental, mesenteric, kidney knob, subcutaneous, intermuscular and intramuscular) on the fatty acid composition in 30 Rasa Aragonesa lambs (15 males and 15 females) of 24 kg live weight and three months of age, were studied. Oleic, palmitic and stearic fatty acids were the main fatty acids making up the fat in all the fat depots from lambs. There were no significant differences (p>0.05) in the fat composition between male and female lambs, even though females had significantly more fat than males in all the fat depots considered (p<0.05). Criteria associated with sex did not affect the composition of fat. Ratio of fatty acids n–6/n–3 values were in the range of recommended average values (< 4), except for intramuscular fat, where values marginally higher were observed (5.5). The internal (omental, mesenteric and kidney knob) fat depots contained more saturated fatty acids than the edibles depots (subcutaneous, intermuscular and intramuscular) (p<0.05). Consequently, the internal fat depot was firmer than fat from the surface of the carcass. Fatty acid of intramuscular fat depot had the highest content of PUFA of all fat deposits studied.

Key words: Fat depot, fatty acid, lamb, Rasa Aragonesa breed, sex.

 

RESUMEN

La calidad de la grasa de cordero es importante en los mercados de canales ovinas porque afecta al grado de aceptación de los consumidores. En España, el consumo de carne de res y cordero es importante, y los corderos jóvenes son muy valorados. Además, la precocidad de las hembras determina su sacrificio a edades más tempranas que los machos para evitar su excesivo engrasamiento. Por tanto, se estudiaron el efecto del sexo y del depósito de grasa (omental, mesentérica, pelvicorrenal, subcutánea, intermuscular e intramuscular) sobre la composición de ácidos grasos en 30 corderos de raza Rasa Aragonesa (15 machos y 15 hembras) de 24 kg de peso vivo y tres meses de edad. Los principales ácidos grasos que componen la grasa en todos los depósitos de grasa de los corderos fueron los ácidos oleico, palmítico y esteárico. No hubo diferencias significativas (p>0.05) en la composición de la grasa entre los corderos machos y hembras, a pesar de que las hembras presentaron significativamente más grasa que los machos en todos los depósitos considerados (p<0.05). Los criterios asociados con el sexo no afectaron a la composición de la grasa. La relación de ácidos grasos n–6/n–3 se encontró en el rango de los valores promedio recomendados (<4), excepto la grasa intramuscular, donde se observaron valores más altos (5.5). Los depósitos de grasa interna (omental, mesentérica y pelvicorrenal) mostraron más ácidos grasos saturados que los depósitos comestibles (subcutáneo, intermuscular e intramuscular) (p<0.05). En consecuencia, el depósito de grasa interna presentó grasa más firme que el de la superficie de la canal. Los ácidos grasos del depósito intramuscular presentó el contenido más alto de ácidos grasos poliinsaturados de todos los depósitos de grasa estudiados.

Palabras clave: Depósito de grasa, ácido graso, cordero, Rasa Aragonesa, sexo.

 

INTRODUCTION

Quantity and composition of fat influence I both carcass and lamb meat quality Pérez et al., 2002). The firmness of fat varies afceording to unsaturated fatty acid (UFA) and saturated fatty acid (SFA) content. Also, high proportions of UFA decrease directly the fat melting point (Berthelot et al., 2001) causing, at the same time, the fat to spoil and turn rancid more readily (Wood, 1984). In contrast, the structure of the SFA helps make the fat firmer. This is particularly important in the case of the subcutaneous fat, because firm covering fat helps preserve the carcasses during cold storage and aging (Wood et al., 2004). Besides, consumers reject fats with high SFA content, because human consumption of these fatty acids is associated with the incidence and onset of cardiovascular diseases (Napolitano et al., 2002).

Several studies have corroborated the notion that a range of factors, including animal breed, age, sex and weight at slaughter, diet, carcass fatness, etc., affects the properties of fat (Wood et al., 2004; Juárez et al., 2007). The fat in the intramuscular, intermuscular and subcutaneous depots greatly affects meat quality because these fat depots contribute to aroma development and to meat juiciness during cooking (Wood, 1984). Commercial terms designed to optimize the fat content of lambs include slaughtering females at an earlier age than males to prevent the carcasses from accumulating excessive amounts of fat because excess fat is the reason suspected for rejection by the Mediterranean consumer.

Under these circumstances, in Spain there is a typical system of lamb production, according to final purpose. Rasa Aragonesa breed is raised for lamb production purposes, and they are popular with Spanish consumers on account of their quality: they are sold under the "Ternasco de Aragón" Protected Geographic Indication label. "Ternasco de Aragón" lambs are slaughtered at 22–24 kg live weight and carcass weight of 11–12 kg, around 90 d after birth. These lambs remain with the mother for approximately one and a half months and consume exclusively mother's milk. After weaning, the lambs are given concentrate food and straw cereal ad libitum until slaughter.

The objective of the present study was to quantify the influence of sex and fat location on the fat composition in various fat depots of Rasa Aragonesa breed lambs slaughtered at commercial weight.

 

MATERIALS AND METHODS

Production system and sampling

Thirty single lambs (15 males and 15 females) of the Rasa Aragonesa breed were used; their mothers (30) grazed on similar native pastures and were fed the same commercial concentrate (14.00 % protein; 7.00 % fiber; 3.50 % total fat; 33.00 % starch; 1.80 % Ca; 0.30 % P) at the Technical Institute and Management of Ruminants in Navarre (North Spain) at 962.3 m above sea level and 120 mm average annual precipitation. After weaning, at 45–50 d of age, the lambs were fed ad libitum a concentrate diet (17.00 % protein; 5.10 % fiber; 4.00 % total fat; 39.00 % starch; 1.20 % Ca; 0.40 % P) and barley straw, until slaughter. A mineral vitamin supplement and water were available.

Male and female lambs were slaughtered at 24.5±0.57 and 23.1±0.72 kg live weight and 89±8 and 91±7 d of age. Fasting lambs were taken to a slaughterhouse certified by the European Union, and slaughtered in compliance with health and sanitary regulations. Carcasses were weighed 24 h after slaughter and their conformation score and carcass fatness were graded according to the European classification system laid down in EEC Regulation No. 461/93 (European Union, 1993), the EUROP conformation scale (converted to a 15 point scale) and the carcass fatness scale (converted to a 15 point scale). Back fat thickness was measured with callipers on the right and left half carcasses, cutting at a point located 4 cm equidistant from the backbone and the last rib.

Immediately after slaughter all dissectable adipose tissue in the omental (OM), mesenteric (MES) and kidney knob (KK) fat depots, was removed and weighed in the slaughterhouse at 10 °C. Additionally, adipose tissue samples were taken from the OM (midportion of the great omentuni), MES (midportion of the rectum), KK (left kidney), subcutaneous (SC) [base of the tail], intermuscular (IN) [between the sternum and the pectoral muscle] and intramuscular (IM) [longissimus dorsi pars lumborum muscle]. The fat was homogenized and vacuum–packed and stored frozen at —30 °C until analysis.

The IM fat from the longissimus dorsi muscle was extracted using quantitative lipid extraction with diethyl ether (Soxhlet method), and expressed as percentage of the total muscle weight.

Preparation of fatty acids

The lipid content of all the fat depots was extracted according to the method described by Bligh and Dyer (1959) with a mixture of chloroform and methanol (1:2) and C1K (0.88 % in water). Fatty acids methyl esters were obtained using a solution hexane and KOH in methanol (Eichhorn et al, 1985). An amount of 200 μL of internal standard (heneicosanoic acid, C21:0) was added to each vial. Fatty acid methyl esters were stored at —80 °C until chromatographic analysis.

Chromatographic analysis

Fatty acid methyl esters were analyzed using a model HP 5890–11 capillary gas chromatograph (Hewlett–Packard Company, USA) equipped with a flame ionization detector and a model HP 7673 automatic injector. The fatty acid methyl esters were separated on a 100 m Supelco SPTM–2560 fused silica capillary column (Supelco Inc., USA), with an internal diameter of 0.25 mm and a film thickness of 0.20 μm. The injector (splitless) and detector were both heated to 230 °C. The column oven temperature program was 100 °C to 158 °Cat 3 °C min^–1, 158 °C to 165 °Cat 1 °C min^–1 with a 10 min final hold. The helium carrier gas flow rate was 1 mL min , with column head pressure set to 30 psi. Injected sample size was 1 μL. Data were collected and detector signals integrated using HP 3365 series II Chem–station software. Peak identification was based on the retention times of reference compounds [Nu–Chek GLC reference standard 534 (Nu–Chek Prep, Inc., USA)].

Description of data and variables

Individual fatty acids content was expressed as a proportion of total amount of the fatty acids identified. In addition, various indices such as saturated (SFA), mono–unsaturated (MUFA), poly–unsaturated (PUFA), n–6/n–3 and desirables fatty acid (MUFA+PUFA+C18:0) (Huerta–Leidenz et al, 1991), were presented.

Statistical analysis

Data were analyzed using the SPSS program, version 11.5 (2003) [SPSS Inc., USA]. Analysis of variance (ANOVA) was used to study the effect of sex and fat depot location (OM, MES, KK, SC, IN, and IM) on the lipid composition, using the statistical model:

where Y_ijk = percentage fatty acids or lipid content related carcass characteristic; μ = least squares mean value; S_i = fixed effect of sex (i = 1: male, i = 2: female); D_j = fixed effect of depot location (j = 1: OM, j = 2: MES. j = 3: KK, j = 4: SC, j = 5: IN, j = 6: IM); S_i × D_j = interaction between the sex effect and the fat depot location effect; e_ijk = random residual. Tukey's test for comparing means was applied in all cases.

 

RESULTS AND DISCUSSION

Characteristics of the lamb carcasses

The carcass characteristics of the male and female Rasa Aragonesa lambs are shown in Table 1. There were no significant differences (p>0.05) in carcass characteristics between male and female lambs. All carcasses were graded in conformational category "O" and fat cover category "little fat" on the European Community scale for grading ovine carcasses. Accordingly, these lamb carcasses all fell within the category of light–weight carcasses from the Mediterranean area (Sañudo et al., 2000). Female lambs exhibited higher backfat thickness values and OM, MES, KK and IM fat than males (p<0.05). However, females were slaughtered with less live weight (23.1 kg) than males (24.5 kg) but similar age: 91 and 89 d. Therefore, higher fat contents in females could be related to the finding that females deposited larger amounts of lipids at an earlier age than males, because they are more precocious (Jacobs et al., 1972).

Effect of sex on the fatty acids composition

There were significant interaction between sex and fat depot location for some of the fatty acids (Table 2). The fatty acids for which there were no interactions are shown in Table 3 and the fatty acids for which interaction was observed are shown in Table 4.

C18:1, C16:0 and C18:0 were the main fatty acids making up the fat in all the fat depots, in concordance with findings reported for sheep by Russo et al. (1999). These were the major fatty acids quantitatively, accounting for 85 % of the total fatty acids identified. C18:1 was the principal fatty acid (almost 45 % of the total fatty acid identified) in the fat of lambs.

No significant differences in the composition of fat between males and females were observed in the lambs slaughtered at approximately 24 kg (Table 3). Malau–Aduli et al. (2000) suggest that the differences in fat composition observed in male and female ruminants might not follow the same pattern at different ages. Therefore, in weaned lambs with a functional rumen, the differences in the content of fatty acids between sexes are not so evident since the lipid fraction in food is converted by the rumen microorganisms into C18:0. But, in unweaned lambs, in which the rumen has not yet begun functioning, SFA deposition is more pronounced in females than in males, due to higher fat deposition in females (Westerling and Hedrick, 1979).

The absence of differences between SFA content in fat of males and females determines similar firmness between males and females. Therefore, only the difference in fat content observed between males and females (Table 1) determines the different appearance of carcasses. Then, in females fat and back fat thickness guarantees carcass preservation for longer time and preserves water loss.

In Table 4 there are shown the values for the effect of fat depot location on the fatty acid composition of the internal OM, MES, and KK depots and the carcass SC, IN, and IM depots in lambs, for variables showing a sex × depot location interaction. Only intramuscular fat depot differences were observed between males and females. Content of C16:0, C20:4 and PUFA was higher in females than in males (p<0.001). C18:0 proportion was higher in males as compared to females (p<0.001), as has also been reported by Madruga et al. (2006) in Brazilian Santa Inés lambs.

Effect of fat location on the fatty acids composition

The values for the effect of fat depot location on the fatty acid composition of those same depots for variables without a sex × depot location interaction, are shown in Table 5. The fatty acid composition of fat varies with the location of fat depots in the body (Banskalieva et al., 2000) and highest SFA contents tend to be found in the internal depots (Zygoyiannis et al., 1985). In our study, SFA content was higher in the OM, MES and KK depots.

The PUFA content was higher in the IM depot compared to the other fat depots, as reported by Castro et al. (2005) for 45 d old male Ojalada lambs. This was mainly brought about by the high C20:4 (Table 4) and C18:2n–6 (Table 5) contents observed in the IM fat depot. The high PUFA content found in the IM depot can be explained by the phospholipids present in the cell membranes (Enser et al., 1996). This depot contains both the fatty acids present in the cell membranes of adipocytes in the fat depot, as well as those in the muscle cells.

Ratio n–6/n–3 in OM, MES, KK, IN and SC fat depots ranged from 1.77 to 3–36 (Table 5). These values correspond with those recommended by the Committee on Medical Aspects of Food Policy (COMA 1994) in order to decrease the incidence of cardiovascular diseases in humans consuming animal fats (values <4). Moreover, IM fat was higher than 4 (5–55).

The C18:0 values in the KK depots of lambs were higher than those observed in the other fat depots (Table 4), as observed by Cramer and Marchello (1964) and Pérez et al. (2002) in Suffolk Down lambs. An increase in the melting point of KK fat is associated with higher proportions of C18:0 (Osorio et al., 2007) and firmness. Bas and Morand–Fehr (2000) found a high linear correlation (R0.90) between C18:0 and the melting point. At the same time, the C16:0 content in the KK depot was lower than in the other fat depots (Table 4). This might be an indication that elongation of 16:0 in C18:0 is affected by the location of fat depots, and specifically, that this activity is more pronounced in the KK depot.

The desirable fatty acid (PUFA+MUFA+C18:0) content was higher (p<0.001) in the KK depot than in the other depots (Table 5), basically on account of the high C18:0 content in this depot. Besides, the high amounts of desirable fatty acid observed in the IM depot were due mainly to the high proportion of PUFA in that depot.

 

CONCLUSIONS

There were no differences in fat composition between sexes in Rasa Aragonesa lambs slaughtered at three month of age. It appears that in light lambs traditionally slaughtered in Spain, the characteristics of both carcass and meat were affected only by differences in the amounts of fat accumulated in males and females. The most remarkable differences between fat depots are those related to the greater content of SFA of the most internal deposits, and the PUFA in the IM fat depot. In light lambs (24 kg live weight), the relationship of fatty acids n–6/n–3 is, in general, within the levels proposed by the COMA to prevent the onset of cardiovascular disease.

 

ACKNOWLEDGEMENTS

The authors wish to thank the Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA) [National Agricultural and Food Research and Technology Institute] for funding provided for this study under the said projects. Particular gratefulness to Mr Bernard Aurrousseau (INRA, Theix France) for contributing his knowledge.

 

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