Ciencia animal

 

Effect of slaughter weight on fat composition in lacha lambs

 

Efecto del peso de sacrificio sobre la composición de la grasa de corderos de raza lacha

 

Alberto Horcada–Ibáñez¹*, María J. Beriain–Apesteguía², Julia Chasco–Ugarte², Gregorio Indurain–Báñez³, Antonio Purroy–Unanua²

 

¹ Escuela Universitaria de Ingeniería Técnica Agrícola, Universidad de Sevilla. 41013. Carretera Utrera, km 1, Sevilla, Spain. *Autor responsable: (albertohi@us.es).

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

³ Neiker Tecnalia, Departamento de Producción Animal, Campus de Arkaute. 01080. Vitoria, Spain.

 

Recibido: Agosto, 2009.
Aprobado: Julio, 2010.

 

ABSTRACT

Due to the fact that domestic consumers prefer low fat meat, the lamb production system in Spain is based on slaughtering low weight animals (12 to 24 kg). Lambs in other European countries are slaughtered at higher weights because consumers prefer more intense flavored and fatter meat. Based on evidence that slaughter weight affects fat characteristics, the effect of slaughtering weight on the fatty acid profile of omental (OM), mesenteric (MES), kidney knob (KK), subcutaneous (SC), intermuscular (IN) and intramuscular (IM) fat in Lacha breed lambs was studied. Forty–two male Lacha lambs were raised and slaughtered at 12 (L12), 24 (L24) and 36 kg (L36) live weight. L12 lambs were fed only on mother's milk until slaughter, one month old. L24 and L36 lambs were fed ad libitum on commercial feed and barley straw from weaning (around 16 kg live weight) until slaughter. Adipose tissue samples were taken at the slaughterhouse, vacuum–packed, frozen and stored. Fatty acid methyl esters were analyzed by capillary gas chromatography, and the relative amounts were recorded. The results were analyzed using ANOVA and Tukey's test (p<0.05). Concentrations of C12:0 and C14:0 decreased after weaning due to the fact that the lambs no longer ingested mother's milk. In all fat depots, L24 showed the highest saturated fatty acid concentration, mainly due to the higher C18:0 content. An increase in C18:1n–9cis in heavier lambs (L24 to L36) was observed, associated to fat reserve accumulation processes. The analysis of these data suggests an increase in desaturation enzymes activity among higher–weight lambs, when ruminal activity was initiated. The most suitable n–6/n–3 and polyunsaturated/saturated ratio in muscle fat for preventing cardiovascular disease were observed in L24 lambs.

Key words: fat deposit, fatty acids, lambs.

 

RESUMEN

Debido al hecho de que los consumidores domésticos prefieren carne baja en grasas, el sistema de producción de corderos en España se basa en el sacrificio de animales de bajo peso (12 a 24 kg). Los corderos en otros países europeos se sacrifican a pesos mayores porque los consumidores prefieren carne de sabor más intenso y con más grasa. Debido a que el peso del sacrificio afecta las características de grasa, se estudió el efecto del peso de sacrificio de corderos de raza Lacha en el perfil de ácidos grasos de la grasa omental (OM), mesentérica (MES), pelvicorrenal (PVR), subcutánea (SC), intermuscular (IN) e intramuscular (IM). Cuarenta y dos corderos machos de raza Lacha se criaron y sacrificaron con un peso vivo de 12 (L12), 24 (L24) y 36 (L36) kg. Los L12 se alimentaron únicamente con leche materna hasta su sacrificio, a un mes de nacidos. Los corderos L24 y L36 se alimentaron ad libitum con alimento comercial y paja de cebada desde el destete (alrededor de 16 kg de peso vivo) hasta el sacrificio. En la planta de sacrificio (matadero) se tomaron muestras de tejido adiposo; se empacaron al vacío, se congelaron y se almacenaron. Se analizó los ésteres metílicos de los ácidos grasos por cromatografía de gases capilar y se registraron las cantidades relativas. Los resultados se analizaron usando ANDEVA y la prueba de Tukey (p<0.05). Las concentraciones de C12:0 y C14:0 disminuyeron después del destete debido a que los corderos ya no consumían la leche de la madre. En todos los depósitos de grasa, L24 mostró la concentración más alta de ácidos grasos saturados, principalmente debido al mayor contenido de C18:0. Se observó un aumento en C18:1n–9cis en corderos más pesados (L24 a L36), asociado a los procesos de acumulación de reservas de grasa. El análisis de estos datos sugiere un aumento en la actividad de enzimas de desaturación en los corderos de mayor peso, una vez iniciada la actividad ruminal. La tasa más adecuada de n–6/n–3 y poliinsaturado/saturado en grasa muscular para prevenir la enfermedad cardiovascular se observó en corderos L24.

Palabras clave: depósitos de grasa, ácidos grasos, corderos.

 

INTRODUCTION

Omental (OM), mesenteric (MES) and kidney knob (KK) fat deposits in sheep are important because they provide energy reserves, mechanical protection to several organs and regulate body temperature. At the same time, intramuscular (IM), intermuscular (IN) and subcutaneous (SC) fat depots contribute to the development of meat sensory attributes (Field et al., 1983) and are a source of essential fatty acids (FA) for humans. The presence of high levels of polyunsaturated FA (PUFA) is related to lower fat stability because FA are very labile to oxidation. Although oxidative processes are important in developing meat characteristic such as aroma during cooking (Wood et al., 2004), they also can cause rancidity, deterioration of meat quality and changes in carcass appearance.

The consumption of large amounts of animal fat has been related to the incidence of human diseases. Wolfram (2003), recommend a n–6/n–3 fatty acids ratio around 4 to reduce the risk of cardiovascular disease. Lamb fat is presented to consumers as having a n–6/n–3 ratio, within the recommended limits (Enser et al., 1996). However, its high saturated FA (SFA) content is of concern to dieticians. Researchers are trying to lower these levels of SFA (mainly C16:0), which may harm to human health. Factors such as sex, breed, environment and breed–production systems affect fat composition in lambs (Juarez et al., 2008). In addition, slaughter weight, age and body fatness can contribute to a variation in the FA composition of fat depots (Wood et al., 2004).

Due to the importance of FA contents in the quality of both, the carcass and the meat, knowledge of the total amount and the quality of fat in the most valuable joints is of great interest. In Mediterranean countries of Europe, consumers of lamb meat reject carcasses with high fat content because of the meat's intense flavor (Sañudo et al., 2000). In Spain, therefore, lambs are slaughtered at 12 kg live weight and one month old (Lechal lamb) or at 24 kg live weight and three months old (Ternasco lamb). Conversely, in Central and Northern Europe consumers prefer heavier, fatter lambs (36 kg) because the meat has more intense flavor. Therefore, Mediterranean producers interested in expanding their markets to Central and Northern Europe, have increased the carcass weight of certain breeds produced in Spain. Thus, the objective of the present study was to evaluate changes in the FA profile of Lacha lambs at three live weights (12, 24 and 36 kg).

 

MATERIALS AND METHODS

Animal management

Forty–two Lacha male lambs were born and raised at the experimental flock of Remendía Technical Institute and Management of Ruminants in Navarra (962.3 m altitude, Northern Spain). The lambs were slaughtered at 11.4±0.70 kg (L12), 24.6±1.41 kg (L24) and 35.3±1.67 kg (L36) live weight and at around 25, 87 and 131 day old (Table 1). Fifteen L12 suckling lambs were fed exclusively on ewe's milk and slaughtered at weaning. Fifteen L24 and 12 L36 lambs were weaned at 16 kg and fed ad libitum with concentrate (17.0 % crude protein, 5.1 % crude fiber, 4.0 % total fat, 39.0 % starch, 1.2 % Ca, and 0.4 % P) and barley straw. The lambs' mothers were fed on a mixture of native pasture (Lolium perenne, L. multiflorum, Trifolium repens and Festuca arundinacea) and commercial feed (14.0 % crude protein, 7.0 % crude, 3.5 % total fat, 33.0 % starch, 1.8% Ca, and 0.3 % P). All animals received a mineral vitamin supplement and water. The procedures were conducted according to Council Directive 86/609/EEC (European Communities, 1986) guidelines on the protection of animals used for experimental and other scientific purposes.

Slaughter procedure

Fasting lambs (12 h) were taken to a certified slaughterhouse. Carcasses were kept at room temperature for 6 h, stored at 4 °C, weighed 24 h after slaughter, and were graded for fatness using the EEC Regulation No. 461/93 (European Union, 1993) system for lamb carcasses. The degree of fatness is related to the amount of subcutaneous and internal fat. Fat degrees ranged from 1 (not greasy) to 15 (very greasy).

After slaughtering, adipose tissue samples were taken from the OM (midportion of the great omentum), 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). Samples were homogenized vacuum–packed, and stored at —30 °C until analysis.

Fatty acid analysis

OM, MES, KK, SC, IN and IM lipids were extracted with the method reported by Bligh and Dyer (1959) using chloroform/methanol. Extracted lipids were stored at —80 °C until methylation. Total fatty acid composition was analyzed by derivatizing FA to methyl esters (FAME) according to Eichhorn et al. (1985). To each vial 200 μL internal standard (eicosanoic acid, C21:0) were added. FAME were stored at —80 °C until analysis.

FAME were separated and quantified in a capillary gas chromatograph (5890–II, Hewlett–Packard Company, USA) equipped with a flame ionization detector and, a HP 7683 automatic sample injector. The operating conditions were as follows: SPTM–2560 fused silica capillary column (100 m, 0.25 mm i.d., 0.2 μm film thickness; Supelco Inc, USA); the injector (splitless) and detector were both heated to 230 °C; carrier gas flow (He) 1 mL ¹ with column head; initial column temperature 100 °C to 158 °C at 3 °C min^{— 1}, 158 °C to 165 °C at 1 °C min^{— 1} with a 10–min final hold; 1 μL of solution was injected. Data were collected and detector signals integrated using a HP 3365 series II Chem–station software (Hewlett–Packard Co., USA). Peak identification was based on retention times of reference compounds [Nu–Chek GLC reference standard 534 (Nu–Chek Prep, Inc., USA)].

Statistical methods

The relative amount of each FA (% total FA) was calculated as a ratio of total FA to a FA peak area. Saturated FA (SFA), monounsaturated (MUFA), polyunsaturated (PUFA), PUFA/ SFA and n–6/n–3 were also calculated. Activities of D9 desaturase were estimated as C18:1n–9cis/(C18:1n–9cis +C18:0) (Malau–Aduli et al., 1998). A one–way ANOVA was carried out to study the effect of slaughter weight on carcass fat characteristics and the FAME profiles of OM, MES, KK, SC, IN, and IM fat depots, using the following statistical model:

Y_ijk = μ + W_i + D_j + W_i × D_j + e_ijk

where: Y_ijk = FAME percentage or carcass characteristic; μ = mean value; W_i = fixed effect of weight (i = 1: 12 kg; i = 2: 24 kg; i = 3: 36 kg); Dj = fixed effect of fat deposit location (j = 1: OM; j = 2: MES; j = 3: KK; j = 4: SC; j = 5: IN; j = 6: IM); Wi × Dj = interaction between weight and fat depot; e_ijk= random residual. Tukey's test (p<0.05) for comparing FAME means on three live weights was applied in all cases. Statistical analyses were carried out using SPSS (SPSS V.11.5, SPSS Inc., USA).

 

RESULTS AND DISCUSSION

In Table 2 it is shown the main FA on OM, MES, KK, SC, IN, and IM lipid profile fat depots from Lacha lambs slaughtered at 12, 24, and 36 kg live weight. Significant differences in fat compositions were recorded for the six fat depots and three weights. Oleic acid (C18:1n–9cis) was the most abundant, accounting for more than 40 % total FA. Palmitic (C16:0) and stearic acids (C18:0) together accounted for 34 to 47 % total FA in all fat depots. This fat composition was in agreement with the results reported by Castro et al. (2005).

There was a decrease in the lauric acid (C12:0) and myristic acid (C14:0) contents as slaughtering weight increased for all fat depots (p<0.001). Similar results were found by Diaz et al. (2005) for SC and IM deposits in Rasa Aragonesa lambs. Higher C12:0 and C14:0 contents was observed in young suckling lambs (L12) fed only with ewe's milk which is rich in short chain FA, such as C12:0 and C14:0 (Bargo et al., 2006).

L36 lambs had higher C18:0 and C18:1n–9cis contents than L12 (p<0.001). A general increase in C18 FA content, observed after weaning, could be due to the fact that lambs begin to consume concentrate and straw, both containing long–chain FA. However, an increase in C18:0 and a decrease in C18:1n–9cis were observed in fat depots after weaning (L12 to L24) (Table 2). This observation is related to low Δ⁹ desaturase activity in L24 lambs and changes in the supplement of concentrate and straw. Conversely, heavier (L36) lambs showed higher C18:1n–9cis and lower C18:0 content than L24. The analysis of these results indicated that an increase of C18: 1n–9cis at the expense of C18:0 in heavier lambs was due to the increase in Δ⁹ desaturase activity, observed in L36. This result is in agreement with that reported by Hocquette et al. (1998) who show a higher C18:0 desaturation enzyme activity in heavier animals.

The n–6 fatty acid in fat depots in L12 to L36 increased (around 70 %), although there was only a 7 % decrease in muscle fat (IM). Conversely, a 60 % decrease n–3 in fat depots was observed. According to Nürnberg et al. (1998), higher concentrations of C18:2n–6 and lower C18:3n–3 in heavier animals are due to the concentrate feed composition supplied to the animals.

The n–6/n–3 ratio is a good indicator of the fat nutritional quality (Enser et al., 2001); the suitable balance for n–6/n–3 recommended by COMA (1994) is 4. In our study, the n–6/n–3 ratio in Lacha lambs agrees with that reported by Cañeque et al. (2005) in Manchego lambs. In all fat depots, a general increase of n–6/n–3 was observed in heavier lambs (Table 2). Regardless of other fat depots, intramuscular fat from L36 showed a more unfavorable n–6/n–3 ratio than L12 (11.91 vs. 6.88; p<0.001). These differences could be a consequence of fatty acid composition in the diet, since C18:2n–6 predominates in concentrate feed supplied to L36 (Raes et al., 2004). According to French et al. (2000), these differences could be due to the FA composition of the mother's diet, since C18:3n–3 is the major fatty acid in grass lipids supplied to ewes.

Changes in the FA ratios (SFA, MUFA and PUFA) between L12, L24 and L36 were also observed (Figure 1).

There was a higher SFA content in lambs slaughtered at 24 kg (Figure 1), whereas L36 showed lower SFA content and higher carcass fatness scores (Table 1). The correlation between carcass fatness score and total SFA in heavier lambs was negative (r = — 0.41; p<0.05). The analysis of these results indicate that fatness is not related to increased SFA as reported by Cañeque et al. (2005), and the influence of factors such as weaning and diet, is important. In general, the proportion of SFA was the same at L12 and L36; however, the SFA lipid profile was different for both slaughter weights. For L12 short–chain FA (C12:0 and C14:0) concentration was higher than in L36, while C18:0 content was highest in L36 (approximately 20 %). Regarding nutritive value, short–chain FA consumption is associated with an increased plasma cholesterol and low density lipoprotein (LDL) levels, linked to a higher coronary disease risks (Grundy, 1987). An increase of 7.4 % in C16:0 was found in meat fat between L12 and L36; this fatty acid is also harmful for human health. No differences in the C16:0 were observed between L12 and L36 in the remaining fat depots.

In all fat deposits, the lowest MUFA content (p<0.001) was in L24 and the highest in L36; this association between MUFA content and lamb' fatness was reported by Castro et al. (2005). Lower and higher carcass fatness scores were also observed in L24 and L36 (Table 1). The analysis of these results suggest that different growth patterns take place for lambs slaughtered at 12, 24 and 36 kg live weight; besides, according to Huerta–Leindez et al. (1996), the state of fatness affects fatty acid composition. Thus, L24 metabolisms of lambs was focused on muscular growth (leaner carcasses) and associated to lower C18:1n–9cis levels, whereas fat increase observed from L24 to L36 was due mainly to increased C18:1n–9cis (around 10 %).

PUFA content was higher in the IM than in any other fat depots (Figure 1), a finding similar to that reported by Horcada et al. (2009) in Rasa Aragonesa lambs. In OM, MES, KK and SC fat depots, higher PUFA content was observed in heavier lambs which, in contrast, showed lower muscle fat PUFA content. Generally, the type of diet tend to cause similar effects on FA composition, regardless of the anatomical location of fat depots (Bas et al. 2000). However, forage consumption in heavier lambs affected the increased PUFA concentration found in OM, MES, KK and SC; this influence was not evident in muscle fat. Therefore, fat deposition processes in the muscle differ with respect to other anatomical locations. The difference observed from PUFA content between fat depots can be explained because fat muscle taken from different body regions may differ in their structure and metabolic characteristics (Thornton et al. 1983). The initial fat accumulation process is related to an increase in the number of adipocytes (Flint and Vernon, 1993). PUFA are basically present in cell membrane phospholipids, and in OM, MES, KK and SC increased PUFA content were observed in 24 kg live weight lambs. This, fat accumulation processes takes place from 24 kg live weight onwards, and it was assumed that at this body weight there is an increased number of adipocytes. The lowest PUFA in meat fat was observed in L36, as IM accumulation starts at later stages. In this respect, Diaz et al. (2005) found that PUFA in intramuscular lamb fat is similar throughout the carcass.

In general, PFA/SFA ratios were in agreement with those reported by Banskalieva et al. (2000) for lambs (0.07–0.25). The adequate PUFA/SFA balance suggested by Hunty (1995) to avoid coronary heart disease risk is 0.4. In our study, 0.03–0.08 ratios were observed in most fat depots, whereas IM fat showed a more favorable ratio (0.13–0.14). In L24 an increased PFA/SFA ratio was observed in fat depots (25 % range); conversely, muscle fat ratio showed a decrease (7 %). This was because fat deposition follows different patterns between muscle and other fat deposits. Thus, muscle phospholipid content is relatively constant and contains mainly PUFA, while levels of the neutral lipids, present primarily in fat depots, depend on fat content (Sharma et al., 1987). The decrease of SFA levels in muscle fat was higher than that of PUFA as fatness increased, leading to a reduction in PUFA/SFA ratio. However, in other fat depots, decrease in SFA levels was less pronounced than PUFA decrease as fatness increased; as a result, PUFA/SFA ratio increased.

 

CONCLUSIONS

Fatty acid profiles from different fat depots in Lacha lambs were dependent on weight, changes in diet and carcass fatness. The most important changes in fat depots composition take place after lambs are weaned, mainly as a consequence of the diet change, from mother' milk to solid food, and the onset of ruminant activity. Fat storage reserve processes in heavier lambs are mainly due to an increase in monounsaturated fatty acid, mostly oleic. As far as polyunsaturated fatty acids are concern, storage patterns differ according to the specific fat depot. Polyunsaturated fatty acid composition in muscle fat remained in similar proportion in all the studied lamb types, whereas heavier lambs showed an increase in fat depots. From a nutritional point of view, the slaughter of Lacha lambs at 24 kg is the best advisable option because this weight is associated to n–6/n–3 and polyunsaturated/saturated ratios, favorable for human diets.

 

ACKNOWLEDGEMENTS

The authors thank the Interministerial Science and Technology Board (CICYT) for funding this study.

 

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