Predicting beef cutscomposition, fatty acidsand meat quality characteristicsby spiralcomputed tomography
N. Prieto1*, E. A. Navajas1, R. I. Richardson2, D. W. Ross1, J. J. Hyslop3, G. Simm1 and R. Roehe1
1Sustainable Livestock Systems Group, Scottish Agricultural College, West Mains Road, EdinburghEH9 3JG, UK.
2University of Bristol, Division of Farm Animal Science, Langford, Bristol, BS40 5DU, UK.
3Select Services, Scottish Agricultural College, West Mains Road, EdinburghEH9 3JG, UK.
*CORRESPONDING AUTHOR: Nuria Prieto. Scottish Agricultural College (SAC), Bush Estate, EdinburghEH26 0PH, UK. Tel.: +44 131 535 3361, fax: +44 131 535 3121. ;
1
Abstract
The potential of X-ray computed tomography (CT) as a predictor of cutscomposition and meat quality traitsusing a multivariate calibration method (partial least square regression, PLSR) was investigated in beef cattle. Sirloins from 88 crossbred Aberdeen Angus (AAx) and 106 Limousin (LIMx) cattle were scanned using spiral CT. Subsequently, they were dissected and analyzed for technological and sensory parameters, as well as for intramuscular fat (IMF) content and fatty acid composition.CT-PLSR calibrations, tested by cross-validation, were able to predict with high accuracy the subcutaneous fat (R2, RMSECV = 0.94, 34.60 g and 0.92, 34.46 g), intermuscular fat (R2, RMSECV = 0.81, 161.54 g and 0.86, 42.16 g), total fat (R2, RMSECV = 0.89, 65.96 g and 0.93, 48.35 g)and muscle content(R2, RMSECV = 0.99, 58.55 g and 0.97, 57.45 g) in AAx and LIMx samples, respectively. Accurate CT predictions were found for fatty acid profile (R2 = 0.61-0.75) and intramuscular fat content (R2 = 0.71-0.76) in both sire breeds. However, low to very low accuracies were obtained for technological and sensory traits with R2 ranged from 0.01 to 0.26.The image analysis evaluated in this study provides the basis for an alternative approach to deliververy accurate predictions of cutscomposition, IMF content and fatty acidprofilewith lower costs thanthe reference methods (dissection, chemical analysis),without damaging or depreciating the beefcuts.
Keywords: computed tomography, carcass composition, beefquality, technological parameters, sensory characteristics, fatty acids.
Introduction
Consumers preferleaner meat withthe minimal fat level required to maintain juiciness and flavour, a preference thought to be due to health concerns (Ngapo, Martin & Dransfield, 2007). In addition, consistent quality, less wastage, convenience and ease in cooking and high level of choice or flexibility in available cuts are of concern to consumers(Aaslyng, 2009).Hence, cattle breeders need to address carcass composition and meat quality traits, which will determine consumer acceptance of beef. Overall, meat quality is difficult to define because it is a combination of microbiological, nutritional, technological and organoleptic components.Moreover, the term“quality” of carcasses has different meaningsdepending on local customs in different countries of the world(Hocquette Gigli, 2005).Hence, it becomes necessary to move focus from the aggregate “quality” to investigate individual components of meat quality, such as visual aspects (e.g. the colour of lean) or eating quality (tenderness, juiciness and flavour), which in turn are affected by intramuscular fat and fatty acid composition (Aaslyng, 2009).
Measurements of meat quality traits presentparticular problems for improvement, as direct measurements require destruction of the sample. Muscle quality is generally considered to be difficult, if not impossible, to measure in the live animal and is expensive and time-consuming to measure completely in samples from the carcass (Clutter, 1995).Tools to predict carcass composition for grading and classification of carcasses generally use dissected composition as a reference, which is usually obtained by manual dissection performed by skilled technicians. Beside the valuable and accurate information provided, it is also a destructive, time-consuming and therefore a costly method. Hence these methods are difficult and expensive to use in research programmes or breeding programmes involvingmany animals, and impossible to use routinely in commercial operations (Kempster, Cuthbertson & Harrington,1982).
Because of these restrictions, alternative methods have been used in beef cattle to predictmeat quality attributes,such as near infrared (NIR) spectroscopy (Andrés,Murray, Navajas, Fisher, Lambe & Bünger, 2007;Prieto, Andrés, Giráldez, Mantecón, & Lavín, 2008; Prieto et al., 2009a). Moreover,partial dissection usingsample joints (Kempster & Jones, 1977), visual assessment of fatness and conformation (Kempster et al., 1982), ultrasound scanning in live animals (Realini, Williams, Pringle & Bertrand, 2001) orvideo-image analysis (VIA) of carcasses (Allen & Finnery, 2001) and live animals (Sakowski, Sloniewski & Reklewski,2002; Hyslop, Ross, Schofield, Navajas, Roehe& Simm, 2008) have been used as a means of assessingcarcass characteristics at slaughter.
More recently, the use of X-ray computed tomography (CT) in carcasses has been investigated in pigs, sheep and beef cattle. CT scanning is a non-invasive technique that can provide in vivo predictions of carcass composition, which are used in pig and sheep breeding programmes (Simm, Lewis, Collins & Nieuwhof, 2001; Aass, Hallenstvedt, Dalen, Kongsro & Vangen, 2009). Very accurate in vivo predictions of muscle, fat and bone weight were reported in both species (sheep:Jones, Lewis, Young & Wolf, 2002; Lambe,Young, Mclean, Conington & Simm, 2003; Macfarlane, Lewis, Emmans, Young & Simm, 2006;pigs: Szabo, Babinszky, Verstegen, Vangen, Jansman & Kanis, 1999). Very accurate predictions of carcass tissue weights were also reported from the CT scanning of carcasses of pigs (Dobrowolski, Romvari, Allen, Branscheid & Horn, 2003; Vester-Christensen et al., 2009) and sheep (Johansen, Egelandsdal, Røe, Kvaal & Aastveit, 2007; Kongsro, Røe, Aastveit, Kvaal & Egelandsdal, 2008). In beef cattle, although the size of the CT scanner gantry prevents CT scanning of live beef cattle or whole carcasses, Navajas et al. (2010, in press) reported that it could be used as an economical and faster alternative to total dissection for determining carcass composition based on the scanning of primal cuts. This allows anon-invasive assessment of composition without affecting the value of primal cuts.More comprehensive and faster scanning ispossible due to the development of CT technology, such asspiral CT scanning (SCTS), which hasbeen recently investigated in animal and meat science.Predictions of beef and sheep carcass composition as well as muscle volume and weights and muscularity in sheep, based on in vivo or post-slaughter SCTS, were found to bevery accurate (Navajas et al., 2006, 2007, 2010, in press).Although multivariate analysis was used to predict sheep carcass composition from CT images (i.e. Johansen et al., 2007; Kongsro, Røe, Kvaal, Aastveit & Egelandsdal, 2009), it has not been applied for estimating beef carcass composition by SCTS.
The prediction of meat quality using CT scanning has been investigated based on the averageCT muscle density, calculated as the average values of the pixels segmented as muscle in the CT images. In sheep,Karamichou, Richardson, Nute, McLean and Bishop (2006a)found strong negative genetic correlations of CT muscle density with IMF content and taste panel scores for flavour, juiciness and overall palatability; although no genetic association with tenderness was identified. Associations of variable magnitude were reported with different fatty acids (Karamichou, Richardson, Nute, Gibson & Bishop, 2006b). A more sophisticated approach was used by Lambe, Jopson,Navajas, McLean, Johnson and Bünger(2009) to quantify the association between CT parameters and IMF in sheep. By fitting parameters of a mixture of four normal overlapping distributions for the full tissue density the accuracy increased by 10% compared to those usingaverage muscle density.
Chemical and physical differences in the tissues between live animals and carcasses are expected due to the post-mortem transformation process, particularly in the case of muscle/meat (i.e. lower water content due to drip losses, differences in tissue density because of low temperatures, histological differences due to ageing, etc.) (Lawrie, 1998). CT scanning of meat may capture the changes of tissue densities and properties and therefore improve the predicting ability of CT data for both composition and quality traits compared to measurements in the live animal. In the case of beef, moderate to low phenotypic correlations were found between average CT muscle density of beef primals and IMF in a preliminary study by Navajas et al. (2009).To the best of our knowledge, there are no studies testing the use of SCTS to predict quality parameters of beef using a multivariate analysis.
The aim of this study was to investigate,using a multivariate approach,the potential of SCTS tissue density values as predictors of beef cuts composition and beef quality characteristicsin crossbred Aberdeen Angus and Limousin cattle.Beef quality traits included in this study weretechnological parameters,eating quality traits,fatty acid profile and intramuscular fat content.
2. Material and methods
2.1. Animals and management
This study was carried out as part of a larger trial in which a total of 88 Aberdeen Angus (AAx) and 106Limousin(LIMx) crossbred heifers and steers were slaughtered in the autumn/winter months of 2006, 2007 and 2008. The AAx and LIMx animals had average live weights of 582 and 609 kg and average ages at slaughter of 546 and 544 days, respectively.
Within the 194 animals, 144 animals were slaughtered in 2006 and 2007 and produced within a two-breed reciprocal crossbreeding rotation using Aberdeen Angusand Limousin breeds at the SAC Beef Research Centre (BRC). The 144 animals from the SAC BRCwere finished during the final 2-4 months of their production cycle on similar diets consisting of 1st cut grass silage and a barley based concentrate (50:50 on a dry matter basis) which was offered ad libitum as a completely mixed ration on a daily basis. The ration analysis averaged 381 g.kg-1 dry matter (DM), 12.0 MJ.kg-1 DM metabolisable energy and 139 g.kg-1 DM crude protein. All animals remained on these diets for a minimum of eight weeks after which they were selected for slaughter according to standard commercial practice (target grades R4L or better). The remaining 50 animals were slaughtered in 2008, sourced from different commercial farms and sired by either Aberdeen Angusor Limousin sires but the breed of the dam was unknown. These 50 animals were selected in the commercial abattoir where all slaughtering took place on the basis of sire breed, sex and the fact that both farm of origin was known and the individual sire identity was recorded on the animal passport.Although the ration formulation was not known, their ages and slaughter dates suggest that their finishing management was likely to be similar to that of the BRC animals.
2.2.Meat samples
After slaughter, the left carcass sides were kept and chilled for 48h, until quartering between the 10th and 11th ribs. After quartering, carcass sides were split into 20 primal cuts, as illustrated in Figure 1.From the sirloins, two other cuts were obtained which will be referred to as 11–12th rib sirloin and 13th rib sirloin, whilst the remaining lumbar section of this cut will be referred to as lumbar sirloin.M. longissimus thoracis et lumborumof thesecuts waschosen for assessing all the traitsin the present studysince most meat quality studies (e.g. Prieto et al., 2009) chose it for being the most homogeneous and representative muscle of the carcass.Colour was measured after 45 min blooming on the 11–12th rib sirloin. Lumbar sirloins and 11–12th rib sirloinswere vacuum packed in the abattoir and transported to the SAC-BioSS CT unit in Edinburgh,where they were CT scanned,and thensent to the University of Bristol for dissection and meat quality analysis. Cuts were kept and transported at temperatures of 1-2°C. The 13th rib sirloins were not vacuum-packed as they were retained for texturalslice shear force (SSF) measurements that were taken at approximately 72 h after slaughter.
[Figure 1 near here, please]
After dissection, samples of the M. longissimus thoracisof the 11–12th rib sirloins werevacuum packed and aged at 1ºC to 14 days post-mortem for assessment by a trained sensory panel. From the dissected M. longissimus lumborum of the sirloins, a 75 mm-long piece of the cranial end was separated, vacuum packed, aged for 10 days and used to assess instrumental texture by Volodkevitch shear jaws.From an adjacent section, 25 mm-thick steaks were vacuum packed and used to determine texture by a second SSF, after an ageing period of 14 days. The next 25 mm of the lumbar sirloin was taken, vacuum packed and frozen for subsequent analysis of fatty acid composition and intramuscular fat content.
2.3. CT scanning and data
At the SAC-BioSS CT unit in Edinburgh,beef cuts were CT fully-scanned using a Siemens Somatom Esprit scanner. The X-ray tube operated at 130 kV and 100 mAs, using Pitch 2. The diameter of the CT images was 450 mm.CT scanning method was SCTS, in which the X-ray tube rotates continuously in one direction whilst the table on which the cuts were positioned is mechanically moved through the X-ray beam. The transmitted radiation takes on the form of a helix or spiral (Jackson & Thomas, 2004). This technology captures very detailed information from a volume of contiguous slices, rather than by collecting individual cross-sectional images. SCTS were collected of each of the sirloins with cross-sectional images that were 8 mm thick. A pilot trial was carried out to evaluate the protocol and check that the sizes of the primal cuts were such that effective and useful CT scanning was possible. Given the size of the primal cuts, a thickness of 8 mm gave a good balance between the quality of the images and the time required for the actual scanning. Furthermore, with this slice thickness, it was possible to have one spiral sequence per cut for most of the cuts, and reduce the risk of overheating the CT tube. The average numbers of cross-sectional images were 19 and 54 for the 11–12th rib and lumbar sirloins, respectively.
The principle of CT is based on the attenuation of X-rays through tissues and objects depending on their different densities. These differences are reflected in different CT values, which are measured inHounsfield units (HU) (Hounsfield, 1992).The frequency distributions of pixel values from -256HU upwards were obtained for these cuts using STAR 4.9 CT image analysis software (Mann, Glasbey, Navajas, McLean & Bünger, 2008). Alternatively, the histogram of HU values could have been obtained directly from the CT scanner or from any standard image processing software.
The pixel distribution values for the range of CT densitiesthat correspond to the soft tissues (fat and muscle) were considered in this study. The range was defined between -254 and 133 HU using as reference values the CT tissue thresholds estimated as part of the development of image analysis to predict carcass composition, described by Navajas et al. (2010).
2.4. Dissection of sirloins
After CT scanning, beef cuts were transported to the University of Bristol where they were dissected into subcutaneous, kidney knob and channel, intermuscular and thoracic fat, muscle, cutaneous trunci,bone and ligaments.The composition traits included in this study were the weights of subcutaneous fat, intermuscular fat, total fat (in this case as subcutaneous fat plus intermuscular fat) and muscle (muscle plus cutaneous trunci)of the 11–12th rib sirloin and lumbar sirloin.
2.5. Sensory analysis
Sensory analysis was carried out by a 10-person trained taste panel (BSI, 1993). The samples were defrosted overnight at 4 ºC and then cut into steaks 20 mm thick. Steaks were grilled to an internal temperature of 74 ºC in the geometric centre of the steak (measured by a thermocouple probe) after which, all fat and connective tissue was trimmed and the muscle cut into blocks of 2 cm3. The blocks were wrapped in pre-labelled foil, placed in a heated incubator and then given to the assessors in random order chosen by a random number generator. The assessors used 8-point category scales to evaluate the following traits: tenderness (1 – extremely tough, 8 – extremely tender), juiciness (1 – extremely dry, 8 – extremely juicy), beef flavour intensity (1 – extremely weak, 8 – extremely strong), abnormal flavour intensity (1 – extremely weak, 8 – extremely strong) and overall liking (1 – dislike very much, 8 – like very much).
2.6. Physical analyses
Meat colour as L* (lightness), a* (red-green) and b* (yellow-blue) (CIE, 1978) was measured at 48 hours post mortem after blooming for 45 minutes, with a portable Minolta® colorimeter (CM-2002, D45 illuminant and 10 º observer; Konica-Minolta Sensing, Inc., Germany).
The slice shear force test was performed on hot cooked meat, according to Shackelford, Wheeler and Koohmaraie (1999). Meat was cooked in a pre-warmed clam shell grill(George Foreman brand) wheretemperature was monitored continuously,using a stabbing temperature probe inserted into the geometric centre of the steak during the cooking process,until it reached 71 ºC, when the steak was removed from the grill and monitoring continued until temperatureplateaued at approximately 76 ºC.The weight before and after cooking was used for calculation of cooking loss. For this test, a single meat sample of 50 mm by 10 mm was sheared orthogonal to muscle fibre orientation and the maximum shear force noted. AStevens CR Texture Analyser (Stable micro-systems, UK)was usedfor 14 days test and a Lloyd Texture Analyser (Lloyd Instruments, UK)for 72 hours test; both instruments equipped with a custom-designed accessory, featuring a flat, blunt-end blade as described by Shackelford et al. (1999). Particular care was taken to avoid fat or connective tissue at the point of shearing.
For the Volodkevitch shear force test, the samples were cooked in a water bath at 80 ºC until a centre temperature of 78 ºC was reached. From each of these cooked sections, 10 replicate blocks (20 x 10 x 10 mm) were cut parallel to the fibre direction and sheared across the fibres with the Volodkevitch jaw (stainless steel probe shaped like an incisor) on a Stevens CR Texture Analyser (Stable micro-systems, UK).
2.7. Fatty acid and intramuscular fat analyses
Fatty acids analysis was carried out by direct saponification as described in detail by Teye, Sheard, Whittington, Nute, Stewart and Wood (2006). Samples were hydrolysed with 2M KOH in water:methanol (1:1) and the fatty acids extracted into petroleum spirit, methylated using diazomethane and analysed by gas liquid chromatography. Samples were injected in the split mode, 70:1, onto a CP Sil 88, 50 m0.25 mm fatty acid methyl esters (FAME) column (Chrompack UK Ltd, London) with helium as the carrier gas. The output from the flame ionization detector was quantified using a computing integrator (Spectra Physics 4270) and linearity of the system was tested using saturated (FAME4) and monounsaturated (FAME5) methyl ester quantitative standards (Thames Restek UK Ltd, Windsor, UK). Total IMF content was calculated gravimetrically as total weight of FA extracted.
2.8. Data analysis
The effect of breed cross (AAx or LIMx) on beef cuts composition and beef quality traits was estimated using the general linear models (GLM) procedure of the SAS package (SAS, 2003). The data were subjected to one way analysis of variance according to the following model: