Phenotypic correlations of backfat thickness with meatiness traits, intramuscular fat, longissimus muscle cholesterol and fatty acid composition in pigs

The aim of the present study was to determine the phenotypic correlations of backfat thickness with meatiness traits and intramuscular fat, cholesterol and fatty acid composition in the longissimus muscle of pigs. For this study, 60 barrows and 60 gilts (Pietrain × Duroc boars and Polish Large White crossbred sows) were slaughtered at 100 kg bodyweight. Lean meat percentage (LMP), loin muscle area (LMA), backfat thickness measured at five locations and average backfat thickness (ABF), and intramuscular fat (IMF), cholesterol (CHLM) and fatty acid composition in the longissimus muscle were determined. Phenotypic correlations of individual backfat thickness measured at five locations and ABF with LMP, LMA and polyunsaturated fatty acids (PUFAs), including C18 :2n-6, were negative and moderate to high, while with monounsaturated fatty acids (MUFAs), including C16:1 and C18:1 were positive and very low. Correlations of individual backfat thickness and ABF with saturated fatty acids (SFAs) and C16:0 were positive (0.29 to 0.56), while for C18:0 were low (0.10 to 0.23). Correlations of IMF and CHLM with LMP, LMA and PUFAs, especially C18:2n-6, were negative and high, while with SFAs and MUFAs were positive and moderate to high. Correlation between IMF and CHLM was high (0.74). The results of the present study indicate that increased IMF content results a significant decrease in carcass meatiness (LMP and LMA) and of PUFAs content and an increase in backfat thickness and contents of SFAs, MUFAs and CHLM.


Introduction
Intramuscular fat (IMF) content and fatty acid composition in pig meat have received considerable interest in view of their implications for human health (Williams, 2000) and for meat quality characteristics (Wood et al., 2003).Intramuscular fat content is an important parameter that influences the sensory characteristics of fresh pork (Fernandez et al., 1999).It has a positive effect on flavour, juiciness, tenderness/firmness and overall acceptability (Fortin et al., 2005;Gao & Zhao, 2009;Schwab et al., 2009).Some authors consider that levels of 2.5% to 3% IMF are required to ensure a desirable quality of fresh pork (De Vol et al., 1988), while other studies have reported positive relationship between IMF and sensory traits up to 3.5% IMF (Fernandez et al., 1999).Currently, IMF levels in the majority of modern commercial breeds have decreased below 1.5% (Gjerlaug-Enger et al., 2010;Hamill et al., 2012).This is largely owing to longterm intensive genetic selection for lean growth and decreased backfat thickness (Sonesson et al., 1998).
The genetic and phenotypic correlations between subcutaneous backfat depth and IMF content show moderate positive values (Newcom et al., 2005).This indicates that reductions in carcass fatness are likely to be accompanied by lower intramuscular fat levels in meat.Variation in fat content in meat has an effect on fatty acid composition (De Smet et al., 2004), since fat deposition is determined by de novo fatty acid synthesis and the uptake of exogenous fatty acids supplied by the diet (Kloareg et al., 2007).Meat of pigs with a higher intramuscular fat content has a higher level of saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA), and a lower level of polyunsaturated fatty acids (PUFA) (Rauw et al., 2012).
The aim of the present study was to determine the phenotypic correlations of individual subcutaneous backfat thickness with meatiness traits and intramuscular fat, cholesterol and fatty acid composition in the longissimus muscle of pigs.

Materials and Methods
A total of 120 crossbred pigs (Pietrain × Duroc boars and Polish Large White sows), comprising 60 gilts and 60 barrows, were used in this study.The animals were housed in 20 pens (six pigs per pen).Feed was supplied ad libitum, and water was provided by nipple drinkers.
In the fattening period (30 ± 3.1 -100 ± 4.2 kg body weight), animals were fed a grower diet (from 30 to 60 kg bodyweight) and a finisher diet (from 60 to 100 kg body weight).The ingredient composition and nutrient contents of the diets are given in Table 1.
Table1 Ingredient composition and nutrient contents of the grower and finisher diets Animals were slaughtered at 100 ± 4.2 kg bodyweight.Carcass evaluation was conducted according to the Polish Pig Testing Station's methodology (Różycki & Tyra, 2010).After chilling at 4 ºC for 24 h, the carcasses were weighed and the right half-carcass was measured.Subcutaneous backfat thickness was measured by Vernier calliper to the nearest 0.1 cm, at five locations: at the thickest point over the shoulder (BFOS), on the back between the last thoracic and first lumbar vertebrae (BFB), at three points on the sacrum -above the rostral edge (BFS I), above the middle (BFS II) and above the caudal edge of the gluteal muscle section (BFS III).These measurements were used to calculate average backfat thickness (ABF) from 5 measurements.Next, the right half-carcasses were dissected and the components were weighed and measured.The height and width of the longissimus muscle at the last rib were measured by Vernier calliper.The loin muscle area (LMA) was determined by the height × width × 0.8, and meat content was calculated as follows (Różycki & Tyra, 2010): where y is weight of meat of right half-carcass (kg); x 1 is weight of fatless ham (kg); x 2 is weight of longissimus muscle (kg); and x 3 is double width + height of longissimus muscle (cm).The weight of the meat of the half-carcass was used to calculate the lean meat percentage in the carcass (LMP).The samples (300 g) of the longissimus muscle were taken from the right half-carcass, in the region of the last rib.Samples were minced, vacuum packed and stored at −20 ºC, pending analysis of IMF, fatty acids and cholesterol concentrations.
The basic chemical composition in the diet was determined by standard methods (AOAC, 2006), while amino acids in the diet were assayed with the Beckman 6300 Amino Acid Analyser (Beckman Instruments, Palo Alto, Calif, USA).Phosphorus was determined via the vanadium-molybdenum colorimetric method (Cavell, 1955) and calcium by emission spectrometry on BUCK Scientific's 210 VGP Atomic Emission Spectrophotometer.
Intramuscular fat content in the longissimus muscle was determined by the Soxhlet extraction method (AOAC, 2006).The fatty acid composition in the lipid extracts was determined by gas chromatography after transesterification using a solution of 14% boron trifluoride (BF 3 ) in methanol.The fatty acid analysis was performed on a Hewlett Packard GC 5890 series II gas chromatograph.Total cholesterol content in the muscle was determined through the methods of Rhee et al. (1982).
Basic statistical characteristics of the results in this arithmetic means (Mean), standard error of means (SEM) and correlation coefficients, including significance (P), were calculated using statistical computational software (Statistica PL, version 8.0).

Results and Discussion
Descriptive statistics for intramuscular fat and cholesterol in the longissimus muscle and slaughter traits of the pigs are shown in Table 2.The values presented in Table 2 are characterized by great variability.The lean meat percentage of pigs in this study ranged from 51.0% to 62.4% (mean 56.6%), while ABF ranged from 1.72 cm to 2.95 cm (mean 2.28 cm).These values are representative of the commercial pig population.In this study, the average IMF content of the longissimus muscle was 2.5% (ranging from 1.73% to 4.14%).This content is in the range of values (2.5% -3.5%) that has been indicated as being sufficient to provide meat with the marbling characteristics appreciated by the consumers and to improve the sensory aspects of meat quality (De Vol et al., 1988;Fernandez et al., 1999).Hwang et al. (2005) and Kim et al. (2008) found the average IMF content in the range of 2% -3% in the longissimus muscle of commercial pig breeds.This value is consistent with the results of our study.The content of fatty acids in the IMF of longissimus muscle of the pigs was variable (Table 3, SEM).The longissimus muscle fat contained about 62.5% unsaturated fatty acids.The mean MUFAs content was 50.5% (range = 46.8% to 53.4%), while PUFAs was 12.13% (range = 8.41% to 15.68%).Similar values were reported by Kim et al. (2008).
Phenotypic correlations (Table 4) of subcutaneous backfat thickness measured at five locations (BFOS, BFB, BFS I, BFS II BFS III) and ABF with LMP and LMA were negative (−0.48 to −0.69; P <0.01, and −0.37 to −0.57; P <0.01, respectively).Bahelka et al. (2007) found that the correlation between ABF and LMP was −0.50, which is substantially smaller than in our study (−0.69).Also, the phenotypic correlations between the three individual subcutaneous backfat layers and LMA reported by Newcom et al. (2005) were considerably smaller (−0.18 to −0.23) than in the present study.
Correlations between backfat thickness and CHLM were positive and differed (P <0.01) from zero, except for BFS I and BFS II with CHLM.Correlations of five individual subcutaneous backfat thickness and ABF with IMF were 0.32 to 0.50 (P <0.01), respectively.Newcom et al. (2005) and Yang et al. (2010) reported smaller correlations between the individual subcutaneous backfat layers and IMF (0.26 to 0.34).The value for the correlation between ABF and IMF reported by Bahelka et al. (2007) (0.33) is also substantially smaller than that observed in the present study (0.49).Substantially greater correlations of individual backfat layers at the 10th rib (outer, middle, and inner) with IMF for Duroc-sired pigs (0.66, 0.76 and 0.71, respectively) were reported by Eggert (1998).Newcom et al. (2005) suggested that the contradictory results may be because of differences in the populations of pigs that were studied (pure bred vs. cross bred pigs), measurement methods and the mean values of these traits.
Correlations of CHLM and IMF with LMP and LMA were negative and high (P <0.01).Eggert (1998) and Newcom et al. (2005) reported smaller correlations between IMF and LMA (−0.56 and −0.22, respectively, vs. −0.72 in our study).Bahelka et al. (2007) reported a correlation between IMF and LMP of −0.53, which is smaller than −0.71 obtained in our study.Loin muscle area is an indicator of lean meat content in a pig carcass; thus, the correlation between IMF and LMP can have a value similar to that between IMF and LMA, which was observed in the present study.
The results of the present study indicate that genetic selection for increased IMF content can result in a decrease in carcass meatiness and an increase in backfat thickness.Schwab et al. (2009) showed that long-term selection for intramuscular fat in Duroc pigs yielded decreased loin muscle area and increased  (1994) reported that increasing the lean meat content of pig carcasses by 1% is associated with a reduction in IMF content by 0.07%.In this study, higher content of IMF was associated with a higher content of CHLM (r = 0.74; P <0.01).A previous study Dorado et al. (1999) showed a significantly high relationship between IMF content and cholesterol levels (r = 0.88).In contrast, Rauw et al.
A greater ratio of SFA to PUFA improves most aspects of meat quality, particularly pork flavour, although this may have a negative effect on the nutritional value of the meat (Teye et al., 2006).Increased intake of SFA is associated with obesity, increased plasma cholesterol and cardiovascular diseases in humans (Chizzolini et al., 1999).Increased palmitic acid (C16:0) is one of the main risk factors for these diseases, whereas stearic acid (C18:0) has little to no benefit to human health (Bonanome & Grundy, 1988).Thus, an increase in the proportion of C18:0 in pork would be preferred in view of human health and pork palatability.Our study has shown that a decrease in percentages of SFAs, especially C16:0 in the longissimus muscle, was associated with significantly smaller carcass fatness and a lower content of IMF and CHLM, while there were low correlations between C18:0 and BF (from 0.10 to 0.23), IMF (0.20) and CHLM (0.16).Phenotypic correlations of SFAs and C16:0 with BF ranged from 0.29 to 0.53 (P <0.05 and P <0.01), and 0.34 to 0.56 (P <0.01), respectively.Correlations of SFAs and C16:0 with IMF and CHLM were 0.44 and 0.50 (P <0.01), and 0.33 and 0.36 (P <0.01), respectively.Similar tendencies of phenotypic correlations of C16:0 and C18:0 with BF were confirmed by Suzuki et al. (2006) and Yang et al. (2010), while correlations of SFAs, C16:0 and C18:0 with IMF were confirmed in the studies of Yang et al. (2010) and Rauw et al. (2012).
Correlations of MUFAs, including C16:1 and C18:1 with BF, were positive and very low, while those with CHLM and IMF were significant (P <0.01; r = 0.54, 0.36 and 0.53, and 0.43, 0.31 and 0.38, respectively).Similar correlations between C18:1 and BF have been reported by other authors (Cameron et al., 1990;Piedrafita et al. 2001).Suzuki et al. (2006) found that the phenotypic correlations of BF and IMF with C16:1 and C18:1 were almost zero (−0.02 to 0.07).Correlations between MUFAs and IMF reported by Yang et al. (2010) andRauw et al. (2012) were 0.42 and 0.47, respectively, which are similar to the estimated correlations in the present study.
The results presented in Table 4 indicate that pigs with greater fat deposition tended to have smaller percentages of PUFAs in the longissimus muscle, as in the results of Cameron & Enser (1991).Correlations of PUFAs and individual PUFAs with all backfat were negative and moderate and were similar to the values reported by Yang et al. (2010).The main PUFA is C18:2n-6 (9.5%; Table 3), which is an essential fatty acid in mammalian nutrition.Suzuki et al. (2006) found a moderate negative correlation between C18:2n-6 in LM and BF (−0.43), and similar correlations were estimated in our study.Also, Wood et al. (2008) reported that the proportions of C18:2n-6 decline in LM as fat deposition increases.A greater content of PUFAs in the meat is beneficial from a human health aspect, but poses increased risk for its shelf life owing to oxidative stability of fat (Wood & Enser, 1997;Martin et al., 2008).
In the present study, the LM with a greater content of IMF contains less PUFAs.Intramuscular fat content was negatively correlated with PUFAs and individual PUFA (from −0.40 to −0.68; P <0.01).Yang et al. (2010) and Rauw et al. (2012) found high negative correlations of IMF with PUFAs and C18:2n-6 (−0.54 and −0.52, and −0.48 and −0.48, respectively), although these were smaller than in the present study (−0.68 and −0.65; P <0.01, respectively).Negative relationships of CHLM in the LM with PUFAs and individual PUFA (r = −0.41 to −0.73; P <0.01) were estimated in this study.In contrast, Rauw et al. (2012) reported almost zero correlations between CHLM and all fatty acids (PUFA, MUFA and SFA).

Conclusions
The results of the present study indicate that increased IMF content results in a significant decrease in carcass meatiness (LMP and LMA) and of PUFAs content, and an increase in backfat thickness and contents of SFAs, MUFAs and CHLM.Intramuscular fat content in pork is receiving greater attention in breeding programmes owing to its role in the sensory aspects of meat quality.A minimal level of IMF is required to maintain the overall acceptance of meat by consumers.However, further increases in the content of IFM may have negative health implications.

Table 2
Descriptive statistics for intramuscular fat and cholesterol in longissimus muscle and slaughter traits of the pigs 3 (n = 120)

Table 4
Phenotypic correlations of backfat thickness, cholesterol and intramuscular fat in longissimus muscle with lean meat percentage, loin muscle area, cholesterol, intramuscular fat in longissimus muscle and major fatty acid composition of intramuscular fat w backfat thickness.De Vries et al.