Characterizing the amount and variability of intramuscular fat deposition throughout pork loins using barrows and gilts from two sire lines

The objective was to determine the amount and variability of intramuscular fat (IMF) in a pork loin attributable to anatomical chop location, sex, and sire line. Pigs were sired by commercially available terminal Duroc boars selected for meat quality (MQ; n = 96) or lean growth (LG; n = 96) and equally split between barrows and gilts. After slaughter and fabrication, bone-in chops were removed from four locations of each left-side loin (A = 6th rib, B = 10th rib, C = last rib, and D = 4th lumbar vertebrae). An adjacent pair of chops from each location was collected and evaluated for visual color and marbling, subjective firmness, moisture and extractable lipid (IMF) (anterior chop), and Warner–Bratzler shear force (posterior chop). Data were analyzed using the MIXED procedure of SAS as a split-plot design. Homogeneity of variances was tested on raw data using Levene’s test of the GLM procedure and found to be heterogeneous. Thus, a two-variance model was fit using the REPEATED statement of the MIXED procedure, grouped by pig. The mivque(0) option of the VARCOMP procedure was used to calculate the proportion of variability that each factor contributed to the total variance. Barrows (3.64%) produced chops with greater (P < 0.01) IMF content than gilts (3.20%), and barrows (2.14) had greater (P < 0.01) IMF variability than gilts (1.23). Chops from MQ pigs (4.02%) exhibited greater (P < 0.01) IMF content than LG (2.82%), and MQ (1.76) had greater IMF variability (P < 0.01) than LG pigs (0.97). Chops from locations A (3.80%) and D (3.77%) had greater IMF than B (3.34%; P < 0.01), and A, B, and D had greater IMF than C (2.77%; P < 0.01). Variances of IMF also differed (A = 1.44, B = 1.59, C = 1.05, and D = 2.18; P = 0.01) across chop locations. Of the variability in IMF, 33.0% was attributed to sire line, 10.16% to chop location, and 4.01% to sex, with 52.83% not accounted for by these three factors. Location A chops were the most (P < 0.01) tender (2.57 kg) and C chops the least (P < 0.01) tender (2.93 kg), while B and D chops were intermediate and not different from each other. No differences in variability (P = 0.40) of tenderness were observed among chop locations (A = 0.31, kg B = 0.24 kg, C = 0.24 kg, and D = 0.23 kg). These results demonstrated that variability in tenderness values did not reflect the variability of IMF. In conclusion, chop location, sex, and sire line all contribute to the amount and variability of pork loin marbling.