Sensory and objective mutton quality characteristics of SA Merino sheep selected for and against reproductive fitness

Abstract

: The effect of divergent selection for ewe multiple-rearing ability over a 15-year period on mutton quality was examined on the M. longissimus dorsi and M. semimembranosus of entire rams (R) and ewes (E) derived from two Merino lines. The selection of these two lines was based on maternal ranking values for multiple-rearing ability in a positive (P) and negative (N) line. In the negative line the replacements were based on the progeny of ewes that rear less than one lamb per joining or lambing opportunity (i.e. failed to lamb or lost all progeny born at least once). Progeny of ewes that reared more than one lamb per joining (i.e. reared twins at least once) were selected for the positive line. Ratings of sensory quality characteristics on the M. semimembranosus of the different genotype lines were obtained from a trained taste panel and related to data on physical and chemical characteristics. The moisture, total lipids, protein, ash, mineral concentrations and fatty acid composition of the M. semimembranosus were obtained. Physical parameters measured on the M. longissimus dorsi were: ultimate pH (pH48), drip loss, cooking loss and Warner-Bratzler shear force (WBS). The influence of selection line on the sensory characteristics of the mutton was generally negligible, with the exception of the sensory attribute of first bite, where meat derived from the positive line was rated to be less tender than that of the negative line contemporaries. Chemically, the meat derived from the four different groups differed significantly in moisture and lipid content. The Positive Ram (PR) group had the highest moisture (ca. 76%) and the lowest lipid (ca. 7%) concentrations, whereas the Negative Ewe (NE) group had the lowest moisture (ca. 70%) and the highest lipid (ca. 10%) concentration. No significant differences were detected in the proximate chemical composition between the P and N lines. The pH48 and WBS values showed significant differences between the four groups. The Negative Ram (NR) group had the highest pH48 and the lowest WBS values. Results indicated a line effect on WBS tenderness. Meat derived from the positive line was less tender compared to the meat from the negative line. Differences between the reproductive lines pertaining to the mineral and fatty acid composition were also noted. A general tendency found, was for the Positive Ewes (PE) to have the highest and the NE to have the lowest mineral concentration. Significant differences were detected between the four groups in the individual fatty acids arachidic acid (C20:0), lignoceric acid (C24:0), eicosenoic acid (C20:1n-9), linoleic acid (C18:2n-6), homo-g-linolenic acid (C20:3n-6), arachidonic acid (C20:4n-6), eicosapentaenoic acid (C20:5n-3), docosadienoic acid (C22:2n-6), docosapentaenoic acid (C22:5n-3) and docosahexaenoic acid (C22:6n-3) concentrations. The PR group had the highest total PUFA (polyunsaturated fatty acids) content (ca. 8%) and the NR the lowest PUFA content (ca. 6%). The positive group had significantly higher C18:2n-6, C20:3n-6, C20:4n-6, C20:5n-3, C22:5n-3 and C22:6n-3 concentrations in comparison with the negative line. Significant difference in the total PUFA composition was also detected between lines, with the positive line showing a higher concentration (ca. 7%) compared to the negative line (ca. 6%).