Measuring the effects of phenotype and mechanical restraint on proteolytic degradation and rigor shortening in callipyge lamb longissimus dorsi muscle during extended aging P.S. Kuber a , S.K. Duckett b , J.R. Busboom a, *, G.D. Snowder c , M.V. Dodson a , J.L. Vierck a , J.F. Bailey b a Department of Animal Sciences, Washington State University, Pullman, WA 99164-6310, USA b Department of Animal and Veterinary Science, University of Idaho, Moscow, ID 83844-2330, USA c US Sheep Experiment Station, Dubois, ID 83423, USA Received 27 December 2001; received in revised form 29 March 2002; accepted 29 March 2002 Abstract The purpose of this study was to determine if tenderness of callipyge (CLPG) longissimus dorsi muscle (LM) could be improved by: (1) extending the aging period to 48 days postmortem or (2) preventing rigor shortening by clamping. In CLPG and normal (NML) chops respectively, initial Warner-Bratzler shear values (WBS) were lower (P < 0.05) in clamped (CL) (5.5 and 3.6 kg) compared to unclamped (UCL) (7.4 and 4.9 kg) LM. In CLPG, an acceptable WBS (3.6 kg) was reached at 48 days PM, whereas, NML lambs reached an acceptable level (3.8 kg) by 3 days PM. Sarcomere lengths (SL) of CL (1.68 mm) were longer (P < 0.05) than for UCL (1.44 mm) and were negatively correlated with WBS (r=0.55; P < 0.1). The appearance of Troponin-T (TNT) degradation product coincided with tender WBS values; 3 days postmortem in NML UCL and 48 days postmortem in CLPG. In conclusion, clamping reduced WBS possibly by reducing rigor shortening. Extended aging resulted in CLPG LM with acceptable WBS values, concurrent with the appearance of TNT degradation products. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tenderness; Callipyge lamb; Troponin-T 1. Introduction The US sheep industry could benefit from the positive production traits of superior feed efficiency and yield of retail cuts offered by callipyge (CLPG) lambs (Shack- elford, Wheeler, & Koohmaraie, 1998). Busboom, Wahl, and Snowder (1999) reported that CLPG genetics have the potential to decrease the cost of lamb to con- sumers and increase profitability in the lamb industry. The specific muscle hypertrophy associated with CLPG genetics results in an increase in the area of the long- issimus dorsi muscle (LM) and in the size of the leg muscles (Clare, Jackson, Miller, Elliot, & Ramsey, 1997; Duckett, Klein, Leckie, Thorngate, Busboom, & Snow- der, 1998; Jackson, Miller, & Green, 1997a; Koohmar- aie, Shackelford, Wheeler, Lonergan, & Doumit, 1995). In addition to the increase in muscle mass, an increase in hot carcass weight and dressing percentage (12 and 8%, respectively), with no difference in live weight at time of harvest between CLPG and normal (NML) lamb (Duckett, Klein, Leckie, et al., 1998a), indicates the potential for higher economic return to US sheep producers. However, Clare et al. (1997) reported that consumers found 40% of all CLPG LM chops unac- ceptable for tenderness. Numerous studies have shown that CLPG LM is tougher initially (1 day postmortem) than NML LM. The phenotypic (CLPG vs NML) dif- ferences resulting in a variation in tenderness are due to higher calpastatin activity in the muscle tissue of CLPG compared to NML lamb (Clare et al., 1997; Duckett, Klein, Leckie, et al., 1998; Duckett, Snowder, & Cock- ett, 2000; Koohmaraie et al., 1995). Calpastatin is the specific inhibitor of the calpain system, which is responsible for the postmortem (PM) proteolytic breakdown of muscle proteins. Calpastatin has been shown to alter both the rate (Koohmaraie et al., 1995) 0309-1740/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0309-1740(02)00089-X Meat Science 63 (2003) 325–331 www.elsevier.com/locate/meatsci * Corresponding author. Tel.: +1-509-335-2880; fax: +1-509-335- 1082. E-mail address: busboom@wsu.edu (J.R. Busboom).