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Effect of Early Postmortem Temperature on Beef Tenderness

University of California¹, Davis 95616

Abstract

Twenty feedlot-fed and six grass-fed Hereford steers were slaughtered. One side of each carcass was conventionally chilled in a cooler of 0 C (LT side); whereas the other side was held at 35 C for 3 h after slaugher, after which it was subjected to the same chilling condition of 0 C (HT side). Postmortem chilling rate, tenderness of rib steaks, sarcomere length, muscle fiber structure and electrophoretic pattern of myofibrillar proteins were determined. For the feedlot-fed animals, with an average backfat of 1.26 cm, HT sides showed greater toughness than LT sides after 3- or 7-d aging. Longissimus muscle of HT side exhibited wide variations in sarcomere length among myofibrils with an average length of 1.66 µm. Some myofibrils had extremely short sarcomeres of 1.37 µm with adjacent less-shortened myofibrils kinked and distorted. In contrast, conventionally chilled LT muscles showed a much longer average sarcomere of 1.87 µm with little variation among myofibrils. The toughening effect of early high postmortem temperature in feedlot-fed carcasses with substantial fat covering appeared to be due to the extreme heat shortening of some myofibrils. These muscle fibers, once shortened, did not improve in tenderness even after extended aging. For the grass-fed animals, LT sides were slightly tougher than HT sides after 3-d aging (the reverse of feedlot-fed), but after 7-d aging little difference was found in tenderness or sarcomere length between LT and HT sides. Regardless of environmental temperature, the grass-fed showed greater toughness than the feedlot-fed and conventionally chilled carcasses. The SDS-gel electrophoresis patterns of myofibrillar proteins did not differ between LT and HT sides for both groups of cattle.

¹ Muscle Biol. Lab., Dept. of Anim. Sci.