Calpains from thaw rigor muscle

Pre-rigor beef M. Longissimus lumborum and diaphragma were frozen at -70 °C and thawed at different temperatures and the activities of extracted calpains and the toughness of heated meat compared with those in chilled muscle. Fresh muscle contained about 14 μg of μ-calpain/g and was unaffected by freezing, but was reduced after thawing. Rapid thawing at 30 °C for 20 min reduced the μ-calpain to 14%. When cooked from the frozen state, extensive shortening occurred and tender meat was obtained. By storing at -3 °C for 1 day, thaw-shortening was prevented, but tougher meat obtained. The μ-calpain decreased to 70% whilst the m-calpain was unaffected. Toughness decreased after further storage at -3 °C, as did the μ-calpain. The latter changes were similar to those during development of rigor mortis and ageing of non-shortened meat stored at 4 °C. Variation in calpain activity, rather than in sarcomere length, are likely to be the cause of toughness variation in thaw rigor muscle.