INFLUENCE OF TEMPERATURE, FIBER DIAMETER AND CONDITIONING ON THE MECHANICAL-PROPERTIES OF SINGLE MUSCLE-FIBERS EXTENDED TO FRACTURE

The tensile properties of single muscle fibres isolated from porcine l ongissimus and iliocostalis muscles conditioned for various lengths of time and cooked at different temperatures for Ih were investigated. T he stress-strain curves of fibres extended to fracture were r-shaped w ith yield points of 2-5% resting length in raw fibres and 10-20% in co oked fibres. The yield point was independent of cooking temperature, f ibre diameter and mechanical skinning of the fibres but decreased with ageing. The stress and strain at which the muscle fibres fractured wa s found to depend on cooking temperature, fibre diameter and condition ing time. Raw muscle fibres isolated from 24 h aged longissimus and il iocostalis muscles required 216 +/- 13 kN m(-2) and 272 +/- 6 kN m(-2) , respectively, to fracture. On cooking the muscles for 1 h at 80 degr ees C the fracture forces increased to 342 +/- 19 kN m(-2) in fibres i solated from the longissimus and 443 +/- 36 kg cm(-2) in fibres isolat ed from the iliocostalis. Ageing the muscles for 72 h halved the force s required to break raw muscle fibres isolated from them. However, fur ther ageing of the muscles had no noticeable effects on the force requ ired to fracture fibres isolated from the longissimus dorsi but caused a further reduction in the stresses needed to break fibres isolated f rom the iliocostalis muscle. Muscle fibres isolated from the iliocosta lis fractured at higher strains than those isolated from longissimus m uscle. However, in both muscles the strain at which the fibres fractur ed increased with cooking temperature and was lowest in fibres isolate d from raw muscles and highest in fibres isolated from muscles cooked at 80 degrees C for 1 h. Ageing the muscles led to a marked decline in the strain at which the fibres fractured and these effects were great er in fibres isolated from the longissimus than in fibres isolated fro m the iliocostalis muscle. In 24 h and 72 h conditioned muscles, fibre s having diameters <60 mu m required significantly greater forces to f racture than those with larger diameters. However, ageing the muscles for 264 h removed this diameter-dependent difference in fracture force . Fibre diameter had no effect on the strain at which the muscle fibre s fractured at all experimental conditions.