Microstructure of Squid Muscle and its Influence on Texture

Abstract

Scanning electron microscopy was used to examine the microstructure of mantle muscle tissue from Illex illecebrosus and Loligo pealei, the two commercially important varieties of North Atlantic squid. Samples were prepared by fixation and dehydration; freezefracture was also employed. Individual muscle fibers were separated by homogenization of tissue followed by filtration. Fiber diameters were measured as were shear forces required to cut across muscle fibers. The influence of processing steps, including holding on ice, freezing and cooking were investigated. Squid mantle fibers averaging 3.6μm in diameter, are roughly triangular in shape and consist of rectangular obliquely striated myofibrils arranged somewhat radially around a central cytoplasmic core. These myofibrils are twisted in a left-hand helix lying at a contraction dependent angle of about 16-17° to the main fiber axis with a repeat distance of 38μm. No discernible stuctural differences were observed as a consequence of species or between the circular and radial fibers comprising the mantle. Cooking produced the most obvious processing changes resulting in larger, more closely packed fibers, fusing of myofibrils, the appearance of granulated heat denatured protein on the fiber surface, transverse breaks across fibers and disappearance of a prominant connective tissue network. These changes resulted in a 58% decrease in shear force, probably as a result of the latter two effects, even though heating was accompanied by thermal denaturation of contractile proteins and a large increase in tissue density. Other processing effects were less obvious; freezing produced a tendency for fibers separated by homogenization to lose their outer membranes and for the underlying myofibrils to become shredded.