Changes in tissue structure and rheological properties of squid were studied as related to cooking time using the mantle of squid just after instant killing. Collagen fibers of integument were tautly shrunk and voids were formed among the fibers by cooking. Muscle fibrils were considerably dehydrated and disconnected. Rupture energy (RE) for breaking raw squid parallel to the circular musculature was greater than that for breaking across the circular musculature. Cooking softened squid flesh and reduced RE both in parallel and transverse breaking of circular musculature. Reduction in RE was considerable in parallel breaking but not notable in transverse breaking.

Histological and physicochemical changes in the mantle muscle tissue of squid (Sepioteuthis lessoniana) during storage were studied. When raw squid mantles were broken parallel to the circular musculature, the rupture energy (RE) was significantly lowered along with the storage. In the case of breaking across the circular musculature, the RE was slightly lowered but the yield point (YP) in the stress-strain curve shifted to a larger strain during storage. In the case of heated squid meat, the YP shifted to a lower strain during storage. The electron microscopic observation of the muscle tissue of raw squid indicated that a number of voids appeared between the muscle bundles and the muscle fibers during storage, and after cooking, obvious detachment was observed between the muscle fibers and muscle bundles. No significant changes in the molecular sizes of major protein components (muscle fibril proteins) were observed, indicating that the muscle fibril proteins were not degraded.

TO CLARIFY differences between raw squid and softened dried squid, dried squid mantle was softened under various conditions, degree of swelling and certain properties were measured, and the musculature was examined by electron microscopy. Swelling the dried squid to the original level of raw squid required softening in three steps: presoaking, alkali-soaking, and post-soaking. The alkali-soaking was mainly responsible for the swelling effect. Resulting properties were largely changed from those of raw squid. Electron microscopy showed much water permeation throughout the muscle fibrils and fibers, while there was almost no permeation of water inside the individual fibrils.

This study evaluated the relationship between squid flesh transparency and muscle tissue microstructure. Squid mantle muscle was stored at 4 degrees C after being transported for 2 h by 4 different transportation methods used commonly in Japan (Group 1: live squid packed in ice-cold seawater; Group 2: live squid packed at 4 degrees C; Group 3: squid killed immediately after harvest and packed at 4 degrees C; Group 4: live squid packed in a fish tank containing seawater). Parameters of muscle tissue transparency were measured by an image analysis of digital images of squid muscle tissue. The mantle muscle tissue was observed under a transmission electron microscope to determine the postmortem structural changes at the cellular level. The ATP content of muscle tissue and rupture energy of squid flesh were also measured. As a result, the transparency of squid flesh and the ATP content of the muscles showed the same pattern of change in degree as time passed. The values of these parameters were highest in the group of squid killed immediately followed in order by those transported live, the refrigerated squid, and squid stored in ice-cold seawater. The mantle muscle tissue started to lose its transparency when the ATP in the muscle tissue started to decline. Disintegration of squid muscle tissue structure at the cellular level during storage under refrigeration for 24 h (4 degrees C) was observed in all methods of transportation. This suggested that destruction of the squid muscle tissue structure by autolysis is remarkably fast. The muscle tissue structure disintegrates due to decomposition of the muscle proteins, and muscle transparency is lost because the entire muscle develops a mixed coarse-minute structure.