Studies on the Mechanism of Softning of Fish Bone by Cooking

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Small fishes are eaten as traditional dishes of marine or "tukudani" with a whole including bones in Japan. This study was cried out to clarify the changes of physical properties and components of bone in vinegar-curing or cooking in water. Softening of fish bone cured in vinegar or heated in water was evaluated by sensory tests and instrumental measurements. The hardness of horse mackerel vertebra decreased in 30 min after curing in an acetic acid solution. In this process, high percentages of calcium eluted from bone, but little protein was eluted. When fish bone was heated in water at atmospheric pressure for 120 min, hardness of bone decreased by measurement of Reometer. During cooking, twenty five percent of proteins eluted from bone into water, but calcium and other minerals hardly eluted in water. In the case of cooking in acidic solution, softening of the bone occurred in short period, both of protein and much minerals eluted from bone. Histological structure observation showed that bone matrix changed remarkably during cooking in water or curing in acidic solution. By the electrical scanning microscopic observation, the bundle of collagen fiber in the surface of bone was showed clearly during cooking in water or in acid solution and an amorphous substance on the surface of bone during acid curing. The cortical bone and the spongy bone became more coarse structure in cross sections by optical microscope.This studies revealed the changes in hardness of fish bone and the behavior of protein and inorganic compounds in bone during cooking. We assumed that the softening of bone occurred in because of separation of protein and minerals in fish bone.