A novel method for producing softened edible seaweed kombu

- A novel softened kombu was produced using protease and phosphate buffer treatment.
- The softened kombu retained the original appearance of kombu after treatment.
- The softened kombu enabled to be mashed with the tongue but with low stickiness.
- Phosphate buffer reduced calcium content in the kombu and distorted the cell wall.
- Proteolysis produced unique mesh microstructures in the cell wall.

Kombu is difficult to soften by conventional cooking. We developed a novel softened kombu, while maintaining its original appearance, for elderly people with chewing or swallowing difficulties. Kombu treated with enzyme and phosphate buffer (EP) resembled conventionally-cooked kombu (C) in appearance, and was soft enough to be mashed completely using the tongue and upper jaw. The firmness of EP was 32 kPa, which was 1/30th that of C, and the stickiness of EP was lower than that of kombu treated only with phosphate buffer (P). The calcium content of EP decreased to 70% of that of C. The cell wall of EP was distorted compared with that of C. Scanning electron microscopy showed that the cell wall of EP was a mesh surface composed of microfibers and void spaces, unlike the dense structure of C and the spongy structure of P. These results suggest that the softening was caused by removing calcium from alginate in kombu using phosphate buffer and distorting the cell wall. The reduction in stickiness was considered to come from the change in the cell wall microstructure by proteolysis that prevented viscous sodium alginate from flowing out.