Changes in Dietary Fiber in Seaweed Foods during Commercial Heat Processing

Hijiki Hizikia fusiformis, brown alga, was steamed for 6h, and kombu Laminaria ocho-tensis, brown alga, and nori Porphyra spp., red alga, were boiled in a seasoned solution for 2h by imitating the manufacturing method of boiled and dried hijiki, and tsukudani (seasoned and cooked seafood) of kombu and nori, and changes in dietary fiber in these seaweeds were studied. The water soluble dietary fiber of all seaweeds increased steadily during heat processing. However, the molecular weight of the main components in the soluble dietary fiber decreased during heating, and especially in the case of nori it diminished markedly to 15% of the unheated sample after 1h. There were some differences in the changes of insoluble dietary fiber content and, as to kombu, the insoluble type was reduced to 40% of the unheated sample by boiling for 1h. Amounts of carbohydrates dissolved from hijiki increased gradually during steaming, and the molecular weights of the main components in this fraction were comparable to those of the soluble dietary fiber in the hijiki.