A study on the Mechanism of “Iromodori” or an Unfavorable Discoloration of “Yakinori” (Roasted “Nori”)

Abstract

“Hoshinori”, or dried sheet made of the laver Porphyra spp., turns into appetizing green when roasted. The green color usually remains for many days when moistened with seasoned vinegar for “sushi” but, depending upon lots of “yakinori”, changes into unfavorable reddishbrown withim a few hours. In order to elucidate the mechanism of this phenomenon called “iromodori” (a kind of discoloration), some examinations were made.
A clear difference was noticed by opal glass spectrophotometry between “iromodori discolored yakinori” and normal “yakinori”: The former exhibited absorption maxima at around 490 and 550 nm, both of which were completely absent in the latter. The responsible pigment was extracted with 8M urea from “iromodori discolored yakinori”, and purified by ammonium sulfate fractionation and Sephadex G-200 gel filtration. The pigment thus obtained exhibited an absorption spetrum having maxima at 495 and 456 nm. It was found by SDS-polyacrylamide gel electrophoresis that the subunit molecular weight of the pigment is about 20, 000. For comparison, some experiments were performed on photosynthetic biliproteins from the laver, and phycoerythrin was found to give comparable results when denatrued.
It was concluded therefore that in some lots of “yakinori”, phycoerythrin may have been denatured by unknown reason(s) to much larger extent than in normal lots, resulting in “iromodori discoloration” of the former lots of “yakinori”.