Abstract
Wild sweetfish (Plecoglossus altivelis) or Ayu in Japanese has cucumber-like and watermelon-like aromas produced in the de-composition of ingested algae, while cultured one has less of these aromas due to the absence of algae in their feed. In this study, microalgae were incubated using treated municipal wastewater (TWW) without any exogenous nutrients and used for the experiment to add the aromas to the cultured sweetfish. In the experiment, the microalgae incubated in TWW were mixed with artificial feed at a ratio of 0 to 30% by weight and fed to the cultured sweetfish for two weeks before the harvest. The incubation in TWW found the dominant class of algae was Chlorophycese, which is belonging to green algae, followed by Diatoma. Of the class of Chlorophyces, the dominant species was Chlorococcales sp., regardless of sampling dates in June and July 2023. In some dates of sampling, Oxyphotobacteria, a bluegreen algae, were detected. Feeding these incubated algae to the cultured sweetfish did not affect the concentration of cucumber-like and watermelon-like aromas such as 3, 6-Nonadien-1-ol and 2, 6-Nonadienal in the fish meat regardless of the amount of fed microalgae but increased them in the fish guts, demonstrating that the sweetfish cultured with the microalgae could obtain such desirable aromas. Additionally, the concentrations of musty off-flavor (2, 4, 6-Trichloroanisole, 2-methylisoborneol, and geosmin) and musk flavor (Galaxolide (HHCB) and Tonalid (AHTN)) components in the fish guts under the microalgae-feeding condition were higher than those without the microalgae-feeding, suggesting that the microalgae incubated in TWW might have these undesirable flavors. This study revealed the feasibility of our idea to give the cultured ayu cucumber-like and watermelon-like aromas by feeding microalgae incubated TWW, although further investigation is needed to determine an optimum condition for microalgae feeding which give these aromas to cultured sweetfish without musty and musk flavors.
