Skimming off the Scum

The scum which rises to the top of the dashi must be completely removed, as it is composed of those oxidised lipids which come into being during the production or preservation process of the bonito flakes. These oxidised lipids have bitter notes to their taste, and, if left in, can give an unpleasant astringency to the resulting dashi. It is to prevent the multiplication of these chemicals that bonito flakes packaged in plastic bags are sealed with carbon dioxide or nitrogen in order to avoid their coming into contact with oxygen. It is important to use as quickly as possible once opened.