Processes for the production of a composition comprising material derived from seeds of plants of the genus Oryza, the process comprise the steps of : (a) providing material derived from seeds of the genus Oryza; and (b) determining the presence or absence and/or measuring the amount of at least one iminosugar in the material of step (a). Also described is the use of material derived from seeds of plants of the genus Oryza, which material comprises at least one iminosugar, as a food or feed supplement for lowering the glycaemic index (GI) value of said food or feed.