Process for preparing natural alkannin

The present invention discloses a preparation method of natural redroot gromwell colouring matter. It is characterized by that said preparation method includes the following steps: using redroot gromwell root as raw material, directly soaking it in n-hexane, through leaching, concentration, filtering and refining, obtaining a black-red thick paste-like oil-soluble colouring matter, through emulsifying, adsorption, spraying and drying, obtaining a purplish red water-soluble solid powder, and further through regulating, it is also able to obtain a blue-black water-soluble solid powder. Said invention can use a set of equipment to simultaneously produce oil-soluble, water-soluble, blue and red series products as ideal food additives.