Abstract: Grain is cleaned in a dry state and then admitted into a washing chamber for the removal of residual contaminants. The grain is then introduced into a centrifugal dryer where excess water is removed. Sufficient water remains in the grain so that it has a moisture content between 18 and 24% by weight. This moisture content is such that the outer layers of individual grains become swollen thereby activating enzymes in the outer layers which are capable of synthesizing vitamins while, however, initiation of germination of the grain is avoided. The grain is now conveyed into a heating chamber where it is heated to temperatures between 25.degree. and 35.degree. C. Thereafter, the grain is admitted into an insulated silo composed of corrosion-resistant material where it remains for a period of 5 to 30 hours while the moisture content and temperature are maintained within the respective ranges indicated above. In the silo, vitamins are synthesized by the activated enzymes.