Particles of a tobacco stream are first expanded by passing across a high-frequency field wherein they are exposed to the action of electromagnetic waves which effect a vaporization of moisture in the capillaries with attendant increase in volume. The thus expanded particles of the tobacco are thereupon conveyed through a drying zone wherein the particles are externally heated by hot air for a short period of time to thus effect rapid drying of strata immediately adjacent to external surfaces of the particles and to thereby stabilize the particles against shrinkage. In the last step, the tobacco stream is forcibly cooled with air to effect a condensation of vaporized moisture in the capillaries. The expansion of tobacco can be enhanced if the tobacco stream is caused to pass through a bath containing freon or another organic fluid having a boiling point lower than water before the tobacco stream is conveyed across the high frequency field.