The process consists of alternately subjecting the mass of loose product to be rehydrated to a treatment with a hot fluid, for an adjustable preset time interval, and with a cold fluid, for another preset time interval, and of alternately and successively repeating the treatment with hot fluid and with cold fluid until the preset rehydration percentage is attained. The apparatus includes an assembly (1) introducing the product into a tank (2) containing hot water and having a screw feeder (3) which feeds the product to a discharge collector (4). Transfer means (5), in the discharge collector, conveys the product to a cooling assembly.