A plant substance blend is periodically extracted in extraction periods comprising a first phase, in which the plant substance blend in the first filtration bag is partially extracted with water into a partial extract, and a second phase, in which the plant substance blend in the second filtration bag is extracted with the partial extract of the first phase into a final extract, whilst in the second filtration bag a new plant substance blend is extracted in all extraction periods and, in the first filtration bag a new plant substance blend is extracted in the initial extraction period and a partially extracted plant substance blend from the second phase of the foregoing extraction period is extracted in the subsequent operational extraction periods, whereas the new plant substance blend- undergoes a treatment of water vapour before extraction.
