the retention temperature t retention in the buffer tank 5 decreases it is necessary for the retention time t to increase according to established digester practice a decrease in the cooking temperature by about 10 u00b0 c results in it being necessary to double the extent of the cooking time it is consequently important to preserve the temperature of the extraction liquor during the dwell time in order not to be unnecessarily forced into long retention times while it should at the same time be borne in mind that temperatures which are too high degrade xylan and should therefore be avoided in a variant of the invention different additions q add can be made to the buffer tank for the purpose of augmenting the effect of this dwell time t see fig2 these additions q add can comprise for example white liquor green liquor black liquor cellulose derivatives such as cmc organic sulphides such as carbon disulphide mercaptides etc aq derivatives etc extraction liquor containing a high content of hemicellulose for example from a digester system using hardwood as the raw material can also be used as an addition in a parallel production line in which softwood constitutes the raw material these additions q add can also be added and in that case first and foremost white liquor green liquor or black liquor after the buffer tank 5 possibly for adjusting the alkali content prior to the next treatment stage in the cooking department system the invention can.