High yield cooking method

A cooking method comprising:

• a) providing wood based cellulosic raw material comprising chips, pin chips, shavings, saw dust, or a combination thereof, and a pre-treating the wood based cellulosic raw material by steaming at 80-120° C. to provide pre-treated material comprising cellulosic solid fraction to form a pre-treated wood based cellulosic material;
• b) cooking the pre-treated wood based cellulosic material in cooking conditions comprising a cooking liquor comprising a chemical charge selected from the range between 1-200 g/l of acidic sulfite as total SO2 or an organic acid selected from lactic acid, acetic acid, formic acid, citric acid, oxalic acid, uric acid and malic acid, to provide cooked cellulosic material in a spent cooking liquor;
• c) grinding and compressing the cooked cellulosic material in the cooking conditions to release high molecular weight lignin (HMW lignin) from the cooked material into the spent cooking liquor, and to provide a compressed cellulose fraction;
• d) recovering the HMW lignin from the spent cooking liquor;
• e) displacing the spent cooking liquor with a displacing liquor containing cooking liquor having 1-200 g/l of acid sulfite as total SO2 or organic acid;
• f) recovering the HMW lignin from a displaced liquor;
• and
• g) recovering pulp from the compressed cellulose fraction;
• and wherein step c) is carried out at a temperature of 120-180° C. and to a consistency of at least 10% by weight.