A cooking apparatus for treating biomass containing lignocellulose, comprises one or more cooking reactors (100, 100') adapted to cook biomass in a cooking chemical which includes formic acid and acetic acid. A heating arrangement (106) is adapted to adjust heat energy transfer to each cooking reactor (100, 100') by adjusting the temperature of the cooking chemical as a function of time and by feeding the temperature-adjusted cooking chemical into each cooking reactor (100, 100').