Readily available lignocellulosic biomass as substrate for biogas plants is gaining popularity amongst biogas plant operators. Results of low‐temperature pretreatment (light cooking) of wheat straw to remove waxes and prepare the biomass for microbial action are described. Benefits of light cooking are low thermal energy demand and low investment cost compared to conventional techniques such as steam explosion. The novelty lies in utilizing the low temperature range 25–100 °C for pre‐soaking the biomass. Two different types of wheat straws were pretreated at varying temperatures and sizes. The results were compared with Buswell's equation for theoretical maximum biomethane yield. Compared to untreated straw, pre‐soaking leads to a significantly higher methane yield. Size reduction combined with light‐cooking does not affect the methane yield in the same manner as pre‐soaking of the biomass.