Mild thermal treatment assists fungal preprocessing of softwood sawdust for production of fermentable sugar

Preheating with hot air at 85 – 125 °C was evaluated for its effectiveness in removing terpenes and terpenoids in softwood sawdust, thereby enhancing fungal preprocessing and subsequent saccharification of softwood-based mushroom substrates. Sawdust from pine (Pinus sylvestris L.) and spruce (Picea abies (L.) H. Karst.) was preheated prior to shiitake (Lentinula edodes (Berk.) Pegler) cultivation. Preheating removed up to 96 % of terpenes in pine- based substrates and up to 50 % in spruce-based substrates. Additionally, preheating decreased total terpenoids content in spruce by up to 78 %. For the pine-based substrate, the mild heating generally led to faster colonisation and improved mushroom yield, with the fastest mycelia colonisation and highest yield observed for 105 °C treatment. This temperature was associated with the lowest content of total terpenes and absence of major monoterpenes. The content of terpenes and terpenoids continued to decrease during cultivation, alongside fungal degradation of lignocellulose. As a result of more extensive lignin degradation, the enzymatic digestibility of cellulose was higher for spruce-based spent mushroom substrate than for pine-based one (up to 89 % vs. 49 % conversion). Enzymatic digestibility showed a negative correlation with the α-pinene content, and a positive correlation with increasing preheating temperatures.