Influence of the particle size of poplar wood chips on the development of mesophilic and thermotolerant mould during storage and their potential impact on dry matter losses in piles in practice

Highlights: • During the storage of wood chips three characteristic phases, P1 to P3, can be identified. • For medium chips highest dry matter losses occurred in P2 (high temperature), and in P3 (low temperature) for small chips. • Lower pile temperatures in P3 lead to high concentrations of moulds in both chip sizes. • The larger the chip format, the lower the maximum temperatures reached in the pile. • For both chip sizes examined, a long P2 led to the highest dry matter losses at the end of storage. -- Abstract: For efficient provision of wood chips from agriculture and forestry these raw materials must be stored in such a way that mass and energy losses are kept as low as possible. The connections between wood chip size, moisture content, pile temperature, mould concentrations and dry matter loss were examined in practice during storage of wood chips from poplar (Populus) from SRC in covered piles for periods of seven to nine months. Similar temperature curves were measured in the piles in all trials. The curves can be subdivided into characteristic temperature-dependent phases (P) – P1 (frost), P2 (high temperature), P3 (low temperature). In piles with small chips, temperatures measured in P2 (up to 64 °C) were always higher by comparison with those measured in medium chip piles (up to 58 °C). Low mesophilic and thermotolerant mould contents (up to 3.8 _lgCFU g⁻¹) as well as low dry matter losses (up to 5.7%) were also recorded. As temperatures drop below 45 °C in P3, mould contents and dry matter losses (up to 16%) increase clearly. In the case of medium chips, highest dry matter losses (up to 10%) occurred already in P2 in presence of high thermotolerant mould content levels (up to 6.2 _lgCFU g⁻¹). In order to store larger chips or chopped pieces with low dry matter losses, the process should be designed in such a way that the duration of P2 is minimised. A later harvest at higher outdoor temperatures in spring can also have a beneficial impact on reducing dry matter losses.