Exploration of ways to utilize wheat chaff through fungal substrate conversion : a base for animal feed or substrate for cultivation of edible mushrooms

Sammanfattning: The shift towards a more sustainable food system emphasises a need for increased circularity and valorisation of available resources. Chaff constitutes one of three main fractions obtained upon harvest of certain crops, during which it is generally discarded on the field. New technologies have enabled collection of the chaff, consisting of 'bait and chaff', which could potentially increase the harvested residues by 30% and thereby, strengthen the interest in utilizing this unexploited biomass resource. Due to the presence of lignin in the plant cell wall, which cannot be degraded by rumen microbiota, delignification is necessary to increase digestibility when used as a feed ingredient. Pleurotus ostreatus has been recognized for its efficient lignin degradation in addition to its ability to produce fruiting bodies (mushrooms) on various substrates. The aim of the study was to explore utilization of wheat chaff through fungal substrate conversion, as potential base for animal feed or substrate for edible mushroom production. The trials were performed at three different scales. Firstly, the mycelial expansion of various filamentous fungi – P. ostreatus, Pleurotus eryngii, Grifola frondosa, Lentinula edodes, Aspergillus oryzae and Rhizopus oligosporus – was compared on wheat chaff with a moisture content (MC) of approximately 75% packed into petri dishes and incubated at 25°C. P. ostreatus displayed the fastest growth followed by P. eryngii which, together with G. frondosa, created the densest mycelium during extended incubation. After 77 days of incubation, the chaff inoculated with P. ostreatus, P. eryngii, G. frondosa had significantly increased in ash content in relation to uninoculated control demonstrating loss of organic material by fungal utilization. In the second trial, the moistened wheat chaff (75% MC) with pH adjusted to approximately 7.25 with CaCO3 was inoculated with spawn of P. ostreatus strain M2191 and incubated in small boxes (5cm × 9 cm) for 46 days at 25°C to assess alterations in lignin, crude protein, and ash content in the wheat chaff. Lignin content decreased significantly from 12.2% to 10.8%, (of total solids), a decrease by 11.5%, in relation to the time zero control samples. Crude protein and ash content did however not show any significant difference. The wheat chaff exhibited structural alterations after treatment with P. ostreatus including bleached and fibrous appearance and granular slime sheets, all signs of ligninolytic activity by P. ostreatus. The third trial was performed with a larger substrate volume in bags (30 cm × 42 cm) to evaluate the potential of wheat chaff as a substrate for mushroom production. Three different bag trials were made with various adjusted pH in the range of 7.0-7.7. Bags were incubated at 25°C during the mycelial colonization and then subsequently moved to a pilot plant with conditions to induce fructification. Successful fruiting body production of P. ostreatus was established in bags from all the trials. From point of inoculation until first harvest a range of 62-85 days were required within the different trials. The study has contributed to the exploration of possible applications for fungal conversion of wheat chaff, as animal feed or mushroom substrate. To further evaluate the utilizations as feed, additional analysis on e.g., animal digestibility is required, as well as exploration of various strains to increase the efficiency and selectivity of the lignin degradation. The cultivation of fruiting bodies on wheat chaff also requires further optimization of culture conditions and substrate characteristics, possibly by supplementation, to reduce the time for the production cycle and increase the yields.