The effect of operation modes on filamentous fungi cultivation using agro-industrial residues-derived volatile fatty acids

As a sustainable organic waste valorization approach, anaerobic digestion has great scientific and industrial attention for years. Volatile fatty acids (VFAs) produced as intermediate products of anaerobic digestion of agro-industrial residues, have the potential to be converted to microbial protein, for example, through filamentous fungi cultivation; however, this process is sensitive to the concentration and type of VFAs. This study examined the inhibitory effect of acetic (AcA), propionic (PrA), butyric (BuA) acids, synthetic VFAs mix and apple pomace and potato protein liquor- VFAs (AP-VFA) effluent on five fungal strains ( A. oryzae, N intermedia, A. niger, R. delemar and R. oligosporous ) in batch fermentation. To alleviate the inhibitory effect of VFAs on fungal cell growth and maximize their utilization by fungi, two fed-batch feeding strategies were developed. In batch cultivations, A. oryzae and N. intermedia exhibited a similar performance in AcA consumption . However, in PrA and BuA media, fungal growth was significantly inhibited leading to an extended lag phase. The fungus A. oryzae demonstrated robust growth in AP VFAs media with higher BuA concentrations, achieving 23% BuA consumption and biomass yield of 0.15 g cell/g VFAs fed . Implementing 12 h and 24 h fed-batch feeding systems resulted in a significant increase in PrA and BuA consumption with biomass yields of 0.42 and 0.40-0.42 g cell/ g VFAs fed for AP-VFAs during 12 h and 24 h feeding, respectively, representing up to a three-fold boost in biomass compared to batch fermentation. These findings suggest that this approach enhances the bioconversion of waste-derived VFA mixtures into fungal biomass. • Aspergillus oryzae showed robust growth and high tolerance to toxic VFAs. • Propionic and butyric strongly inhibited fungal growth in batch cultivation. • Fed-batch feeding modes were developed to mitigate VFA-induced growth inhibition. • Fed-batch enhanced acids consumption and boosted biomass yield up to six-fold. • Adjusting feeding interval was crucial for bio-based VFA biovalorization.