Colonization of solid particles by Rhizopus oligosporus and Aspergillus oryzae in solid-state fermentation involves two types of penetrative hyphae: A model-based study on how these hyphae grow

• The first model to describe concentration profiles of penetrative hyphae with depth.
• The first mesoscale model to reproduce data of biomass growth in a real SSF system.
• The model recognizes that some fungi form two types of penetrative hyphae.
• There are long scouting hyphae and shorter vegetative hyphae.
• Pressure drop inside the hypha and medium firmness may limit hyphal length.

Among all hyphal types formed on substrate particles during solid-state fermentation, penetrative hyphae are the least studied. Important questions regarding penetrative hyphae remain to be answered: for example, what determines the final length of a penetrative hypha and why do some fungi produce two types of penetrative hyphae, long and short penetrative hyphae? In this work, we develop a discrete model to describe the biomass distribution of penetrative hyphae as a function of depth inside solid particles and apply this model to Rhizopus oligosporus and Aspergillus oryzae, growing in different media. We use our model to suggest different modes of growth and roles for long and short penetrative hyphae: the long ones, also called scouting hyphae, explore the composition of the particle and search for new colonization sites across the particle, while the short hyphae, also called vegetative hyphae, act as anchors for the colony and as enzyme producers. The same factors, namely the firmness of the solid particle and the pressure drop inside the hypha, may determine the final hyphal length of both hyphae. Scouting hyphae may be longer due to a lower pressure drop along the hypha. These hypotheses regarding growth of penetrative hyphae now require experimental investigation.

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