Solid-state fermentation, using the fungus Aspergillus niger, has been employed for the production of citric acid from kumara, a starch-containing root crop. A multi-layer packed-bed reactor was designed and operated in an attempt to understand mass and heat transfer during the fermentation. Although only a limited understanding was obtained, the multi-layer packed-bed reactor improved the mass transfer considerably compared with a single-layer packed-bed with the same substrate loading. In addition, the multilayer reactor allowed precise measurement of the gradients for carbon dioxide, citric acid, starch and fungal biomass.