Citrus processing waste is a leftover from the citrus processing industry and is available in large amounts. Typically, this
waste is dried to produce animal feed, but sometimes it is just dumped. Its main component is the peel, which consists
mostly of pectin, with D-galacturonic acid as the main monomer. Aspergillus niger is a filamentous fungus that efficiently
produces pectinases for the hydrolysis of pectin and uses the resulting D-galacturonic acid and most of the other
components of citrus peel for growth. We used engineered A. niger strains that were not able to catabolise
D-galacturonic acid, but instead converted it to L-galactonic acid. These strains also produced pectinases for the
hydrolysis of pectin and were used for the conversion of pectin in orange peel to L-galactonic acid in a consolidated
process. The D-galacturonic acid in the orange peel was converted to L-galactonic acid with a yield close to 90%.
Submerged and solid-state fermentation processes were compared.