Bringing Mould to Work: Upgrading Citrus Peel and Sugar Beet Pulp

by Peter Richard

For regular households, mould in the food is hardly considered good news. For us research scientists it’s another story. Researching the mould on your onion can open the way to produce chemicals from agricultural wastes that can replace chemicals made from fossil oil.

115 million tons of citrus fruit and 270 million tons of sugar beet are produced annually worldwide which leaves a lot of citrus peel and sugar beet pulp. These raw materials are rich in pectin but of not much use. Sugar beet pulp is dried and used as animal feed. It has to be dried since it otherwise would rot; however drying is very energy consuming. Citrus peel can also be used as animal feed but is often just dumped which is problematic because of the odour. The pectin from citrus peel is sometimes extracted and used as a gelling agent in food, such as jams or marmalades. For this application only 40 – 50 thousand tons are used, i.e. there is a lot of pectin in the world that is unused.

OnionThe mould Aspergillus niger, the black mould, is a common contaminate of foods. If you find black spots on the onion in your kitchen, this is most likely Aspergillus niger. This mould is however also a useful microorganism that is used to produce citric acid. Since the 1930’s citric acid is produced in industrial scale using Aspergillus niger. It is used in food as an acidifier and especially much in soft drinks. Almost all the commercial citric acid, which is currently about 1.6 million tons per year is produced by this mould.

Aspergillus niger ferments also very efficiently pectin which is a polymer made out of D-galacturonic acid monomers. The mould produces pectinase enzymes that hydrolyse the pectin to D-galacturonic acid (oxidised sugar) which is then used as an energy source for growth. We thought that if we want to make something useful out of pectin or D-galacturonic acid, this is a suitable organism. We succeeded to genetically modify the mould so that it would convert D-galacturonic acid to different products, for example galactaric acid which is also called mucic acid.

With the engineered mould the process to produce galactaric acid is very simple. Galactaric acid is not soluble in water at acidic pH which facilitates the downstream processing. The substrates are all soluble and the mould operates well at acidic conditions.

Currently galactaric acid is produced chemically by oxidising the sugar D-galactose using nitric acid. This process is environmentally challenging. Galactaric acid is currently used in skincare products.  In the past, i.e. in the 1930’s it was also used as a food component in self-rising flour. In Mobile, a city in the south of the USA, D-galactose from a wood processing plant was oxidised chemically to produce about 600 tonnes galactaric acid per year.

Besides these applications galactaric acid can be converted chemically to other compounds like adipic acid or furan dicarboxylic acid, FDCA. Adipic acid is used for Nylon. FDCA can also be polymerised. The polymer has good gas barrier properties and can be used for bottles for carbonated drinks. This could replace the PET bottles that are currently used.

Reference: Mojzita et al.Metabolic engineering of fungal strains for conversion of D-galacturonate to meso-galactarate. Appl Environ Microbiol. 2010, 76(1):169-75.

100_0046If you want more information about the fungal pathways and D-galacturonic acid catabolism contact the author Dr. Peter Richard. He works at VTT Technical Research Centre of Finland Ltd. as Principal Scientist in Metabolic Engineering research team. His main interests are metabolic engineering of yeast and mould for the production of fuels and chemicals.

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