In Germany, more than 500,000 metric tons of old baked goods are produced each year which, without additional processing, are not suitable for further consumption or as animal feed. Until now, their use has been primarily energetic, for example in combustion processes or in biogas plants. Old bakery products such as bread, rolls or cakes contain large quantities of starch. The starch can be converted into the basic chemical HMF, which offers potential for a wide range of applications. “In our subproject, we determined the application potential for HMF in more detail, as regionally available old bakery products represent a meaningful resource over and above energetic utilization,” explained the Project Manager at the Fraunhofer WKI, Dr. Steven Eschig.

The project team at the University of Hohenheim developed a process for the so-called hydrothermal treatment of old bakery products, through which moist biomasses are converted under heat and slightly increased pressure. From the old bakery products and the starch contained therein in large quantities, HMF is created in aqueous solution and carbon. “The process parameters, such as pH value, temperature and duration, are selected in such a way as to achieve the highest possible yields of HMF,” explained Markus Götz, employee in the specialist area of Andrea Kruse at the University of Hohenheim, who is leading the project. Carbon is produced as a by-product of the hydrothermal treatment. It can be used as a biofuel or as soil fertilizer. Simultaneously, it is a good adsorbent and can therefore be utilized as activated carbon.

“Here at the Fraunhofer WKI, our task was to isolate the HMF from the aqueous solution and to process it further,“ said Dr. Eschig. He and his team discovered that methyl isobutyl ketone (MIBK) works better as an extraction agent than chloroform (CHCI₃) and that the addition of sodium chloride has a positive effect on the quantity extracted. In addition, they were able to prepare and characterize polyesters using furandicarboxylic acid.

HMF is a versatile starting material, as it can serve as a substitute for formaldehyde, for example in formaldehyde-free resins and bio-adhesives. Furthermore, it can form chemical bonds which can be dissolved again when the temperature is increased. This enables the production of materials with self-healing properties. The possibility of the reversible dissolution of the chemical bonds can also be used for switchable adhesives, as a result of which new recycling possibilities are created.

Via chemical modifications, so-called dialcohols (reductive) or dicarboxylic acid (oxidative) can be obtained from HMF. They can be used as building blocks for polymers, for example in the manufacture of coatings or fibers. The production of the plastic polyethylene furandicarboxylate (PEF) as a PET substitute has already been tested. PEF made from renewable raw materials is not only ecologically advantageous; it is also lighter and more durable and therefore of great interest to the beverage industry.

The researchers were able to demonstrate that old bakery products are suitable for higher-value applications and represent an attractive alternative for the industry as part of a bio-based recycling economy. The project was funded by the German Federal Ministry of Education and Research (BMBF) via Project Management Jülich.

Joint press release of the University of Hohenheim and the Fraunhofer WKI.