The green side of asparagus: a new approach to bioplastics production

Asparagus season has officially been over in Germany for about a month now – but at the Institute for Bioplastics and Biocomposites (IfBB) in Hanover, it is still going strong. The "SpaPlast" research project shows how asparagus peels can be used as a filler for bioplastics. The project is funded by the Lower Saxony Ministry of Food, Agriculture and Consumer Protection.

Maximilian Bergholz, a research associate at the institute, tells K-MAG in this interview about how these otherwise unused peels are converted into a valuable raw material. We talk about the background to this idea, the manufacturing process and the potential benefits of the asparagus bioplastic.

Mr. Bergholz, how did the idea of producing plastic from asparagus peels come about?

Maximilian Bergholz: As popular as asparagus is as a culinary product, its peel has been insignificant until now - even though it makes up around a quarter of the entire spear. In Germany alone, for example, around 30,000 tons of asparagus peels are returned to the fields every year, where they rot without any additional benefit. In fact, according to Statista, asparagus production is about 120,000 tons per year in Germany, a quarter of which is peel.

The idea behind SpaPlast came from the fact that asparagus fiber is chemically similar to wood fiber, one of the most common fillers in polymers. With the asparagus peels, therefore, a residual material is used that is already there anyway, so that wood can be saved as a raw material.

Can you use any type of asparagus peel for this?

Bergholz: Generally speaking, yes. Even though trials have so far been conducted exclusively with white asparagus, the use of green asparagus is also imaginable. However, due to slight differences in cellulose, hemicellulose and pectin content, adjustments in further processing may be necessary.

Approximately how many kilos of asparagus peels are needed to produce one kilo of plastic?

Bergholz: Wood-filled plastics such as the so-called wood-plastic composites achieve a filler content of 50 to 80 %, which is also realistic for flour made from asparagus peels with suitable extruders. Accordingly, 500 to 800 grams of dry asparagus peels are required to produce one kilogram of plastic compound. However, because freshly harvested asparagus peels have a water content of over 90 %, this estimate must be increased tenfold to 5 to 8 kilograms.

Can you explain the manufacturing process in more detail? What special techniques or procedures are used in the process?

Bergholz: At the beginning there is the collection of the asparagus peels and rapid drying before the peels would start to mold. If possible, drying should be done with waste heat from a large plant to keep costs low and the overall balance ecologically positive. The dried peels are ground to a particle size of about half a millimeter in diameter before being incorporated into the melted plastic matrix in the extruder.

Does the bioplastic made from asparagus peels have any special properties or advantages compared to other bioplastics?

Bergholz: One of the biggest advantages of asparagus plastic is that, as a residual material, it does not take up any additional land. As an alternative to wood flour, this can lead to a relief of the wood market as well as counteract overforestation. In addition, there is a suspicion that asparagus peels lead to eutrophication, at least to a small extent, when they are spread on the field, without being able to quantify their nitrogen content. The extent to which they improve the material properties of the plastic has yet to be evaluated. However, initial trials suggest that they are in no way inferior to wood-filled plastics.

Where has the asparagus bioplastic already been used so far?

Bergholz: So far, the project is still to be regarded as a feasibility study, although on the basis of the promising initial results, the aim is to develop it further until it is ready for the commercial market.

What advice would you give to people in the industry who are now interested in this topic?

Bergholz: There is already a lot of interest in SpaPlast. In addition to various media, asparagus companies, manufacturers of peeling machines and the wood processing industry have become aware of the project. We advise interested partners to get involved in the development of logistics concepts at an early stage so that the material can be transported efficiently from decentralized asparagus farms to central processors, where it can be dried at least as efficiently. If this challenge succeeds, the result will not only be an innovative bioplastic, but also new sales channels for all involved.

What do you wish for SpaPlast in the future?

Bergholz: For the future of SpaPlast, opportunities for further process and material development are desirable in order to fully exploit the potential of the project. Work on the use of asparagus as a residual material will continue at IfBB.