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Once all the peppers have been picked, the last leg of the leaves’ and stems’ journey takes them to the compost heap. Researchers from Jülich, Aachen and Bonn don’t think that this needs to be the case. They are investigating whether valuable substances can be extracted from the unused plant leftovers for use in medicine, cosmetics and the food industry. In the end, the vegetable farmer could profit: he sells not only the fruit, but also the accessory agents.

They are crisp, fresh and healthy: bell peppers. The fruit from the nightshade family has long since landed in salads, frying pans or on the grill in all colours all year round. It’s no wonder that farmers in Germany are growing more peppers. Classically, they sow the peppers in the greenhouse, take good care of them, harvest the bell peppers and sell them. Leaves, stems and roots end up as compost. Dr. Anika Wiese-Klinkenberg from the Plant Sciences (IBG-2) section finds that these are “squandered resources”.

Valuable substances

Die Wissenschaftlerin hält eine Paprikaschote in der rechten Hand und in der linken Hand eine Paprikapflanze. Sie trägt ein rotes Shirt und ist vor rotem Hintergrund abgebildet.Plant physiologist and paprika researcher Dr. Anika Wiese-Klinkenberg.
Copyright: Forschungszentrum Jülich / Sascha Kreklau

In the TaReCa joint research project, the plant physiologist, together with colleagues from RWTH Aachen University and the University of Bonn, is developing technologies to utilise the plant’s leaves and stems as well: “After the fruit harvest, we use the plant as a production factory for valuable substances known as secondary metabolites.” These include pigments, fragrances and flavours, but also poisonous and bitter substances as well as antioxidants. Such plant constituents are particularly interesting for medicine and the cosmetics and food industries.

The plants use these substances to attract insects or fend off vermin such as viruses, fungi or bacteria. They produce the bioactive substances when they are under stress, for instance, such as from too much sun. “For example, plants cannot put up an umbrella to protect themselves from too much ultraviolet light and therefore form secondary metabolites that absorb the ultraviolet light,” explains Wiese-Klinkenberg. The Jülich scientists want to produce such stress reactions in a controlled manner so that the plant produces more secondary metabolites and the residual mass from paprika production is upgraded.

Scientists know of over 100,000 such secondary plant substances. These include the flavonoids cynaroside and graveobioside A, which are also formed by the paprika and in which the Jülich researchers are interested. “We also want to examine the paprika leaves for other interesting ingredients, however.”

Promising results

In recent years, the number of patent applications involving the substance cynaroside has increased. “We therefore assume that cynaroside is an ingredient with promising market potential for the cosmetics and pharmaceutical industries,” explains Wiese-Klinkenberg. Graveobioside A is known to act against insect egg deposition: In the long term, it could be used as a natural biocide.

In order to stimulate the production of the two substances, the scientists put the plants under stress in climatic chambers: with common salt in the nutrient solution, with low temperatures and ultraviolet light, or they deprived them of their nutrients. “The salt stress was the most effective. Through the combination of salt addition and other stress factors such as cold, we have succeeded in increasing the cynaroside content in the leaves tenfold. That was a surprising success,” says the researcher excitedly. The researchers were also able to measure a triple increase in the concentration of graveobioside A in certain salt stress combinations: “We get about 20 mg graveobioside A per gram of dry weight. That’s a good yield.”

Drei Paprikaschoten vor neutralem Hintergrund: links und rechts rote, in der Mitte eine gelbe.Greenhouse paprika cultivation in Germany grew from 41 hectares in protected cultivation in 2008 to 108 hectares in 2018. In 2018, the gardeners harvested an average of 136 tons per hectare, which is around 14,700 tons in total (figures: Federal Statistical Office ). However, this covers only a fraction of domestic demand. The imports amounting to 394,000 tons (2018, source: Federal Statistical Office) come mainly from Spain, the Netherlands, Hungary and Turkey. The world’s largest pepper producer in 2017 was China with 17.8 million tons. Spain, the largest European grower, ranked 5th with 1.3 million tons (figures: Food and Agriculture Organisation of the United Nations - FAO).
Copyright: Pixabay License

The scientists are now working on better coordinating the interaction of optimal salt dosage, treatment duration and various stress factors, because “too much” means that the plant dies and drops its leaves, “too little” means too little yield. “In addition, the vegetable farmer also needs a measurement method to control how stress works,” says the researcher. “For this, we use non-invasive methods with photos and colour analyses, because the colour of the leaves, for example, changes with the stress.”

Recognising market potential

Vegetable farmers in particular could benefit from these research approaches: they sell not only the fruits, but also the accessory agents from the rest of the plant. “In order to assess the market potential of individual ingredients and possible areas of application, the TaReCa joint research project involves not only plant researchers, horticulturists and process engineers, but also economists,” explains Wiese-Klinkenberg. For example, they can calculate whether the farmer should dry the leaves and stems himself or whether a large contractor should collect them: the drying process would significantly reduce transport costs, but the vegetable farmer would have to invest in a drying oven.

The peppers in the salad, the flavonoids extracted – what remains is the fully exploited pepper plant, which no longer ends up in the compost, but in a biorefinery, where it is converted into platform chemicals for many other industrial products and has, thus, finally completed its service. The value chain is thus extended, economic added value is generated and a valuable substance is produced in a resource-saving manner – an example of a successful bioeconomy. And this not limited to the peppers. “Long term, we want to apply the new processes and technologies to other horticultural plants,” says Wiese-Klinkenberg. Her next favourite: the cucumber.

About TaReCa

TaReCa – the acronym stands for the joint project “Tailoring of secondary metabolism in horticultural residuals and cascade utilization for a resource efficient production of valuable bioactive compounds”. TaReCa is funded by the Federal Ministry of Education and Research.

Katja Lüers