Hunting for high-value substances in biowaste

We are all familiar with tossing out biowaste, diligently filling our green bins for municipal composting and taking the garden waste to the recycling centers. While this might seem like a lot of biowaste, it is nothing compared to the massive quantities generated by food, supplement and dye manufacturers. Think of the apple pulp produced by a cider mill, tomato skins from a canning factory or coffee grounds from a café chain.

Traditionally, this surplus has been treated as waste, carted away and burnt. Then the world recognized the potential of converting biowaste into fuels as an alternative to fossil fuels, and now people are beginning to see another layer of value in biowaste that can be extracted.

– Biowaste contains many different natural substances, which we refer to as high-value substances. They can be extracted from biowaste and used in a way that is more sustainable and therefore more environmentally friendly than if the industry were to produce them from scratch, explains Lars Porskjær Christensen, professor of chemistry and head of the Department of Physics, Chemistry and Pharmacy.

Sugars, antioxidants and fatty acids

Christensen’s chemical research focuses on methods to extract interesting high-value substances, especially bioactive substances from plants like carrots, that could be valuable to the industry.

Examples of high-value substances include unsaturated fatty acids, dyes, proteins, antioxidants, medicinal substances and sugars.

• Unsaturated fatty acids can be extracted from plant biowaste (like nuts, seeds, olives) and fish, and added to dietary supplements.
• Dyes can be extracted from purple carrots and Spirulina cyanobacteria and are e.g. used for food colouring.
• Proteins can be derived from red clover, sugar beet tops and various other plants and used in food and animal feed.
• Antioxidants can be extracted from various types of plant waste and used in foods and cosmetics.
• Lignin, found in cellulose-rich plant materials like sugarcane, corn stalks and trees, is used as a strengthening component in building materials and as a building block in various synthetic products.
• Special sugars such as fructans are present in many plants and can be extracted from sources like onions, garlic and wheat. Fructans include inulin, a polysaccharide extracted from chicory roots, which is used in supplements and foods.

Medicinals substances in red clover and carrots

Medicinal substances can be extracted from plants like red clover. In addition to plant proteins, red clover contains phytoestrogens, which are used as dietary supplements for menopausal women. While high-value substances are usually first extracted from the biowaste left over after processing the plants, the red clover requires the opposite approach: The phytoestrogens are extracted first and afterwards the plant proteins.

Another interesting example is extracting bioactive substances from purple carrots, specifically anthocyanins used for dyes. The bioactive substances in carrots are closely related to unsaturated fatty acids and are extracted after the dyes have been removed from the purple carrots. The carrots use these bioactive substances as a defense against fungal attacks, and they have shown promising results in animal studies for cancer and type 2 diabetes prevention.

Greener chemistry

While numerous start-ups attempt to succeed with high-value substance extraction, researchers and engineers are focused on refining extraction techniques to make them more sustainable and economically viable.

Most extractions of natural substances require a solvent to draw high-value substances from biomass. Previously, researchers used mineral-based, environmentally harmful solvents, but modern chemistry aims for greener methods.

At SDU, researchers are using a newly acquired supercritical extraction machine, funded by the SDU Climate Cluster, to develop new eco-friendly methods for extracting high-value substances from biomass and biowaste without using harmful solvents. Instead, the machine utilizes pressurized CO₂, sometimes combined with more environmentally friendly solvents like ethanol or methanol.

– The principle is to place the biomass or biowaste in a container, fill it with CO₂ and apply pressure. Under pressure, CO₂ becomes liquid and acts as a solvent, extracting high-value substances into the liquid CO₂. When the pressure has been released, the biowaste and high-value substances separate, explains Lars Porskjær Christensen.

So far, the machine has processed biomass from cannabis plants, which contain cannabinoids like psychoactive THC, used for pain and anxiety treatment, and CBD, which has pain-relieving and anti-inflammatory properties. Besides these active substances, cannabis biomass also contains high-value substances like unsaturated fatty acids and vitamin E.

High-value substances in cannabis, along with many others from biomass and biowaste from other plants, can be extracted effectively with the supercritical extraction machine with a focus on achieving sustainability in the process.