How insect larvae should make biorefineries more sustainable

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The "black soldier fly" is a secret star among insects. More precisely: their larvae. They grow quickly and are undemanding when it comes to food. The best prerequisites, therefore, for turning organic waste into valuable products in insect farms. "Compared to other insects, soldier flies are often more frugal, they are satisfied with organic residues and thrive under different environmental conditions," explains Susanne Zibek from the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB) in Stuttgart. She heads the InBiRa (insect biorefinery) project and, together with an interdisciplinary team and project partners, wants to show that soldier fly larvae have even greater potential for the chemical industry.

Could the voracious larva become a substitute for petroleum products or tropical fats such as palm kernel and coconut oil "Made in Germany"?

Biorefineries are complex plants and operate on an industrial scale. In contrast to other refineries, which mostly process petrochemical products, biorefineries use products made from renewable, organic raw materials. Up to now, biorefineries have used sugar, starch, plant residues such as grass silage, fungi or microorganisms as a basis. For the first time, InBiRa is looking at insect larvae that are fed with residues from the region. Since up to now insect farms have often fed specially produced insect food, this would be an important step towards a sustainable circular economy. From the residue stream, the industrious fly larvae produce four material streams: Fats, proteins, chitin and residues such as faeces. These four fractions are used in the biorefinery to create products for further industrial processing, especially for the chemical industry, which then uses them to manufacture end products for everyday consumption - from fuel to hair shampoo.

How fly larvae upcycle organic waste

A mother soldier fly lays up to 500 eggs, from which the larvae quickly hatch. The insect offspring can upcycle organic residues into fats and proteins within only ten days. In the next step, they are processed into larval dry matter and then split into their components. "Fractionated", as engineer Zibek says. The dry matter consists of almost 50 per cent each of proteins and fats. More concrete information on the ratio of feed, amount of larvae and yield is not yet available. For comparison, another research team led by Hui Wang from Wuhan University concluded in 2017 that mealworms and soldier fly larvae converted more than 50 per cent of their maize straw feed into insect biomass. 8.5 grams of larval biomass provided 1.95 grams of crude fat to produce 1.76 grams of biodiesel, 6.55 grams of protein and 111.59 grams of biofertiliser."

Since Zibek first analysed soldier fly lipids twelve years ago and recognised the similarity of their fatty acid composition to coconut and palm kernel oil, the topic has not let her go. Fatty acids have a chain of carbon atoms as their structural framework, the so-called carbon chain.

The length of this chain is decisive for the chemical and physical properties of a substance, such as the melting point. Insect fat, with its high proportion of fatty acids with medium chain length - such as lauric acid, which occurs in larger quantities and has a chain length of C12 - is similar to tropical vegetable fats such as coconut fat and palm kernel oil. Native fats such as rapeseed or sunflower oil, on the other hand, have more longer-chain fatty acids, such as C18.

However, German oleochemistry needs many medium-chain lipids for the products in demand and therefore imports large quantities of tropical fats and oils. With insect fats from domestic production, one could become less dependent on imports. The current supply chain problems in many industries have shown that establishing domestic production of such industrially important raw materials could make sense.

Insect fats instead of petroleum and tropical fats

One of the InBiRa focal points is fat-based biosurfactants, which act as washing-active substances in cleaning products and as emulsifiers in cosmetics. Insect fats would be more sustainable alternatives to modern surfactants made from petroleum products and tropical fats. Since both petrochemical substances and tropical products, for which rainforests are often cleared, are harmful to the climate and ecologically questionable, more sustainable alternatives are urgently required. Surfactants made from regionally produced insect fat would hark back to the time before petrochemicals and tropical raw material imports; after all, soaps were once made from animal bones and fats. Besides biosurfactants, biofuels and industrial lubricants are other important areas of application for biofats.

The first step in the InBiRa project is to further explore how the changing composition of food affects growth and the chemical composition of insect lipids, says Susanne Zibek. After all, the composition of the leftovers from food and catering, organic waste bins and other sources changes daily. The insect proteins, which are also valuable, are being tested in other projects, for example as a basis for films and coatings.

Use of the chitinous skins of the larvae

Another project focus is the use of the chitin skins of larvae and adult dead flies. The biopolymer chitosan for the textile industry can be obtained from the chitin in several process steps. Since chitosan also has an antibacterial effect, it is also interesting for the medical sector and is suitable as a thin film for new types of packaging for fruit and vegetables.

"At the end of the project, I hope that after the pilot plant we will be able to set up small industrial plants and exploit our technologies from the InBiRa project together with the collaborative partners," says Zibek.

(jle)