Sustainable protein from tomato leaves, fungi and insects

The way we produce and consume protein today places an unsustainable burden on our natural resources, and the problem is getting worse as demand for animal-based protein grows. A protein transition is essential if we are to ensure a sustainable diet for everyone on the planet in future. Wageningen University & Research is looking for ways to increase the availability, diversity and acceptance of existing and emerging sources of protein. Circular systems are an example of how this might be achieved.

In an optimal food system, humans and animals would not be competing for food, and any resources suitable for direct consumption would indeed be used as food. Any protein sources unsuitable for human consumption could be upcycled using, for example, micro-organisms or insects. Here are a few examples of projects in which WUR is working on circular protein production:

Protein from tomato leaves

There is value in high-protein crop residues, but this value is often not extracted and used in the food chain. Residues might be composted or, at best, used as animal feed. The leaves of all plants, such as those of sugar beet or tomato plants, contain the protein rubisco. “At least a quarter of all the protein in the leaf is rubisco,” says Marieke Bruins, Senior Scientist in Protein Technology. “This protein is essential for capturing and storing CO₂ from the atmosphere. It’s readily soluble and not attached to anything else in the cell, so you can squeeze it out of the leaf.” But the disadvantage to tomato leaves is that they also contain toxins. Bruins and her colleagues had previously already developed a patented process for extracting protein from leaves. They’ve now also done tests to see if this process prevents toxins ending up in the final product, making it safe to eat. The method is currently being used by Cosun, a beet producer, to extract protein from sugar beet leaves. But Bruins hopes to see it being rolled out more widely. “There’s such an abundance of leaves on our planet, and ultimately we’d like to make use of every resource available to us. At present, tomato leaves are cut off and just swept up into a pile. The best case scenario is that they end up in a compost heap. And just think of the quantity of leaves generated as a result of greenhouse horticulture.”

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One advantage of rubisco protein is that it can be easily processed into a gel. “That’s not a quality you typically find in many other plant-based proteins, such as soy,” says Bruins. Producers will often add egg as a way of achieving a particular structure, which might be helpful if it’s to be used in meat substitutes or plant-based dairy products. But that means it’s not vegan. “The only other plant-based protein that can make a good gel is potato protein,” says Bruins. “You can also use starch or methylcellulose, but the product will then be high in carbohydrates, which is often not what you want.” According to Bruins, rubisco’s gelling quality is what makes it really appealing to food producers. “Its nutritional value is also very good. It’s just that the extraction process is relatively expensive. That’s why it can’t compete with soy, which also has good nutritional value and is much cheaper. What we want to do now is find the right balance, which might be 10 percent rubisco protein for the gelling properties, supplemented with cheaper protein like soy for its nutritional value.”

There’s another challenge for the researchers, which is that only a portion of the protein is actually made available for use. "A substantial amount is lost as part of the purification process, which is carried out to achieve a colourless product. Many of the proteins take on a brown/green colour when they’re extracted, and that’s something you want to avoid. But avoiding this means you end up with a lower yield of protein. We’re going to try and improve on this.” Bruins is also working with researchers from the Plant Breeding group as part of efforts to improve the extraction process. “Crops like tomatoes and sugar beet were originally developed for their primary product: the beet itself, and the tomato. We want to see if we can make adaptations that optimise secondary protein extraction from the leaves of these crops too. It’s less about the absolute amount of protein, and more about the proportion we can actually extract. Because right now we’re only using a small proportion.”

Bruins is also researching whether the process works for other plants too. “We’ve noticed that yields vary between plant species and varieties. We want to see if we can make sense of that and optimise it.” The researchers are also looking into opportunities for using fibre from leaves and stems. These might be used for the production of packaging materials, for example. “It would be great if we get to the point where none of the plant is wasted.”

Mushroom-forming fungi as an alternative source of protein

Another potentially interesting and sustainable source of protein is mushroom-forming fungi (basidiomycetes), and in particular the mycelium, which is the network of fungal threads normally discarded. Little research has so far been done into this. “These fungi have the advantage of being able to grow on lignocellulose, which is something that few other organisms can do,” says Karin Scholtmeijer, research associate in Plant Breeding. Lignocellulose is a woody material found in all plants, and it’s the largest renewable type of biomass on Earth.

Unlike existing types of fungal protein (such as tempeh or Quorn) which are cultivated on substrates that are themselves suitable for human consumption, Scholtmeijer hopes to produce proteins from renewable forms of biomass that have not previously been used in this way. “As a bonus, you don’t need any additional land, because you can grow them in buildings. And if you grow only the mycelium, and therefore don’t have to wait for the mushrooms to appear, you have a shorter production cycle.” Scholtmeijer’s research has included looking at the quantity of protein found in different species of fungi. The researchers grew 60 species of edible mushroom-forming fungi on a variety of substrates such as beet pulp, rice straw, cocoa pods and wood chips. “We’re seeing vast differences in protein levels across all these species. The fungi also produce a lot of free amino acids, which is useful in terms of nutritional value because these are easily absorbed in the intestine.” Other healthy components in the fungi include fibre, vitamins B and D, and components that boost the immune system. They also come in a variety of different textures, aromas, colours and flavours.

But before you start using them to make something, it’s important to understand how consumers feel about these types of products. Scholtmeijer and her team have carried out research into this with colleagues from the Social Science group. “What we found is that people are certainly prepared to eat it, but preferably as a ‘clean label’ food, meaning as unprocessed as possible,” she says. “But if we tell them more about the sustainability aspects, they’re willing to try it in other forms, such as a powder.”

Future research will include selecting the best fungi from the trial for further investigation, partly to determine their precise composition. “We want to see if we can use breeding techniques to produce better varieties with a higher protein content,” says Scholtmeijer. “Mushrooms aren’t quite such a good source of protein as people say, because the measurement of their protein content is based on their dried weight and mushrooms/fungi have a high water content. So you’d really have to eat a lot of mushrooms if you were trying to match the protein content of a chicken breast.”

According to Scholtmeijer, there are still a few practical hurdles to overcome before these fungi become part of our staple diet. “If you grow them on wood chips, for example, you will need a way of separating the two, because people probably don't want to find wood chips in their food.”

Breeding insects

Insects can make a valuable contribution to the protein transition because they can be reared on residues and then processed into products that can be eaten by both people and animals. “We can’t keep using soy to feed our growing population of farm animals,” says Esther Ellen, project leader for Insect Breeding. “Insects have a similar nutritional value but they’re more sustainable because they don’t require much land or water, and they can be reared on residues.”

However, rearing insects on a large scale for protein production is still challenging. Yields will fluctuate, for example. “At present, insect production is focused on producing larvae and rearing those larvae,” says Ellen. “The rearing is stimulated by optimising their food. But no one is looking at the individual differences or assessing which species are most efficient.”

Breeding systems could play a key role in the scaling up of insect protein production. Ellen is working on breeding programmes and is researching whether genetic selection could improve protein production in insects. “We’re looking not just at larvae growth, but also at how efficiently larvae use their food and at health and welfare.”

Ellen and her colleagues have looked at which characteristics are actually hereditary, and how strong the heredity factor is. “You need to know that so you know what you could select for. One of the things we’ve found is that it’s possible to select for body weight and ultimately for protein yield.”

The researchers also used a computer simulation to look at what would happen if you did in fact select for these characteristics. The initial results are promising: it seems that you can indeed produce more protein using less feed and fewer animals. The breeding programme will be tested on the black soldier fly, a commonly used insect species. “It’s trickier breeding insects compared to other animals like cows, for example, because with insects you can’t identify individuals,” says Ellen. “So you don’t know who the parents are.”

At present, the insects are housed in large groups and they all mate with each other. The breeders would therefore need to make some significant modifications to the insect housing, and keep the parents separate, for example. An added complication with black soldier flies is their insistence on having group sex, says Ellen. “You could try putting a male with 10 females and seeing how the offspring of that male perform, in order to develop lines that grow better or are less prone to illness, for example. Breeders are open to this kind of thing and we’re actually organising Masterclasses where we share knowledge with them and invite them to participate in the process.”

Insect breeding – like the insect sector as a whole – is still in its infancy, but Ellen is pleased that we’re already thinking about how we want to approach it. “For example, we don’t want to select for production alone. We also want to think about inbreeding, health and welfare. We have an opportunity now to establish good practice early on, and to learn from experiences of breeding in traditional livestock farming.”

Thanks to new EU regulations, insect protein can now also be used in animal feed for poultry and pigs. Some insect products have also been approved for human consumption. There are some restrictions, though. The residues used in production must be clean, for example. “At present, insects are being fed with things that you could also feed to farm animals. It would be good if we could eventually rear them on by-products that can’t be used as food for humans or farm animals, such as manure. Then we’d really have a circular system. But existing legislation doesn’t allow that.” Work is currently underway to identify and assess the safety risks of various categories of by-products that can’t be used directly as food for humans or farm animals.