Tomato leaves are surprisingly high in protein: up to 27 percent. This makes the leaves a potentially interesting source of protein, particularly at a time when demand for plant protein is increasing. However, large-scale application is not yet possible. PhD research by Marietheres Kleuter shows which obstacles still need to be overcome to turn tomato leaves into a valuable raw material.
"Waste doesn't exist" - that is the premise of a circular economy. Smart and creative solutions are needed to make better use of residual streams such as plant waste. Wageningen University & Research is therefore researching ways to turn this kind of waste into something valuable.
Kleuter: "Much of the protein in tomato leaves consists of Rubisco. That is an important enzyme for photosynthesis: it helps the plant capture CO2 from the air. Ideally, you can turn this protein into a dish similar to tofu. But that's a way off. In our experiments, the extracted protein remained green due to the chlorophyll. Which isn't ideal - people aren't used to green protein - but if you use it in shakes or as a dietary supplement, for example, the colour is less of a problem."
It will probably be some time before there are tomato plants in the greenhouse that not only produce fruit but also useful proteins from their leaves. "There are still many factors that make extraction difficult," Kleuter explains. "An important one is that the cell walls in the leaves form a physical barrier, making it difficult to extract the protein. The composition of the cell walls also changes as the leaves get older. That reduces the yield of extracted protein, especially from the older leaves."
Degradation in older leaves
There are more reasons why it is best to extract protein from young tomato leaves. As the leaves age, the proteins break down into smaller molecules called peptides. This is due to enzymes that break down proteins - called proteases. "In our trials, we turned off two genes that are responsible for the production of such proteases," Kleuter explains. "That worked: protein degradation was inhibited, but not until the very end of the growth period."
This was the starting point for extensive research into how proteins in tomato leaves are built up and then broken down. This was done through a so-called proteomics analysis. The analysis showed that many more genes play a role in protein content and the ability to extract that protein. These are important insights for agriculturalists. When breeding new tomato varieties, they can now look more specifically for traits that ensure the protein content remains high even as the leaves age. Which is important because using young leaves is difficult in practice. These leaves are very important for photosynthesis - and thus for the growth of the tomatoes themselves.
New protein crops
Kleuter's research links up with broader work within Wageningen University & Research aimed at finding new sources of plant protein. Such protein crops are vital due to the continued growth of the global population and the fact that the current use of animal protein is not sustainable.
A good example of this is duckweed. Under the name water lentils, this crop has now been approved by the EU as a source of food and protein. This was preceded by years of research at WUR. "Ongoing studies are also looking at how well proteins from existing varieties and variants (accessions) of agricultural and horticultural crops can be extracted in practice, for example in sugar beet," she concludes.
