Using Plant Leaf Proteins as a Source of Dietary Protein
The planet’s resources are limited. As the world’s population increases, scientists are looking into new ways to reduce food waste, and maximize the value of agricultural production. A common theme is the transformation of what is traditionally seen as waste into a new and valuable product, a little like how we can collect kitchen scraps to turn into compost, or use spent coffee grounds to grow gourmet mushrooms. One waste product that is produced in enormous quantities worldwide are the plants left over after crops have been harvested. Think about the leaves of corn, wheat, rice, millet or sugar beet. While these leaves and stalks can simply be left to decompose in the field, or alternatively animals can be allowed to graze on the plants, the plant matter also contains useful compounds such as cellulose that can be converted into bioethanol, and plant protein for human or animal nutrition.
But it is not a simple matter of putting the plant leaves into food. The protein content of leaves is rather small compared to other foods. For example, edible leaves such as raw spinach (11457) contains 2.9 g protein per 100g, raw kale (11233) contains 4.3 g protein per 100g, and Swiss chard (11147) contains 1.8 g protein per 100g. Meeting protein needs with leaves requires a lot of munching! On the other hand, vegetarian protein source cooked lentils (16070) contain 9.0 g protein per 100 g, with 200 g in one serving, and ground beef (13047) contains 19.4 g protein per 100g (5-digit numbers in brackets are references to the USDA food database). If plant leaves are to be used as a source of protein, it makes sense to concentrate the proteins first. Tenorio and colleagues discuss the key steps in the extraction of plant proteins for human nutrition.
The process is somewhat similar to the production of cheese or particularly tofu. First of all, a mechanical process breaks open the plant leaf cells to release the proteins. Then, the proteins are precipitated either by the use of heat, pH or enzymatic treatments (analogous to the use of rennet and salt in cheese production, or nigari or calcium sulfate for tofu production), then the protein can be concentrated from the solution. The most common protein that is extracted is called rubisco, while leaf pulp and an insoluble curd remain over from the process (unlike cheese and tofu, the solubilized protein in the whey is the most useful as many green-colored proteins remain fixed to the fiber in the curd). The current processes are capable of extracting about half of the protein in leaves, and it is currently not yet economically feasible. The authors provide a schematic overview of the protein content in the various fractions that are produced when protein is extracted from leaves. Each part of the process has the possibility to be further researched to determine the best conditions to extract more protein for each plant leaf type.
While it is too early for veggie burgers made from protein recovered from crop leaves and stalks to hit the shelves, as we move further into the 21st century, it may make an important contribution to providing enough protein for human nutrition.
Tamayo Tenorio, A.; Gieteling, J.; de Jong, G.A.H.; Boom, R.M.; van der Goot, A.J. Recovery of protein from green leaves: Overview of crucial steps for utilisation. Food Chemistry 2016, 203, 402-408, http://dx.doi.org/10.1016/j.foodchem.2016.02.092.
Tan L, Tang YQ, Nishimura H, Takei S, Morimura S, Kida K. Efficient production of bioethanol from corn stover by pretreatment with a combination of sulfuric acid and sodium hydroxide. Prep Biochem Biotechnol. 2013;43(7):682-95. doi: 10.1080/10826068.2013.773338. http://www.ncbi.nlm.nih.gov/pubmed/23768113