Which opportunity for plant leaf proteins?

In recent years, more and more research has been carried out into finding new alternatives to animal protein sources, such as pulses, cereals, oilseeds and others, with a view to meeting the future needs of an ever-growing population. In this context, proteins from plant foliage, traditionally used as animal food or integrated into animal feed, may represent an alternative that has not yet been fully explored as a sustainable plant protein.

Plant leaf protein: the most abundant protein in the world

The idea of using proteins extracted from plant leaves for human consumption dates back to the 1960s, when there was already growing interest in protein from a widely available source such as leaves. However, the first extraction processes produced a protein concentrate from leaves with a dark green colour and a somewhat herbaceous, bitter taste.

However, the first extraction processes produced a protein concentrate from leaves with a dark green color and a somewhat herbaceous, bitter taste. It later became clear from research that it was necessary to extract only one of the two protein fractions within the leaf, as one is green-colored with strong-off notes, and the other one is light-colored with a good sensory profile, which could be of interest for human consumption.

This is known as the ‘soluble’ fraction, which mainly comprises the enzyme RuBisCo, which is crucial for photosynthesis and for its role in fixing atmospheric carbon. The ‘soluble’ fraction is the most interesting one, as it has better organoleptic and sensory properties, with a light colour and high protein quality, unlike the ‘insoluble’ fraction which is green in colour and has a grassy taste. According to the study entitled ‘Plant leaf proteins for food applications: Opportunities and challenges’ under consideration here, the amount of protein in leaves is limited, as it can vary from 1% to 8%, depending on the source and its stage of growth. From a nutritional point of view, however, these proteins appear of interest due to their content of essential amino acids, with the exception of methionine and lysine, which may be limiting. In terms of digestion, while leaf proteins are less digestible than animal proteins, they are in line with other commercially available vegetable proteins such as pea, soya, broad bean and others. Few allergic reactions have been reported in the literature, but none of them life-threatening.

Known extraction methods include crude extraction, which involves pulverising and pressing the leaf to obtain a liquid which is then used to coagulate the protein, thermal and enzymatic extractions, alkaline extraction, sonication or ultrasonic extraction, ultrafiltration, as well as thermal and acid precipitation. To date, all these methods have been tested on a laboratory scale, and not adapted for large-scale application, as the operational costs for extraction and purification limit their scalability.

Challenges and players

In short, while protein from leaves has the potential to be an abundant and available source of protein, a number of factors limit its large-scale production, in terms of cost and process scalability, as well as end-consumer appeal. Nevertheless, the variety of leaf protein sources allows for the development of different product types to meet customised nutritional needs in the market for functional ingredients. Moreover, the same variety of sources also allows for more extensive experimentation at the level of extraction techniques, which is an interesting subject for research and scientific progress.

A number of start-ups are already working on the development of leaf proteins: for instance, Rubisco Foods – a Dutch company working on the extraction of proteins (>50%) from the biomass of water lentil and alfalfa, as ingredients with high nutritional value for protein enrichment in baked goods, savoury products or even beverages. Also The Leaf Protein Co., an Australian company, has developed a method to extract protein through filtration, separating the chlorophyll from the other components of the leaf. The company has successfully extracted proteins from Atriplex (saltbush) and Pereskia as a ‘proof-of-concept’ of their technology.