As global demand for protein increases, so does the demand for sustainable protein sources. Grasses and legumes such as alfalfa and clover have long been used as protein sources for ruminants, but monogastric animals such as pigs have difficulty metabolising the fibres and protein from this type of plants. There are also environmental benefits of the green fodder crops with a long growing season because they bind carbon in the soil and have less leaching of nitrogen to the aquatic environment. In addition, they have a lower need for pesticides than, for example, cereal crops. 

But as long as only ruminants can benefit from the proteins in grasses and legumes, it is limited how much the green fodder crops can alleviate the problem of the high demand for sustainable protein for monogastric animals. But running them through a biorefinery can change that.

“It is old news that you can broaden the feed potential of protein from fodder crops with the help of biorefining technology. Here, protein is separated from the plant fibres into a green protein concentrate. And this protein can be digested by monogastric animals such as pigs, while the fibrous mass (pulp) has shown good results as feed for ruminants or as use for biogas,” says post doc Henrik Thers from the Department of Agroecology.

Recognised method

To assess the potential of different types of green biomass, the material is screened to see whether there is protein to be processed, how much ,and how it is fixed within the plant. Researchers from the Department of Agroecology and the Department of Animal Science at Aarhus University have now validated the most widely used method for screening green biomass.

“A recognised method for analysing green biomass is called the Cornell Net Carbohydrate and Protein System (CNCPS). It is a method commonly used to assess the quality of a biomass in relation to the feeding of ruminants. We have validated the method so that in the future it will be easier to assess the potential of different crops without actually having to extract in the biorefinery first. In other words, we can use this method as a kind of shortcut, and once we have validated it, it will be faster for us to assess green biomass from many different plant species and cultivation methods,” explains Henrik Thers.

The CNCPS method divides crude protein into five fractions, which are called A, B1, B2, B3, and C. According to the researchers, it is especially the B fractions that are interesting in relation to the protein concentrate that can be produced at the biorefineries.

Validation of recognised method

The aim of the project was to divide the green biomass content of crude protein into the five CNCPS fractions and to investigate how they are subsequently distributed between the three biorefining products: protein concentrate, pulp, and brown juice. The latter is a residual product from the protein precipitation process.

“Basically, the green biomass consists of water-soluble protein, non-soluble protein, and a lot of elements that are not protein at all. The water-soluble protein is what we can extract when we in the biorefinery push the liquid out of the green mass. We made a CNCPS screening of the green biomass as well as the protein concentrate and pulp for a number of different species of feed crops to validate the method,” explains Henrik Thers.

Not as simple as it may seem

Using the CNCPS method, the researchers can screen the green biomass and find out how much A, B1, B2, B3, and C it contains, and by actually extracting the protein in the same biomass in a biorefining process, they could find out, how much of each protein fraction ends up in the pulp, the protein concentrate, and in the brown juice.

“We found that part of the water-soluble protein ended up in the protein concentrate, just as expected, but not all of it. Some remain in the pulp because it still contains about 65% water or more. In addition, we found that it is not equal percentages of the B2 fraction that end up in the concentrate for the different plant species we have studied,” says Henrik Thers. 

It turns out that more B2 is extracted from legumes than from the two grass species studied. And B2 is one of the most attractive protein fractions in terms of feed value. 

“This is an important discovery, because so far it has mostly been grass that has been grown because it gives the highest dry matter yield. But legumes have the advantage that more of the good protein B2 is extracted, which is why they are better than the grasses in relation to the protein concentrate,” explains Henrik Thers.

Large differences between species

It thus turns out that in relation to the protein concentrate, legumes such as alfalfa and clover have an advantage, but according to Professor Søren Krogh Jensen from the Department of Animal Science, the feed value of the protein concentrate is largely the same.

 “The pulp, on the other hand, is more sensitive to species differences and the plants' developmental stages in relation to their feed value. Some plant species are born more difficult to digest than others, and there are some plant species that become more stem-like, fiber-rich, and thus less digestible than others,” he explains.

In other words, the difference between species and stages of development plays a greater role in relation to the feed value of the fibrous pulp for ruminants and the amount of protein that can be extracted in the biorefining process, than the composition and digestibility of the protein extracted.

Big advantages the cattle farmer

"The pulp from the biorefining generally has a high feed value for ruminants, and this means that it can replace ordinary clover grass silage at the cattle farmer," says Søren Krogh Jensen.

And according to the researchers, it can have quite large benefits for the cattle farmer to grow green biomass for biorefining. When grass is harvested, cut, collected, and driven home to be ensiled, there is a risk of loss during each process or due to the weather. In addition, it can be difficult to control the dry matter content of the silage.

“But whether harvested in rain or sunshine, the pulp silage has a stable dry matter content of approx. 35% depending on how the press is set. It is thus a very constant product of uniform quality, and there is less risk of loss in the field. Of course, dry matter is delivered to the protein concentrate, but you get paid for it, and you do not get any earnings from the loss in the field during a traditional silage harvest,” explains Søren Krogh Jensen.

The tool is in place

Basically, biorefining is gaining more and more ground, also commercially. And therefore, there has been an increased need to validate whether the CNCPS method can also be used to assess the potential for extracting protein from different species of crops.

“We have looked at the different species and their cultivation, but we also imagine a wide range of other things that should be investigated. There are a large number of crops that have not been studied yet, and we also imagine that different ways of fertilising or growing the crops may play a role. So, we are far from finished, but now we have the tool in place. And now we can screen many more plant species, mixtures of species, production methods, etc. faster and cheaper in the future.” says Henrik Thers.