During the production of bioethanol, valuable proteins for nutrition can be obtained from residual materials.

Monica Cornejo heaves two white plastic buckets onto a table in her laboratory. Then she opens the lids and points to the contents: both buckets contain a simple, coarse powder. One resembles sandy clay, the other is grainier and darker – both are residues from a bioethanol plant. TU researcher Cornejo wants to find out how valuable ingredients can be separated from the brown-beige crumbs: proteins for human nutrition.

The world population continues to grow and the demand for protein-rich nutrition is increasing disproportionately. Meeting this rapidly growing demand with animal proteins such as milk and meat is challenging. This is because, year after year, large areas of fertile arable land are being lost, not least due to climate change. That is why experts are increasingly looking for previously unused protein sources of plant origin. With the “BioProHuman” project, the Institute of Environmental Technology and Energy Economics (IUE) at the Hamburg University of Technology is now focusing on the residues from bioethanol production.

In Germany alone, almost 700,000 tons of bioethanol are produced each year. As a renewable fuel, it is added to gasoline to reduce the CO₂ emissions of cars. In Europe, the main raw material used for production is grain such as wheat. After it has been ground, it is mixed with water and enzymes and heated slightly. This converts the starch from the grain into sugar. The sugar solution produced in this way can be fermented with the help of yeast cultures to produce ethanol. This is then separated from the fermentation broth by distillation. What remains is the so-called stillage. This is a watery mixture of substances that contains, among other things, nutrients, organic components, non-fermented sugars and also proteins that come from both the processed grain and the yeast.

Protein-rich residues

The amount of stillage produced is astonishing: with an annual German production of almost 700,000 tons of bioethanol, around seven million tons of stillage are produced. Some of it is already being used – for biogas plants to produce biomethane or as animal feed. If certain valuable proteins could be separated from this mass in a concentrated form, they could be used specifically for human nutrition and thus contribute to an improved supply of vegan proteins for the population and also to a higher added value for the existing bioethanol biorefineries.

In a previous project, researchers at the IUE at the TU Hamburg have already succeeded in isolating proteins from so-called thin stillage. This is the relatively thin liquid part of the total amount of stillage produced, which can be separated using decanter centrifuges. However, a large proportion of the proteins present there are found in the solids contained in the liquid. “The primary goal of our project is to separate the proteins from these residues and obtain them in an enriched form,” emphasizes Monica Cornejo.

The challenge: the solid residues of the stillage contain a variety of other unwanted components, including lignin, which ensures the structure and stability of the plants. It acts as a kind of cage for the proteins to be separated. “To get at these proteins, we first have to break down these cages,” explains the researcher. “This is possible with the help of certain chemicals, but we prefer to use gentler, more environmentally friendly methods.”

Liquefying proteins by means of hydrolysis

One of the methods she is investigating is called hydrothermal hydrolysis. In this process, the solid residues are treated with hot water under high pressure with the aim of converting the proteins they contain into the liquid phase so that they can then be selectively isolated. The other approach is enzymatic hydrolysis, which uses enzymes, or biocatalysts, to break down the proteins in the lignin cages. During the course of her project, Monica Cornejo aims to systematically examine the parameters under which each method works best and how they can be combined in the most favorable way in terms of technical, economic and ecological criteria.

In the first phase of the project, she has already determined the protein content of various solid fractions. To do this, she uses sophisticated analytical methods such as high-performance liquid chromatography (HPLC). In the laboratory, Cornejo opens the lid of the device. The HPLC column, only slightly larger than a ballpoint pen, can be seen. A pump drives a solution of hydrolysed distiller's solubles through this column. It is filled with a special material known as a stationary phase. Various molecules in the solution interact with this stationary phase to different degrees. Depending on their chemical properties, such as polarity and size, the molecules are therefore retained to different degrees. This means that they leave the column at different times.

This allows the amino acids in the solution to be separated and precisely determined. Monica Cornejo points to a diagram on the screen. It shows which amino acids – the building blocks of proteins – occur particularly often. “Among other things, we have found that stillage solids from wheat have a significantly higher protein content than those from corn, which is used in particular in the USA for ethanol production.”

Basis for protein bars

The BioProHuman project will end in the fall of 2026. If the experiments are successful, the process developed from the analyses could then be put into practice together with industry. The idea: “Such a protein separation should, if possible, be integrated into existing bioethanol factories,” hopes Prof. Martin Kaltschmitt, head of the IUE. This would also allow a more extensive heat integration from the distillation to be used for this separation process; this could help to save energy and money. This would further develop the ethanol factory into a biorefinery that not only produces fuel but also proteins for human nutrition. The remaining organic residues could then be further processed in a downstream biogas plant to produce biomethane and fertilizer. This means that, ideally, everything would be recycled and nothing wasted.

In any case, the proteins obtained could be used in a variety of foods, such as in protein shakes and bars for athletes. They could also serve as a basis for meat substitutes: Vegan sausages, schnitzels or cheeses are trendy, and the market for them is growing. “If our project is successful, it could not only improve the protein supply, but also show that energy and food production are not mutually exclusive, but can go hand in hand,” hopes Monica Cornejo. Then in the future it could be not just about “food or fuel” but “food and fuel”.

More Information

Institute of Environmental Technology and Energy Economics (IUE)

IUE Projects