A by-product of mechanical oil extraction is flaxseed press cake, which offers great potential for the food industry due to its high protein and fibre content as well as a residual fat content of around 10%. Nevertheless, it is currently used predominantly as animal feed. The main reason for this is its natural content of cyanogenic glycosides, which can release toxic hydrogen cyanide (HCN) through enzymatic hydrolysis.

For human consumption, Regulation (EU) 2023/915 sets a maximum limit of 150 mg HCN/kg for whole or ground flaxseeds. However, the hydrogen cyanide content in flaxseed press cake often significantly exceeds this limit, meaning that safe use in food products requires suitable cyanide reduction methods.

Comprehensive research approach to flaxseed and its by-products
The Linovit research project examined flaxseed and flaxseed press cake from multiple perspectives. DIL e. V. - German Institute of Food Technologies, Ölmühle Moog GmbH, and INRES – Renewable Resources at the University of Bonn collaborated to analyse the quality-determining and quality-reducing components of flaxseed and its processing products. The project was supported by funds of the Federal Ministry of Food and Agriculture (BMEL) based on a decision of the parliament of the Federal Republic of Germany via the Federal Office for Agriculture and Food (BLE) under the Federal Programme for Ecological Farming.
While INRES investigated the effects of location and cultivation conditions on the formation of hydrogen cyanide precursors, Ölmühle Moog focused on seed pressing, oil quality analysis, and product development with flaxseed press cake. DIL developed technological processes for targeted cyanide reduction in press cake and analysed its functional properties.

Process for hydrogen cyanide reduction
At DIL, a three-stage process was developed to significantly reduce hydrogen cyanide content. The reduction of cyanogenic glycosides occurred in two steps: first, they were enzymatically hydrolysed to hydrogen cyanide, and then the released HCN was removed from the product. This continuous process is suitable for industrial scaling and consists of:
•    Incubation: Press cake is mixed with water and incubated under controlled heat to activate occurring enzymes and achieve controlled hydrolysis of the glycosides.
•    Extrusion: High temperatures and mechanical shearing cause the evaporation of hydrogen cyanide, which is removed through a degassing unit.
•    Drying: The final drying step stabilises the product and ensures long shelf life.
Using this method, the hydrogen cyanide content was reduced from 390 mg/kg DM to below 40 mg/kg DM, well below the legal limit of 150 mg/kg.

Challenges in hydrogen cyanide analysis
Another result of the project was the identification of analytical challenges in determining the hydrogen cyanide content in flaxseed press cake. Variations in sample preparation, enzymatic hydrolysis conditions, and detection methods led to considerable differences in analytical results.
A laboratory comparison involving eight analytical laboratories, along with a round table discussion with experts from research, industry, and analytical institutions, revealed that no standardised method currently exists for measuring hydrogen cyanide in flaxseed press cake. The complete hydrolysis of glycosides and the choice of enzyme source were identified as critical factors affecting the accuracy and reproducibility of the measurements.

Potential for the food industry
The developed process opens new opportunities for using flaxseed press cake as a safe ingredient in food production. In addition to successful cyanide reduction, the project examined the impact of processing on technofunctional properties. Results showed that extruded flaxseed press cake remains suitable for producing texturised proteins for meat alternatives.
Future research should focus on optimising extrusion parameters to preserve protein functionality. Furthermore,  the development of standardised analytical methods is necessary to ensure reliable assessment of hydrogen cyanide content. Additionally, strategies for the safe removal and handling of released hydrogen cyanide are required to enable large-scale industrial application.