Rapeseed contains not just oil, but also high-quality protein. However, protein extracts from rapeseed have an intense, bitter flavor. A team led by food chemist Thomas Hofmann has now identified the substance that is responsible for this bitter taste. This is the first step towards making rapeseed suitable for human consumption.
According to the Food and Agriculture Organization of the United Nations (FAO), the demand for food will approximately double by 2050 due to the growing world population. “In this context, bottlenecks are to be expected, particularly in protein supply,” says Thomas Hofmann, head of the Chair of Food Chemistry and Molecular Sensor Technology at the Technical University of Munich (TUM).
It is therefore important to identify new plant protein sources for the human diet. Hofmann, who is also the director of the Leibniz Institute of Food Systems Biology, believes that rapeseed is a good local source.
Rapeseed contains high-quality protein
Rapeseed contains not just oil, but also high-quality protein, which contains many essential amino acids. Worldwide, around 1.12 million tons of crude protein are generated each year in the production of rapeseed oil. Although farmers have long used the rapeseed meal from oil production as a protein feed for fattening animals, rapeseed has so far played no role as a protein source for humans.
One reason is that the substances contained in rapeseed strongly affect the taste of the protein isolates obtained from it. These substances include, for example, very bitter-tasting secondary plant constituents. Hofmann and his team therefore investigated which bitter substances are responsible for the unpleasant taste of rapeseed protein.
The key substance that makes rapeseed protein taste bitter
The researchers investigated three different protein isolates using mass spectrometric analysis methods and taste tests. The first isolate was an extract of all the proteins contained in rapeseed meal. The second isolate contained mainly cruciferin and the third napin – these are the two main storage proteins of rapeseed. All three protein extracts had a protein content of 80 to 90 percent.
As the investigations have shown for the first time, a compound called Kaempferol-3-O-(2””-O-sinapoyl-ß-sophoroside) is the key substance that makes protein extracts from rapeseed inedible. The cruciferin isolate in particular contained 390 milligrams of this bitter substance per kilogram. Although the rapeseed meal and napin isolates contained less than one-tenth as much of the substance, they still tasted bitter in the sensory test.
Starting point for new processes
“Since we now know the cause of the bitter taste, it will be much easier to develop suitable technological methods or cultivation strategies which can be used to make tasty, protein-rich foods from rapeseed,” said co-author Corinna Dawid, who heads the Phytometabolomics research group at TUM.
C. Hald, C. Dawid, R. Tressel, T. Hofmann:
Kaempferol 3-O-(2””-O-sinapoyl-?-sophorosides) causes the undesired bitter taste of canola/rapeseed protein isolates
J Agric Food Chem, 67: 372-378, DOI: 10.1021/acs.jafc.8b06260