Why stored linseed oil tastes bitter−And what you could do about it

Freising, April 21, 2022 - A team of scientists led by the Leibniz Institute for Food Systems Biology at the Technical University of Munich, in cooperation with the Chair of Food Chemistry and Molecular Sensory Science at the Technical University of Munich, has now uncovered new molecular details relevant to the bitterness of stored linseed oil. The new findings should help to develop suitable technological processes or breeding strategies that preserve the good taste of the edible oil for longer.

Compared to other vegetable oils, linseed oil is particularly rich in vital omega-3 fatty acids and can therefore contribute to a healthy diet. Freshly pressed, it has a delicate, nutty flavor. However, depending on storage conditions, it relatively quickly develops an unpleasant bitter off-flavor, which works against consumer acceptance.

Oxidation products under suspicion

Earlier analyses had already led to the assumption that, in addition to oxidized fatty acids, ring-shaped peptides (cyclolinopeptides) contribute considerably to the bitter taste in the aged oil. The cyclolinopeptides consist of eight to nine amino acids and can be divided into six classes (1 to 6). Until now, however, it was unknown which of the 25 human bitter receptor types they stimulate.

In order to investigate this in more detail, the scientists first determined the concentrations of the various cyclolinopeptides in fresh linseed oil and eight-month-old linseed oil stored at room temperature using spectroscopic analysis methods. They also determined the respective content of the various oxidation products. Subsequently, Tatjana Lang and Maik Behrens of the Leibniz Institute investigated the effect of the isolated non-oxidized as well as oxidized peptides on the different bitter receptor types. For this purpose, they used a cellular test system established at the Leibniz Institute.

Only two human bitter receptor types respond

"As assumed, the proportion of oxidized peptides increased significantly due to storage," reports food chemist Oliver Frank from the Chair of Food Chemistry and Molecular Sensory Science. First author Tatjana Lang adds, "Surprisingly, however, only two of the 25 bitter receptor types reacted to the peptides - especially TAS2R14." According to the scientists, almost all of the peptides that were able to activate the receptors contained one or two oxidized methionine building blocks in their ring structure. Methionine is a sulfur-containing amino acid whose oxidation leads to methionine sulfoxide or methionine sulfone.

As the study shows, a methionine sulfoxide-containing class 4 cyclolinopeptide was mainly detectable in relevant amounts in the stored oil. At the same time, it strongly activated the receptor TAS2R14. "This oxidation product therefore seems to be dominantly responsible for the bitter off-flavor compared to others," says principle investigator Maik Behrens. "Consequently, it would be conceivable to optimize the flavor quality of stored linseed oil by removing or reducing the content of this peptide class in the oil through breeding or technical measures," he continues. 

The genes that encode cyclolinopeptides in flax are known, the researchers said. Likewise, there are flax seed varieties such as "Flanders" that contain fewer class 4 cyclolinopeptides compared to other varieties and could potentially be used as the basis for new breeds.

Publication: Lang, T., Frank, O., Lang, R., Hofmann, T., and Behrens, M. (2022). Activation Spectra of Human Bitter Taste Receptors Stimulated with Cyclolinopeptides Corresponding to Fresh and Aged Linseed Oil. J Agric Food Chem. 10.1021/acs.jafc.2c00976. pubs.acs.org/doi/abs/10.1021/acs.jafc.2c00976

Funding: This research was supported in part by the German Research Foundation (Deutsche Forschungsgemeinschaft - DFG) (BE 2091/7-1 to MB).

Scientific contact:

Dr. habil. Maik Behrens
Section II, Head of Research Group Taste & Odor Systems Reception
Leibniz Institute for Food Systems Biology
at the Technical University of Munich (LSB)
Phone: +49 8161 71-2987
E-mail: m.behrens.leibniz-lsb(at)tum.de
Profile: https://www.leibniz-lsb.de/en/institute/staff/profile-dr-habil-maik-behrens/

Press contact at LSB:

Dr. Gisela Olias
Knowledge Transfer, Press and Public Relations
Phone: +49 8161 71-2980
E-mail: g.olias.leibniz-lsb(at)tum.de

Information about the Institute:

The Leibniz Institute for Food Systems Biology at the Technical University of Munich (LSB) comprises a new, unique research profile at the interface of Food Chemistry & Biology, Chemosensors & Technology, and Bioinformatics & Machine Learning. As this profile has grown far beyond the previous core discipline of classical food chemistry, the institute spearheads the development of a food systems biology. Its aim is to develop new approaches for the sustainable production of sufficient quantities of food whose biologically active effector molecule profiles are geared to health and nutritional needs, but also to the sensory preferences of consumers. To do so, the institute explores the complex networks of sensorically relevant effector molecules along the entire food production chain with a focus on making their effects systemically understandable and predictable in the long term.

The LSB is a member of the Leibniz Association, which connects 97 independent research institutions. Their orientation ranges from the natural sciences, engineering and environmental sciences through economics, spatial and social sciences to the humanities. Leibniz Institutes devote themselves to social, economic and ecological issues. They conduct knowledge-oriented and application-oriented research, also in the overlapping Leibniz research networks, are or maintain scientific infrastructures and offer research-based services. The Leibniz Association focuses on knowledge transfer, especially with the Leibniz Research Museums. It advises and informs politics, science, business and the public. Leibniz institutions maintain close cooperation with universities - among others, in the form of the Leibniz Science Campuses, industry and other partners in Germany and abroad. They are subject to a transparent and independent review process. Due to their national significance, the federal government and the federal states jointly fund the institutes of the Leibniz Association. The Leibniz Institutes employ around 21,000 people, including almost 12,000 scientists. The entire budget of all the institutes is more than two billion euros.

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