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A research team from the Leibniz Institute for Food Systems Biology at the Technical University of Munich has analyzed the aroma of walnut kernels and deciphered the underlying "odorant code". As the team shows for the first time, the typical walnut aroma is created by the combination of two odorants that are present in the nuts in roughly a one-to-one ratio. The first substance is sotolon, which smells like Maggi Seasoning sauce and which, as a single component, characterizes the aroma of lovage, for example. The second compound is called (2E,4E,6Z)-nona-2,4,6-trienal. It is known from oat flakes and is responsible for the typical odor there.
British scientists had already olfactorily characterized numerous volatiles from walnuts about 50 years ago. However, none of the compounds they found had a specific walnut note. Thus, the researchers concluded that the characteristic walnut aroma is based on a combination of odorants. Despite this finding and further experiments, however, it had still not been clarified which odor-active compounds are decisive for the aroma of walnuts.
50 odor-active compounds identified
To pursue this question, the team from the Leibniz Institute in Freising, led by Martin Steinhaus, analyzed fresh, dried walnut kernels. In total, the team identified 50 eligible odor-active compounds. However, subsequent quantitative analyses revealed that only 17 of the substances found were present in the nuts in odor-relevant concentrations, i.e., concentrations that could be perceived by the nose.
Based on these results, the team conducted additional aroma reconstitution and omission experiments as well as sensory tests in which they evaluated different combinations of the odor-relevant compounds. In doing so, the researchers demonstrated for the first time that the mixture of sotolon and (2E,4E,6Z)-nona-2,4,6-trienal best reproduces the characteristic walnut aroma.
The one-to-one ratio is important
As the quantitative analyses demonstrated, the two aroma-determining odorants were each present in the nut kernels at a concentration of about 10 micrograms per kilogram. "In our sensory tests, the walnut note intensified even further when we increased the natural concentrations of both odorants up to tenfold," reports Christine Stübner, a doctoral student who worked on the study. "However, it was important to maintain the one-to-one ratio," she continues.
"We have thus deciphered the odorant code of walnut aroma almost half a century after research began. Based on our findings, new breeding strategies can now be developed to improve walnut aroma. But probably the most exciting result is that the combination of two compounds that individually characterize the odor of different foods, creates a completely new food odor," says Martin Steinhaus, who has been section and working group leader at the Leibniz Institute for several years.
As the food chemist explains, the effect is easy to try out. To do this, put a tablespoon of oatmeal in a glass, add a few drops of the well-known seasoning sauce, shake it a bit and smell the mixture.
Publication: Stübner, C. A. and Steinhaus, M. (2023). Sotolon and (2E,4E,6Z)‑Nona-2,4,6-trienal Are the Key Compounds in the Aroma of Walnuts. J Agric Food Chem 71, 7099-7108. 10.1021/acs.jafc.3c01002. https://pubs.acs.org/doi/pdf/10.1021/acs.jafc.3c01002
More Information:
About the Odorants:
In chemical terms, the odorant sotolon is 3-hydroxy-4,5-dimethylfuran-2(5H)-one. It is present in odor-active amounts in many different foods, and its odor is strongly reminiscent of the smell of the seasoning developed by the Swiss Julius Maggi in 1886. Sotolon characterizes the aroma of some herbs and spices, including lovage and fenugreek, as well as the curry powder commonly used in Europe. It also contributes significantly to the aroma of Asian seasoning sauces, such as Japanese shoyu and Thai nam pla. The oatmeal-like smelling (2E,4E,6Z)-nona-2,4,6-trienal is much less well-known as an odorant. In oatmeal it dominates the olfactory impression, but it is also relevant for the aroma of black tea.
Contacts:
Scientific Contact:
PD Dr. Martin Steinhaus
Head of Section I and the Research Group Food Metabolome Chemistry
Leibniz Institute for Food Systems Biology
at the Technical University of Munich (Leibniz-LSB@TUM)
Lise-Meitner-Str. 34
85354 Freising, Germany
Phone: +49 8161 71-2991
E-mail: m.steinhaus.leibniz-lsb@tum.de
Press Contact at the Leibniz-LSB@TUM:
Dr. Gisela Olias
Knowledge Transfer, Press and Public Relations
Phone: +49 8161 71-2980
E-mail: g.olias.leibniz-lsb@tum.de
https://www.leibniz-lsb.de
Information about the Institute:
The Leibniz Institute for Food Systems Biology at the Technical University of Munich (Leibniz-LSB@TUM) 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 Leibniz-LSB@TUM is a member of the Leibniz Association (https://www.leibniz-gemeinschaft.de/en/), 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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PD Dr. Martin Steinhaus
Head of Section I and the Research Group Food Metabolome Chemistry
Leibniz Institute for Food Systems Biology
at the Technical University of Munich (Leibniz-LSB@TUM)
Lise-Meitner-Str. 34
85354 Freising, Germany
Phone: +49 8161 71-2991
E-mail: m.steinhaus.leibniz-lsb@tum.de
Stübner, C. A. and Steinhaus, M. (2023). Sotolon and (2E,4E,6Z)‑Nona-2,4,6-trienal Are the Key Compounds in the Aroma of Walnuts. J Agric Food Chem 71, 7099-7108. doi 10.1021/acs.jafc.3c01002. https://pubs.acs.org/doi/pdf/10.1021/acs.jafc.3c01002
https://de.statista.com/statistik/daten/studie/688452/umfrage/anbau-von-walnuess... European countries with the largest area under walnut cultivation from 2019 to 2021
https://de.statista.com/statistik/daten/studie/478411/umfrage/erntemenge-von-wal... Harvest volume of walnuts worldwide by country 2022/23
Walnuts
Martin Steinhaus
Martin Steinhaus
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