Researchers from the Technical University of Munich’s Leibniz Institute for Food Systems Biology have conducted a comprehensive analysis of the aroma found in walnut kernels, unraveling the underlying code of its scent. In a groundbreaking discovery, they have determined that the distinct aroma of walnuts is formed through the intricate interplay of two odorants, which are present in approximately equal proportions within the nuts. The first compound, known as sotolon, emits a fragrance reminiscent of Maggi Seasoning sauce and is responsible for the characteristic aroma of herbs such as lovage. The second compound, (2E,4E,6Z)-nona-2,4,6-trienal, is typically found in oat flakes and contributes to their recognizable scent.
Around half a century ago, British scientists had already made strides in identifying various volatile compounds present in walnuts using olfactory analysis. However, none of these compounds possessed the distinct walnut note. Consequently, the researchers deduced that the unique walnut aroma is the result of a combination of odorants. Despite subsequent experiments, the specific odor-active compounds that play a pivotal role in creating the walnut aroma had remained elusive until now.
Fifty odor-active compounds identified
Under the guidance of Martin Steinhaus, the research team from the Leibniz Institute in Freising undertook a comprehensive analysis of both fresh and dried walnut kernels, delving deeper into the investigation. Their meticulous efforts led to the identification of an impressive total of 50 odor-active compounds. However, upon conducting quantitative analyses, it was discovered that only 17 of these compounds existed in concentrations significant enough to be detected by the human nose.
Armed with this valuable knowledge, the team embarked on further experiments to shed light on the intricate nature of walnut aroma. A series of aroma reconstitution and omission experiments were conducted, alongside sensory tests that involved assessing various combinations of the odor-relevant compounds. In an unprecedented achievement, the researchers successfully demonstrated that the optimal blend of sotolon and (2E,4E,6Z)-nona-2,4,6-trienal perfectly recreates the unmistakable fragrance associated with walnuts. These groundbreaking findings have been published in the esteemed Journal of Agricultural and Food Chemistry.
The one-to-one ratio is important
Through meticulous quantitative analyses, the research team determined that both aroma-defining odorants were present in walnut kernels at a concentration of approximately 10 micrograms per kilogram. The team’s sensory tests further revealed that intensifying the natural concentrations of both odorants up to tenfold resulted in a heightened walnut note. However, maintaining the crucial one-to-one ratio was vital for capturing the authentic aroma.
Christine Stübner, a doctoral student involved in the study, explains, “We have successfully deciphered the odorant code of walnut aroma, nearly fifty years since research on this topic commenced. Our findings pave the way for the development of novel breeding strategies aimed at enhancing walnut aroma. However, the most captivating revelation is the creation of an entirely new food odor through the combination of two compounds that individually characterize the scents of different foods.”
Martin Steinhaus, an esteemed section and working group leader at the Leibniz Institute, adds, “This effect can easily be experienced firsthand. Simply take a tablespoon of oatmeal, add a few drops of the familiar seasoning sauce, give it a gentle shake, and inhale the resulting mixture.” With this straightforward experiment, the profound impact of the odorant combination becomes apparent.
Source: Leibniz-Institut für Lebensmittel-Systembiologie