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Volume 13, Issue 17
The durian fruit is notorious for its unpalatable aroma, and yet the fruit is incredibly popular throughout Southeast Asia and amongst travellers. Holding the title of “the world’s smelliest fruit” attracts attention including that of Martin Steinhaus from the Aroma Research Group at the Deutsche Forschungsanstalt für Lebensmittelchemie (German Research Center for Food Chemistry). He spoke to The Column about his group’s research into the compounds responsible for the fruit’s uniquely unpleasant aroma.
The durian fruit is notorious for its unpalatable aroma, and yet the fruit is incredibly popular throughout Southeast Asia and amongst travellers. Holding the title of “the world’s smelliest fruit” attracts attention including that of Martin Steinhaus from the Aroma Research Group at the Deutsche Forschungsanstalt für Lebensmittelchemie (German Research Center for Food Chemistry). He spoke to The Column about his group’s research into the compounds responsible for the fruit’s uniquely unpleasant aroma. -Interview by Lewis Botcherby
Q. This research is a continuation of your past research which aimed to structurally identify major odourâactive compounds in durians. What initially led to your research on the subject?
A: In 2002, I spent a couple of weeks in Thailand performing fermentation trials in the course of a project on white pepper aroma. At this time I ate my first durian. The aroma was so different from any other kind of fruit I knew that I was absolutely curious which compounds would be responsible for it. Back home I searched the literature, but the data I found were not consistent. I kept the problem in mind until in 2011 I got the chance to decide on a topic for a Chinese Ph.D. student, Jia-Xiao Li. That was when we started our project. Jia-Xiao’s advantage: He was used to durian consumption from his early childhood and was a big fan of the fruit. Not all his laboratory mates here in Germany agreed!
Q. Durian has a very complex mixture of odorants. Did this represent any challenges from a chromatographic sense to fully characterize?
A: Analyzing odorants is always challenging. For a long time, many people believed odours could be characterized by volatile analysis. However, most volatiles are simply odourless and the odourâactive ones are often trace compounds. Activityâguided screening using gas chromatography–olfactometry (GC–O) techniques is, therefore, a prerequisite to preselect the crucial compounds. The real challenge is to correctly assign their structures. GC coupled to mass spectrometry (MS) and applied to the entire volatile fraction is not helpful because of coelutions: The MS you get at the retention time of an odour is most likely not the MS of the odorant, but the MS of a coeluting compound present in much higher concentrations, but absolutely odourless.
The crucial approach to solve this problem is prefractionation. Using different techniques such as acid-base extraction, silica gel chromatography, and covalent chromatography, the volatiles extracted from a food are separated into fractions and the odourâactive compounds are localized in the fractions by GC–O before the individual fractions are subjected to GC–MS analysis. A valuable fractionation tool that helped us tremendously in the durian project was selective isolation of thiols by covalent chromatography using mercurated agarose. Another helpful prefractionation approach is two-dimensional GC. Using a heart-cut interface, the GC eluate is transferred for ~1 min to a second GC, where the eluate portion is rechromatographed on a column of different polarity before it enters the MS system. A sniffing port in the first dimension serves as monitor detector, a sniffing port in the second dimension aids in the correct localization of the target peak in the MS chromatogram. Most important: Once MS analysis resulted in a structure proposal, the structure essentially needs to be confirmed by GC–O analysis of a reference compound.
To confirm and quantify the importance of the individual compounds, their exact quantitation is the next step, typically achieved by stable isotope dilution assays. Concentration data are then compared to threshold data by simply dividing the concentration of each compound by its odour detection threshold, which results in odour activity values (OAVs).
Q. How novel was this approach? And in hindsight would you have done anything differently to improve it or perhaps save yourself time?
A: The basic approach has been used in our laboratory for approximately 30 years and has been applied to more than 200 different food aromas. In a recent metaâanalysis we have summarized and evaluated the data (1). The outcome revealed an amazingly small number of only 226 compounds exhibiting an OAV >1 in at least one foodstuff. Considering the fact that >11,000 volatiles have been characterized in food so far, this means that only ~2% of the known food volatiles are odour-active. The abundance of the 226 odour-active compounds in the food samples differed extremely. A bunch of
16 “generalists” for example occurred in >25% of the total number of foods analyzed. One idea to save time is, therefore, to develop a targeted approach for the simultaneous quantitation of the most prevalent odourâactive compounds in our diet. For the durian project, however, this would not have been very effective, because a majority of odour-active compounds in durian were “specialists” and were hardly ever found in another food before.
Q. Did you find any compounds that could not be confirmed?
A: Of the 44 compounds detected by GC–O in the extract obtained from durian, only three minor compounds remained unidentified.
Q. Were you at all surprised at the odour activity values you discovered and are they comparable to any other fruits?
A: Were we surprised? Yes and no. No, because the odour activity values somehow reflected the overwhelming odour intensity experienced when consuming durians.
Yes, because odour activity values >100,000 are rarely found for food odorants. In the above mentioned metaâanalysis on odourâactive compounds in food, we found that only 17 OAVs in that range have ever been reported (1). In fruits, OAVs are typically lower.
In Haden mangoes, for example, the most potent odour-active compound was identified as fruity smelling ethyl 2-methylbutanoate and exhibited an OAV of 2100 (2). In durian, the same compound showed an OAV of 1,700,000!
Q. Did the high OAV values require any instrument adjustments?
A: The instruments were always run in the optimum target compound concentration range. To meet this requirement, adjustments were made on the level of sample amount or extract concentration. Optimum workup conditions were determined in preliminary tests.
Q. The two compounds that mimicked durian pulp odour (namely ethyl [2S]-2-methylbutanoate and 1-[ethylsulfanyl]ethane-1-thiol) are described as fruity and roasted onion. These don’t sound like they would be unpleasant to smell and yet they successfully mimic the smell. Why is this? Is it in some way related to the ethanethiol, which is also prominent?
A: Ethyl (2S)-2-methylbutanoate actually has a very pleasant fruity smell that changes only a little with increasing concentration. Roasted onion-like smelling 1-(ethylsulfanyl)ethane-1-thiol, on the other hand, becomes rather offensive when the concentration increases and the smell gets close to that of ethanethiol.
Q. Are you planning any future research in this area?
A: Monthong is just one variety of many durian found in the market in Southeast Asia. And some of them are even stronger in smell! Actually, at the moment we are talking about a new project on durians with Thai colleagues. In the meantime, we are focusing on some other fruits with extraordinary odour characteristics such as cempedak and longkong.
Q. Is there anything you would particularly like to research the aroma of? Surströmming perhaps?
A: Why not? I assume the chemistry behind surströmming could be very interesting!
Martin Steinhaus is Head of the Aroma Research Group at the Deutsche Forschungsanstalt für Lebensmittelchemie (German Research Center for Food Chemistry), an institute of the Leibniz Association located in Freising, Germany. His basic research interest is the identification of key aroma compounds in food. In recent years, one focus of his work has been fruits, herbs, and spices, particularly from the tropics. Another focus is hops and their influence on beer aroma. Further areas of interest include the role of sulfur compounds as potent food odorants and their analysis, the influence of food processing on the key aroma compounds, and the molecular background of off-flavours in food and non-food materials. Martin graduated in food chemistry from the Ludwig Maximilian University of Munich and received a Ph.D. from the Chemistry Department of the Technical University of Munich in Garching, Germany.