Why Does a Sweet Tropical Fruit Have an Onion-Like Smell?

A sweet, ripe tropical fruit with an onion-like smell: the Southeast Asian fruit cempedak combines these seemingly mismatched aromas.

A study of its distinctive aroma showed that three sulfur-containing substances matter alongside the many compounds responsible for its fruitiness. The researchers investigated how it differs from the closely related jackfruit, going beyond detecting compounds to reconstructing the aroma and removing selected components.

The smell that distinguishes two similar fruits

Cempedak is a tropical fruit tree in the mulberry family, like jackfruit. Its large fruit contains rows of seeds surrounded by soft, edible flesh. It was the aroma of this flesh that interested the researchers.

Both fruits share sweet-fruity, malty, and cheesy notes. Cempedak adds a sulfurous, onion-like character. Earlier studies had identified fruity esters and compounds reminiscent of cooked rice, but had not adequately explained this feature.

The researchers therefore undertook a broad search for compounds contributing to cempedak aroma, quantified them, and compared the results with jackfruit data. Their final step was to test how the overall aroma changed when candidate sulfur compounds were omitted.

Examining both the extract and the air above the flesh

The samples were fruits grown in Thailand and airfreighted to Germany. When necessary, they were kept at room temperature for one or two days, with ripeness assessed by softness and characteristic aroma.

Volatile compounds were extracted from the flesh and separated by gas chromatography while people sniffed for odors. The extract was then diluted to determine how far each compound remained detectable. This approach revealed 49 odorants.

However, highly volatile compounds can be lost while an extract is concentrated. The researchers therefore also directly analyzed the air above the flesh in a container. This additional investigation revealed six compounds, bringing the total to 55, of which 47 were identified.

The candidates with onion-like, sulfurous odors were 2-(methylsulfanyl)propane, 2-(methylsulfanyl)butane, and 2-(methylsulfanyl)pentane. These substances share similar structures. The propane compound was found through the headspace analysis. Using more than one measurement approach helped reveal the fuller aroma profile.

The most abundant compound is not necessarily the most odor-active

The researchers measured the concentrations of the identified compounds and divided them by their odor detection thresholds in water. This ratio is the odor activity value, or OAV. Values above 1 identify candidates that may contribute to the aroma.

The highest value, approximately 410,000, was found for fruity-smelling ethyl 3-methylbutanoate. This means that its concentration was about 410,000 times its detection threshold in water. It does not mean that the whole fruit smells 410,000 times stronger, or 410,000 times stronger than another fruit.

Compounds such as 3-methylbutanal were considered contributors to the malty note, and 3-methylbutanoic acid to the cheesy note. Within the sweet, fruity impression are elements that can be described as malty, cheesy, or reminiscent of cooked rice.

The sulfur candidates also cannot be judged by abundance alone. The two enantiomers of 2-(methylsulfanyl)butane had OAVs of approximately 480 and 470, and the propane compound approximately 110. The pentane forms also exceeded their thresholds, but at about 2.5 and 4.3 their values were much lower than those of the other two compounds.

Even within this sulfur-containing family, combining concentration in the fruit with the nose’s sensitivity suggests that the magnitude of each compound’s role may differ.

Reconstructing the aroma with 41 compounds, then omitting three sulfur compounds

Next, the researchers built a cempedak aroma model using the 41 compounds with OAVs of at least 1. An aqueous solution containing sugars and organic acids formed the base, with odorants added at levels corresponding to their concentrations in the flesh.

Seventeen trained assessors smelled the flesh and models, rating nine attributes, including fruity, onion-like/sulfurous, malty, and cheesy, on a 0–3 scale. The complete model’s aroma profile closely matched that of actual cempedak flesh.

Removing the three 2-(methylsulfanyl)alkanes together lowered the onion-like/sulfurous rating. For this attribute, the model moved closer to the jackfruit flesh assessed alongside it.

Chemical analysis also showed that these sulfur compounds were not present at aroma-contributing concentrations in jackfruit. Because the concentration differences between fruits and the effects of omitting the compounds pointed in the same direction, the researchers concluded that this group was key to distinguishing the two aromas.

Compounds that add individuality to a shared fruity aroma

Sulfur compounds were not the only differences from jackfruit. Fruity ethyl 3-methylbutanoate and ethyl hexanoate, among others, were higher in cempedak, whereas ethyl butanoate and some other compounds were higher in jackfruit. The proportions of their shared odorants differed too.

Against that background, removing the sulfur compounds weakened the characteristic onion-like impression. The study’s central contribution was narrowing down what supports the individuality of two otherwise similar fruits.

The reconstitution model also involved practical adjustments and approximations. Because quantitative sugar and acid information was unavailable for cempedak, jackfruit values were used. Some unavailable enantiomers were replaced by other isomers or substitutes at adjusted amounts to match their OAVs.

The sensory evaluation involved 17 assessors in one session, and the main paper did not report statistical tests for these aroma differences. Because the three compounds were omitted together, it also remains unclear whether each is individually essential. Nevertheless, the combined extraction, headspace analysis, quantification, and reconstitution provide concrete evidence explaining the distinctive smell.

Summary

Cempedak aroma centered on strong fruity odorants, layered with compounds reminiscent of malt and cheese. Removing three sulfur compounds from a 41-compound model weakened the characteristic onion-like/sulfurous impression and brought it closer to that of related jackfruit.

Even an odor that seems out of place in a sweet fruit can help define its character. An aroma’s individuality comes not only from its dominant sweetness, but also from a few additional compounds.

Reference

Grimm JE, Steinhaus M. (2020). Characterization of the Major Odorants in Cempedak—Differences to Jackfruit. Journal of Agricultural and Food Chemistry, 68, 258–266 (published online in 2019). Original paper / Full text included in the author’s dissertation

The authors declared no competing financial interest.