Research shows that some people can't help but be disgusted by so-called Brassica vegetables due to microorganisms found in their mouths in combination with the defense mechanism of these plants.
Brassica vegetables (more commonly known as cruciferous vegetables, think of broccoli, kale, and Brussels sprouts) naturally contain a substance known as S-methyl-ʟ-cysteine sulfoxide, which serves as a sort of defense mechanism. When plant tissue is broken, an enzyme contained in the plant can break down this substance very quickly, releasing, among other things, a smelly, volatile sulfurous odor. This is how the plant protects itself.
Research has shown that people have similar enzymes produced by bacteria found in their saliva. These enzymes strengthen the aforementioned odor. Raised levels of it can cause people to dislike cruciferous vegetables.
Scientists also determined that the amount of enzymes in saliva differs from person to person. However, it was unclear whether children, like adults, also have varying amounts of this enzyme and to what extent these differences influence their food preferences. A supplementary study published in the Journal of Agricultural and Food Chemistry aimed to clear things up.
First, the science team identified the primary odor-active chemicals in raw and steamed broccoli and cauliflower using gas chromatography-olfactometry-mass spectrometry. After that, 98 children aged 6 to 8 were asked to rate the key odor components together with their parents.
The test subjects were presented with several substances and had to assess their smell. Dimethyl trisulfide, a substance that smells sulfurous and rotten, was pointed out by the parents as well as the children as the substance with the most disgusting smell. All subjects then gave up some saliva for further testing.
That saliva was then mixed with raw cauliflower powder, after which the researchers looked at which volatile substances were released as a result of the interaction between the cauliflower powder and enzymes in the saliva. There appeared to be large differences between the amount of sulfur-like substances that came out of the culture trays filled with saliva from different test subjects.
The researchers concluded that the saliva of children also contains varying amounts of S-methyl-ʟ-cysteine sulfoxide-degrading enzymes. Moreover, after some questioning about food preferences, children whose saliva produced the most sulfur-like substances when interacting with cauliflower powder also disliked cruciferous vegetables the most. Therefore, their food preferences seem to be partly determined by their oral microflora, i.e., the bacteria that live in their mouths.
More interesting findings
The study yields another interesting conclusion. While the extent to which saliva produced sulfur-like odors after interaction with cauliflower powder could vary widely from person to person, the children's saliva often turned out to generate almost as many odorous substances as that of their parents.
According to the researchers, above mentioned similarities can be explained by the fact that children and their parents must have approximately the same oral microbiome. This is probably not only the result of genetics but also due to the fact that people that live together encounter similar microbes through pets, diet, and other factors.
Another interesting finding was that children whose saliva created large amounts of sulfur volatiles disliked raw cruciferous vegetables the most, but this correlation was not seen as strongly in adults, who may have learned to endure the flavor over time. These results provide a new potential explanation for why some people like these vegetables while others (particularly children) don't.
How to get children to eat cruciferous vegetables
The fact that adults, on average, seem more resilient to the odors produced by eating cruciferous vegetables means that through time they have gotten more tolerable to the taste.
Cruciferous vegetables are deemed very healthy, and as repeated exposure can change people's fondness for them, it would be worth it for parents to keep trying to get their children to like them. Perhaps experimenting with preparation methods may be of help, as certain methods can mask the chemical reaction somewhat, facilitating the adjustment phase.
Sources and further reading:
Do we really have to wash fruit and vegetables? (Universal-Sci)
In-Mouth Volatile Production from Brassica Vegetables (Cauliflower) and Associations with Liking in an Adult/Child Cohort (Agricultural and Food Chemistry paper)