Your first memory of eating Brussels sprouts and broccoli is likely not a very happy one. Many children dislike these sorts of vegetables, known as Brassica, and some may even find them disgusting. There are a couple of reasons why broccoli can taste really bad, especially for children who are more sensitive, including bitter-taste compounds and gene variants.

Now, scientists have found yet another factor that makes these plants unpalatable: enzymes in broccoli can combine with bacteria in our saliva to produce very unpleasant sulfurous odors. The higher the levels of these compounds, the more likely children were to say they dislike the vegetables. Furthermore, the levels of these volatile compounds were found to be similar in parent-child pairs, which suggests the oral biome is shared.

In the mouth, broccoli can produce putrid odors in some people

Broccoli, cauliflower, and Brussels sprouts all contain a glucosinolate compound that makes them taste bitter. But to some people, their taste can be especially foul. For some time, scientists have known that the TAS2R38 gene is responsible for regulating how humans sense bitterness in food, with huge evolutionary implications.

The bitter taste, along with sourness, is thought to be protective, an early sign that is supposed to communicate ‘be careful, this food may be toxic’. This warning system is quite robust, being capable of identifying thousands of different compounds, some of which could poison and even kill us.

Sensitivity to bitter compounds is a little bit higher in very young humans. Children have around twice as many taste buds as adults, for instance. Also, there’s quite a bit of genetic variance in how people express TAS2R38.

Of course, broccoli isn’t toxic. On the contrary, it’s a ‘superfood’, very rich in nutrients and antioxidants, while being low on calories. It just so happens that our body mistakes it for something that may be toxic, and this sensitivity is within a spectrum, meaning there’s significant variation among people. To some people broccoli and other vegetables like it are palatable, to others it’s simply not approachable.

Normally the Glucosinolates get all the attention, but Damian Frank, a Research Fellow in Food Chemistry and Sensory Food Scientist at the University of Sydney, found that another compound called S-methyl-ʟ-cysteine sulfoxide shouldn’t be overlooked when it comes to Brassica bitterness. When these compounds combine with enzymes in the plant’s tissue and people’s saliva, they produce sulfurous odors.

Frank and colleagues investigated differences in sulfur volatile production in saliva from 98 child/parent pairs. Using gas chromatography-olfactometry-mass spectrometry, the researchers first measured the main odor-active compounds in raw and steamed cauliflower and broccoli. They then mixed saliva samples from each participant with raw cauliflower powder and analyzed the produced volatile compounds. Each sample was then associated with taste ratings self-reported by the parent or child.

Unsurprisingly, dimethyl trisulfide, which smells rotten, sulfurous and putrid, was the least liked odor by both children and adults. But what was intriguing was that there were large differences in sulfur volatile production between child/parent pairs while children had very similar sulfurous odor production to their parents. This makes sense since people tend to have similar microbiomes when sharing the same diet, household, and ancestry.

“There were big differences between the amount of volatiles formed between individuals. But there was a significant correlation between children and adults; the parents of children with high enzyme activity tended to also have high activity. This suggested similarity in the amount and type of bacteria present,” Frank told ZME Science.

Although children whose saliva produced the highest amount of sulfur volatiles predictably disliked raw Brassica vegetables the most, this relationship wasn’t as strong for their parents. This is perhaps due to less taste sensitivity with age and an acquired tolerance of the flavor with repeated exposure through life. That being said, many parents likely hate broccoli as much as their kids do.

“Sometimes the parent has to overcome their own dislike to give their child “healthy” food like brassicas. They want to be a good parent and do the right thing, but it goes against the grain!” said Frank.

The researchers also measured common genetic differences in bitter sensing receptor genes among the participants, the results of which will be published soon. These will likely help explain why some people like Brassica vegetables and others, well, not so much.

“Not sure whether I will be doing further work in this interesting area.  But a better characterization of the type of bacteria present in individual oral microbiomes is a worthwhile research area. Also more research on how bacteria in the mouth affect taste and perception is super fascinating,” Frank said.

The findings were reported in the Journal of Agricultural and Food Chemistry.