Hate Broccoli? Your Oral Microbiome Might Be the Reason

The words "eat your vegetables" can be the stuff of nightmares for some children, particularly if said vegetables are of the Brassica genus, including broccoli, cauliflower and cabbage. A new study published in the Journal of Agricultural and Food Chemistry suggests that the oral microbiome may contribute to a love or hate for these veggies. The research was conducted at CSIRO, Australia’s Science Research Agency.

Why you should eat your greens

We know that vegetables in the Brassica genus are abundant in nutritional benefits. They contain metabolites, vitamins and antioxidants that exert antimutagenic effects, meaning they are effective in the prevention of DNA changes that can underpin cancer development. And yet, we're not eating enough of them. "In most countries [including Australia], children and adults consume much less vegetables than recommended by nutritional experts. Brassica vegetables are generally the least liked of all vegetables yet have significant positive health impacts," Damian Frank, research fellow in food chemistry and sensory science at the University of Sydney, told Technology Networks.

Children are notoriously polarized to the Brassica genus – while some kids can't get enough cabbage or broccoli, others loathe them with a deep passion. This divide between Brassica adoration and hatred can also extend into adulthood.

The strong polarization observed in this particular group of veggies has attracted scientists for many years.

Previous work suggests that children prefer sweeter tastes and have a reduced tolerance for bitterness compared to adults. Vegetables in the Brassica genus possess glucosinolate (GLS) compounds, which are broken down into isothiocyanates, associated with bitterness and pungent tastes. But Brassica vegetables also produce odor-active sulfur volatiles, due to the breakdown of a compound called S-methyl-ʟ-cysteine sulfoxide, or SMCSO. We know that sulfur is pungent and stinky.

The enzyme that is required for the breakdown of SMCSO into its sulfur counterparts is cysteine sulfoxide lyase, which exists in the plant's tissue. Certain bacteria that are present in the mouth – part of the oral microbiome – have also been shown to possess cysteine lyase activity.

Adults can have different levels of the enzyme in their saliva. However, whether children have different levels too, and whether this might have an effect on their food likes and dislikes, hasn't been well studied – until now.

Barriers to Brassica intake

Frank and colleagues are undertaking a large study that explores the potential barriers to Brassica intake. Part of this research has involved looking for differences in the production of the odor-active sulfur volatiles (the breakdown components of SMCSO) in the saliva of 98 paired children and adults, and whether this is related to an acceptance of Brassica vegetables.

To conduct their study, the team adopted a method called gas-chromatography-olfactometry-mass spectrometry. This technology uses gas chromatography to separate volatile compounds that can then be identified using mass spectrometry. In the context of this study, the researchers first wanted to identify odor-active compounds in raw and steamed broccoli and cauliflower. They then asked the children/adult pairs to rate the odor compounds, before taking saliva samples from them.

"The PTR-MS (proton transfer reaction mass spectrometer) can measure volatiles in food in real time. We incubated fresh saliva from children and adults with cauliflower powder and measured the stinky sulphur volatiles that were produced," explained Frank. They found variation in the level of volatiles produced; for some individuals the amounts were low, and for others there were "rather a lot", Frank described.

Interestingly, children often had similar levels to their parents. Frank and colleagues suggest that this is likely due to the children and parents sharing similar microbiomes, which means they might have similar levels of the bacteria present that possess smelly sulfur-producing cysteine lyase activity. Children that produced the highest amounts of the sulfur volatiles were found to show the most dislike towards raw Brassica vegetables, however this relationship was not mirrored in adults. It implies that adults perhaps learn to tolerate, or even like the flavors over a period of time.

Food liking: A fascinating area

"Other research groups have shown strong evidence that bacteria present in saliva are primarily responsible for the processes examined in our study. If you inactivate saliva (by filtering out bacteria or heat denaturation) these reactions are inhibited," Frank explained. A caveat to the work is that the researchers did not have the resources available to characterize the microbiological composition of the study.

So how can the data from this study be used to help us understand food preferences and encourage a balanced diet containing vegetables? "This is just one part of the puzzle," Frank said. "There may be food preparation methods that minimize these reactions. There may be breeding methods that can minimize the amount of the substrate (SMCSO) present in Brassicas. But since there are also health benefits, I would not recommend removing it from vegetables. We don’t know enough about why differences in oral microbiota exist between individuals." He added that there may be other dietary and lifestyle factors that are important and modifiable.

The next step for this area of research, Frank said, is to look at the relationship between saliva composition, oral microbiome and in-mouth flavor perception. "Food liking is a fascinating area of research. Whether we, or other groups conduct this, there is much to learn," he concluded.

Damian Frank was speaking to Molly Campbell, Science Writer for Technology Networks.

Reference: Frank D, Piyasiri U, Archer N, et al. In-mouth volatile production from brassica vegetables (cauliflower) and associations with liking in an adult/child cohort. J. Agric. Food Chem. 2021. doi: 10.1021/acs.jafc.1c03889.