Sweetener without aftertaste

If you buy stevia sweeteners today, you can assume they consist primarily of two steviol glycosides: rebaudioside-A and rebaudioside-D. They are up to 450 times sweeter than sucrose, but they also leave your coffee tasting faintly of metal. And yet there is one steviol glycoside that does not have that bitter aftertaste: rebaudioside-M, or reb M for short. The only drawback: it occurs in exceptionally low concentrations in the stevia plant. Simply extracting it is therefore not a viable option. Marjan De Mey, Professor of Biotechnology at Ghent University, consequently developed a synthesis method based on micro-organisms, together with Manus Bio, a spin-off of MIT.

‘You need to search for the genetic information that drives the production of reb M,’ De Mey explains. ‘We were already very familiar with part of that, because stevia is a terpenoid. We know from plants that, alongside a central metabolism for growth, they also have a secondary metabolism for communicating with their environment. And the latter is particularly interesting, because beyond their biological function in nature, we find that these metabolites also exhibit a wide range of bioactivities that can be of practical use to humans: flavour, fragrance, and even therapeutic functions.’

Bacterial factories

They then modified the genetic information so that the production of reb M was boosted and made possible within a micro-organism. ‘Plant cells have a nucleus and organelles,’ says De Mey. ‘A great many of the molecules are membrane-bound to the endoplasmic reticulum, an organelle that bacteria do not possess. So you also need to ensure that a bacterium knows how to handle this. Once you know which steps are required to produce reb M in a bacterium, the real work can begin.’

De Mey continues: ‘Here it is not the protein structure but the protein function that we want to understand.’ She explains that protein production can proceed via different kinetics. It is therefore important to monitor the ratios of intermediates, so that no toxic bottleneck can form anywhere. If you can ensure those ratios are correct, you have a cell capable of producing reb M. That is all well and good in a laboratory, but then you still need to scale it up.

That scale-up has since been achieved, and in the United States reb M is already available as the sweetener NutraSweet Natural. In Europe, we shall have to be patient a little longer, says De Mey: ‘The application has been submitted, but partly due to the pandemic, progress at EFSA (the European Food Safety Authority, ed.) is extremely slow. Once the application is approved, it is simply a matter of extending the distribution from the United States.’

‘Because you use an aqueous solution, you have no need for organic solvents’

Green sugars

The production of reb M also has ecological advantages. Because an aqueous solution is used, there is no need for organic solvents, as is the case with the synthesis of aspartame. Moreover, production in micro-organisms is not subject to seasonal variation, unlike the extraction of stevia plants. This production can also be carried out in a circular manner, De Mey explains: ‘At present, the bacterium still grows on a sugar substrate. But in principle, you can grow the micro-organism on any carbon source, which means it can be deployed using a local waste stream.’

To avoid losing touch with society, De Mey enjoys collaborating with philosophers and artists. ‘That little voice in the back of your head gets stirred up. Are we doing the right thing? I think it is important that my PhD students are critical about their research, but also about their impact on society and our entire ecosystem.’

Health

The key question, of course, remains whether these artificial sweeteners are healthy or not. Bruno De Meulenaer, Professor of Food Safety at Ghent University, offers a word of caution: ‘Every substance is toxic, depending on the dose. That applies to water, sugar, and therefore to sweeteners as well. The advice regarding chemical food safety is very straightforward: never fall into extremes. It is not a good idea to eat only foods containing artificial sweeteners. Let alone products that contain just one particular artificial food substance. In that case you are not spreading your risk, and you run the chance of consuming too much of a particular substance. This applies not only to sweeteners, but to every substance.’

Studies on the health effects of artificial sweeteners are springing up like mushrooms. One study claims they cause you to die sooner, another that they cause cancer. These are often observational studies, and an important caveat is warranted here, De Meulenaer believes: ‘Observational studies can be very powerful, provided they are conducted in a rigorous manner. Making such claims on the basis of a questionnaire is utterly irresponsible.’ And that is precisely what happened in these two studies. Finally, De Meulenaer wishes to add one last note: ‘The presence of artificial sweeteners must be labelled. If you as a consumer are not comfortable with that, you can make your own decision not to consume those sweeteners.’