Many plants protect themselves from predators with bitter substances. But consumers do not like bitter vegetables. Bitter substances have therefore been bred out over the years, but the plants are then completely defenseless. Rethinking is announced.
Autumn time is harvest time. Many popular types of vegetables taste even better today than they used to, because the less tasty bitter substances have gradually been bred away. But this breeding away has made the vegetables more defenseless – now they taste better to predators such as slugs or mushrooms. “Allotment gardeners might have fewer worries with some old varieties,” says Nicole van Dam from the German Center for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig. With white cabbage, for example, there is a variety that contains less bitter-tasting mustard oil glycosides, but is more sensitive to infestation with roundworms (nematodes). Bitter substances have also been bred away from other plants such as chicory and Brussels sprouts.
According to the German Society for Nutrition (DGE), the food industry made constant efforts to eliminate bitter substances in plants through breeding and cultivation methods and to create milder varieties, especially between the 1980s and 2010s. “For example, by offering milder types of beer and mixed beer drinks, the per capita consumption of beer, which has been falling steadily since 1980, has been partially compensated for by attracting new consumers.”
In general, defense mechanisms have been lost in many varieties because the main focus during breeding was on appearance for a long time. “If you pull the teeth out of the plants, you have to protect them differently,” explains van Dam. “You need more pesticides.” An exception are organic varieties, in the breeding of which it is also important that they get by with little pesticides and artificial fertilizers. The latter intensifies the problem anyway: “Fertilizing with compost, for example, stimulates the immune system of plants, this training is missing with artificial fertilizers.”
However, the metabolic pathways for bitter substances do not only affect the body’s defenses and taste, as van Dam adds. “The production costs the plants energy, they may grow more slowly.” Gardeners are faced with decisions such as: Do I want the large, juicy tomato with little self-defense or do I prefer the smaller, somewhat bitter tomato that is also more defensive and requires less poison?
Especially with the need to reduce the excessive use of pesticides, more attention is currently being paid to natural defense mechanisms in breeding. There are even options for vegetables in which the bitter substances they contain are undesirable because of their toxic effect in rare cases: with zucchini, for example, one could breed in the direction of bitter leaves and not bitter fruits. “When the plant is still young, predators are kept away and you have to use fewer pesticides, and that only when the fruit comes.”
A number of videos on social media show babies being given something bitter to try – and immediately grimacing in disgust. And just recently, researchers showed that babies pull a crying face when they hear cabbage aromas while they are still in the womb – while they smile when they hear carrots. The scientists at the University of Durham in north-east England explain in the journal “Psychological Science” that the fetuses are probably already getting used to it.
“There is an innate aversion to bitter,” explains Maik Behrens from the Leibniz Institute for Food Systems Biology at the Technical University of Munich. “Just like there is a fondness for sweet.” In this way, babies avoid ingesting toxic bitter substances and then, as they grow up, learn which bitter things they can and cannot eat without hesitation.
Although there are many toxins that are not bitter at all – like that of the death cap mushroom – and also numerous harmless bitter substances, says Behrens. “But there are also a lot of very toxic bitter substances.” In addition, there were carcinogenic bitter substances, for which an accumulation in fatty tissue could be a problem, especially with regular consumption.
The bitter substances, which are toxic in large quantities, include the cucurbitacins, which sometimes occur in cucumbers, melon, pumpkins and courgettes – for example when ornamental gourds are accidentally crossed or when there is heat stress. A few years ago, the case of a 79-year-old who died after eating a home-grown zucchini caused a stir. The vegetable mix cooked with it tasted terribly bitter, but he ate it anyway, the man told the doctors. His wife survived. In general, such deaths are extremely rare. “The dose makes the poison,” says Behrens.
Artichokes, hops, gentian, endive, chicory, radicchio, dandelion – many plants contain bitter substances, according to the German Society for Nutrition. The spectrum of effects is broad and ranges from health-promoting to toxic or even deadly.
But what exactly are bitter substances? The substances summarized below are not a chemically uniform group. The only thing they have in common is that they activate the sensory cells responsible for “bitter”, as Behrens explains. 25 bitter receptors are known so far, four to eleven of which are active in the individual receptor cells.
In addition, researchers found that bitter receptors are not only found in the mouth, but also in many other body tissues, even in the heart and brain. In the gastrointestinal tract, they are involved in processes such as gastric acid production and the release of hormones, and in the large intestine in the development of diarrhea as a defense measure of the body, says Behrens. In the respiratory tract, bitter substances therefore ensure the release of substances that act against bacteria. The movement of the cilia in the lungs is accelerated – and particles such as bacterial cells are transported out more quickly.
Scientists have also discovered that certain bitter substances relax the smooth muscles of the bronchi, which expand as a result, says Behrens. “This effect can possibly be used for drugs against asthma, which may work better than those previously used.” Many other functions in the individual tissues are still completely unclear.
An effect that has been known for a long time is widely used: bitter substances can apparently have both an appetite-suppressing and an appetite-stimulating effect – which is one reason why aperitifs often have a bitter note. Consumers are usually not bothered by this at all, similar to coffee or the craft beers that have come into fashion with their distinct bitter aromas. Brussels sprouts, chicory, radicchio and bitter lemon are also established in European food culture, as the DGE explains.
“Most people may grimace in disgust when they first try coffee, but the positive experience – caffeine wakes me up – gradually covers that up,” explains Behrens. Getting used to it also plays a role with other foods, such as beer – in the north, bitter varieties are traditionally preferred. “There is a tolerance for bitter that depends on one’s attitude and experience.”
The taste complexity of food increases when, for example, not only sweet impressions but also bitter notes emerge, for example with dark chocolate, explains the DGE. Bitters have recently been making a comeback anyway, which is not only evident in the range of beers in retail. Findings from studies contribute to the renewed interest. “There is increasing evidence of a variety of positive health effects of bitter substances if they occur in the food matrix – for example as secondary plant substances in vegetables and fruit – and are not consumed in isolation (and possibly in highly concentrated form).”
One group, the saponins found in legumes, herbs such as rosemary and sage, and oats, form insoluble complexes with cholesterol and bile acids in the gastrointestinal tract. “As a result, they may contribute to a reduced cholesterol concentration in the blood,” says the DGE. There may also be anti-carcinogenic and antibiotic effects. In concentrated doses, however, toxic to lethal effects could also occur.
In general, experts advise against taking it in the form of food supplements, because the effect often depends on the interaction of different substances and metabolic pathways of the plants. “Vegetables and fruit are always better than a dietary supplement,” emphasizes van Dam. And for many bitter substances, it is not yet sufficiently known what toxic or carcinogenic effects they may have – taking them in powders or tinctures is risky for that reason alone.
Too much heat can be a shock to the body and, in the worst case, fatal – is there a dangerously strong “bitter”? There is no comparable shock effect, but some of the more than a thousand synthetic and natural bitter substances that have been recorded so far taste so extreme that even the biggest bitter fanatic avoids them. One example is denatonium benzoate, which Behrens says is used to denaturate alcohol, to impregnate cables, and in anti-nail biting tinctures.
The amarogentin of the yellow gentian, which is used as a measuring standard for bitter taste, is also extremely bitter. If one gram of amarogentin were dissolved in 58 million liters of water, it would still taste bitter.