A matter of taste
A matter of taste
Kathrin Ohla takes the test herself: smelling and seeing affect the taste.
Gingerbread, cinnamon biscuits, speculaas: these biscuits are much tastier around Christmas than on a mild October afternoon or in the first spring sun. But why is that? Kathrin Ohla can answer this question. She investigates taste – how it is influenced by other sensory impressions and what happens in the brain when tasting.
A plate full of Christmas cookies at a photo shoot in early autumn? That’s not everbody’s cup of tea. Kathrin Ohla, psychologist at the Institute of Neuroscience and Medicine (INM-3), is also torn. An outside temperature of 20 degrees Celsius and a neutral office do not immediately evoke a Christmassy mood. The other senses are simply not yet attuned to Christmas – but it is exactly these that are important for tasting. “The sense of taste alone would give us little plea-sure,” says Kathrin Ohla. “Putting it simply, it only knows sweet, sour, bitter, salty and umami, which comes from Japanese and can best be translated as savoury.” It is therefore not possible to recognise a particular foodstuff solely by its taste. For example: beer. The taste recognises only “bitter” – a vapid impression. It is only the combination of senses that make beer what it is. Our eyes see the white, foaming beer head, our nose smells the hops, our mouth feels the tingling of the sparkling fluid. And it is precisely these other senses that make Christmas cookies taste better by candlelight and the scent of cinnamon, with mulled wine and the Christmas tree than at the barbecue party at the quarry pond.
Image above: Kathrin Ohla takes the test herself: smelling and seeing affect the taste.
Ohla and her team have investigated the interplay of different senses in a series of studies. They all confirm: above all, seeing and smelling influence the taste. For example, in pink-coloured, slightly sugared natural yoghurt, many people taste aromas of strawberry or wild berries although it does not contain any at all. Via their scent, aromas such as banana or vanilla, for example, can enhance the sweet taste. The link between smell and taste is particularly strong, because during chewing, molecules are released from the food which we inhale retronasally through the nose – that is, directly from the mouth into the nose. This is why, in everyday life, we often confuse smelling and tasting.
Even newborns have preferences regarding taste: “If you give a drop of sugared water onto their lips, they react positively, while with a bitter substance or lemon juice, they make the familiar lemon face,” Kathrin Ohla says enthusiastically. This innate preference or dislike is vital. “This is how we distinguish nutritious from harmful food,” according to the researcher. “Bitter”, for example, is a potential indication that a thing might be poisonous – after all, most poisons taste bitter. A sweet taste, on the other hand, signals that a food contains carbohydrates and thus quickly available energy. Umami indicates protein-rich food and thus a good source of energy, and it is also preferred from birth. A sour taste, in contrast, has an ambivalent function: on the one hand, it indicates that valuable vitamin C could be contained, while on the other hand, it warns against spoiled food such as sour milk. The situation is similar with salty foods. Although salty taste indicates important electrolytes, too much salt is harmful and is therefore rejected. It is the dose that is crucial.
Nutritious or unpalatable?
So taste is often about the question: nutritious or unpalatable? Keep eating or spit it out? But how soon do we know? How quickly does our brain process information regarding taste and then return it to our tongue? In order to answer this question, Kathrin Ohla and her colleagues carried out a number of studies using electroencephalography (EEG) to measure the brain waves of volunteers. The test persons tried different samples during the studies and worked on several tasks at the same time: for example, they indicated when they tasted something, what they tasted and how pleasant and intense the taste was. The evaluation of the data was then carried out with the help of maschine learning algorithms trained to allocate taste sensations to brain-wide response patterns. The researchers correlated the time required to detect and to differentiate between tastes with the time that participants needed to make the same decisions. The brain data enabled them to recognise when taste processing in the brain begins and how it is linked to perception.
The most important insight: taste is processed much faster by the brain than had previously been assumed. “The first measurable signal is the trigger for our behaviour with respect to taste, so whether something is tasty or perhaps not so good,” says Ohla. This means that no downstream processing steps are necessary in the brain. It takes about 175 milliseconds for the test persons to notice that they taste something. “Prior to our measurements, researchers assumed that this would take about three times as long,” reports Ohla. For comparison: we have a first visual impression after about 100 milliseconds, hearing takes about 80 milliseconds. “However, the taste molecules do not directly encounter receptors the way that, for example, light does on the retina during vision,” said Ohla. “They must first dissolve in the saliva on the tongue until they reach the taste buds and the receptors in them. That alone takes roughly 50 milliseconds.”
The researchers also showed that there are differences in how quickly we perceive certain tastes. We taste sour and salty faster than sweet and bitter – and the brain also decodes the tastes with a corresponding time delay. We can distinguish bitter and sweet the moment we taste them. It is different with sour and salty: although the study participants noticed a little faster that they tasted something, it then took them longer to find out what it was – and they often confused the two tastes. “Evolutionarily, it makes sense that we can immediately and reliably distinguish bitter – that is, potentially toxic – from sweet, that is nutritious,” says Kathrin Ohla. “However, it is still unclear why more time passes between perceiving and distinguishing salty and sour tastes. We will have to look into that more closely.”
So we can realise very quickly that the salad dressing does not taste good. But it is not always certain to judge whether there is too much vinegar or salt. Too much bitter almond aroma in the cookie dough, on the other hand, is clear. And even if her findings do not play a direct role in baking cookies, Kathrin Ohla has a tip for this case: bitterness can be “masked” with a little sugar. Then it's just a matter of the right mood to ensure tastiness.
Janine van Ackeren
What actually constitutes a gourmet? And why don’t many Chinese like cheese, while it can’t be intense enough for the French? “Basically, we primarily like what we know best. However, taste ‘learns’ in the course of time: Bitter substances in the form of coffee, beer or chicory can taste quite good to us after some accustoming,” explains Jülich psychologist Kathrin Ohla. So we learn through our own experience and through our environment what tastes good, what does us good and what harms us. The sense of taste, however, does not remain constant for life. The sense of taste declines as one gets older. This is mainly due to the fact that the number of taste sensory cells on the tongue decreases. However, we do not notice this, because there is no comparison in tasting – in contrast to seeing, for example: if we can no longer decipher a text in the daily newspaper, we quickly notice that our eyes have deteriorated.
Extensive tests have so far been necessary in order to scientifically test how well someone can taste. Test persons had to taste many differently concentrated taste samples over several hours. Kathrin Ohla has simplified this with a new test procedure. A flexible algorithm calculates from the patient’s answers which sample should be used next. This is repeated until the software has determined the taste limit. The number of samples required to determine this limit is thus greatly reduced. Test duration and stress are reduced accordingly. “The test delivers a reliable result in a few minutes as far as the taste sensitivity of a test person is concerned,” says Ohla. The algorithm even runs on smartphones and tablets. “This quick and easy test is ideal for use in the clinic, for example for patients who cannot complete long tests due to their state of health, or in cohort studies where many people need to be tested,” says Ohla. “We get requests from colleagues all over the world.”
She also wants to use the test for her basic research, for example to find out where the brain processes information concerning taste. In one project, she and her team plan to work with stroke patients. The first step will be to examine what damage the stroke has caused in the brain. The patients then complete the taste test – separately for each side of the tongue, since the nerves also lead separately to the brain. Selective impairments in taste can then be related to individual brain damage. This allows for insight into how taste information flows from the tongue through different stations in the brain. “In this way, we hope to find areas in the brain that are essential or dispensable when it comes to tasting. That is not understood enough so far,” says Ohla.
Janine van Ackeren
Image: Forschungszentrum Jülich, Graphic: SeitenPlan