How many taste sensations do you have? Until recently, most of us were taught at school that there are four basic tastes: sweet, sour, salty and bitter. Scientists have been able to show that we have specific receptors on our tongues for each of these distinctive flavours.
However in 1908, Kikunae Ikeda, a Tokyo-based chemist, claimed to have discovered a new taste: umami. Trying to work out what gave his dashi soup its distinctive savour, he eventually concluded that it was the amino acid, glutamic acid. This is found in several popular Japanese food ingredients. Bonito flakes are a great source, as is the sodium salt of glutamic acid - commonly used as the food additive monosodium glutamate (MSG). However Ikeda’s new flavour was not taken seriously until several decades later when glutamic acid receptors were finally discovered on the human tongue.
Now the “brothy” flavour of umami is everywhere. Chefs rave about “umami bombs” – packets of taste sensation that lift dishes from the mundane to the sublime. Umami flavours have also been pin-pointed in western ingredients, like Parmesan cheese.
So much for people, but what about animals? For pet nutritionists, making pet food palatable to cats and dogs is of critical importance. If pets won’t eat nutritionally balanced pet food, they will not get the benefit of a healthy diet. So researchers are keen to understand exactly what pets can taste and, more importantly, what flavours they actually like.
Cats, for instance, are obligate carnivores: they must eat meat. This inevitably affects the kinds of tastes they can sense. For instance, a collaboration between the Monell Chemical Senses Center
and WALTHAM showed that cats don’t have a functional sweet taste receptor. In addition, cats are also known to avoid tastes that are bitter to humans. How they respond to umami, however, is only beginning to be understood. Given that it is often described as a “meaty” taste, it would seem logical that it would be a feline favourite.
It has now been established that cats do have glutamate receptors. At a molecular level, however, it has not been clear just how these interact with umami tastes. Doing experiments directly with cats, however, is time-consuming and expensive. Another approach was needed.
So a research team from Institut National de la Recherche Agronomique
(INRA) in France, together with WALTHAM, decided to explore novel experimental strategies to screen new compounds that activate taste receptors – known as “tastants”. The results have just been published in the journal PLoS ONE.
The research team wanted to measure the binding of selected amino acids to the cat umami taste receptor without involving actual cats. The answer was to “grow” the active part of cat taste receptor in bacteria. Essentially this is a protein that acts almost like a Venus fly trap, snapping-up umami compounds and then triggering a nerve response to create the taste sensation.
The big question, however, was whether these “artificial” receptors would work in the same way as in the cat.
The scientists ran a number of experiments with the newly created receptors to see if targeted amino acids would bind with them, exactly in same way that tastants bind with receptors in real life.
Using complex laboratory analysis they managed to demonstrate that the experimental receptors did bind with the target amino acids. The team then compared their results with taste tests performed with cats and found that they were indeed comparable with the values obtained from behavioural experiments.
This not only means that more tastants can be screened without the need to work directly with cats, but also that only small quantities of amino acids are needed.
“The approach will be valuable in producing large amounts of functional proteins,” says WALTHAM’s Dr. Scott McGrane, one of the paper’s authors. “We can use these in studies to better understand the mechanisms of umami taste perception. The technique also offers an interesting new experimental strategy to screen potential new umami tastants or umami enhancers.”