Scientifically Speaking | The expanding world of taste (beyond the common four)

How many tastes can our tongues identify? The answer to this seemingly simple question might not be what you think.

Most of us grew up learning about four primary tastes: sweet, sour, salty, and bitter. These basic tastes are detected by our taste buds. But taste is only part of the story. Flavour, the overall sensory experience of eating, is a complex interaction between taste, smell, texture, and even visual signals.

There’s no debate about the four traditional primary tastes, but for nearly a century there was no consensus on whether umami could be considered the fifth primary taste. So, what exactly is umami?(Pexels)

The classic tastes

Taste is a biological function, designed to help us identify and select food that is both nutritious and safe. Sweetness signals the presence of sugars, which offer a quick energy source. This sensation is universally appealing, drawing us to fruits, honey, and a variety of desserts that have been cherished throughout human history. On the other hand, sourness helps us detect acids in food, often warning us against spoiled or unripe foods.

Saltiness, the taste marker for sodium, is important for maintaining ion and water balance in the body. This taste enhances flavours and is a key player in our diet, though excess salt consumption is also tied to poor heart health. This is a growing concern worldwide (including in India).

Bitterness, on the other hand, typically signals toxins, warning us of harmful substances. While bitterness can be off-putting, it can also be an acquired taste which adds complexity to our palate. Today, many of us appreciate the bitterness in coffee, dark chocolate, beer, and certain vegetables like bitter gourd.

Umami, the fifth taste

There’s no debate about the four traditional primary tastes, but for nearly a century there was no consensus on whether umami could be considered the fifth primary taste. So, what exactly is umami?

In 1908, Japanese chemist Kikunae Ikeda experienced an intriguing taste while enjoying a bowl of dashi, a traditional Japanese broth made with kombu seaweed. This taste was neither sweet, salty, sour, nor bitter. Ikeda named this new taste "umami," derived from the Japanese word "umai," meaning delicious. Through meticulous research, Ikeda isolated glutamate as the source of this savoury sensation. In 1909, he published his findings in the Journal of the Tokyo Chemical Society, describing umami as the distinctive taste that arises from fish, meat, and similar foods.

Despite his groundbreaking work, Western scientists remained sceptical for decades. The delayed acceptance of umami can also be attributed to the longstanding tradition of recognising only four basic tastes — sweet, sour, salty, and bitter — which had cross-cultural consensus for millennia. Overturning this ingrained belief required substantial evidence.

This evidence began to emerge around the turn of the century when researchers identified specific glutamate receptors in both rats and humans. A taste receptor means that there’s a biological machine in our mouth for detecting it.

This biological evidence was crucial in finally acknowledging umami as the fifth taste. However, the recognition of umami as a basic taste was not just a matter of scientific evidence. It also required cultural readiness and shifts in societal attitudes towards Asia and Asian science since the taste had been discovered in Japan.

The enigmatic taste of kokumi

While umami has now taken its place among the primary tastes, another Japanese concept is emerging as a potential new taste: kokumi. Kokumi is harder to define than umami. In Japanese, the term "koku" describes foods with a rich, mouth-coating sensation often provided by fats, though it does not itself taste like fat.

Kokumi compounds, such as glutathione, enhance and extend other flavours rather than being detected independently. Research into kokumi began with studies on garlic by Japanese scientist Yoichi Ueda in 1990, who found that adding diluted garlic to soups enhanced their flavour. Kokumi compounds are found in long-cooked meats, aged cheeses, and fermented foods, and they modify the sensory experience by making foods taste richer and fuller.

The concept of kokumi may be intertwined with human evolution and modern culinary practices. Our ancestors likely favoured cooked and fermented foods for their complex aromas and richer mouthfeel, which are often associated with kokumi. Today, chefs and food scientists are exploring kokumi to create healthier food with perceived richness without adding calories.

A taste for fat

Another fascinating area of taste research is the sense of fat. Does fat have an actual taste? While fats are known for their texture and mouthfeel, scientists are investigating whether fatty acids — their chemical units — should be considered a primary taste. Like carbohydrates, which give sweetness, proteins, and umami, now many think that fats might have their own specific taste.

Receptors for fatty acids have been identified, but there is debate over whether the sensation of fat is distinct from other tastes like umami. Recognising fat as a primary taste could help in understanding dietary preferences and developing healthier foods, but it requires clear evidence that fat activates specific taste receptors independently of other sensations.

Considering fat as a taste could help us better understand why we crave certain foods and how we can create satisfying, healthier alternatives.

A taste for starch

A research paper published in the journal Chemical Senses in 2016 by Trina Lapis and her team found that humans can taste glucose oligomers, a type of carbohydrate, as a "starchy" flavour, separate from the well-known sweet taste. Unlike simple sugars detected by the sweet taste receptors, glucose oligomers are sensed by a different process, hinting at the presence of a separate taste receptor. The jury is still out, but if this is indeed a new taste, it may have evolved to help humans identify and consume starchy foods, which are a source of energy.

Does water have a taste?

Even water, traditionally considered tasteless, is being re-evaluated. In a scientific article in the journal Nature Neuroscience in 2017, Yuki Oka and his team at the California Institute of Technology offered the provocative idea that water has a detectable taste. Their research showed that water triggers a strong taste response in mice, and disabling their sour-sensing receptors made them unable to distinguish water. When they substituted a light oil instead of water, the mice drank it too. If water had no taste, they argue, light oil which is tasteless, should be indistinguishable in normal mice.

Of course, mice are not people, so we will have to see if this work extends to humans. If water is indeed a taste, it adds another layer to our understanding of sensory experiences. It challenges the notion that some substances are purely neutral and highlights the complexity of our taste system. This research opens the door to further studies on how we perceive the most basic elements of our diet and what these perceptions mean for our overall health and well-being.

The future of taste

Our understanding of taste is evolving, from the classic four tastes to the recognition of umami, and now the rising possibilities of kokumi, fat, starch, and even water as primary tastes. Our sense of taste will keep surprising us as scientists and chefs redefine what we know and experience.

Anirban Mahapatra is a scientist and author, most recently of the popular science book, When The Drugs Don’t Work: The Hidden Pandemic That Could End Medicine. The views expressed are personal