Sour Taste’s Day in the Sun

By Karen Kreeger, Director of Communications

Monell scientists have been researching this somewhat overlooked taste for decades, from the genetics of sour liking to a pair of sour taste receptors. Why would we need the ability to taste sour, or acidity in the first place? Many chemosensory scientists maintain that it first evolved as a warning system in early vertebrates for foods that had turned or were otherwise toxic or unhealthy to eat.

Given this cautionary notion of sour taste, why would any human ever down a glass of lemonade on a hot July day? Yet, many do and to some parents’ surprise their young children even go for the sour candy versus the sweet choice. Many species just plainly do not like sour, yet some do, such as humans, but sour taste has received relatively little attention by chemosensory investigators.

We spoke with Monell member Paul Breslin, PhD, a senior author on a recently published study charting the evolution of sour taste among many types of vertebrates.

Q: How were you able to construct a comparison of the liking/disliking of acidic or sour foods in different species?

A. My coauthors and I read through dozens and dozens of papers and noted how different species reacted to acidic taste. One of my contributions is a comprehensive discussion on sour taste across species in a 2013 conceptual review on the overall evolution of human taste. From this, we compiled a database of vertebrate species whose ability to detect acidity in food had already been found in the published literature. We then used this information to note which species disliked, liked, or were uncertain about sour taste. With that we mapped the evolution of the like/dislike response to acidity among vertebrate species, including humans.

Q. What did compiling data on the liking or disliking of sour taste reveal?

A. An amazingly small number of species have been studied regarding their ability to detect acids. For example, we were able to find data for only 33 out of roughly 5400 species of mammals on Earth. Our evolutionary reconstruction of sour taste began with sturgeons, an ancient species of fish. Most organisms do not like sour taste. Animals, including humans, that do enjoy sour taste triggered by acidic foods are not the rule, but the exception. This status is held by a mixed bag of mammals, including some primates, apes, and pigs.

Almost all species hate sour taste, but those that do may need it for a few different reasons. For instance, humans have lost the ability to make vitamin C, which is high in many fruits. The majority of sour-tasting substances lack nutritional value, with the exception of vitamin C, short-chain fatty acids, and certain amino acids. Sour taste may have been a necessary guide to vitamin C-rich fruits for early humans.

The mixture of acids with sugars could have also helped humans determine fruit ripeness by detecting the right mixture of sweet and sour tastes. As such, this taste combination is perceived as working together in the flavor of many fruits. The acids and sour tastes in fermented foods are also part of the human diet around the globe, an area of current, fairly intense study.

Q. How does this research relate to the new discoveries by Monell colleague Yali Zhang on how fruit flies discern different concentrations of acid to produce contrasting feeding behaviors?

A. The gene for the protein that senses acidity that Yali’s lab is working on is very old, from an evolutionary perspective. It was originally described in older fish species that likely used it to detect the pH, a scale of acidity, of their aquatic environment. His group identified a protein in fruitfies called Otopetrin-like (OtopLa), which has a human counterpart, as a long sought-after sour taste receptor. OtopLa forms a proton-selective ion channel specific in flies that is like the human sour taste receptor. Mutant flies without OtopLa are averse to low concentrations of acid (sour) as well as repulsed by higher concentrations (really, really sour). The human version of the OtopLa protein helps us sense sour foods.

Q. Why do you think some children like sour taste so much?

Fermented foods are sour and have been important and ubiquitous across many human cultures for millennia. Fermented foods are probiotics. Think yogurt. They help the gut microbiome stay healthy by crowding out the bad bacteria. Humans tend to have fragile guts, even more so in children. Kids might be attracted to sour foods like pickles as a way of getting the right organisms to help create a healthy gut microbiome. They are, in a way, inoculating themselves without realizing it.