Above: Cherry tomatoes (Luc Viatour / www.Lucnix.be)

Did you know? Botanically, tomatoes are fruits just like apples or oranges. But in 1883, the U.S. Supreme Court ruled that tomatoes are vegetables, at least for the purposes of applying import duties.

The French used to call tomatoes “pommes d’amour” (love apples) because these delicious red fruits were believed to enhance romantic love. Sadly, there’s not much to love about supermarket tomatoes today. All too often, they simply add a slice of red to your favourite sandwich without providing any actual flavour.

But there is hope for this bland state of affairs! Scientists have discovered that the modern supermarket tomato is missing some important flavour components called volatile compounds. And this discovery may help turn a tale of plant breeding, flavour perception and chemistry into a love (apple) story.

All tomato varieties have been selectively bred from their wild ancestors. This includes older heirloom varieties that are popular in farmer’s markets and backyard gardens. For centuries, humans bred tomatoes to improve characteristics such as taste and size.

Before the 1940s, North Americans ate fresh, homegrown tomatoes (today’s heirloom varieties) only when they were in season, at the end of the summer. However, increasing consumer demand for cheap, year-round tomatoes spurred plant breeders to create varieties with high yield and durability, so they could be shipped over long distances.

Did you know? Distinct cells on your tongue can respond to five different taste categories: sweet, salty, sour, bitter and umami (savoury). In contrast, cells in your nose can detect hundreds of distinct smells.

In the 20th century, selective breeding has therefore focused on providing an abundance of sturdy tomatoes, well-suited to commercial growing and processing. Modern, commercially-grown tomato plants produce lots of tomatoes quickly by filling the fruits up with water. Tomatoes are also picked before they’re ripe because hard green tomatoes don’t get squished during shipping.

Unfortunately, somewhere along the line, the flavour was lost.

What exactly gives a tomato great flavour? Food scientists have discovered from tomato taste tests that high sugar content is important, along with some acidity to balance the sugar. Specialized cells in your tongue’s taste buds detect food molecules like sugar. These cells send a signal to your brain that contributes to your perception of yummy sweetness.

But it’s not just taste signals from your tongue that determine flavour. Your brain also interprets smell signals sent from cells in your nose, a process called olfaction. The smell of food makes a huge contribution to how you perceive its flavour. That’s why food tastes so bland when your nose is stuffed up from a cold!

There are actually two ways that your nose responds to odorants (smelly components) in food. When food odorants go up your nostrils when you inhale through your nose, it’s called orthonasal olfaction. When food odorants reach your nose through the back of your mouth, it’s called retronasal olfaction.

Did you know? Small tomatoes, such as cherry tomatoes, usually contain more sugar than the bigger varieties, so they’re a tastier option during the winter months.

The retronasal route is how your nose detects important flavour components in food called volatile compounds (often simply called volatiles). Volatiles are chemicals that evaporate quickly. Food volatiles are released into your mouth when you chew, before evaporating and traveling up your nose. Tomato volatile compounds are a diverse group of chemicals, made from fats, amino acids (protein building blocks) and vitamin A precursors called carotenoids.

It turns out that tomato volatiles are just as important as sugar for creating a delicious tomato. In 2012, a research group led by Harry Klee at the University of Florida published the results of their work on tomato volatiles. They constructed a chemical profile for different varieties of supermarket tomatoes, as well as for some heirloom tomatoes. The chemical profiles included the levels of sugar, acid (such as ascorbic acid, or vitamin C), and dozens of volatile compounds.

Taste testers scored the tomatoes for qualities such as sweetness, acidity, flavour intensity, and general likeability. Then, researchers correlated how much testers liked each tomato variety with its chemical profile, to see which chemicals were most important. As expected, sweetness was a critical factor for tomato likeability. However, the perceived sweetness wasn’t only due to the sugar content.

Did you know? The concentration of tomato volatiles plummets at cooler temperatures, because the enzymes that make them become inactive. So keep your tomatoes out of the refrigerator for better flavour!

Six different tomato volatiles enhanced the perception of sweetness. Also, the most important volatiles for tomato flavour aren’t the same ones that are important for tomato aroma. This highlights the difference between retronasal and orthonasal olfaction, as well as the complexity of flavour perception.

On average, modern supermarket tomatoes have lower levels of volatiles compared to heirloom varieties. Remember that modern tomato plants were bred to produce lots of fruits, all pumped up with water. Production of volatiles, and therefore flavour intensity, was (unknowingly) sacrificed in pursuit of commercially successful tomatoes.

Harry Klee’s lab is working to identify the enzymes responsible for generating the volatiles that make a tasty tomato. That will make it possible to breed those enzymes back into established commercial varieties. And maybe then we can have year-round fresh tomatoes that live up to their “love apple” heritage!

References

General news and science websites

The Recipe for a Tastier Tomato (Erik Stokstad, ScienceNOW) Harry Klee, molecular horticulturalist (Paul Gabrielsen, University of California, Santa Cruz) Tomatoes: Heirlooms vs. Hybrids (Jennifer Skene, QUEST) Tomato: The Apple of Peru (McGraw Hill Higher Education) Press releases UF scientists identify natural compounds that enhance humans’ perception of sweetness (Mickie Anderson, University of Florida)

Classroom activities

Can You Taste Without Your Nose? (Institute of Food Technologists)

Scholarly publications

Tieman D et al. 2012. The Chemical Interactions Underlying Tomato Flavor Preferences. Current Biology. 22:1035-1039.

Krysta Levac

After an undergraduate degree at the University of Guelph, I earned a PhD in nutritional biochemistry from Cornell University in 2001. I spent 7 years as a post-doctoral fellow and research associate in stem cell biology at Robarts Research Institute at Western University in London, ON. I currently enjoy science writing, Let's Talk Science outreach, and volunteering at my son's school. I love sharing my passion for science with others, especially children and youth. I am also a bookworm, a yogi, a quilter, a Lego builder and an occasional "ninja spy" with my son.



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