Why Heirloom Tomatoes Taste Better: The Juicy Science Behind Tomato Flavor

I don’t think I’d be making waves if I confidently declared that heirloom tomatoes taste better. After all, most of us have had the experience of biting into a mealy, flavorless grocery store tomato. It simply doesn’t compare to the sun-warmed richness of an heirloom picked at peak ripeness.

And I don't think this preference is owed to nostalgia alone.  Large flavor studies comparing hundreds of tomato varieties have confirmed what gardeners have long suspected: heirloom tomatoes often score higher for both flavor intensity and overall liking than modern varieties—even when grown and harvested under the same conditions [1].

So while some may dismiss the appeal of heirlooms as little more than marketing hype, there is real scientific evidence behind their reputation. In fact, researchers studying the chemistry of tomato flavor have even begun to uncover why these varieties taste so distinctive.

In this week’s post, we're going to explore the science behind tomato flavor and examine how traits like color, fruit structure, and ripening genetics influence the taste of heirloom tomatoes.

Join me as we dig into the juicy science behind heirloom tomato flavor.

Nothing beats the flavor of a vine-ripened heirloom tomato.  Now, with the help of science, we're beginning to understand exactly why these varieties taste so good.  Variety: Orenburg Giant

The Three Components of Tomato Flavor

At its most basic level, tomato flavor is created by the interaction of three types of compounds: sugars, acids, and a diverse group of aromatic molecules known as volatile compounds.

Sugars such as glucose and fructose provide sweetness, while organic acids—primarily citric and malic acid—create the bright tang we associate with fresh tomatoes. But the distinctive character of tomato flavor comes largely from volatile compounds, which produce the complex aromas we perceive as fruity, floral, green, spicy, or savory.

Scientists have identified more than 400 volatile compounds in tomatoes. Fortunately, we don’t need to understand all of them to appreciate tomato flavor. Researchers believe that around twenty of these molecules account for most of the aromas humans actually detect when we bite into a ripe tomato [2].

Aunt Ruby's German Green tomato is beloved by heirloom enthusiasts for its smoky, spicy, and vanilla-like aroma, notes that are conferred by its unique profile of volatile compounds.

Large flavor studies comparing hundreds of tomato varieties have revealed something even more striking: nearly half of the chemicals known to influence overall flavor intensity and consumer liking occur at significantly lower concentrations in modern commercial tomatoes than in heirloom varieties [3].

In other words, many of the compounds that give tomatoes their rich, complex flavor have gradually been reduced during modern breeding.

But while heirlooms have retained these flavor-enhancing volatiles, what's even more fascinating is how dramatically the compounds can vary between varieties. In some cases, the concentration of certain aroma molecules differs by more than three thousand-fold [3], helping explain why one heirloom may taste bright and citrusy while another carries hints of melon, honey, or even smoky richness.

This remarkable variability is one reason gardeners often find heirloom tomatoes far more interesting—and flavorful—than more uniform modern tomatoes.

How Tomato Genetics Influence Flavor

The balance of sugars, acids, and aromatic compounds in a tomato is largely determined by its genetics. Small genetic differences can influence how much sugar a fruit accumulates, how acids develop during ripening, or which aroma molecules are produced as the fruit matures.

Many of these aromas originate from a handful of plant compounds that break down during ripening.

With heirlooms, flavor goes deeper than taste.  Aroma is often our first interaction with a tomato, and in Andrew Rahart's Tomato, it hits hard, often captivating before the knife hits the cutting board.

Depending on the variety, these compounds can produce aromas described as citrusy, grassy, honeyed, melon-like, smoky, or even chocolate-like.

Interestingly, many of the traits gardeners notice first—things like fruit color, green shoulders, fruit size, or even the number of seed cavities inside a tomato—are connected to these same biochemical pathways. As we’ll see, characteristics that appear purely cosmetic often turn out to have surprisingly large effects on flavor.

In many cases, the traits that make heirloom tomatoes visually distinctive are also the ones that help produce their complex and memorable taste.

Flesh Color and Carotenoids

The dazzling diversity of heirloom tomatoes—bright reds, vibrant oranges, cheerful yellows, greens, and even purples—is one of the things that makes them so beloved among gardeners. But this diversity is far more than superficial. Color turns out to be one of the most important clues to a tomato’s flavor.

Many gardeners have noticed that yellow tomatoes tend to taste milder than their red counterparts, a quality that has led to the widespread belief that they are lower in acid. In reality, yellow tomatoes are not consistently lower in acid than other tomatoes. Instead, their distinctive flavor comes from differences in the pigments known as carotenoids, the compounds responsible for much of a tomato’s color. 

Yellow tomato-lovers appreciate the mild, sweet flavors of varieties like Hensley's Settlement, which often come with unique fruity and tropical notes.

Red tomatoes accumulate large amounts of lycopene, while many yellow tomatoes produce little or none of this pigment. When researchers altered the red tomato, San Marzano, so that it produced yellow fruit instead, they discovered that the change affected far more than color. The mutation also reduced several apocarotenoid volatile compounds—aromatic molecules derived from carotenoids that contribute to the classic “tomato” aroma [4].

With fewer of these dominant tomato aromas present, the overall flavor of the fruit becomes softer and less intense. This milder profile can trick the palate into perceiving the tomato as less acidic, even when its chemical acidity is similar. At the same time, the reduced intensity of the classic tomato aromas can allow more delicate notes to become easier to detect.

That Golden Glow

Not all yellow tomatoes achieve their color the same way, however. Some heirloom varieties—including Dixie Golden Giant and Kellogg’s Breakfast—owe their golden color to a different carotenoid profile altogether. These tomatoes accumulate orange-tinted prolycopene, a pigment associated with the so-called tangerine mutation. In these varieties, researchers have measured levels of the aromatic compounds geranylacetone and 6-methyl-5-hepten-2-one (MHO) that are several times higher than those found in comparable red-fleshed tomatoes [5].

Kellogg's Breakfast boasts a vibrant orange color that is facilitated by high concentrations of pro-lycopene, giving it intense aroma and improved nutritional quality.

Both compounds contribute floral and citrus-like aromas, helping explain the bright, fruity character often noted in these golden heirlooms. Interestingly, both geranylacetone and MHO have also been shown to enhance the perception of sweetness [6], even when the actual sugar content of the fruit remains unchanged. In other words, the distinctive aroma chemistry of these varieties can make them seem sweeter to our senses—a subtle but powerful example of how aroma and taste work together to shape the flavor of a tomato.

Skin Color and the Tomato Epidermis

If you’ve read our article explaining how tomatoes get their color, you may remember one surprising detail: red and pink tomatoes are actually the same color on the inside. The difference lies not in the flesh, but in the thin outer layer of the fruit—the epidermis, or skin.

Most red tomatoes have yellow-tinted skin covering the fruit. This yellow skin acts as a filter that warms the color of the red flesh beneath it, giving the tomato its familiar deep red appearance.

In pink tomatoes, that yellow pigment is missing. The skin is clear instead of yellow, which softens the color of the red flesh and gives the fruit its characteristic pink tone.

Although pink tomatoes are derived from a simple change in skin color, the effects are more than skin-deep.  As we will soon learn, this minor change has a major influence on flavor. Pictured: Dester tomatoes.

This small genetic change might seem purely cosmetic, but research suggests it can have a surprisingly large influence on flavor.

The yellow pigment in tomato skin is produced through the same biochemical pathways that generate many aroma compounds in the fruit. When that pigment is absent—as in clear-skinned or pink tomatoes—the chemistry of those pathways shifts.

Researchers studying these tomatoes found higher levels of several aroma compounds that contribute floral, spicy, and slightly smoky notes [4]. One of these compounds, 2-phenylethanol, is especially interesting because it can also enhance the perception of sweetness, making the fruit taste richer even when the sugar content is unchanged [7].

Clear-skinned tomatoes were also found to produce higher amounts of certain “green” aroma compounds derived from fatty acids, which contribute fresh, garden-like notes to tomato flavor.

In other words, changing the color of the skin doesn’t just alter how a tomato looks—it can subtly reshape the entire aromatic profile of the fruit.

Chlorophyll, Green Shoulders, and Uneven Ripening

Color in tomatoes isn’t determined by carotenoids alone. Another important pigment is chlorophyll, the same green molecule plants use to capture sunlight during photosynthesis.

In some heirloom tomatoes, however, this green pigment persists longer during ripening. When chlorophyll remains in the flesh and mixes with red carotenoids, it produces the deep, dusky tones that give purple or black tomatoes their characteristic color.

Black Krim tomatoes derive their color from the mixing of red lycopene and green chlorophyl, leading to a dusky purple color.  They are best known for their rich flavor, which makes them one of the most popular heirlooms around.

This effect is often associated with a genetic trait known as the greenflesh mutation, which slows the breakdown of chlorophyll as the tomato ripens. Fruit carrying this trait can maintain active chloroplasts in their tissues for longer periods, allowing the fruit to continue performing photosynthesis as it develops.

That continued photosynthesis can contribute directly to flavor. Studies have shown that tomatoes with persistent chlorophyll often accumulate higher levels of sugars, producing fruit that tastes richer and sweeter [5].

The Uniform Ripening Mutation

Not all tomato mutations have worked in favor of flavor.

In the 1940s and 1950s, plant breeders introduced a trait known as uniform ripening. Tomatoes carrying this mutation ripen more evenly across the fruit, eliminating the green shoulders that were once common in many varieties. The mutation also made it easier for growers to judge harvest maturity and helped reduce problems like sunscald and fruit cracking. 

Today, nearly every commercial tomato carries this mutation.

Cherokee Purple tomatoes are best known for their beautiful green shoulders. While some dislike that the shoulders are slow to ripen, this feature serves an important purpose, as we will soon learn.

But the convenience came with an unexpected tradeoff.

Researchers later discovered that the uniform ripening mutation reduces the number of chloroplasts that develop in tomato fruit [8]. While the tomatoes still grow to normal size and ripen normally, they accumulate 10–15% less sugar and produce less of the red pigment lycopene. The result is fruit that often appears paler and tastes noticeably milder.

This discovery helps explain a long-standing observation among gardeners: tomatoes with green shoulders and uneven ripening often taste better.

Those traits may not produce the most visually perfect fruit, but they are signs of active chlorophyll and chloroplast development—features that contribute directly to the sweetness and complexity of tomato flavor. In other words, some of the characteristics that make heirloom tomatoes look a little rough around the edges are also the ones that help make them taste exceptional.

Fruit Shape, Locule Number, and the “Ugly Tomato” Effect

Another trait that subtly influences tomato flavor is fruit shape.

Modern commercial tomatoes are typically bred for smooth, uniform fruit that are easy to harvest, pack, and ship. Heirloom tomatoes, by contrast, are often a little less tidy. Many develop ribbed shoulders, irregular folds, or other quirks that make them look almost misshapen compared with the perfectly round tomatoes found in grocery stores.

But these visual differences often reflect something happening inside the fruit.

Tomatoes contain seed cavities known as locules, which are surrounded by the gelatinous tissue that holds the seeds. This gel contains many of the sugars, acids, and aromatic compounds associated with tomato flavor.

Tomato varieties vary tremendously in how these locules are arranged. Some fruit contain only a few large seed cavities. Others—particularly many heirloom beefsteaks—contain numerous smaller locules distributed throughout the fruit.

Beefsteak tomatoes are known for their numerous seed cavities, or locules, which allow the tomatoes to remain juicy without getting sloppy.

Increasing locule number often comes with side effects that commercial breeders have historically tried to avoid. Fruit with many locules frequently develop ribbed shoulders, irregular folds, or “cat-facing,” traits that make tomatoes harder to grade, pack, and ship. As a result, modern breeding programs have often selected against these characteristics in favor of smoother, more uniform fruit.

But those same traits can influence how flavor is experienced. When a tomato contains many smaller locules spread throughout the fruit, the flavorful seed gel is distributed more evenly through each slice. By contrast, tomatoes with only a few large seed cavities often concentrate much of that gel in the center—you've experienced this if you've ever had the inside of your tomato spill out on the cutting board when you cut it.

In other words, the irregular shapes seen in many heirloom tomatoes may reflect an internal architecture that helps preserve juiciness and distribute flavor more evenly throughout the fruit.

Sometimes the best-tasting tomatoes are the ones that, from the outside, look a little—well, ugly.

Yield vs. Flavor: The Tomato Sugar Penalty

If color, ripening, and fruit structure explain how tomatoes develop flavor, another factor helps explain why many modern tomatoes taste less intense than heirlooms: yield.

Plants operate under a simple biological constraint—resources are limited. The sugars that give tomatoes their sweetness are produced by photosynthesis and must be divided among the plant’s growing tissues. When a plant produces more or larger fruit, those sugars are distributed across a greater amount of tissue.

For this reason, tomato researchers have long observed a trade-off between fruit yield and sugar concentration. Increasing the genetic activity of certain sugar-producing pathways can raise sweetness in the fruit, but often results in smaller harvests or reduced fruit size [9]. In other words, concentrating flavor can come at a cost.

Pink Brandywine tomatoes are perhaps the best example of the yield vs flavor effect.  While they are widely regarded as one of the best-tasting tomatoes around, they are also known for being low-yielding—a perfect example of the quality vs. quantity tradeoff.

Large-scale studies comparing heirloom and modern tomato varieties support this observation. Researchers in the Klee laboratory found that modern hybrids generally contain lower levels of both sugars and organic acids, two of the most important contributors to tomato flavor [10].

So how did this happen?

Part of the answer lies in how tomatoes are bred and sold. The primary customers of commercial seed companies are farmers, and farmers are typically paid by the pound or by the crate—not by flavor.

At the same time, flavor is surprisingly difficult to measure in a breeding program. Yield, disease resistance, and fruit uniformity can be evaluated quickly in the field. Flavor, by contrast, requires chemical analysis or taste panels—processes that are far more time-consuming and expensive.

Box Car Willie and its cousin Mule Team, represent a class of tomatoes best described as transitional—not quite heirloom, yet not commercial.  In every respect, they straddle the line, yielding almost as good as hybrids with a flavor almost as good as older heirlooms.

Over time, this imbalance naturally pushed breeding programs toward traits that were easy to measure and economically rewarded, such as yield, shelf life, and durability during shipping. Traits that are harder to quantify—like complex flavor—often received less attention.

Heirloom tomatoes followed a different path. These varieties were typically maintained by gardeners and small-scale growers who selected plants based largely on how the fruit tasted, rather than how many pounds each plant produced.

The result is a remarkable diversity of tomatoes that may not always produce the highest yields, but often deliver something far more memorable: exceptional flavor.

Heirlooms may not always be the highest yielding, the most uniform, or the nicest looking, but they do promise one thing—flavor.  Pictured: Red Brandywine

Final Thoughts: Why Heirloom Tomatoes Taste Better

When you step back and look at these traits—color, ripening patterns, fruit structure, and yield—a pattern begins to emerge.

Many of the characteristics that make heirloom tomatoes look unusual are the same ones that contribute to their remarkable flavor. Their vibrant colors reflect different aromatic compounds. Their green shoulders and uneven ripening signal higher sugar production in the fruit. And their ribbed or irregular shapes often reveal complex internal structures that distribute flavorful gel throughout each slice.

Even their more modest yields can play a role. When plants produce fewer fruits, sugars and acids are concentrated into each tomato rather than spread across a larger harvest.

In other words, many traits that breeders once viewed as imperfections—odd shapes, uneven ripening, lower yields—are often the very things that make heirloom tomatoes so delicious.

Want to experience the difference yourself? Explore our collection of heirloom tomato seeds and discover varieties preserved for generations by gardeners who cared most about one thing—flavor.

References

1. Tieman et al. (2017) A chemical genetic roadmap to improved tomato flavor. Science, 355(6323), 391–394. PubMed
2. Martina et al. The Genetic Basis of Tomato Aroma (2021) Genes (Basel). 2021 Feb 4;12(2):226. PubMed
3. Tieman et al. (2012) The chemical interactions underlying tomato flavor preferences. Curr Biol. 2012 Jun 5;22(11):1035-9. PubMed
4. Dono et al. (2020). Color Mutations Alter the Biochemical Composition in the San Marzano Tomato Fruit. Metabolites, 10(3), 110. https://doi.org/10.3390/metabo10030110
5. Aono et al. (2021). High-Throughput Chlorophyll and Carotenoid Profiling Reveals Positive Associations with Sugar and Apocarotenoid Volatile Content in Fruits of Tomato Varieties in Modern and Wild Accessions. Metabolites, 11(6), 398. PubMed
6. Vogel et al. (2010). Carotenoid content impacts flavor acceptability in tomato (Solanum lycopersicum). Journal of the science of food and agriculture, 90(13), 2233–2240. PubMed
7. Baldwin et al. (2008). Interaction of volatiles, sugars, and acids on perception of tomato aroma and flavor descriptors. J Food Sci. 2008;73(6):S294-S307. PubMed
8. Powell et al. (2012). Uniform ripening encodes a Golden 2-like transcription factor regulating tomato fruit chloroplast development. Science (New York, N.Y.), 336(6089), 1711–1715. PubMed
9. Klee, H. J. (2010). Improving the flavor of fresh fruits: Genomics, biochemistry, and biotechnology. New Phytologist, 187(1), 44–56. PubMed
10. Goff, S. A., & Klee, H. J. (2006). Plant volatile compounds: sensory cues for health and nutritional value?. Science (New York, N.Y.), 311(5762), 815–819. PubMed