The Chemical and Genetic Basis for Flavor in Tomatoes

There is no universal agreement on what a “perfect” tomato tastes like.  There can be significant differences in flavor ranking based on personal preferences – level of sweetness or acid content, for example. Fruit texture and skin thickness can also influence “likeability”. However, recent research, including the use of replicated tasting panels, has helped highlight, on average, what factors enhance or detract from fruit desirability. Here is what we know about the flavor in tomatoes.

The flavor is detected on the tongue (taste) and through the nose (aroma) and is a complex interaction between multiple chemical factors.

A refractometer is used to take a Brix reading, which measures sugar content.

Our tongue is capable of discerning differences in sweet (sugars – sucrose and fructose), sour (acids – glutamate, citrate, malate), salty, bitter, and umami.  Although there are tomato lovers that detest tomatoes that are “too sweet”, sugar content/Brix is the factor most highly correlated with a favorable flavor rating.  The conventional wisdom is that high sugar, in balance with a moderate acid content, is the foundation for what most tasters identify as a favorable flavor in tomatoes. But there is more …

We now understand that volatile aromatic compounds (VOCs), with retronasal detection, also greatly influence flavor.  Researchers at the University of Florida have helped rank the importance of the various tomato VOCs based on carefully controlled taste panel results.  They have found that although there are over 400 VOCs in tomatoes, less than a couple of dozen have a significant influence on flavor.  It should be noted that some common VOCs have a positive influence on flavor and others have a negative impact.

One family of important VOCs (apocartenoids) is derived from the breakdown of carotenoid pigments.  The apocarotenoid flavor compound derived from lycopene is different from the one derived from beta-carotene and both are different from the cis-lycopene apocarotenoid found in tangerine tomatoes. Most of these carotenoid pigments have an effect on both color and flavor.  A 2010 study ranked flavor in this order: wild type (85% lycopene/15% beta-carotene) > crimson (100% lycopene) > tangerine > yellow.  This comparison used isogenic lines that varied only in the noted carotenoid pathway alleles.  Another study found that the primary beta-carotene-derived apocarotenoid VOC (beta-Ionone) was second only to sugar content in its positive correlation to flavor.  Some apocarotenoid VOCs also increase the perception of sweetness, without changing sugar content.  Since sugar content and fruit size are negatively correlated, any enhancement of “sweetness” without increasing sugar content will help improve the flavor of larger fruited types.

The research group at the University of Florida, with numerous scientific colleagues, has recently published scientific papers that compare the genes expressed and/or the chemical composition of wild relatives, heirloom types, and modern hybrids.  They found a significantly higher concentration of favorable VOCs in some of the heirloom types and most of the wild relatives, compared to modern hybrids. One of the more interesting findings was that during domestication (wild > cultivated) there was the loss of the favorable allele for a key gene controlling the production of several favorable VOCs.  This favorable allele was identified in a specific accession of S. pimpinellafolium and is now a target for crossing into modern tomato breeding lines.

There are several other examples of loss of favorable flavor alleles or a combination of alleles during domestication and modern breeding. This is generally attributed to a general loss of genetic variability (i.e. genetic drift) during a domestication period with intense selection for large fruit phenotype, and to modern breeding focused almost exclusively on yield, disease resistance, and firm fruit that are easy to ship. Pre-domestication genetic resources are available, e.g. the UC Davis Tomato Genetic Resource Center, which contains the desired combinations of these favorable flavor alleles.

Understanding the chemical and genetic basis for tomato flavor has been an important and necessary step in developing breeding strategies for flavor improvement. There is broad consumer recognition of a general deterioration of flavor in modern hybrids compared with heirloom types, and there has been well-deserved criticism of tomato breeders for focusing on yield, disease resistance, and “transportation-friendly” traits, at the expense of flavor in tomatoes. The good news – this is rapidly changing.  Most tomato breeding programs are now working hard to combine the flavor profiles of heirloom types with the improved yield, plant health, and fruit quality of modern breeding lines. Early progress looks very promising.  The basic research on flavor in tomatoes that was described above, such as at the Klee Lab at the University of Florida, will provide a roadmap for further improvement, that targets genes controlling the synthesis of specific VOCs. The content of favorable chemicals can be increased, and the content of unfavorable chemicals decreased, by utilizing molecular markers associated with superior alleles for key genes. This collection of genomic tools will also lead to more efficient mining of useful alleles for flavor genes from related “wild” species.

Most tomato breeders are very optimistic about simultaneously improving flavor, fruit quality, fruit yield, and plant health/disease resistance in tomatoes.