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AUTHOR

David Trinklein
University of Missouri
Division of Plant Sciences
(573) 882-9631
trinkleind@missouri.edu

The Quest for More Flavorful Tomatoes

David Trinklein
University of Missouri
(573) 882-9631
trinkleind@missouri.edu

Published: September 3, 2021

ripe tomatoes

Tomato is America's most popular home garden vegetable. This is due, in large part, to the fact that tomatoes grown some distance from where they are purchased normally are harvested when the fruits have barely started to develop color (i.e., "breaker' stage). After being shipped hundreds or thousands of miles, they are artificially ripened by exposing them to ethylene gas when they reach their retail destination. From the standpoint of flavor, these so-called "gassed" tomatoes simply cannot compete with locally grown, vine-ripened produce. Aware of the public's desire for more flavorful tomatoes, plant scientists are actively involved in trying to incorporate superior flavor as a trait in their new releases.

green tomato

Tomato at "breaker" stage of ripeness. Credit: California Tomato Commission

First, it is important to recognize there is a difference between flavor and taste. Although often interchanged, the two are not synonymous. Taste, frequently referred to as our gustatory sense, is the perception of sweet, sour, bitter, or salty flavors detected through the taste buds on our tongue. Flavor, on the other hand, is a sensation of the interaction between taste and smell. In short, flavor equals taste plus aroma. In tomato fruits, the ratio of sugar to acids is recognized by the tongue, whereas volatile flavor compounds are perceived by our olfactory sense, via the nose. Therefore, perception of tomato flavor involves the integration of two of our basic senses: taste and smell.

Second, the flavor of a tomato will only be as good as its genetic make-up and the environment in which it is grown will allow. Plant geneticists often use the equation I = H x E as a short hand expression of this phenomenon. The equation suggests that an individual is the product of its hereditary makeup (genes) and the environment in which those genes are allowed to express themselves.

Using tomato fruit size to illustrate the above, small-fruited (cherry) tomatoes will never develop large fruits no matter how favorable the environment in which they are grown. In short, they lack the genes that cause fruits to become large. Conversely, normally large-fruited varieties grown under poor environmental conditions will not develop large fruits, even though they are genetically capable of doing so. In the latter case, the production environment fails to allow the genes for large fruit size to express themselves fully.

cherry tomatoes on vines

Cherry tomato

Although the search for the optimal production environment is far from over, at this point plant scientists are focusing much of their efforts on improving the hereditary makeup of tomato in attempt to improve flavor. Tomato is an example of a vegetable crop that has been the subject of extensive plant breeding and improvement. Unfortunately, factors affecting flavor and consumer preference often have fallen by the wayside in favor of traits such as disease resistance, fruit firmness and other factors contributing to total, marketable yield. That trend, however, is changing.

Until recently, breeding for improved tomato flavor has been directed mainly toward adjusting the amount of sugar and acid (i.e., sugar:acid ratio) produced by tomato fruits. Citric, malic and ascorbic are the primary acids produced by tomato fruits, whereas glucose, fructose and galactose are the three main sugars. Tomatoes with high sugar and high acid levels are judged by most to have the best flavor. This includes many varieties of cherry or salad tomatoes which most consumers judge to be sweeter than larger tomatoes. Recently, it was determined that the sugar in cherry tomatoes has a higher fructose content than larger tomatoes. Since fructose tastes sweeter to the human tongue than does sucrose, the perception that cherry tomatoes are sweeter holds merit.

Alternatively, tomatoes with low sugar and high acid content are considered to have a sour (tangy) flavor, whereas high sugar and low acid results in bland-tasting tomatoes. Fruits with both low sugar and low acid are relatively flavorless, and are typical of those that have been harvested at the breaker stage of maturity and artificially ripened.

While there still is work to be done in improving the sugar:acid content of fruits, recent efforts on improving tomato flavor have centered around attempting to understand more about the inheritance of tomato's volatile "flavor" compounds. As a tomato is chewed, these aromatic volatile compounds enter the olfactory system via the back of the mouth. There, molecules of the compounds bind to a receptor, transmitting a signal through the olfactory system. In turn, these signals interact in various parts of the brain with taste signals sent by the tongue to create the perception of flavor. The result in tomato fruits are flavor impressions such as sweet, fruity, tangy, tart, etc. Other descriptions include "classic tomato flavor" or "old-fashioned tomato flavor."

Plant scientists are attempting to determine how all of these chemicals interact to create the unique flavor of a tomato. To date, over 400 aromatic volatile compounds have been identified in tomato fruits. However, only about 20 are believed to contribute to tomato flavor. The formation of these compounds appears to be very complex. As a result, their inheritance (i.e., number and nature of genes involved) is not well understood. Suffice to say, the ultimate goal is to identify the genes (and the nature of their inheritance) that control synthesis of the volatile flavor compounds and to use this knowledge to produce tomatoes with superior flavor.

Dr. H.J. Klee, Professor of Horticulture Science at the University of Florida, has been a pioneer in this effort. During the past two decades Dr. Klee has focused on identifying the complexities of how flavor molecules are synthesized and released. One of his goals is to produce a tomato variety with heirloom flavor but without the production problems typically associated with heirloom varieties.

tomatoes on cutting board with knife

'Garden Treasure', one of Dr. Klee's releases for improved flavor. Credit: Proven Winners

In an attempt to gain information about the public's perception of his new varieties, Dr. Klee has initiated a "citizen science initiative." For a modest donation to his research program, gardeners receive seeds of three of Dr. Klee's recent creations, plus 'Better Boy' as a reference variety. Gardeners who take advantage of this offer are encouraged to submit their impressions of the tomato varieties via an online survey.

Since the initiation of the citizen science program, seeds of Dr. Klee's hybrids have been sent to over 14,000 gardeners in all 50 states plus 40 countries. Readers of this article who would like to become involved with the citizen science initiative may do so by visiting the following website: https://hos.ifas.ufl.edu/kleelab/new-garden-cultivars/trial-agreement/.

In closing, tomato flavor was largely ignored in the quest to breed more productive, higher yielding varieties that ship well. The argument can be made that regaining those factors that were responsible for good flavor in tomato is likely to come with a reduction in yield. For the home gardener, this is of minor concern, since a few more plants will make up the difference in yield. Such is not true for commercial growers for whom yields are closely tied to profits. The end result might be that more flavorful tomatoes will need to be treated as gourmet items that command a higher price than "regular" tomatoes. The willingness of the average consumer to accept the increased cost of more flavorful tomatoes has yet to be determined.

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REVISED: September 3, 2021