Klee Lab

Research

Variety Trials

The first step in a flavor improvement program starts with a simple question: what do people like and what’s in the varieties that people do like? In order to answer this question, we took a giant step back to “heirloom” tomatoes. There is absolutely no definition of the word “heirloom”. But most people accept that an heirloom is an old variety that predates the time of intensive commercial breeding. Most, though not all, are inbred (that is, they breed true to type). Contrary to popular belief, not all heirlooms taste good. We grew a broad cross-section of heirlooms, gave them to a panel of consumers and asked them to rate each variety. Don’t worry about the scale we used. It’s complicated. All you need to know is that the higher the score, the better it tasted. Note that there is some variation between years for varieties that were tested multiple times. All of these varieties were grown in our spring season at the University of Florida Suwanee River Research and Education Center in Live Oak, FL. Expect some variation in other parts of the country and from season to season. One can only hope that some of the really terrible varieties do better somewhere!

This work was published in: Science (2017) 355 (6323):391-394, and in Current Biology (2012) 22: 1–5. It was supported by a grant from the National Science Foundation and from support of the Florida Agricultural Experiment Station.

Variety	Liking
Maglia Rosa Cherry	30.5
Garden Gem	28.5
Early Red Chief	22.4
B hybrid	21.0
Tasti-Lee	20.8
St. Pierre	20.7
Peron Sprayless	20.0
W hybrid	18.6
Better Boy	18.3
Wisconsin 55	17.2
Garden Treasure	16.8
Livingston's Globe	16.4
Super Sioux	16.3
Bloody Butcher	16.2
Orange Queen	15.8
Valencia	15.5
Three Sisters	15.5
FLA 8059	14.2
Red Calabash	13.9
Czech Bush Red	13.6
Red House Free Standing	12.5
Matina	12.5
Giant Belgium	11.5
Dixie Golden Giant	11.0
Bear Creek	10.7
German Queen	10.3
Florida 47	10.1
Marmande VFA	9.3
Brandywine	9.1
Livingston's Stone	8.9
Kentucky Beefsteak	8.8
Black	8.6
Flora-Dade	7.6
Watermelon Beefsteak	7.5
Martian Giant	7.3
Japanese Black Trifele	6.2
Santiam	4.6
FLA 8735	1.9

When we bite into a tomato, what our brains register as a "tomato" is actually a complex interaction between sugars (glucose and fructose), acids (citric, malic and ascorbic) and multiple volatile compounds. While several hundred volatiles have been identified in tomato, only about 15-20 actually impact our perception of the fruit. This is because most of these compounds fall below the odor threshold. This threshold is determined by both the concentration of the substance and our ability to detect it. Thus, a compound that is present in quite high levels that we detect poorly will not register. Conversely, a substance to which we are quite sensitive will be perceived in very low amounts. For an excellent and understandable explanation of this concept, see Baldwin et al. HortScience 35:1013-1022 (2000). Much work has gone into defining the "important" volatiles that together make up tomato flavor and the list provided below should be considered only an approximation. Odor thresholds vary markedly between individuals and can be greatly influenced by the way in which the volatile is presented. A list of the major volatiles, their concentrations and their characteristic odors follows:

Volatile	Conc. (ppb)	Log odor units	Odor Characteristics
cis-3-Hexenal	12,000	3.7	tomato/green
ß-ionone	4	2.8	fruity/floral
hexanal	3,100	2.8	green/grassy
ß-damascenone	1	2.7	fruity
1-penten-3-one	520	2.7	fruity floral/green
2+3-methylbutanal	27	2.1	musty
trans-2-hexenal	270	1.2	green
2-isobutylthiazole	36	1.0	tomato vine
1-nitro-2-phenylethane	17	0.9	musty, earthy
trans-2-heptenal	60	0.7	green
phenylacetaldehyde	15	0.6	floral/alcohol
6-methyl-5-hepten-2-one	130	0.4	fruity, floral
cis-3-hexenol	150	0.3	green
2-phenylethanol	1,900	0.3	nutty/fruity
3-methylbutanol	380	0.2	earthy, musty
methyl salicylate	48	0.08	wintergreen

Tomato CCD genes

Several of these volatiles (ß-ionone, ß-damascenone and 6-methyl-5-heptene-2-one) are produced by oxidative cleavage of carotenoids. Additional carotenoid-derived volatiles just below this threshold include geranyl acetone and pseudoionone. We have isolated a pair of closely related tomato carotenoid cleavage dioxygenases (CCDs) that cleave a wide range of carotenoid substrates at the 9,10 and 9’,10’ positions to release a variety of volatile compounds that are essential components of tomato flavor. These results provide a direct link between enzyme and volatile.

CCD Paths

The 2-Phenylethanol pathway

Several important flavor volatiles (methylsalicylate, phenylacetaldehyde, 2-phenylethanol and 1-nitro-2-phenethane) are derived from the shikimate pathway. We have isolated several of the genes responsible for synthesis of these important volatiles. The most important enzymes for the phenylalanine-derived volatiles are the aromatic amino acid decarboxylases (AADCs). They perform the first step in the pathway: conversion of phenylalanine to phenethylamine. For methylsalicylate, a methyltransferase, SAMT, is the most important enzyme.

One complicating factor that is not well understood involves conjugation of the flavor volatiles to sugars such as glucose. These glycosides are no longer volatile and therefore cannot contribute directly to flavor.

2-phenylethanol pathway