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Wednesday, June 27, 2007 04:47 pm

Homegrown tomatoes

Sugars and acid levels largely determine flavor

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Untitled Document There’s a lot of science behind the observation that homegrown tomatoes taste better than store-bought ones. “It’s not very hard to taste a tomato and decide whether you like or dislike the flavor,” says Jennifer Schultz Nelson, a horticulture educator with the University of Illinois Extension, “but what do you really taste? Tomato flavor is a combination of two major factors and one minor factor. From there, things get complicated quickly.”
The two big players in tomato flavor are sugars and acids, which occur in the largest quantities.
Fructose and glucose are the two primary sugars produced in the tomato. Obviously sugars are the source of the tomato’s sweetness. The acids in the tomato, mainly citric and malic acids, are the source of the tart flavor. “Looking at the possible combinations of sugar and acid levels in tomatoes, researchers have been able to make some generalizations,” she says. “Tomatoes with high sugar and high acid levels are generally considered to have good flavor. People tend to categorize tomatoes with low sugar and low acid levels as bland. “A tomato with high sugar but low acid content would most likely be called sweet, and one with low sugar but high acid content would be considered tart by most.”
The minor factor in flavor is volatile compounds. These occur in minute amounts, but researchers have found that they are the greatest contributor to what we label “tomato” flavor, she adds. The tongue cannot detect these volatile compounds; instead, they are picked up by the olfactory nerve in the nose. “We usually forget that the sense of smell contributes to tasting the flavor of foods until we are congested and can’t smell anything,” Nelson says. “Then it seems like many things taste more bland than usual. This is because our olfactory nerve cannot detect the volatiles in the food we’re eating.”
More than 400 different volatile compounds in the tomato have been quantified by researchers. Of these, only 30 occur in quantities greater than one part per billion. Only 16 of these compounds have been associated with significant contributions to tomato flavor. “How does all this relate to the difference in flavor between homegrown and artificially ripened tomatoes?” she says. “The different conditions in which each group is grown have significant effects on the levels of sugar, acid, and volatile compounds in the tomatoes produced.” 
Flavor is not necessarily the first consideration for commercial growers. Generally speaking, such traits as disease and pest resistance rank higher in importance. Also, a commercial producer must consider how well a variety can survive harvest and shipment to market. “This is one reason why commercial tomatoes are typically picked very underripe, at a stage called ‘mature green,’ meaning in another 24 hours or so it will show some pink coloring and be at the ‘breaker’ stage,” Nelson says.
“Tomatoes that are still green will store a lot longer, and travel better than ripe tomatoes.”
Before these tomatoes travel to market, they are artificially ripened with the use of ethylene gas, which is naturally produced by ripening fruits of all kinds. Exposing the mature green tomatoes to ethylene will trigger the ripening process, permitting the grower to send red tomatoes to market. Tomatoes picked at the breaker state do not need ethylene to ripen, because the process has already begun. Curiously, these breaker tomatoes are the ones sold in stores as “vine-ripened.”
Tomatoes destined for processing into canned products are allowed to ripen fully on the vine but must be tough enough to not break during harvest and transport to the canning facility. They are definitely not the tender, juicy homegrown tomatoes people savor each summer.
“Exposure to sunlight is crucial for sugar production in tomatoes,” Nelson says. “Picking mature green or breaker-stage tomatoes reduces their time in the sun and reduces the levels of sugar in the tomatoes.”

For more information about the University of Illinois Extension’s Sangamon-Menard unit, go to www.extension.uiuc.edu/Sangamon.
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