2007 Year in Review

Home-grown Tomatoes

There's a lot of science behind the observation that home-grown tomatoes taste better than store-bought ones, said a University of Illinois Extension horticulture educator.

"Next time you enjoy a great-tasting home-grown tomato, consider not only how wonderful the flavor is, but also how wonderful and amazing it is that such a small, common fruit houses incredibly complex processes to produce that flavor," said Jennifer Schultz Nelson.

"It's not very hard to taste a tomato and decide whether you like or dislike the flavor. 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, she pointed out, are sugars and acids. They are considered to be major factors in flavor because they occur in the largest quantity.

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 are mainly citric and malic acids. These are the sources of the tart flavors.

"Looking at the possible combinations of sugar and acid levels in tomatoes, researchers have been able to make some generalizations," she said. "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 factor that contributes most to what we label as ‘tomato’ flavor, she added.

“Volatiles are the key factor that helps us distinguish flavor differences among varieties with similar sugar and acid content,” Nelson said.

The tongue cannot detect these volatile compounds. They are picked up by the olfactory nerve in the nose, otherwise known as the sense of smell.

“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 said. “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.”

Over 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 contribution to tomato flavor.

“How does all this relate to the difference in flavor between home-grown and artificially ripened tomatoes?” she said. “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.”

When breeding or choosing a commercial variety of tomato to produce, flavor is not necessarily the first consideration. Generally speaking, traits like disease and pest resistance usually rank higher in importance. Also, a commercial producer must consider how well a variety can survive harvesting and shipping to market.

“This is one reason why commercial tomatoes are typically picked very under-ripe, 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,” said Nelson.

“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 using ethylene gas. Ethylene is naturally produced by ripening fruits of all kinds. Exposing the mature green tomatoes to ethylene will trigger the ripening process, so red tomatoes are delivered to market.

"Those tomatoes that were picked at the breaker state do not need the ethylene to ripen, since they have already begun the process," Nelson noted. "Interestingly enough, 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, yet must be tough enough to not break during harvest and transport to the canning facility. They are generally drier and have thicker skin and flesh than varieties intended for fresh consumption. They are definitely not the tender juicy home-grown tomatoes people savor each summer.

"Exposure to sunlight is crucial for sugar production in tomatoes," said Nelson. "Picking mature green or breaker stage tomatoes reduces their time in the sun, and reduces the levels of sugar in the tomatoes.

"Some studies have related the amount of potassium to acid levels in tomatoes. Fertilizing with greater amounts of potassium resulted in higher acid content in most varieties."

Nelson said that without question, research has shown that artificially ripened tomatoes have significantly lower levels of volatile compounds than home-grown, fully ripened tomatoes.

"The million-dollar question that remains unanswered is how to artificially induce green tomatoes to produce the volatile compounds," she noted. "An even more complex question is identifying the genes that regulate production of the volatile compounds. The goal is to allow breeders to select for plants with the best genetic profile for producing these compounds in developing new varieties."

Farther down the road, the dream of some researchers is to genetically engineer tomato plants to produce more volatiles by physically inserting the genes responsible for their production, she added.

by  Editor, theCity1.com
June 25, 2007