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August News from the MMG Edible Demonstration Garden

Why a Tomato Turns Color and Ripens

Tomatoes are green at the beginning because they produce chlorophyll. Mature green tomatoes also produce a gas called ethylene that initiates the ripening process. As a tomato ages, chlorophyll begins to break down and carotenoids which produce the red or yellow color begin to take over. Industrial producers of tomatoes use ethylene during trucking to ripen fruit off of the vine. Because green tomatoes produce ethylene, home gardeners can also choose to ripen their tomatoes off the vine by picking them at the mature green stage when they show a little color, wrapping each tomato in newsprint, and storing at temperatures between 65-75 degrees for up to 3-5 weeks. Weather conditions and avoiding common growing problems might be reasons to do this. However, most of us want a vine-ripened tomato so what conditions effect tomato ripening?

Conditions Effecting Tomato Ripening

The most important condition is the number of days to maturity, counted from the planting of starts to harvest. This varies from as low as 60 days (Early Girl) to as high as 80 (Brandywine). Assuming you planted your starts in early May, by mid-to-end July, Early Girl tomatoes are rushing to ripeness while it will be late August before you see the same in a Brandywine. The second condition is temperature. It is not light that stimulates tomatoes to ripen but warm, not too hot temperatures. The ideal temperature for ripening is 65°F-75°F. Tomatoes stop ripening above 86°F. Weeks of very hot temperatures will slow down the ripening process. So what does a perfectly ripened tomato look and feel like?

Determining ripeness by looking and touching

First of all, the color should be deep and uniform. In addition, a ripe tomato should look glossy and slightly shiny and when cut in half, will have a fragrance. When squeezed gently, it will feel a tiny bit soft, in between firm and soft. When removing the tomato from the vine, you can grasp and twist it off and if ripe, it will show only a slight resistance to being picked. Of course, there are exceptions to these criteria for ripeness. Heirloom tomatoes like Cherokee Purple ripen before they completely turn color. But you can tell by a gentle squeeze and the heft of it that it is juicy and ready to be picked. Cherry tomatoes will often crack if left on the vine too long so pick them just before they look perfectly ripe. And of course, there are tomatoes like Green Zebra that never turn red or yellow. For those tomatoes, know your days to harvest number and once reached, gently squeeze and feel the tomato to determine ripeness. Remember, tomatoes have their own built in ripening agent so if you have to pick tomatoes before they are perfect, just put them in a place away from a window, shoulders up where it is ideally 65°F-75°F. You will still get that luscious tomato we all yearn for this time of year.


Author: Joan Kozlowski, Co-chair, Edible Demo Garden