If you have an unripe avocado, banana or cantaloupe, you can place it in a paper bag with some apples. The ethylene gas the apples produce will encourage the fruits to ripen -- but that won't work with a seedless grape. To get perfect seedless grapes, pick ones that are soft, sweet and plump right from the vine.
The perfect ripeness of seedless grapes is something only you know. Some people prefer slightly less sweet grapes while others prefer a grape at its full sweetness. Once you've harvested your grapes, some varieties can last up to eight weeks if you keep the fruit at 85 degrees Fahrenheit in a location with 85 percent humidity.
All varieties of grape are preclimateric -- they don't ripen once they're taken off the vine. Professional grape growers know to check the ripeness and sweetness of grapes before picking them because sweetness vary over time, and can be significantly different at different times of the day. Grapes picked early in the day, for example, may taste different than those picked in the afternoon. Unlike some other fruits, grapes won't ripen further after you pick them.
Monitoring grapes closely is the only way to determine the best time to harvest, explains Edward Hellman of Texas A&M; explains. He says, "There is no single stage of development or point in time that the fruit can be universally considered 'ripe.'" As grapes reach maturity, and the cells swell, the fruit gets larger and softer. The sugar increases, while acidity in the fruit decreases. Depending on how you plan to use the grapes -- wine making, use in recipes or just enjoying them as a snack -- different harvesting times might be appropriate. Keep in mind that a color change isn't a reliable indicator of ripeness -- grapes will still become sweeter after the color has changed.
Many factors affect the speed at which your grapes ripen. The weather, for example, plays a role in how grapes mature. If it has been especially cloudy, or if the grapes are shaded, they will ripen more slowly than those in direct, consistent sun. The grapevine's leaves use sunlight to produce sugars, so the sun must reach the leaves -- the sun that reaches the fruit itself is less important. Pests, like grape phylloxera, can also slow ripening by removing nutrients from the roots and damaging them. Protect your grapevines from heavy rain, as well. Grapes skins can split if the plant takes up too much water.
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