Sour, unripe grapes can spoil a dish and you won't want to eat them on their own. Because grapes are used in so many things, from salads and desserts to wines, and juices, it's important to choose quality grapes that will make the item the best it can be. Checking grapes carefully when you buy or harvest them from your backyard grapevines will help you pick the highest-quality fruits. Grapes do not continue to ripen once they're cut from the vine, so pick or buy them at their peak.
When grapes are ripe and of good quality, they should be either blue, red or light green. If the grapes are still dark green, they are not yet ripe. The variety you recognize as "green" grapes are categorized as white grapes. These grapes are ripe when they turn a light chartreuse, a subtle yellow-green. If a grape is brown or turning brown, it is either going bad or did not get the required nutrients while it was growing. Look for a uniform color across the grapes on the bunch or the vine. If colors vary, the grapes are getting an uneven amount of nutrients, which will lead to lower quality grapes.
There is no standard grape size. Size depends on grape variety, including whether it is a seedless type. But size can be a good indicator of grape quality if you know what size is best for the cultivar. Look for a standard size across all the grapes on the vine. If the grapes on the vine are vastly different sizes, the grapes are not getting an even amount of nutrients and will be a lower quality grape.
When grapes are growing, they are firm. Look for grapes that are firm but give and spring back when lightly pressed. Check their quality by giving several grapes a gentle squeeze. If the grapes are soft or have soft spots, they are overripe and going bad. If grapes are still on the vine and getting soft, then they were not picked soon enough or they did not receive the proper amount of nutrients.
To some extent, the color, taste and firmness can all indicate a grape is good. The only thing left to do is taste it. Even if grapes give all indications that they are good, the taste may prove otherwise. Sometimes, grapes grow properly but do not come become sweet. When this happens, try letting the grapes ripen longer, but if they don't get sweeter, they may not have gotten the right nutrients. When tasting, try a single grape from the bunch.