Rinse your grapes thoroughly under cool running water and pat them completely dry with paper towels. Dry grapes are crucial because any moisture will prevent the sugar coating from sticking properly.

Pour the granulated sugar, sour sugar, tartaric acid (if using), and salt into a small bowl. Whisk these together for about 30 seconds to break up any clumps and distribute the sour elements evenly.

Dip a single clean fingertip into the coating mixture and taste it. This is your chance to adjust the tartness or sweetness before committing to the full batch.

Place your dried grapes into a medium-sized bowl that gives you plenty of room to toss them around.

Sprinkle about one-third of your coating mixture over the grapes.

Using a fork or small whisk, gently toss the grapes for about one minute, making sure each grape makes contact with the coating mixture. Work slowly and deliberately so you don't break the grapes open.

Sprinkle the remaining coating over the grapes while continuing to toss. You want a visible layer of sour sugar on each grape without so much coating that it clumps together.

Give the grapes one final gentle toss and let them sit in the bowl for two minutes. This resting period allows the moisture from the grapes to slightly dissolve the outer sugar layer, creating that signature Sour Patch texture where sour comes first, then sweet.