Step 1

Place the berries in an 8x4-inch loaf pan. Distribute them around evenly.

Step 2

Pour ½ cup of the grape juice in a large bowl. Sprinkle the gelatin in a thin, even layer over top (no need to stir) and let sit for 5 minutes. The powder will absorb into the juice and become very thick.

Step 3

Meanwhile, bring the remaining 1½ cups juice and the sugar to a boil in a small saucepan; whisk until the sugar is dissolved. Pour the mixture over the thickened gelatin and whisk to combine.

Step 4

Carefully pour the warm gelatin mixture over the berries. Chill in the refrigerator for 4 to 5 hours, or until very firm.

Step 5

To unmold, quickly run hot water over the sides of the pan (tilting it so that the inside of the pan doesn't get wet). Invert an oblong platter over the pan and flip over. The terrine should slide right out; if it doesn’t, try running a blunt knife along the edges of the pan to loosen, or run the pan under hot water briefly again. Blot any excess liquid on the platter with a paper towel. Refrigerate the terrine until ready to serve. Cut into 1-inch slices with a sharp knife.