Grape Juice Composition

The relative proportions of the compounds that make up grape juice are constantly changing as the berries ripen, so the time of harvest greatly affects composition. Most of the sugary solution that results when grapes are squeezed or crushed derives from the contents of the vacuoles of the cells of the pulp or flesh, although heavy crushing and pressing adds further solution from vacuoles of the skin and vascular strands, thereby mixing many different compounds into the must. Thus the composition of the juice that issues when berries are crushed changes with the pressure and time of crushing; the first, free-run juice has fewest suspended solids and skin extracts; further pressing yields juice with more phenolics and potassium salts, and hence lower total acidity.