Besides being sour, underripe blueberries are typically green or purple and can feel harder to squish, whereas ripe ones are sweet and soft, usually boasting a white-gray sheen.

However, given the unique nature of each blueberry, identifying the ripest ones can be challenging. Fortunately, immersing them in a bowl of water can provide assistance.

Once dropped into a bowl of water, ripe and sweet berries will sink to the bottom, while any underripe, sour berries will float. The science behind this phenomenon is quite simple.

Per Allrecipes, ripe blueberries sink because their higher sugar content causes them to have a greater specific gravity than water. For the opposite reason, underripe ones float.

Since the 1980s, hydrodynamic blueberry sorting machines have tested the ripeness of harvested berries based on this phenomenon. At the time, it was considered common practice.

Today, most blueberries go through quality checks on a series of conveyor belts. This water-free sorting method helps limit losses and can help keep blueberries fresh for longer.