All plants and the plant products like vegetables and fruits are made up of turgid living cells. Turgor is the pressure buildup in the plant cell by the fluid, which causes the cell membrane and the cell contents exert an outward pressure against the cell wall. When this structure is destroyed, the cells break and produce a noise due to the sudden release of turgor pressure. That's why a fresh carrot or celery will produce a "snap" sound when we break it. A vegetable that is not fresh lose moisture and their cell's turgidity slowly. Upon losing the turgor they will become limp and wilts.
Explanation:
A plant cell has a rigid outer cell wall made up of cellulose, hemicellulose,  pectin and lignin. The cell membrane inside the cell wall is semi permeable which allows only certain components to move freely through this membrane. Water can pass in or out of a cell through the cell wall and the cell membrane by osmosis and transpiration.  Another structure inside the plant cell called vacuole acts as a  bag of water and creates turgor in the cell.
Turgor pressure or turgor is the pressure buildup in the plant cell by the fluid, which causes the cell membrane and cell contents to exert an outward pressure against the cell wall. The cell wall is made to withstand this pressure. When the vacuole filled with water presses against the cell wall, it makes the cell wall nice and firm. Thus the turgor makes the vegetables like carrot and celery firm. So a fresh carrot or celery will produce a snap sound when we break it because their cells break and produce a noise due to the sudden release of turgor pressure inside the cell.
Once the vegetables are harvested or damaged, they transpire and lose moisture slowly. As a result, the vacuole which is not full of water anymore does not press against the cell wall and the cell loses its turgidity. Upon losing the turgor the plant will become limp and wilts. That's why a vegetable that is not fresh will not make the snap sound when we break it and may bend.
In order for a fruit or vegetable to remain crunchy and crispy, it is important to maintain the turgor inside their cell. To maintain the turgor, the vegetables should not be damaged so that the cell wall does not break and should stay in place. Also there should be enough water inside the cell, so avoid placing them in direct sunlight.
