Why does it become sweet when potatoes freeze?

There are vacuoles in potato cells. They are filled with cell sap, which contains water and various substances dissolved in it, including sugar and starch. When freezing, the cell sap turns into ice and destroys the vacuole. In addition, potatoes often freeze when stored in a cold place for a long time. Over time, the starch turns into sugar, which also explains the sweet taste. The reverse conversion of sugar into starch does not occur.