Because by the freezing action the starch of the potato is in part converted into sugar.
Why are apples, pears, grapes, etc., in their unripe state sour, and in their ripe condition sweet?
In the unripe fruits mentioned starch is present; in the ripe fruits it is absent; in the process of ripening the starch is converted into sugar, and the fruit becomes sweet.
What are acids?
Acids are substances which excite the taste of sourness when applied to the tongue; they change the blue juices of vegetables to red, and combine with alkalies to form neutral compounds.
What is an alkali?
An alkali is a body that possesses properties the converse of those of an acid. It has a highly bitter, acrid taste, changes the blue juices of vegetables to green, or the juices of vegetables which have been changed red by an acid, back again to blue. Potash and soda are the representatives of the alkalies.
When sulphur is burned in the air what is the product formed?
Sulphurous acid.