Because the temperature is low in winter, or the temperature drops after frost, vegetables must find ways to protect themselves if they want to prevent their own cells from being frozen.

Green vegetables use the low freezing point of sugar water to protect themselves, because vegetables contain starch (almost all green plants contain starch). Starch is not only not sweet, but also not easily soluble in water. But when the temperature is low, the starch in the vegetables is hydrolyzed into maltase under the action of amylase in the body, and then turned into glucose through the action of maltose. Glucose is easily dissolved in the cell fluid water, which lowers the freezing point of the cell fluid, making the cells less likely to freeze and protecting the vegetables. Because glucose is sweet, green vegetables also have a sweet taste.