Reason Why You Plunge Harvested Fruit into Cold Water

Q. I wrote to you about ripening my plums after I picked them from the tree. I did what you told me to do. Why did you tell me to plunge the fruit into cold water after harvesting?

A. Plums will finish ripening very nicely at room temperature after they are picked. We pick them early, when still hard, and do not let them ripen fully on the tree to avoid most of the damage caused by birds. 
Stark Saturn peach with bird damage

Birds will usually begin “tasting” fruit when they are close to harvest. When we start to see bird damage and the fruit is close to harvest, it is a good indicator that the fruit could be harvested and they will continue to “ripen” off of the tree.

These types of fruit are called “climacteric” since they continue to ripen after harvest. Examples of climacteric fruit are most of the stone fruits like peach, nectarine, plum and plum relatives like pluots. Cherries, even though they are stone fruit, will not continue to ripen after harvest.

The speed at which these fruits ripen depends on the fruit temperature; warmer temperatures cause faster ripening. When you harvest fruit in the heat of the summer, the fruit will have a lot of excessive heat. 

This extra heat is from the environment as well as the fruit's respiration or burning of sugars. We call this heat, “field heat”. It can be very destructive to fruit harvested in the field if it is not removed very soon after harvest or prevented. 

For this reason we try to get the temperature of the fruit lowered as quickly as possible soon after harvesting. You do not want to just put it in the fridge or it will stop ripening.

            One way to do this is just plunge the fruit into icy water to remove the field heat and get it closer to room temperature. After it cools to room temperature, you can let the fruit continue to ripen.