Cucumbers sweat in plastic wrap due to condensation. The plastic wrap traps moisture released by the cucumber's respiration, leading to water droplets forming on the surface.
Cucumbers, like all fresh produce, continue to respire after being harvested. Respiration is a natural process where the cucumber takes in oxygen and releases carbon dioxide and water vapor. When a cucumber is wrapped tightly in plastic, the water vapor it releases becomes trapped. Because the plastic wrap prevents the moisture from escaping into the surrounding air, the humidity inside the wrapping increases. As the temperature fluctuates, especially in a refrigerator, the water vapor condenses on the surface of the cucumber and the inside of the plastic wrap, creating the appearance of 'sweating.' This condensation is simply the water vapor turning back into liquid water due to the cooler temperature. The plastic wrap acts as a barrier, preventing the moisture from dissipating and leading to the visible droplets.
To minimize sweating and extend the shelf life of cucumbers, consider using a breathable produce bag or wrapping them loosely in a paper towel inside a perforated plastic bag. This allows some moisture to escape while still protecting the cucumber from drying out too quickly.