Relative humidity and condensation
Humidity refers to the amount of water vapour that is held in the air. This varies according to temperature, with warm air able to hold far more water vapour than cold air. So for example, at 30°C a cubic metre of air can hold a maximum of 31g of water as vapour. At 0°C that same cubic metre will hold less than 5g of water as vapour.
Relative humidity refers to the amount of water vapour in the air compared to the maximum it can hold at that temperature. So a cubic metre of air at 30°C, which contains 20g of water as vapour is approximately 64% of the maximum amount, ie 64% RH.
As temperature drops, air is less able to hold water vapour. The vapour will therefore condense out as liquid water. This has implications for packaging and transport.
For example, if sealed cartons of warm vegetables are placed in a cool room, the air inside the cartons will lose moisture as it cools, resulting in condensation on the product and the inside of the carton.
Conversely, when cold cartons are moved into ambient conditions they cool the surrounding air. Depending on the RH, water vapour from the cooled air will then condense on the outside of the cartons.
Temperature fluctuations therefore almost inevitably result in condensation. This can weaken packaging materials and increase water loss from packed product. Condensation can encourage development of rots and increase the chance of products splitting.
Relative humidity is particularly implicated in fungal infection and growth. Dry conditions prevent spores from germinating, and even if germination is successful the exposed tissue may be too dry to permit infection. Most fungi cannot grow if RH is below 85-90%. However, humidity this low is not suitable for products susceptible to moisture loss, such as leafy vegetables and carrots, both of which should ideally be held at >95% RH.