11.6.2 Relative humidity

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Humidity levels are often combined with temperature manage-ment. Lowering temperature in coolstores generally lowers relative humidity because water condenses on heat exchangers during cooling. Storage problems may arise due to water loss rather than fruit temperature alone, and so it is generally desirable to maintain a high relative humidity (>95%). With kiwifruit, there is generally only a 1–2% water loss during six months’ storage if high-humidity conditions are maintained, compared to 4% or more when there is no effort to raise coolstore humidity.

Some fruits, flowers and vegetables are extremely susceptible to water loss (e.g. wilting of lettuce and deterioration of lychee), and in such cases the relative humidity needs to be held close to 100%. With valuable products like Cymbidium orchids, each flower stem may be fitted with a vial holding water to supply it throughout the storage period. The problem is compounded because flowers and leaves have a much greater surface area with many stomata whereas bulky fruit generally have a waxy coat and lack stomata. Their moisture loss occurs via calyces and fruit pedicels.

 

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