Almost every reefer booking specifies a temperature. Far fewer specify the two settings that decide whether the fruit arrives dry, wet or shrivelled: relative humidity and fresh air exchange. Carriers apply defaults when nothing is stated, and the default is rarely right for the crop in the box. This is how we set both on Egyptian produce shipments.

What the humidity control on a reefer actually does

A modern reefer unit does not humidify. It dehumidifies. When you set a relative humidity target, the machine is permitted to run the compressor beyond what the temperature alone requires in order to condense moisture out of the return air. If you set a humidity target lower than the cargo naturally produces, you get extra drying and extra weight loss. If you leave humidity control switched off entirely, the box settles wherever the cargo and the set point put it, which for most fresh produce is high, around 90 to 95 percent.

For the great majority of Egyptian fruit and vegetable shipments, high is what you want. The moisture the fruit loses is the moisture you are paid for, a point covered in detail in our reference on transit weight loss by crop.

Settings by crop group

• Citrus, oranges and mandarins: humidity control off or set at 90 to 95 percent. Dry air accelerates rind shrivel and creasing on thin-skinned varieties.
• Pomegranate: 90 to 95 percent. Pomegranate husk scald is driven partly by dehydration, and drying the box makes it worse rather than better.
• Mango: 85 to 90 percent. High enough to protect the fruit, low enough to reduce free condensation on carton tops during pull-down.
• Grapes: 90 to 95 percent, with sulphur pads. Never combine active dehumidification with SO2 pads.
• Onions and garlic: 65 to 75 percent, dehumidification enabled. These are the exception. Wet air causes root regrowth, neck rot and mould on cured bulbs.
• Potatoes and sweet potatoes: 90 to 95 percent, no dehumidification.
• Frozen and IQF product: humidity control is irrelevant and fresh air must be fully closed.

Fresh air exchange, the setting that gets ignored

Fresh air exchange, sometimes called the vent setting, replaces air inside the container at a stated rate in cubic metres per hour. Respiring produce generates carbon dioxide and ethylene, and both accumulate in a sealed box. Opening the vent flushes them out. It also lets warm, humid outside air in, which the unit must then cool, so the setting is a genuine trade-off rather than a free safety margin.

As a working guide, citrus and pomegranate run at 15 to 25 cubic metres per hour, mango at 20 to 35, onions and garlic wide open at 40 to 75 or on ventilated dry containers with fixed vents, and frozen cargo fully closed at zero. Ethylene-producing and ethylene-sensitive crops loaded together need the higher end of the range, and in most cases should not be loaded together at all. Our ethylene sensitivity reference sets out which combinations are workable.

Getting it onto the booking

Set point, humidity target and fresh air exchange rate should all appear on the shipping instruction and on the loading record, not just in an email. If the container arrives with the wrong vent position and the trace later shows elevated carbon dioxide, a documented instruction is what separates a supportable claim from an argument. Where the choice of box itself is open, our guide to reefer or ventilated dry container covers the first decision.

If you would like these three settings written into your next Egyptian shipment against your own arrival specification, send us the crop, the destination port and the transit time. Message the export desk on WhatsApp at +20 10 9911 1918.