Field heat is the enemy
A crate of strawberries picked at midday in Egypt can be sitting at 28 °C. Every hour it stays there, it spends respiratory reserves it will need later — sugars burn off, texture softens, and the clock on its shelf life runs at several times normal speed. That loss is permanent. No amount of refrigeration downstream gives it back.
This is why serious growers pick in the cool of the early morning and why the packhouse sits close to the field rather than in an industrial zone an hour away. The goal is to get the product's core temperature down before it has time to consume itself. In the trade this is called removing field heat, and it is the step that separates fruit arriving in good condition from fruit arriving merely alive.
Pre-cooling: three methods, different jobs
Cooling a pallet in a cold store is slow, because still air is a poor conductor and the pallet's centre is insulated by everything around it. Pre-cooling forces the pace:
- Forced-air cooling. Air is pulled through vented cartons rather than around them, cutting cooling time from days to hours. The standard method for berries, grapes and stone fruit.
- Hydro-cooling. Product is showered or immersed in chilled water. Fast and gentle, suited to citrus and root vegetables; unsuitable for anything that absorbs water or moulds easily.
- Vacuum cooling. Pressure is dropped so surface moisture evaporates and takes heat with it. Very fast for leafy produce with a high surface area; irrelevant for dense fruit.
The method matters less than the discipline. What you should ask a supplier is not “do you pre-cool” but “how long between picking and reaching target core temperature, and how do you record it”.
Correct temperatures are product-specific
The most expensive mistake in reefer shipping is treating “cold” as one setting. Several important crops are damaged by temperatures that would be perfectly safe for others.
| Product | Typical shipping temperature |
|---|---|
| Oranges, mandarins | 4–8 °C |
| Lemons | 10–13 °C — chill-injures below about 10 °C |
| Mango | 10–12 °C — chill-injures below about 10 °C |
| Strawberries | 0–2 °C, pre-cooled before loading |
| Table grapes | 0–1 °C, usually with SO₂ pads |
| Pomegranate | 5–7 °C |
| Onions | 0–2 °C, low humidity, well ventilated |
Chilling injury is worth understanding because it is invisible on loading day. A lemon shipped at 4 °C looks fine leaving Alexandria and arrives with pitted skin and stained membranes two weeks later. The damage was done early; it simply took time to show. The same applies to mango, which goes from firm to grey-fleshed without ever looking obviously wrong in between.
The practical consequence: mixed citrus loads have to be planned around the coldest-tolerant item in them. Oranges and lemons do not want the same reefer setting, and splitting the difference damages both.
Ethylene: who produces it, who suffers from it
Ethylene is a ripening hormone that some fruit emits and others react to badly. Mango, melon and stone fruit are strong producers. Leafy vegetables, cucumbers and unripe fruit are sensitive to it — exposure brings yellowing, softening and premature ripening.
In a sealed reefer, a producer and a sensitive product share the same air for the whole voyage. Keep them apart, or accept that one of them will arrive early-aged. Where mixing is unavoidable, ethylene absorbers and a higher fresh-air exchange rate reduce but do not eliminate the effect.
Loading matters as much as the setting
A reefer does not cool the cargo. It cools air and moves it, and the cargo is cooled only where that air can reach. Two rules follow:
- Do not block the floor channels. Air is delivered under the load and must return over the top. Pallets sitting flat on the T-bar floor without clearance create a dead zone that never cools.
- Do not load above the red line. The return air path across the top of the load is not spare space; it is part of the circuit.
Vented cartons should have their vents aligned pallet to pallet, and shrink wrap should never cover them — wrapping a pallet tightly for stability while sealing every vent is a common and self-defeating habit.
What to check when the doors open
- Pulp temperature, immediately. Probe fruit from several positions in the load, including the door end and the top rear, which are the two worst spots. A single reading from the front pallet proves nothing.
- The data logger. Download it before unloading. It shows set point versus actual and every excursion during the voyage — the difference between a supplier problem and a carrier problem.
- Packaging condition. Collapsed or damp cartons mean condensation, which means a temperature swing.
- Uniformity. Open cartons from different pallets. Variation across the load points at loading or airflow; consistent condition throughout points at a single upstream cause.
When frozen is the better answer
Fresh is right when you are supplying a retail shelf in season and can move volume quickly. But fresh carries a shelf-life clock that starts at harvest and cannot be paused, and prices move with the season.
For a processing line, a year-round programme or a buyer who needs a fixed specification every month, IQF frozen removes the clock entirely. Many of our buyers run both: fresh through the season, frozen to fill the rest of the year. You can compare the ranges on our fresh produce and frozen (IQF) pages.