A refrigerated container does not cool cargo down — it only holds temperature. Most reefer claims we investigate trace back to cargo stuffed warm, the wrong set point, or the fresh-air vents closed when the cargo needed to breathe. This guide covers how a reefer works, the correct temperature and ventilation settings for the most common temperature-sensitive cargoes, and the cold-chain discipline that keeps your consignment saleable on arrival.
How a reefer container really works
A reefer maintains, not creates, cold. Cold air is blown through the T-floor at the bottom, rises through the cargo and returns to the unit at the top. If cargo is stuffed above its carrying temperature, the machine may take days to pull the core temperature down — time your produce does not have. That is why pre-cooling the product before stuffing is the single most important step in reefer logistics. The container itself, however, should usually NOT be pre-cooled with its doors open in humid ambient air: the resulting condensation wets cartons and labels. Pre-cool the cargo in a cold store, then stuff quickly.
Three settings you must get right
- Temperature set point: the carrying temperature agreed with your commodity specialist — always state °C or °F explicitly on the booking and bill of lading.
- Ventilation (fresh air exchange): measured in cubic metres per hour (CBM/hr) from 0 to 240+. Respiring produce (fruits, vegetables) needs open vents to flush out ethylene, CO₂ and moisture; frozen and chilled non-respiring cargo ships with vents CLOSED.
- Relative humidity: most reefer units allow a humidity set point (typically 65–95%). High humidity suits leafy vegetables; lower humidity limits mould on onions, garlic and cocoa.
The most common reefer error: leaving fresh-air vents open on frozen cargo. Open vents let warm moist air in, the evaporator ices up, defrost cycles multiply, and the temperature drifts upward. Frozen and deep-frozen cargo always sails with vents closed.
Recommended settings by cargo
| Cargo | Temperature | Ventilation | Humidity / notes |
|---|---|---|---|
| Bananas (green) | +13.2 to +14.0 °C | 25–40 CBM/hr open | 85–95% RH; never below +13 °C — chill damage |
| Mangoes | +10 to +13 °C | 25–40 CBM/hr open | 85–90% RH; below +10 °C causes chilling injury |
| Grapes | -0.5 to 0 °C | 15–25 CBM/hr | 90–95% RH; often shipped with SO₂ pads |
| Onions & garlic | 0 to +2 °C (or ambient +4 °C) | Vents wide open (max) | 65–75% RH — low humidity is critical against mould |
| Potatoes | +4 to +7 °C | 20–40 CBM/hr open | 85–90% RH; colder converts starch to sugar |
| Apples & citrus | -1 to +4 °C (variety-dependent) | 15–30 CBM/hr open | 90–95% RH |
| Fresh meat (chilled) | -1 to 0 °C | Closed | 85–90% RH; strict cold chain, vacuum packed |
| Frozen meat & poultry | -18 °C or colder | Closed | No humidity control needed |
| Fish & seafood (frozen) | -18 to -20 °C | Closed | Tuna often -25 to -30 °C for quality |
| Ice cream | -25 °C or colder | Closed | Extremely temperature-sensitive; never above -20 °C |
| Dairy & butter | 0 to +4 °C (chilled) / -18 °C (frozen) | Closed | 80–85% RH for chilled |
| Chocolate & confectionery | +12 to +18 °C | Closed | 50–60% RH — moisture causes sugar bloom |
| Pharmaceuticals | +2 to +8 °C (validated lanes) | Closed | GDP compliance; data loggers and calibration certificates required |
| Flowers & plants | 0 to +2 °C | 15–30 CBM/hr | 90–95% RH; pre-cool immediately after harvest |
Stuffing rules that protect the cargo
- 1Pre-cool the cargo to its carrying temperature in a cold store before stuffing — never rely on the container to pull temperature down.
- 2Check the reefer's PTI (Pre-Trip Inspection) certificate before accepting the empty unit — the machine must be defect-free and calibrated.
- 3Verify the set point, vents and humidity on the controller against your booking instructions before the first carton goes in.
- 4Keep cartons inside the red load lines and below the load-height limit; air must flow over and through the stow.
- 5Leave 10–12 cm clearance along the T-floor channels and never block the return-air grille at the top of the unit.
- 6Use slip sheets or dunnage where floor channels are covered; block stow tightly to prevent movement but never tight against the unit bulkhead.
- 7Minimise door-open time during stuffing, especially in hot humid yards — every open minute adds heat and condensation load.
- 8Plug the reefer in immediately after stuffing and record the running set point and supply-air temperature on your checklist.
Monitoring the cold chain in transit
Every modern reefer records supply and return air temperatures, defrost cycles and door openings on its internal controller, downloadable after the voyage. For pharmaceuticals and high-value perishables we recommend placing independent calibrated data loggers inside the stow, specifying the carrier's remote reefer-monitoring service where offered, and agreeing in writing on who inspects the unit at transhipment ports. Any temperature deviation claim is won or lost on these records — preserve them the day the container is delivered.
Frequently asked questions
What temperature should a reefer container be set to?
It depends entirely on the commodity: frozen cargo typically sails at -18 °C or colder, chilled meat at -1 to 0 °C, dairy at 0 to +4 °C, pharmaceuticals at +2 to +8 °C, bananas at +13.5 °C, and chocolate at +12 to +18 °C. Always confirm the commodity-specific set point before booking.
When should reefer ventilation vents be open or closed?
Open the fresh-air vents for respiring cargo such as fruits and vegetables (typically 15–40 CBM/hr, maximum for onions and garlic) to flush out ethylene, CO₂ and moisture. Keep vents fully closed for frozen cargo, chilled meat, dairy, chocolate and pharmaceuticals.
Should I pre-cool the reefer container before stuffing?
No — pre-cool the CARGO, not the container. The reefer is designed to maintain temperature, not to cool warm cargo. Stuffing product that is already at its carrying temperature is the foundation of reefer quality.
What is a PTI certificate?
A Pre-Trip Inspection certificate confirms the reefer unit passed a full functional test — compressor, fans, sensors, defrost and alarms — before release. Never accept a reefer container for a sensitive cargo without a recent PTI.
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