Condensation occurs because moisture is always present in the air and hygroscopic (water-attracting) materials such as coffee normally contain a certain amount of moisture as well. Coffee with a moisture content in excess of 12.5% (ISO 6673) should never be shipped, whether in containers or bagged, as beyond this point the risk of condensation and therefore fungi growth occurring becomes unacceptably high. The only exceptions could be specialty coffees that traditionally have a high moisture content, such as Indian monsooned coffees.
This is not to suggest that a moisture content of 12.5% is commercially acceptable for all coffee - for certain coffees, certain origins and certain buyers it is definitely not *. The figure of 12.5% simply represents a known technical point at which the risk of damage from condensation and growth of mould during storage and transport becomes unacceptably high. Shippers who normally ship their coffee at moisture percentages below 12.5% should definitely continue to do so.
An increasing number of buyers now include a maximum permissible arrival moisture content in purchasing contracts. Increasing preoccupations with food health and hygiene in consuming countries suggest strongly that exporters will be well advised therefore to acquaint themselves with their buyers' requirements in this regard.
Coffee is often loaded in tropical or otherwise warm areas for discharge at places where the temperatures are very much lower. Warm air holds more water vapour than cold air; when warm, moist air cools down to dew point, then condensation occurs. Dew point is the temperature at which a sample of saturated air will condense.Put differently: coffee travelling from producing countries during the Northern Hemisphere summer experiences much less temperature change than when travelling during the Northern Hemisphere winter. Vessels may then arrive when snow and ice conditions are prevalent, particularly in Northern Europe and Scandinavia.** Of course such conditions are entirely beyond anyone's control, including the shipping company. On other routes cargo may experience multiple climate zones during transit. For example from the Pacific Ocean ports of Guayaquil (Ecuador) and Buenaventura (on Colombia's West Coast) to the US East Coast (Atlantic Ocean). When passing Cape Hatteras in the State of North Carolina on the US East Coast vessels may in winter sometimes experience a drop in outside temperature of up to 20 degrees Celsius, in just four hours!
The only answer to all such weather-related events is to exercise the utmost care when lining and stuffing containers, and to ensure correct stowage on board ship.
For more on this and containerization generally go to www.tis-gdv.de = Transport Information Service of the German Insurance Association. Alternatively, contact your local shipping company representative for information on container stuffing and related issues.
However, to explain briefly...
During transit the temperature outside the container gradually cools down and the steel container allows the chill to conduct from the outside of the panels through to the inside. On arrival the container has cool roof and side panels, and moist warm air in the space above the cargo and within the stow. Most of the moisture will have been given up by the coffee beans themselves.
When the temperature of the panels falls below the dew point of the air inside the container, condensation starts and will continue until the dew point of the interior air falls to that of the air outside.
Apart from making sure that the coffee has an acceptable moisture content, condensation cannot really be avoided and all one can do is try to prevent the condensation falling onto the coffee as droplets. If temperature changes are gradual and enough time passes then the coffee beans will absorb the excess moisture from the air within the container and the container will again be 'dry'. But temperature differences of 8 to 10 or more degrees over short periods of time almost inevitably will result in condensation taking place. In severe cases water droplets, mostly consisting of dislocated moisture from the coffee itself, form on the interior roof and side panels, and then drip on to the cargo causing water damage and mould.
In summary, differences in temperature plus the time factor and the speed of events combine to release moisture from the coffee. Given enough time the coffee surface will reabsorb the moisture. If events unfold too fast or there is too much moisture, then the coffee cannot reabsorb what it gave up and condensation will continue as long as the temperature difference between the steel of the container and the air inside it is greater than 8 degrees. A simple demonstration: a glass of cold beer 'sweats' because its temperature is below the dew point of the surrounding air. The moisture on the outside of the glass comes from the surrounding air, not from the beer or the glass itself. When the glass warms up, its temperature eventually reaches that dew point, which causes the moisture on the outside to dry again: it evaporates back into the surrounding air.
In producing countries condensation occurs when containers are stuffed at high altitude locations with high temperatures during the day that fall rapidly at night, leading to the same scenario. The risk is increased if full containers are left outside in the radiant heat of the sun, so containers should not be stuffed too far ahead of the actual time of shipment.* In certain areas there are shippers who habitually ship at higher moisture contents but the Coffee Guide is not in a position to express an opinion on this.** Container vessel in Finland during winter, covered in snow... Photograph courtesy Maersk Line Cargo Care.