Sugar is a moisture-sensitive cargo. Sugar becomes like treacle when wet and corrodes the steelworks, therefore cargo holds must be scrupulously clean and dry.
Once loaded, the mass is immobile but its volume may go down by 5%, because of the vibrations of the ship. Unrefined sugar will rarely harden.
Raw sugar absorbs moisture from the outside air and experience has shown that sugar should not be ventilated during transit. A drawback is that alcohol vapors produced will not be carried off. Care should be taken with hold-lighting and open fire in connection with the alcoholic vapors. During stuffing the risk of explosion is eminently present.
Raw sugar is particularly hygroscopic due to its high ash content and readily releases water vapor and must thus be protected from all forms of moisture (seawater, rain, condensation water). At relative humidity below the flow moisture point, raw sugar responds little to the water vapor content of the ambient air and the water content remains close to 0% (0.25 – 1.1%). Once the flow moisture point is reached at a relative humidity of 80%, the raw sugar readily absorbs water vapor, resulting in an abrupt rise in the sorption isotherm, and the sugar crystals deliquesce.
This marked hygroscopicity of raw sugar is attributable to the film of (strongly hygroscopic) molasses remaining on the sugar crystals. Moisture reduces the concentration of the sugar. The change in sugar concentration is determined by polarization measurement (optical measurement method).
Relative humidity > 70% result in agglomeration (sticking together), syrup formation, tackiness, loss of flowability and mold and yeast growth, which cause fermentation. At below 50%, the raw sugar may harden, cake and lose its flowability. Monolithic layers may arise which cannot be unloaded without prior loosening. Moisture and agglomeration damage may be avoided by incorporating additives. Subject to compliance with the appropriate temperature and moisture/humidity conditions, the maximum duration of storage is not a limiting factor as regards transport.
High relative humidities or fog have no negative impact upon unloading operations because the raw sugar is further processed into white sugar. On the other hand, losses do arise due to caking if the relative humidity drops. Hardening has, for example, been observed to occur even during unloading at low temperatures and low humidities. In dry weather at a low relative humidity, raw sugar has been found to be so hard that problems arose during unloading, while no such problems arose in humid, hot weather. Raw sugar has also been found to be unusually hard due to low humidity in frosty conditions.
Favorable travel temperature range: no lower limit – 25°C. Agglomeration (sticking together) is promoted by relatively high temperatures (> 25°C) due to release of water vapor. Sugar should not be stowed near heat sources. Temperature variations should, as far as possible, be avoided as the resultant release of water vapor and re-crystallization may result in caking. Raw sugar with a loading temperature of above 48°C should not be accepted as it may cake on cooling.
Raw sugar requires particular humidity/moisture and possibly ventilation conditions. As bagged raw sugar is protected from water vapor exchange by a plastic lining, it does not normally need to be ventilated. However, if it is transported as bulk cargo, note should be taken of the behavior of a cargo block in the event of temperature changes.:
- Transport from temperate latitudes to the tropics = travel from cold to hot:the cargo block is heated from the outside, resulting in water vapor transport from the outside to the cold core, which causes caking of the outer layers while the inner layers display wetting phenomena (syrup formation).
- Transport from the tropics to temperate latitudes = travel from hot to cold:if the cargo block cools down from the outside, resulting in water vapor transport from the warm core to the outside, wetness and mold may occur there while inside the release of water vapor by the warm core results in caking phenomena (loss of flowability).
- On short voyages, ventilation need not be provided or, if provided, must not be too intense, so that the initial relative humidity and temperature conditions are maintained for as long as possible. On longer voyages, return air ventilation may be provided to prevent mold growth, but the removal of moisture may result in caking.