Moisture, especially as a result of excessive water content, excessive relative humidity, or seawater, rain or condensation, together with heat and lipases (fat-cleaving enzymes) result in the fat in the solid in question being cleaved into glycerol and fatty acids; the fatty acids break down further into carbon dioxide and water, resulting in rancidity and self-heating which, in extreme cases, may give rise to spontaneous combustion. Thermophilic (heat-loving) microorganisms also participate in this process. Self-heating on exposure to moisture proceeds in several stages:
- The general biological phase:
Freshly harvested goods of vegetable origin in particular often have a water content which is higher than container dry. This water content is then at equilibrium with a rel. humidity of > 75%, i.e. above the mold growth threshold. This moisture primarily activates mesophilic molds and bacteria, i.e. those which thrive in moderate temperatures, together with those parts of the plants which are still physiologically active, resulting in the release of heat and moisture. Mesophiles start to develop at 10 - 30°C, the optimum being
20 - 37°C and the upper limit 35 - 50°C.
- The microbiologically particularly active phase:
At temperatures of between 40 and 75°C, the physiologically tougher thermotolerant and thermophilic (heat-loving) molds and bacteria participate in the further evolution of heat. Growth of these organisms begins at 25 - 50°C, the optimum being 50 - 65°C and the upper limit 75°C (thermotolerant microorganisms) and even 80 - 95°C (thermophilic microorganisms).
In addition to microbial biotic activity, the heat released in conjunction with microbial activity promotes oxidation of the unsaturated fatty acids, which is in turn associated with evolution of heat. As a result, the nutritional value of the proteins is first of all impaired, which is recognizable from the incipient brownish discoloration of the goods.
- The thermophilic decomposition phase:
55°C is the critical temperature from which continuous temperature monitoring is required. If the temperature remains constant, there is no danger of further spoilage due to heating. The thermophilic decomposition phase proceeds at temperatures of 65 - 83°C, the vegetative cells decomposing, i.e. after the preceding overlap of biological and chemical evolution of heat, the microorganisms die, microbial spoilage of the goods comes to an end and no further temperature increases occur above 90°C (longish temporary plateau). All vegetable and animal products containing (residual) oil are poor heat conductors with poor upward heat dissipation. Any heating of an organic product is associated with the release of water vapor and a reduction in the moisture content of the goods. The released moisture rises through the cargo as white steam and condenses on the surface of the cargo, forming a fire-extinguishing buffer on top of the cargo. Chemical degradation reactions increase in rate from 85 - 115°C.
- The pyrophoric gas phase:
After the temporary plateau, the chemical reactions in the cargo are resumed with renewed vigor, resulting in further heating. The cargo assumes a dark brown color. An unpleasant pungent smell penetrates to the outside as a result of protein decomposition.
From approx. 80°C, volatile gases are released at the surface. These gases, which have a very low autoignition temperature (60°C), ignite when present in sufficient concentration and burn on the surface of the cargo with small bluish flames. However, they lack the energy to ignite the cargo. In this phase, continuing dry distillation from temperatures of 150 to approx. 200°C gives rise to gases such as phosphine, carbon monoxide (CO) and pyrophoric carbon and results in considerable losses in nutritional value, while the subsequent abrupt further increase in temperature to approx. 260°C and above is associated with a smell of charring. The presence of CO gas is considered the most reliable indication of a fire. Levels of 0.002 - 0.005 vol.% of CO in the air are deemed normal, with values rising to above 1 vol.% in a cargo fire. The lethal (fatal) dose is approx. 0.1 vol.%. Once temperatures exceed 250°C and especially 280°C, it must be anticipated that the autoignition temperature of the cargo dust will be reached. The autoignition temperature of most organic cargoes is 300 - 500°C. Usually, however, the plateau in the decomposition processes at approx. 90°C means that spontaneous combustion of the goods does not occur.