Forensics

Investigation on Self-Heating of Farm Produce

Measuring temperature

It is possible to make a quantitative assessment of self heating by a series of temperature measurements.  This not only helps to prove if self-heating has occurs, but also reveals the quantity affected and the severity of the damage.

The Phenomenon

Self-heating is a process by which heat is generated within a body of material, by some internal process, resulting in a rise in temperature. There are a number of ways in which this can occur, but this pamphlet is confined to self-heating in natural produce, such as soya beans, maize and palm kernel expeller.

In farm produce, self-heating begins with biological activity. Fungi and yeasts proliferate and their activity results in a rise in temperature to as high as 60°C. In most situations this is all that occurs, with the material degrading and the micro-organisms eventually dying off. However, if the produce is present in large quantity then chemical processes can take over, with the temperature rising further, perhaps until the material catches on fire. When this final stage is reached, spontaneous combustion can be said to have occured.

Conditions

Measuring temperature

A cross-section through a heap of soya bean material, exhibiting the blackening of a large mass over a long time. This appearance is characteristic of self-heating.

Given the processes described above, significant self-heating in produce is a result of the combination of the following.

High moisture :  Micro-organisms require moisture. This is one of the reasons why most produce standards stipulate a maximum moisture content.

Size :  The larger the quantity the less the heat loss and the greater the rise in temperature.  For example it is imposibble for an exposed bag of soya beans to self-heat, but pallet-loads of bags close together can catch on fire.

Time :  Self-heating usually take weeks to months for significant temperature rise.

Damage

beans

Soya beans after laboratory heating at 200°C for one hour.

As the temperature of farm produce rises the quality decreases. There are a number of ways in which this might be judged. The material becomes discoloured and darkens and does not smell as fresh. When it blackens there is an accompanying caramel odour and this can become so strong that animals will not eat the material. Chemical tests show a decrease in protein.

Described economically, with increasing temperature and time at temperature, produce decreases in value until eventually it has no value and it costs money to have it taken away.

palmkennel heatedsoya silo
A section through a heap of palm kernel expeller that had been in a fire. The thin band of burnt material on top of the heap is characteristic of external fire damage. Soya beans heated to the temperatures shown for one hour. Longer time at elevated temperature will also resulting in darkening. A silo in which the soya beans were so affected by self-heating that they would not flow by gravity from the bottom. The roof of the silo had to be cut away and the blackned beans removed by grab and later by excavator lowered inside.

Our Services

AThere is a variety of ways in which insurance policies may be worded on the subject of self-heating and spontaneous combustion and we do not propose to enter into this area. Suffice to say that we can do the following :

  • Identify when self-heating has occurred.
  • Distinguish self-heating and spontaneous combustion from fire from an external source.
  • Carry out surveys to determine the extent and magnitude of self-heating.
  • Determine if the conditions of 'fire' or spontaneous combustion have occurred.

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