The Problem

OTA-producers in coffee

Controlling mould growth through moisture management in the marketing chain

If drying is complete, the situation is quite straightforward. At a moisture content (m.c.) corresponding to an A_w level below about 0.68, OTA-producing fungal growth is not possible. Prevention of re-humidification assures that any further fungal development cannot take place.

However, the manner in which moisture in commodities is measured, and the way some coffee trading chains operate, can make definite assessments difficult, given actual conditions in the field.

Moisture content, as measured by any standard method, yields the average for a population of beans. The individual beans of this population do not all have this moisture content - some will be dryer and some will be wetter. We know little about this distribution.

A moisture content value ostensibly supporting no growth may contain a few beans wet enough to support slow growth and if stored in the coffee chain for long enough could spoil. In addition, respiring organisms, including insects, produce water from carbohydrates that could slowly accelerate local deterioration. Lastly, high inoculum levels are known to overcome conditions normally stringent enough to prevent microbial growth, a poorly understood phenomenon sometimes referred to as the 'inoculum effect'.

The maximum moisture content for exportable coffee as defined by the International Coffee Organization (ICO) is 12.5% (ICC Resolution No. 420, May 2004).

If this level of moisture is maintained throughout the market chain, using proper sampling techniques and moisture measurement devices, there should not be a significant risk of OTA contamination in coffee.