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Maintaining the quality of stored coffee and cocoa

Use of visibly diseased pods may result in off flavours and taints and mycotoxin contamination in cured beans further down the chain. (Image source: Terry Mabbett)

Drying and storage conditions applied to coffee and cocoa commodity ‘chains’ are essential in maintaining the long term integrity of these crops

Failure to achieve and maintain bean moisture contents below established levels may cause rapid loss of bean integrity, quality and commercial value. Damage and deterioration is often caused due to feeding by insect pests and the metabolic activity of microbes and especially a small, select but highly damaging group of fungal moulds which synthesise mycotoxins.

The physical integrity of freshly harvested coffee and cocoa is unavoidably compromised by on-farm processing. Processed beans will freely furnish the taste, aroma but their porous structure makes them prone to risks. What can easily exit the beans can just as easily enter and this means cured coffee and cocoa is at high risk of damage from biotic (biological) as well as abiotic (physical and chemical) agents. Being flavour dependent commodities, they are economically sensitive to any damage or deterioration. Mycotoxins are acutely toxic signature chemicals produced by specific fungal moulds.

Aggravating risks to coffee and cocoa in storage and transit is the rapidity and magnitude of any change in temperature and humidity. These physical factors set the scene for the conditions experienced during movement from place of production to point of use. Coffee and cocoa are largely grown in countries with equatorial tropical climates, but for historical and economic reasons are mostly manufactured and consumed in cool temperate countries. As processed commodities they have a much longer ‘shelf-life’ than, for instance, cereal grains and oilseeds and are stored for much longer periods. However, the longer the storage period, the greater the risk of damage and deterioration.

Interacting risk factors

Factors that pose a potential threat to the integrity and quality of stored coffee and cocoa beans are physical and chemical (abiotic) and pests and pathogens (biotic).  Primary physical factors are temperature and relative humidity which cause secondary problems such as condensation and surface moisture. Examples of chemical factors that can compromise the quality of stored coffee and cocoa are:

•             Chemical residues of pesticides applied in the field or in store to control insect pests

•             Contaminating oils in jute sacks used to pack the beans

•             Chemicals stored in the same area as cocoa and coffee, especially if volatile with intrinsically strong odours

Biological factors such as insects, pests and fungal moulds are invariably present but require the interactive intervention of physical factors before they can grow into economically- damaging problems.  Insect and pest infestations are triggered by high temperature and humidity.  

The different types of deteriorative organisms that affect coffee and cocoa in store require different levels of relative humidity to initiate and maintain normal growth and development. Bacteria, fungal moulds (including mycotoxin synthesisers) and mites require a relative humidity level in excess of 90 per cent, 70 per cent, and 60 per cent, respectively. Depending on class and species insect pests require humidity within a 30 to 50 per cent range.

By Terry Mabbett

To continue reading the rest of this article, please see the April/May 2013 issue of Far Eastern Agriculture