Source of Ochratoxin A in Coffee

Starting in the 1970s various studies have shown that contamination of coffee with OTA is predominantly caused by Aspergillus ochraceus [321-323]. However, further studies have concluded that other Aspergilli species may also be responsible for the contamination of coffee beans with OTA (Table 2). At the present time it is not clear, which species are responsible for the OTA contamination of coffee. Levels of contamination as well as the species that produces OTA vary from region to region. Most studies have focused on the yellow spored A. ochraceus, since this species has frequently been proposed to be the major source of OTA in green coffee. More recently, the black spored Aspergilli A. niger and A.

carbonarius have been isolated from coffee by several groups, which indicates that these two species might also be potential sources of OTA in coffee [323-326].

Table 2. Examples of fungal species found in coffee samples.

Fungal species Maturation of coffee Origin References A. ochraceus

A. westerdijkiae green coffee beans India [332]

A. lacticoffeatus green Robusta beans Venezuela [55]

A. sclerotioniger green Arabica beans India [55]

A. carbonarius green coffee beans green Robusta beans

Several national and international surveys on the presence of OTA in green and roasted coffee as well as in coffee-containing products have been conducted. Table 3 gives an overview of the published studies. Usually, neither the origin of the beans, the Coffea species (Coffea arabica vs. Coffea robusta) nor the way green coffee was processed (dry vs. wet processing) were taken into account. Romani et al. could show that coffee beans from Africa are generally contaminated with higher levels of OTA than coffee beans from Central or South America [324]. Moreover, a survey by the UK Ministry of Agriculture, Fisheries and Food (MAFF) indicated that Robusta green coffee was much more likely to contain OTA than

Arabica coffee [334]. Bucheli et al. [337] further stated that strip-picking of coffee cherries, which tends to produce more defected beans, together with the use of cherry sun-drying (‘natural processing’) might explain the higher levels of OTA in green Robusta coffee.

Recently, Leong et al. [338] reported that in the case of Vietnamese green coffee beans significantly more Robusta than Arabica beans were infected with fungi. Interestingly, A.

niger was found to be the dominant species infecting Vietnamese coffee beans, but A.

carbonarius most probably was the source of OTA contamination [335].

Table 3. Systematic investigations on the occurrence of OTA in coffee.

Year of

Due to the legislative pressure of industrialised nations, several surveys not only for the presence of OTA in green and roasted coffee as well as in final coffee products have been undertaken by the importers and coffee-producing countries. The fate of OTA during the processing of green coffee beans and during coffee manufacturing is also being investigated

with increasing interest, since the identification of the stage and conditions under which OTA is produced during green coffee production and handling might give indications about the precautions that have to be taken to reduce the incidence of OTA contamination [348].

Infection with ochratoxin A-producing moulds and subsequent formation of OTA could occur on the coffee tree, during the ripening process of the cherries or may also take place within the developing coffee bean during cherry maturation [348]. In addition, it has also been suggested that a direct contamination of coffee beans may occur by OTA generated by saprophytic moulds in the soil and translocated directly via the plant into the coffee beans [349, 350].

Several authors concluded that OTA formation takes place during harvest and post-harvest processing of coffee beans. Moisture contents vary from 50–70 % in ripe cherries, 35–50 % in coffee raisins to 16–30 % in cherries that are dried on the plant [351]. In order to prevent mould contamination and fermentation the moisture content must be less than 12 % (water activity aw of 0.65–0.7) at the end of the drying step. Since coffee loses flavour at less than 9 % moisture content, while at more than 13 % there is an increasing risk of OTA contamination, drying of coffee beans, subsequent storage as well as transport and environmental conditions are critical to coffee quality and potential OTA contamination [352]. It has been shown in several studies that the presence of OTA in coffee is normally a result of insufficiently controlled harvesting procedures, precarious drying and inadequate storage conditions, which allows the growth of toxigenic fungi [323, 328, 353]. Coffee is a very hygroscopic material that is able to reabsorb moisture from the environment during storage and transport. In this context Palacios-Cabrera et al. [357] showed that cycling environmental conditions may indirectly favour OTA production. Since in a real coffee transport situation there is always a risk of increasing moisture content as a consequence of condensation, it became evident in recent studies that the presence of OTA in coffee is always an indication of serious failures in the practices of harvesting and storage. Moreover, recent studies investigated the growing requirements and potential OTA production of the three most important Aspergilli species on coffee raw material [326, 354, 355]. It has also been shown that OTA contamination could derive not only from inappropriate storage and transport of coffee, but also from coffee cherry drying as demonstrated by Bucheli et al. and Joosten et al.

[326, 356]. Their findings suggested that drying cherries under humid conditions is a rather

risky route for OTA contamination, since cherries always contain plenty of water in the initial stage of drying, which in turn can support growth of A. carbonarius.

The contamination by OTA is reduced during the ongoing processing of green beans and the manufacturing of coffee. Although a small proportion of OTA is eliminated during the cleaning of green coffee, the most significant reduction is achieved during roasting, when OTA is either eliminated with chaff or destroyed, and by subsequent brewing processes during the production of soluble coffee [357, 358].