Wheat Production in Europe 2 - Post-harvest
6. Wheat production in Europe 1 - post-harvest
Introduction
In this article we look at the post-harvest situation (i.e. storage and transportation), consider some aspects of primary processing (normally milling), and discuss the implications of both pre- and post-harvest to the development of HACCP plans.
Grain at harvest and entering initial storage contains a wide range of potential spoilage and toxigenic organisms. The population present depends largely on the field conditions and harvesting process, and will be further modified during storage. Poor post-harvest management can result in rapid deterioration including the production of mycotoxins. As discussed previously, the main post-harvest mycotoxin in wheat is ochratoxin A produced in northern latitudes by Penicillium verrucosum, although there is potential for toxin production by Fusarium species, fungi normally associated with the growing crop, under very poor storage conditions.
1. Harvest
Harvest is the first stage in the production chain where moisture management becomes the dominant control measure in the prevention of mycotoxin development. Moisture management will involve accurate and prompt measurement methods and procedures in place to efficiently undertake bulk drying where necessary.
Another equally important control measure at this stage will be an effective assessment of the crop for the presence of disease such as FEB. This will need to be accompanied by an efficient strategy for separation of diseased material from healthy grain.
2. Interactions of fungi in storage
Until recently, surprisingly little work has been carried out on the ways that spoilage fungi interact with each other in the stored grain ecosystem, and the effect that this has on mycotoxin production. Recent studies have, in fact, indicated that important inter- and intra-specific interactions occur, depending on the species present and the prevailing environmental conditions. Most importantly, the dominance of certain species has been shown to shift under changing conditions, particularly with changes in water content. For example, Cairns et al. (2003) carried out a study which looked at competition for resources and niche overlap between P. verrucosum and a range of other spoilage fungi, and concluded that the system was in a state of dynamic flux with niche overlap altering in direct response to temperature and Aw level. In general, the results indicated that the fungi present tended to occupy separate niches, based on resource utilization, and this tendency increased with drier conditions. Other work carried out by the same group has looked at the influence of species interaction and changing environmental conditions on mycotoxin production. Most importantly, it was found that the production of ochratoxin A (OTA) by P. verrucosum was strongly influenced by competition with other fungi. In one study, production was significantly inhibited by the presence of both Fusarium culmorum and F. poae (Magan et al., 2003).
Insects may also be present in the stored grain ecosystem, and may interact with fungal species. Insect damage may make the grain more susceptible to fungal colonization and mycotoxin production. Insects may also act as disseminators of fungal spores, and generally show high tolerance to the presence of some mycotoxins. However, to date, there seems to be no studies on the toxicity of OTA to storage insects, or the possible influence of the presence of insects on OTA production.
3. Use of preservatives
At present, the use of chemical preservatives in wheat-based food production only becomes important in the latter processing stages, e.g. the use of propionates in breads. This is likely to remain the case, given the current pressure to reduce the use of chemical additives generally in the food industry. However, recent work has taken an alternative view by looking at the potential of using antioxidants, essential oils from plants and other natural products from bacteria and fungi. There are many economic and technological hurdles associated with this type of approach. However, in tests on wheat grain, butylhydroxyanisole (BHA), propyl paraben (PP), cinnamon oil and resveratol gave greater than 90% reduction in mycotoxin accumulation. Resveratol (an antioxidant) in particular showed a wide spectrum of mycotoxin control, although this is a relatively expensive product (Marin et al., Food Tech. Dept., Ud Lleida, unpublished data).
4. The storage situation
Generally, grain stored at a moisture content equivalent to less than 0.70 Aw (< 14.5% moisture by weight) will not be subject to fungal spoilage and mycotoxin production. However grain is often harvested at moisture levels far in excess of this and is often traded on a wet weight basis. In addition, there are still some technological challenges associated with bulk drying and storage of grain, and instances of poor practice and negligence. The mycotoxins hazard is therefore associated with a significant risk in grain production, in the post-harvest situation.
Harvested wheat grain may pass through the hands of a number of "owners" on its way to the primary processor. In perhaps the simplest case it will remain on-farm in store or buffer storage for short time periods before being passed directly to the processing facility. In other cases it may pass through the hands of grain merchants or to third party drying facilities if it has been harvested wet and no on-farm drying facilities are available. In these cases it will be stored at a number of different locations with transportation steps in between. During all of these stages the grain could become susceptible to fungal spoilage if the storage conditions are not strictly controlled. In most instances the key to this is drying of freshly harvested material down to 0.70 Aw and maintaining the grain in this condition. The most important control measures relevant to storage stages may be listed as follows:
Regular and accurate moisture determination.
Efficient and prompt drying of wet grain. This will relate to holding time/temperature prior to drying as well as the actual drying conditions.
Infrastructure for quick response, including provision for segregation and appropriate transportation conditions.
Appropriate storage conditions at all stages in terms of moisture and temperature control, and general maintenance of facilities for prevention of pest and water ingress.
Ability to efficiently identify and reject material below specified standards in terms of both fungal disease and, at some stages, mycotoxin levels (e.g when passing onto a third party).
Operation of approved supplier systems. This involves setting specifications for acceptance/rejection.
5. Processing - milling and bread making
The overall post-harvest situation becomes even more complex if the stages relating to processing are included. For wheat, primary processing will normally be milling to produce flour for bread making. The flour produced at the mill is susceptible to the same mycotoxin hazards as grain under similar environmental conditions. Therefore the production, storage and transportation of flour requires essentially the same type of management as is required for grain. However, due to the way the bread making industry operates, the majority of flour produced will be in storage for a short period of time. Preparatory stages at flour mills include removal of defective and foreign material, and this could in principle act as a control measure for mycotoxin contamination. Current research work is looking at the influence of the milling process on mycotoxins. It is possible that the removal of certain grain components during milling could result in a reduction in toxin levels in contaminated grain. This will be reported in a later Fact Sheet.
The bread making (baking) process itself, based on modern methods such as the Chorleywood Bread Making Process (CBMP), does not appear to present any significant risk factors in relation to mycotoxin development.
6. Overall implications for the development of a HACCP plan.
6.1. Pre-harvest
Most of the pre-harvest controls for wheat cultivation detailed in the pre-harvest fact sheet are principally in place to combat FEB rather than the risk of mycotoxin development. As such they are unlikely to figure as CCP’s for the purposes of any HACCP plans that may be developed specifically for mycotoxin control. Furthermore, most of the controls discussed would normally come under the remit of pre-requisite programs (eg GAP) that underpin, but do not directly constitute control points in HACCP plans. However, impending legislation on acceptable limits (i.e provisional EU limits of 750microgrammes kg-1 for DON), and the general increase in awareness of mycotoxins, may focus attention on this hazard separately from the associated disease state.
One area of particular interest is the development and use of resistant wheat cultivars. In recent studies by Mesterhazy (2002), wheat varieties most resistant to FEB were shown to reduce DON production to near zero. In fact, resistance seemed to depend to a great extent on inhibition of toxin production directly, since the most aggressive disease causing fungal strains were also those producing the highest levels of DON. This author suggested that an increased availability of such resistant varieties, coupled with the use of appropriate fungicides were key in an integrated approach to mycotoxin control associated with Fusarium. Such a strategy, with emphasis on mycotoxin control, could represent a CCP in the pre-harvest stage.
The findings outlined in the pre-harvest fact sheet indicate that a complex situation exists, both between the fungal species capable of producing FEB and mycotoxins, and also in the relationship between disease and toxin development itself. This situation is further complicated by the varying effects of the fungicides currently in use. In fact, the situation may be rather too complex to provide the opportunity to impose mycotoxin control at this particular point, at least at the present time. Interestingly however, the research outlined in the pre-harvest fact sheet indicates that it is possible in some instances to use fungicides that specifically target toxin production. Equally there is some evidence that using other fungicides can, in certain conditions, stimulate toxin production. So in principle, fungicide application in the field could become a critical control point (CCP) for mycotoxin control in a holistic HACCP plan for wheat production. Similarly, recent evidence seems to indicate that certain biological control agents (BCAs) are particularly effective at reducing mycotoxin levels associated with FEB. It is conceivable therefore that they could be used as a specific control measure for mycotoxins within an integrated control strategy, in which they too would figure as a CCP for mycotoxin production. However the use of BCAs as an integrated approach together with fungicide application may be problematic, and has not been investigated to date.
The application of HACCP type principles is at its most problematic in the pre-harvest stages. There are a number of reasons for this:
The field situation with regard to FEB and mycotoxin development is complex and scientific knowledge is still lacking.
At present, no control measures exist that can guarantee elimination of FEB and mycotoxin production.
Economic factors impinge on farming practices. In particular, field management, cropping systems and chemical input may directly reflect economic pressures rather than best practice for disease elimination.
In food processing plants, where HACCP was first developed, definitive control measures can be devised and implemented to close limits. The same levels of control cannot be exercised in the field situation. One obvious example is temperature and moisture levels (i.e the weather conditions). In fact, some experts believe that the attempt to apply HACCP to the pre-harvest situation is fundamentally flawed.
The above points are all certainly valid, and represent real challenges to the development of holistic HACCP plans. This however should not prevent us from attempting to extend the philosophy and potential of HACCP into all stages of food production.
6.2. Post-harvest
Although complex, the post-harvest stages in the wheat commodity chain, including drying, storage, transport, milling and baking are far more conducive to a "classic" type HACCP analysis than the pre-harvest stages. Unlike pre-harvest, these stages are characterized by the ability to apply definitive control measures, to set critical limits and to initiate monitoring procedures. In particular, flour milling and baking can be viewed as straightforward food processing procedures immediately accessible to the HACCP approach.
Although the commodity flow diagram for the drying, storage and transport of grain can become convoluted and complex, the important controls throughout are relatively few. As we have seen previously they centre around:
Moisture control
Identification and segregation of unacceptable material
Operation of supplier compliance systems
The use of novel preservatives could in principle represent controls, although there are significant hurdles associated with this technology.
Whether any of these represent CCP’s in the development of a HACCP plan, or whether they represent controls within pre-requisite programmes is probably ultimately a matter of opinion, and less important than the underlying process and thinking that goes into developing an integrated risk management strategy. If we apply the test that a CCP must specifically be in place to eliminate or reduce a named hazard, then the only clear-cut CCP likely for mycotoxin control is a supplier compliance system that states an acceptable limit for a named toxin. It would, however, be conceivable to devise HACCP schemes where the drying stage is cited as a CCP for mycotoxin control, due to its central importance, although the step is also in place to prevent fungal spoilage. It could equally be argued that moisture management is a central requirement of Good Storage Practice, and therefore within the remit of a pre-requisite progamme.
There is an argument that the combination of HACCP with pre-requisite programmes is conceptually flawed. For example, Frank (2000) has argued that HACCP and Good Agricultural Practice (GAP) should not be combined because GAP assumes that all steps are of equal importance, so that resources cannot be targeted, while HACCP highlights critical steps, which is effectively taking the opposite view. The overall effect can be misleading and contradictory to the notion that pre-requisite programmes "simplify" HACCP schemes. There also seems to be an inherent danger in the reliance of HACCP on pre-requisite schemes: most major foodborne illness outbreaks associated with food processing plants turn out to be caused not by failures in HACCP schemes, but failings in the accompanying pre-requisite arrangements, such as effective cleaning.
References
Cairns, V., Hope, R. and Magan, N. (2003). Environmental factors and competing mycoflora affect growth and ochratoxin production by Penicillium verrucosum on wheat grain. Aspects of Applied Biology 68. In Press.
Magan, N., Hope, R., Cairns, V. and Aldred D. (2003). Post-harvest fungal ecology: impact of fungal growth and mycotoxin accumulation in stored grain. European Journal of Plant Pathology. In Press.
Mesterhazy, A. (2002). Role of deoxynivalenol in aggressiveness of Fusarium graminearum and F. culmorum and in resistance to Fusarium head blight. European Journal of Plant Pathology 108, 675-684.