Example 1: Yellow maize kernels - South East Asia

Introduction.

There are two crops possible in much of South East Asia: a major rainy season crop and a minor dry-season crop. The former is characterised by problematic post-harvest handling resulting in a high risk of mycotoxin contamination, whilst the latter is at low risk post-harvest, but more prone to pre-harvest contamination. There is often a surplus of maize and this is exported as an animal feed ingredient, thus generating valuable foreign exchange.

Exports of yellow maize produced in South East Asia, for use as animal feed, were under serious threat in the mid-1980s, due to difficulty in meeting regulatory limits for aflatoxin set by the major importing countries (e.g. = 20 µg/kg aflatoxin B₁ in the EU), and there was an urgent need for effective control measures. A project was carried out to address this problem and the findings (Nagler, M. J. et al 1987 and Jones, B. D. 1986) are used as a basis for Example 1.

Task 1 - The HACCP team

An appropriate HACCP team will be composed of: a HACCP specialist, a mycotoxicologist, a cereal grain specialist, a socio-economist, a mycologist, a drying engineer, and representatives of the maize industry in the public and private sectors.

Task 2 and 3. - Product Description and Intended Use.

The product description and intended use is given in Table 3.

Tasks 4 and 5 - The Commodity Flow Diagram (CFD), Verified

The CFD will be established using information provided by members of the HACCP team, notably the cereal grain specialist, and representatives from the Department of Agriculture. It will be verified by visiting major maize production centres and interviewing farmers, traders, and silo and feed mill managers and observing their practices. An example of a typical commodity flow diagram is given in Figure 8.

Table 3. Product description and intended use of yellow maize kernels

Target level:

£ 20 µg/kg aflatoxin B₁ for export to the EU and Japan.
£ 50 µg/kg aflatoxin B₁ for domestic animal feed

Task 6: Mycotoxin hazard analysis and identification of possible control measures.

Hazard Analysis

a) Identification of mycotoxin hazard

Maize is very susceptible to aflatoxin contamination and this toxin has been classified as a human carcinogen and is the subject of regulation world-wide. Other mycotoxins which may be present include: zearalenone, one or more of the trichothecenes, and the fumonisins. Maize can be contaminated with more than one mycotoxin, and sometimes contains a cocktail of five or six. However, few countries have set regulatory limits for mycotoxins other than aflatoxin, so the HACCP team may well just concentrate on control of aflatoxin in the first instance.

In this example, aflatoxin is the only mycotoxin carried through to task 7.

Fig. 8. HACCP Flow-diagram: Yellow Maize in Southeast Asia

b) Identification of steps in the Commodity Flow Diagram (CFD) where mycotoxin contamination is most likely to occur.

Steps 1, 2, and 3: on farm, through growing and including harvest

Pre-harvest aflatoxin contamination is associated with drought stress and insect damage (Fortnum, B. A., 1986 and McMillian, W. W., 1986) during the final growing period. The dry-season crop is more prone to these conditions, but was found to be only moderately susceptible to significant levels of pre-harvest aflatoxin contamination. Surveillance studies, and field drying studies (Nagler, M J. et al, 1988) both indicated that levels of aflatoxin were very low at harvest in the rainy season crop, certainly in the locations under study and over the three year study period.

It is concluded that the risk of pre-harvest aflatoxin contamination is low, especially for maize produced in the rainy season.

Step 4: on-farm inspection of cobs.

Pre-harvest contamination with Fusarium mycotoxins will manifest as cobs showing obvious signs of ear rot. Incidence of ear rot was observed in maize produced in both the dry and the rainy season.

Step 5: on-farm accumulation of cobs and storage.

Surveillance studies and on-farm storage studies both indicated that aflatoxin B₁ levels rose to unacceptable levels, 60 to 90 µg/kg, when cobs were taken directly from the field and stored over a 1 to 6 month period, as was the usual practice.

It is concluded that aflatoxin contamination is very likely to occur at this step.

Step 6: shelling

No aflatoxin contamination is likely at this step. However if the percentage of broken grains produced was high, then this could pre-dispose the grain to contamination at a subsequent step.

Step 7: drying and accumulation at Primary Trader

Aflatoxin levels of freshly shelled maize rise very rapidly if the 'safe' moisture content is not attained within 48 hours. Surveys during the rainy season confirmed that aflatoxin contamination is extremely likely at this step.

Step 8: drying and storage at a secondary trader

Aflatoxin surveys showed that maize frequently became more contaminated with aflatoxin at this step.

Step 9: feed mills and export silos.

Feed mills and silos, even when buying maize at a 'safe' moisture content, buy maize of varying histories and a wide range of aflatoxin contents. Hence aflatoxin, produced at earlier steps in the CFD, can occur at this step.

Some silo owners have invested in high capacity mechanical driers and buy cheaper, 'wet' maize. Delays in introducing the maize to the driers, as witnessed by long queues of lorries and the use of 'holding silos' does introduce a high risk of aflatoxin contamination at this step.

c) Possible Mycotoxin Control Measures

The most effective aflatoxin control measure is drying to a moisture content that will not support the growth of toxigenic mould and the production of mycotoxins. For longer-term storage, further drying is required to prevent the growth of all moulds. A related control measure is the maintenance of a 'safe' moisture content.

Field-drying of the rainy season crop for up to 20 days was found to be very beneficial because moisture content of the cobs reduced from 35% at field maturity to less than 22%, allowing immediate shelling and reduced breakage. The lower moisture content made post-harvest drying easier, and did not result in any significant increase in aflatoxin contamination. Although this was true for this study, this may not always be found to be the case.

Drying and storage trials showed that aflatoxin contamination could also be prevented by two-stage drying of shelled maize. If the maize was initially dried to 16% (no part >16.5%) then it could be stored safely for at least one week. This finding was consistent with the fact that Aspergillus flavus and A. parasiticus cannot grow and produce aflatoxin at a water activity a_w £ 0.82 at 25°C. Partial drying would allow a Primary Trader to part-dry maize kernels and then safely sell-on to a Secondary Trader who could complete the drying.

Segregation of acceptable from unacceptable batches of maize was another useful control measure. Although segregation by means of representative sampling and aflatoxin testing was employed as a control measure, it would preferably be used for verification only, once a high level of aflatoxin control had been attained at earlier steps in the CFD.

The use of mould resistant varieties, irrigation to prevent drought stress and use of insecticides or predators to control insects, are examples of GAP which can be effective in limiting pre-harvest mould and mycotoxin contamination.

It was considered to be GAP to shell cobs at an appropriate moisture content using a sheller that produced a low percentage of broken grains.

Tasks 7 to 10: Development of a HACCP Plan

A worksheet summarising the HACCP Plan for yellow maize kernels for animal feed is given in Table 4. The development of the plan at each step in the CFD is given below.

Step 1: Farm, growing in the field - GAP

Pre-harvest mould contamination can be alleviated by the use of relatively resistant varieties, e.g. varieties which have good sheaf cover and have cobs which droop early, allowing rain to run off easily. Insect, rodent and bird control can also be effective in reducing physical damage to the cob. Damage to cobs increases susceptibility to mould attack.

Step 2: Farm, at field maturity - GAP

Cobs have a very high moisture content, in the order of 35%, at field maturity in the rainy season. Harvesting cobs at high moisture makes it extremely difficult to dry to a moisture content low enough for safe storage, or low enough for shelling, without suffering mould damage and mycotoxin contamination. It is advisable not to harvest at field maturity, unless damage is likely to be higher by delaying harvesting, e.g. high incidence of pest damage.

Step 3: Farm, at harvest - CCP1

Although this step was not identified as a step where aflatoxin was very likely to normally occur, it was found that a control measure introduced at this step could reduce the likelihood of subsequent mould contamination, to an acceptable level. Field-dried maize could be shelled directly, with a low percentage of breakage, and then dried relatively easily to a 'safe' moisture content. Step 3 is therefore determined to be a CCP with field-drying being the control measure for rainy season maize. The critical limit for this CCP is £ 22% moisture content and monitoring is by farmer testing. With training, traditional techniques such as biting kernels, or weighing freshly shelled kernels in the hand, can be used to assess the moisture content.

In this study, the increased risk of pre-harvest aflatoxin contamination during field-drying was heavily outweighed by the resulting reduced post-harvest contamination. This may not always be the case in other locations, and under different climatic conditions. Hence the more generalised CCP would be to harvest at an appropriate time.

Step 4: Inspection on farm - CCP2

This step was identified as a CCP with segregation of obviously mouldy cobs as the control measure. This CCP with reduce the percentage of mouldy cobs to an acceptable level, and hence reduce the levels of any mycotoxins produced pre-harvest. It with also reduce the likelihood of biodeterioration and subsequent mycotoxin production, which can occur when mouldy cobs are stored. An appropriate critical limit could be rejection of cobs showing mould damage over >10% of the surface. This CCP is best monitored by trained harvesters.

Step 5: Accumulation and storage of cobs on-farm - CCP3.

This step is identified as a CCP with two alternative control measures. The first is to dry the cobs to £ 16% m.c. within two days of harvest, prior to storage. However, if this is not possible, or storage is not desired, then the cobs should be shelled within one week of harvest, and preferably within two days. These control measures will prevent any significant subsequent production of aflatoxin. Critical limits, other than the final moisture content, can be set in terms of sun-drying time which will produce the required final moisture content.

It is advisable to avoid the use of polypropylene bags until maize is dried to 14% m.c.

For medium-term farm storage (1 to 6 months), Good Storage Practice (GSP) is required to prevent mould contamination. Examples of such practices include: a sound roof, good ventilation, raised floor, insect and pest control.

Step 6: Shelling - GAP

Minimising the percentage of broken kernels produced during shelling is considered Good agricultural Practice. Broken kernels allow easier infection by aflatoxin producing moulds and this can lead to higher levels of aflatoxin contamination if subsequent CCPs move out of control. Hence, if GAP is not applied correctly at this step, the effect will manifest itself as a more extreme product disposition if corrective actions are required at a subsequent CCP.

To minimise breakage during shelling, the moisture content of maize cobs must be in the correct range for the mechanical sheller used. If the maize cobs are wet, say at a moisture content in excess of 20%, they will be too soft for many shellers and damage will be high. Conversely, maize cobs which are very dry, say at a moisture content below 15%, may be brittle.

Step 7: Primary Trader - CCP4.

Drying freshly shelled kernels to £ 16% m.c. within 48 hours is the control measure adopted to establish this step as a CCP. However, Primary Traders currently rely on sun-drying, which is least reliable when it is needed most, during the rainy season. Critical limits are set for sun-drying to attain some measure of control, but to attain the required degree of control, mechanical drying is required. Unfortunately this is rarely financially viable at the Primary Trader step, but can be at Step 8. Primary Traders therefore need to move maize swiftly on to Secondary Traders during bad weather.

Primary Traders usually carry out short-term storage of maize, to allow them to accumulate sufficient maize to trade with the Secondary Trader. Good Storage Practice is required in order to prevent re-wetting, e.g. a store with a sound roof and the use of pallets to prevent water being soaked up from the ground.

Step 8: Secondary Trader - CCP5.

This step is determined to be a CCP with drying to a moisture content of 14% (no part >15%), prior to storage as the control measure.

Some Secondary Traders have mechanical driers that are used to supplement sun-drying and these are essential when sun-drying is impossible.

It is important that Good Storage Practice is in place. As well as measures to prevent re-wetting, insect and rodent control will be required if mycotoxins are to be prevented during medium and long-term storage.

Step 9: Feed mills and export silos -CCP6.

If the CCPs at the previous steps could be implemented fully, then verification rather than a CCP will be appropriate at this step. However, it will take time to fully and satisfactorily implement this HACCP Plan in the commercial sector, so a segregating CCP is appropriate. The critical limit for the CCP is set at the required target level of aflatoxin and monitoring will be by means of representative sampling and aflatoxin analysis using semi-quantitative testing.

Those export silos with a policy of buying wet maize and using mechanical driers should match procurement with drying capacity. Delayed drying will result in mould contamination, heating, and rapid aflatoxin production.

Good storage practice is necessary at this step to prevent re-wetting and damage due to pests.

Task 11: Establish Verification Procedures

Validation procedures will be established for each of the CCPs and overall verification will be provided by the fully quantitative aflatoxin results on representative samples of the batches incoming to domestic feed mills, or on representative pre-loading samples of maize destined for export.

The control measures were validated on the 10 tonne scale, in replicates of 10, at each of two locations in major production areas. Maize produced according to this HACCP plan averaged less than 5 µg/kg in both locations, whilst maize produced without these control measures in place in the commercial sector averaged just under 200 µg/kg.

The HACCP Plan will be audited quarterly and amended as necessary.

Task 12: Establish documentation and record keeping

The HACCP Plan will be fully documented, with appropriate record keeping at each step.

Table 4. HACCP Plan Worksheet: Aflatoxin in yellow maize kernels for animal feed.

* Leave un-harvested in the field after field-maturity
** GSP = Good Storage Practice

References

Fortnum, B. A. (1986) 'Effect of Environment on Aflatoxin Development in Preharvest Maize'. Aflatoxin in Maize: Proceedings of the Workshop, El Batan, Mexico, April 7-11 1986 CIMMYT ISBN-6127-12-7. pp145-149

Jones, B.D., Kenneford, S., Nagler, M.J., Meadley, J., Buangsuwon, D. (1986) 'Efforts to Control the Levels of Aflatoxin in South-East Asian Maize'. International Biodeterioration Spp. 22 89-94.

McMillian, W. W. (1986) 'Relation of Insects to Aflatoxin Contamination in Maize Grown in the Southeastern USA'. Aflatoxin in Maize: Proceedings of the Workshop, El Batan, Mexico, April 7-11 1986 CIMMYT ISBN-6127-12-7. pp194-199

Nagler, M.J., Jewers, K., Wong-Urai, A., Tonboon-Ek, P., Buangsuwon, D., Lorsuwon, C., Siriacha, P., Meadley, J. (1987) 'Production & Quality Control of Maize with a Low Aflatoxin Content during the Rainy Season in Thailand'. Proceedings of the 9th ASEAN Technical Seminar on Grain Post Harvest Technology. Singapore 26-29 August 1986. Ed. de Mesa, B.M. ASEAN, Manila.

Nagler, M.J., Buangsuwon, D., Jewers, K., Faungfupong, S., Wong-Urai, A., Nagler, C., Tonboon-Ek, P. (1988) 'The Role of Leaving Maize Unharvested in the Field after Field-Maturity (Field-Drying) in Controlling Aflatoxin Contamination'. Proceedings of the 10th ASEAN Technical Seminar on Grain Post Harvest Technology. Bangkok 19-21 August 1987.