Minimizing microbial contamination

by World Grain Staff
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Microbial activity is something millers don’t always give much consideration to until there is a major problem.

When addressing this issue, the first thing to do is look at the potential sources of contamination. The mill’s basic raw material is likely the primary host, particularly in warm, moist conditions when grain has been left in the field in the hope that the sun will dry it rather than spending money running it through a grain dryer.

It has been found in recent years in Europe that many of the contaminants such as vomitoxin (DON) originate in the field, and that harvesting early and then drying down before storage will alleviate the problems of high levels being present and the millers rejecting the delivery.


What can millers do to reduce the risk of contamination? They can make sure their silo storage is in good condition by checking for leaks or damage to the silo, particularly the roof areas and especially around the inspection hatches and ventilation covers. A regular schedule of inspection will have been adopted at most mills and will have proven invaluable, especially at mills where incoming wheat arrives by oceangoing vessel every few months.

Where grain is delivered by road it is likely that the mill will have silos capable of handling excess storage just after harvest, thus taking advantage of the lower price. The silos likely will not be used year round, so an intensive cleaning regime can be put in place just before harvest, including fumigation, so that storage silos are in the best condition to receive the new crop.

Secondly, you need to be very diligent at the intake point to ensure that wheat being delivered does not undo the good work you have just done on your silos. Any grain showing contamination in excess of the accepted levels agreed by farmers, merchants, millers and their various national associations should be rejected, although more than likely this wheat will simply be blended with other wheat to reduce the overall contamination levels and then be sent to another miller or used as feed wheat.

Patterns have emerged in the United Kingdom in recent years highlighting areas of the country where DON levels, for example, have been higher than others. With this knowledge, it is possible to be selective in your buying. But in many cases, and especially in the international grain market, you are buying blind and must therefore test your incoming grain thoroughly.

Good hygiene throughout the mill is vital, and cleaning schedules should be set up if they are not already in place to cover all areas of the facility. In feed mills where heat treated meals are produced, it is common practice to have an "airlock" at the entrance to the heat treatment plant area where anyone entering the building will change clothes and shower down before entering and leaving the building. Some flour mills have adopted this policy, but it is not yet accepted practice in the industry.

Again, referring to heat treatment plants for feed, it is custom and practice to preheat the plant before use, ensuring that the plant reaches the required temperature of 80 degrees C well before any meal is introduced. This ensures that no meal can avoid the full treatment heat. It is also common practice as an extra precaution to recycle the start-up material just in case the incoming material, which is at a lower temperature, causes the overall process temperature to drop below 80 degrees.


One item that is often a major source of microbial growths, yet one which we ignore regularly, is our water supply, particularly in countries where water treatment is limited.

A good water treatment plant is a necessity in some countries, and it is recommended that all millers at least check the incoming water supply at regular intervals, even if they do not feel that a treatment plant is necessary from the outset. Water companies are obviously protective of their water and will tell you that the supply meets the national standards for supply, but invariably those standards do not cover microbial growths and relate only to heavy metals. Check the specification of the water being supplied to your mill if you draw from a utility company as opposed to using your own bore hole.

If you do use your own bore hole, make sure it goes deep enough to provide a clean potable supply of clean water and draw samples regularly through the year as the water table rises and falls.

Personal hygiene is important, although I have not heard of anyone passing microbial contamination from themselves into the process. In feed mills, especially where personnel are handling medicinal additives, it is important for millers to have their work wear laundered away from their home. Problems have arisen where the employees’ clothing, when washed with the clothes of young children, have caused skin irritations and rashes on young babies, and for that reason a good work wear contract is of value. Again, random swabbing and testing of work wear is valuable as part of a regime to minimize microbial risks.

The other major source of microbial contamination is through insect activity. This will be evident from testing swabs of flour outloading spouts in flour mills but perhaps is not so easy to track in feed mills, where insect activity may be quite high but most product passes through pellet mills and conditioners which will destroy most microbial growths if operated at a high enough temperature. In such cases, due diligence is called for and samples should be taken both before and after the pelleting process to ensure that what you "think" is happening is actually taking place. Simple things such as returns from cyclones into finished feeds can destroy all your good housekeeping work and introduce microbial growths into an otherwise clean product stream.

Cyclones and dust filters should be inspected weekly and cleaned whenever necessary. Filters handling warm air can be ideal breeding grounds for microbial growths and cyclones can become blocked and create an internal environment which will encourage growths to flourish. Again, swabbing before and after filters and cyclones will tell you where any problems are likely to arise.

The incidence of microbial contamination in feed processes is relatively low, but when such growths appear in feed products, the finger of blame will invariably be pointed at the millers before too long. That’s why it pays to have your house in order.

A good HACCP study, fully supported by test and result documents from an accredited laboratory, will go a long way to alleviate any initial problems that may arise from any public incidents. Mixer uniformity tests are useful to show that product is homogenous, and also in feed mills before and after pelleting tests can be of significant value.

Maintaining adequate, detailed records of your activities, sampling and treatments will be invaluable not only when a problem arises but also when the question is asked, "Why are we spending so much on testing?" Good records will tell the astute miller where the key sampling areas are and where he or she can save some money.

Regular observation of operations will highlight areas where practices need improving. Clean and tidy mills are those which are well managed, and those mills in turn seem to have the fewest number of reportable incidences of microbial growths.

Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J.B. Bradshaw Ltd. He has extensive experience in flour and feed milling in Africa, the Americas, Europe and the Caribbean. He may be contacted at