Flour mill contamination

by Mark Fowler
Share This:

News reports on foodborne illness and the long awaited publishing of the rules to implement the United State Food and Drug Administration (FDA) Food Safety Modernization Act (FSMA) have increased public awareness and concerns about food safety.

Microbial and bacterial contamination of wheat flour has seldom been a concern due to the fact that it has low water activity level. Water activity (Aw) refers to the availability of water in a food or beverage and represents the amount of water that is available to microorganisms. Pure water has an Aw of 1.00. A water activity level of greater than 0.95 is required to support the growth of bacteria.

Wheat flour generally has an Aw level of 0.87 or lower depending on flour moisture and temperature. The concern with wheat flour is that bacteria can be carried or stored and emerge from dormancy when the flour is further processed into a food product. In the milling process, prevention and reduction of microbial or bacterial contamination are good alternatives with elimination options growing in acceptance, but adding significant cost to production.


While most of the food safety stories in the media are linked to E. coli or Salmonella bacteria, Deoxynivalenol (DON), generally referred to as vomitoxin, is a mycotoxin that may be produced in wheat kernels. These kernels are usually infected by Fusarium head blight (FHB) or scab prior to harvest while in the field. The best method to manage DON contamination is to prevent it from entering the milling process.

To avoid DON contamination, the best practice is to implement quality checks of the incoming wheat and reject supply sources that exceed the safe level. The FDA restriction for DON in flour is a level less than 1 ppm. The common practice in the wheat industry is to set the maximum acceptable level of DON in wheat at 2 ppm.

Quick tests, such as ELISA test kits for DON, are available from several laboratory supply companies, which allow milling companies to quickly and effectively screen incoming wheat deliveries. In some situations it is necessary, due to supply or origination constraints, to receive and process wheat in excess of 2 ppm of DON. In these scenarios, it is possible to remove DON-contaminated wheat kernels during the wheat cleaning process using gravity tables and optical sorters. This is not an ideal solution as it concentrates the DON in the wheat screenings, preventing it from use in animal feed rations and creates an additional economic loss for the mill.


Whereas DON-infected wheat kernels can be identified and removed in the wheat cleaning process, other forms of microbial contamination cannot be seen. Several treatments of wheat during the cleaning process are being investigated and researched to determine their effectiveness in lowering the microbial load of flour.

One processing method that has proven effective is aggressive scouring or abrasion of the wheat to remove the outer layers of bran prior to milling. Referred to as debranning or peeling, this processing step can be added during the wheat conditioning process.

Drawbacks of using abrasion to remove the bran include the high amount of energy required to abrade the wheat and controlling the rate of bran removal. This method cannot completely remove the bran from the crease of the kernel which may be the part of the wheat berry most susceptible to microbial contamination. Debranning also has the drawback of concentrating the microbial contamination in the bran portion removed, creating the challenge of how to dispose or treat the byproduct.

Another method to control bacterial growth being extensively researched is the treatment of tempering water with ozone or chlorine. Ozone is an unstable and poisonous allotrope of oxygen. It is an effective oxidant widely used as an industrial air deodorizer, water purifier and in food processing as a disinfectant.

It has been used in grain storage as a fumigant to control insect infestation. A primary benefit of using ozone is that it is a natural solution. It degrades into oxygen, leaving no chemical residue in the grain, flour or processing equipment.

A more common practice in the milling industry is the treatment of tempering water with chlorine. Chlorination is a lower-cost solution and more effective method as compared to ozone. However, it may leave behind hazardous residues and poses greater risk to the working environment and employees.


Once the wheat has been milled into flour, the options to eliminate microbial contamination become more expensive. However, of greater concern to the end-user is the potential to impact the functional properties of the flour while treating it to eliminate or reduce the bacteria present.

Irradiation of flour using gamma rays has been proven effective and is an approved method of reducing microbial counts in some food products. The challenge with gamma ray radiation is that studies have shown a negative impact on dough quality characteristics such as gluten strength and development time. Other forms of irradiation that have been tested or are currently being studied include radio frequency and ultraviolet. Neither method has been proven effective.

Heat treatment of the flour after milling is a proven strategy to reduce bacterial and microbial contamination. Heating can have significant impact on flour functionality and, in fact, is used as a treatment to change flour functionality. Water absorption, gluten strength and dough stability are all quality characteristics that can be impacted using heat treatments. For these reasons, the use of heat to reduce microbial contamination poses significant risks and potential at the same time.

Excessive heat will cause the browning of the flour, denaturing of the functional proteins and, of course, significant moisture loss. The heating of higher moisture flour or the use of dry steam has proven more effective in reducing microbial count with minimal impact on flour performance. The most critical aspect of a heat treatment process is to assure even and thorough distribution of heat into the flour to maintain uniformity of the finished product.


Regardless of the process or stage in flour production where the prevention, reduction or elimination of bacteria is implemented, maintaining a clean environment including proper food safety protocols is critical in the effectiveness of any treatment procedure. A comprehensive HACCP program or similar food safety standard is necessary to assure the treated flour is not re-contaminated further down the process.

Proactive prerequisite programs including sanitation schedules, effective cleaning practices, chemical and pest control as well as the sanitary design of equipment, storage bins and work areas is a critical part of any treatment to reduce bacterial contamination and food safety policy.

Proper filtration and maintenance of compressed air and air make up (or air stabilization) systems are central to maintaining a clean environment. Pneumatic conveying is the primary method of moving finished flour and carries the risk of creating an environment that supports mold growth by triggering condensation in dead spaces and equipment. A comprehension food safety program includes the packaging lines, storage and transportation of the product.

The flour milling industry has a long history of proactively pursing methods of delivering a safe and reliable product to consumers. Current good manufacturing practices, wheat cleaning methods, magnets and the numerous steps of grinding and sifting may effectively eliminate physical hazards in the finished flour, but the risk of microbial contamination still exists.

The low water activity and the inability for bacteria to grow may not be sufficient for our current and future customers and consumers. The belief that further processing, baking or cooking will kill any bacteria present in the flour may no longer be enough to protect the industry and assure a safe product. The new FSMA rules and the increased consumer awareness of food safety may be causing a fundamental shift in the standards required by the milling industry to deliver a safe, quality and essential ingredient.