Growing emphasis is being placed on the microbiology of wheat flour. Flour is mostly used to make doughs or batters that are cooked and subject the product to sufficiently high temperatures for a time, adequate to reduce microbial load, including pathogen risks for consumers. In some instances, however, products may be consumed during preparation or have inadequate cooking needed to make the product safe. Raw flour consumption isn’t recommended due to the potential of pathogen content that may result in food borne illness.

Wheat grown in fields is subject to microbial contamination from soil (including airborne soil), water, air, birds and other animals. During growth and ripening, the presence of moisture can cause fungal development, leading to mycotoxin production as well as undesirable sprout damage. The presence of moisture may also support other microbiological activity.

Harvest, transport, storage and handling equipment also represents sources of potential microbiological contamination. Humans are also a source of potential contamination as they work with and handle the grain from the field through the milling operation and final processing into a consumable finished product. The final processing may take place through additional industrial processing, including bakery mix manufacturing or final preparation of mixes or ingredients in a restaurant or at home.

Typically, wheat processing facilities do not include processing steps that would effectively kill microbial contamination. Scourers in wheat cleaning operations are helpful in reducing microbial load but do not eliminate or effectively sterilize the wheat. Anti-microbial agents added to the tempering water are being considered to reduce microbial load but may not constitute a kill step. Similarly, anti-microbial agents may be considered for addition to finished flour to reduce microbial load. Technologies including heating, forms of radiation, etc., may be used on flour where any potential negative impact on flour functionality or labeling is not a major issue.

Engineering and design issues in wheat processing facilities have made microbiological control difficult as processing systems themselves are a source of microbial contamination. Non-aseptic laboratory tempering and milling has not shown increased microbial contamination as some millers observed in their commercial systems.

Lab study results

The average decrease from wheat to flour in laboratory milled hard red winter wheat crop survey samples was 1.4, 1.1 and 1.4 Log10 CFU/g for Aerobic Plate Count (APC), Enterobacteriaceae (Eb) and Yeast and Mold (Y&M), respectively. If microbial counts are increasing in your processing system, evaluation of system design and sanitation practices must be addressed. Elimination of idle pockets of tempered wheat, mill stock as well as condensation in the processing zones must be eliminated through re-engineering or improved sanitation practices.

Source: Jeff Gwirtz

The location of microbial contamination has not been clearly identified, yet it is believed to be on the surface of the wheat kernel and potentially concentrated in the kernel crease and on the germ surface. The microbial measurements Aerobic Plate Count (APC), Enterobacteriaceae (Eb) and Yeast and Mold (Y&M) were measured for tempered wheat, straight grade flour and millfeed. Average Tandem Mill stream production and ash were used to calculate cumulative flour ash. Average cumulative bran content was calculated using the assumption bran contained 7% ash and pure endosperm contained .020% ash. A graph of the resulting calculations reflecting the level of bran retained in the flour produced is provided in Figure 1. It’s important to note that not all endosperm contains 0.20% ash and not all bran contains 7% ash, but these values are being used for purposes of this article.

Source: Jeff Gwirtz

Table 1 shows the average and standard deviation for microbiological measurements for tempered wheat, straight grade flour and millfeed from six replications of hard red winter wheat milling on the Bühler Tandem Mill. As one might expect, the microbiological load for millfeed was much higher than observed for tempered wheat or straight grade flour. This suggests bran carries most of the microbial contamination found in flour via bran contamination as a result of grinding, sifting and purification done to separate bran, germ and endosperm. The presence of microbial contamination in the straight grade flour given the assumption that the endosperm contributes 0% of the microbial contamination and bran and germ contains all potential contamination was calculated. Two simple methods for estimating the microbial contamination resulting from bran (including germ) were developed.

First, we can assume the microbial content of the millfeed is the same as the microbial content of the bran in the straight grade flour. Second, using the microbial content of tempered wheat and assuming bran and germ make up 17% of the kernel weight carrying 100% of the microbial contamination as indicated previously. A third method developed considers the amount of flour carried in the millfeed, which from previous work translates to approximately 24% to 26% flour or endosperm, indicating the microbiological contamination observed in millfeed understates the actual bran contamination level.

Source: Jeff Gwirtz

Table 2 shows the results of the three calculation methods compared to the actual flour microbial analysis. As shown, the first method of calculation provides for a lower estimate of contamination in the straight grade flour. The second method provides a higher estimate of the contamination in the straight grade flour. The third method adjusts the millfeed microbial level to reflect the flour or endosperm content of millfeed using the calculated contamination level for flour bran content.

These preliminary observations and data indicate that flour microbial contamination is indeed associated primarily with surface contamination of the wheat kernel. A relatively clean sanitary production environment will allow production of a flour lower in microbial contamination than observed in the incoming wheat. The observed higher level of Eb and Y&M measured in straight grade flour over calculated estimates suggests air used in conveying mill stocks may contribute to increased microbial counts in flour.

A 600-tonne-per-day flour mill may use 600 cwts/day air in moving product and suction. Air may be a source of microbial contamination and efforts to minimize potential of contamination should be evaluated. Examine air inlet sources, keeping them up off the ground level and in locations less likely to contain dust from grain unloading or feed and flour loadout may be helpful. Cleaning well repaired filters for air intake rooms as well as positive displacement pumps are essential and extend pump life.

Dr. Jeff Gwirtz is CEO of JAG Services Inc., an international consulting company in Lawrence, Kansas, US, 785-341-2371; He is also is adjunct professor in the Department of Grain Science and Industry Kansas State University, Manhattan.