The Microbiology of Cereals and Cereal Products

Dichloran rose bengal chloramphenicol (DRBC) agar is recommended as a general-purpose medium for direct plating of grain kernels and for plate counts of flours, meals, and processed products for total counts. Dichloran with 18% glycerol (DG18) is also recommended for these uses, especially for direct plating kernels for xerophilic molds, which prefer low a_w and dry conditions. For plating dry grains and cereal products, DG18 may actually be better than DRBC and the medium of choice.

Two differential media may also be used for detection and enumeration of specific molds in cereals. Aspergillus flavus–parasiticus agar (AFPA) can be used as a differential medium for direct plating of kernels and plate counts of A. flavus and A. parasiticus. Czapek agar with iprodione and dichloran added (CZID) is widely used as a differential medium for detecting and enumerating Fusarium species from cereals. The simplest way to evaluate the internal microflora of seeds and kernels is the direct plating method, which involves surface sanitizing seeds or kernels in full strength or 50% household bleach for one minute to kill surface microflora. The kernels or seeds are then rinsed in sterile distilled water and dried on sterile paper towels.

The seeds or kernels are then placed directly on an agar surface in a Petri dish and incubated at 25 to 30°C to allow molds located in the interior of the seed or kernel to grow out. The number of kernels with internal mold is counted and the results are expressed as a percentage of infected kernels. The amount of internal infection of the grain is an indicator of quality and storability of the grain. The technique can also give some information about the safety of the grain if AFPA or CZID have been used, indicating whether or not potentially toxic A. flavus, A. parasiticus, or Fusarium species are present.

Methods for detecting mycotoxins are summarized in Table 3 (above). Chromatographic methods have been used from the beginning of mycotoxin research and are still used for detecting, quantifying, and confirming the presence of mycotoxins. These methods have evolved, been improved, and have become more sophisticated. Chromatographic methods in use include thin-layer chromatography, high-performance liquid chromatography, liquid chromatography combined with mass spectrometry, gas chromatography, and gas chromatography combined with mass spectrometry.

Various detection methods, such as fluorescence, ultraviolet absorption, and others have been combined with chromatographic methods. New methods based on the production of antibodies specific for individual mycotoxins have also been developed and include enzyme-linked immunosorbent assays and immunoaffinity columns. These methods allow for specific and precise detection and quantification of specific mycotoxins. This has lead to test kits for mycotoxins which are rapid and simple to use and can be used in the field, country elevators, grain-buying stations, feed mills, and processing plants.

Drs. Bullerman and Bianchini are professors in the Department of Food Science and Technology at the University of Nebraska-Lincoln. For more information, contact Dr. Bullerman at [email protected].