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MPM: V-3. Grains and Grain Products

Macroanalytical Procedures Manual (MPM) Main Page

A. Methods for Wheat, Corn, and Other Grains (V-15)

(1) Scope

This method describes procedures for the determination of rodent excreta pellets in wheat and other cereal grains and for the determination of damage to grains caused by insects, molds, and decomposition.

(2) Applicable Documents

  1. CPG 7104.03 Defect Action Levels for Wheat
  2. IOM Chapter 4, Chart 4 - Wheat Carload Sampling.
  3. CPG 7114.19 Defect Action Levels for Popcorn

(3) Defects

The principal defects found in grains involve damage during storage caused by rodents, insects, and molds. Field damage by fungal pathogens (including wheat or corn scab fungus, corn blight fungus, and black tip fungus) may become a problem during adverse climatic conditions.

(4) Procedure: Determination of Rodent Excreta in Wheat

  1. Sample Preparation -- The sample consists of a representative number of subsamples taken from a lot. Determine the weight of each subsample to the nearest gram.
  2. Visual Examination -- Examine wheat by spreading a small amount at a time on a sheet of white paper, or use any other convenient device that will permit the rapid and accurate visual examination of an appropriate portion of the sample at a time. Remove and weigh the rat and mouse excreta pellets and fragments. Include as rodent pellet material any whole pellets or broken pellets, provided they are large enough to be readily identified as such by their size, shape, surface coating, or presence of rodent hairs.
  3. Report -- For each subsample, report weight in mg of rodent excrement found and calculate mg per kg. Also report the average mg/kg for all the subsamples.

(5) Procedure: Determination of Insect-Damaged Wheat Kernels

  1. Sample Preparation -- Use subsamples from (4)a. Using a Jones sampler or Boerner divider or equivalent device, mix and reduce each subsample to about 200 g. Weigh out 100 g of each subsample for visual examination. If examination is by X-ray, take slightly more than 100 g of each reduced subsample and brush the wheat, a small amount at a time, on a 5- or 8-in. No. 12 sieve. Use a stiff bristle brush to work any surface insects, dust, dirt, or broken kernels through the sieve. Weigh 100 g of the brushed wheat from each subsample as the test portion for X-ray examination.
  2. Visual Examination -- Examine for insect-damaged wheat kernels according to instructions given in Figure V-3. Remove and count insect-damaged kernels.

    Figure V-3

    Classification of Insect Damage in Wheat Kernels (Cross-Section View)
  3. Report of Visual Examination -- Report number of insect-damaged kernels for each subsample and the average for the subsamples.
  4. X-Ray Examination -- Spread the test portion of wheat, one grain thick, on a support interposed between a source of X-rays and a radiographic film. Expose to soft X-rays using a standard wedge according to the instructions for obtaining maximum contrast as described in Chapter IV, "Special Techniques". Examine the radiograph for insect-damaged kernels using any suitable commercially available X-ray film viewing screen. In general, the wheat kernel appears white or gray on the negative. Any cavity within the grain appears as a dark region and an insect within the cavity appears light in color. Mark the insect-damaged kernels and count the total per 100 g.
  5. Report of X-Ray Examination -- Count number of insect-damaged kernels per 100 g of wheat. Determine number of kernels of wheat per g; calculate and report the percent of insect-damaged kernels by number.

(6) Procedure: Determination of Decomposition Caused by Molds and Other Factors in Wheat

  1. Classification of Decomposed Wheat Kernels -- Classify decomposed wheat kernels as follows:

    1. (i) Moldy -- Classify as moldy, those kernels which fit any of the following categories:

      • Kernels which are affected by scab to the extent that the bran coat is broken open and there is evidence of mold or other disease or damaged condition
      • Kernels which have a dull, lifeless and chalky appearance (so-called "tombstone") as a result of disease
      • Kernels which have mold in the germ or have mold in the crease of the kernel (Note: Carefully remove the bran coat covering the germ; scraping the bran coat too deeply could remove the mold)
      • Kernels which are affected by black-tip fungus to the extent that the fungus growth is on the germ and extends into the crease of the kernel
      • Kernels materially affected by white, pink, gray, black, or green mold
      • Kernels which have ergot (black or dark purple sclerotized grains)
    2. (ii) Otherwise-decomposed -- Classify as otherwise decomposed those kernels which fit any of the following categories:

      • Kernels which have been materially discolored and damaged by external heat
      • Kernels damaged as a result of heating caused by excessive respiration or fermentation
      • Kennels which have been materially decomposed by other causes (describe the condition)
  2. Visual Examination-- Weigh out 50 g of the reduced sample from (5)a and examine the kernels using the naked eye or with low power magnification. If the magnification exceeds 10X, this should be stated in the method section of the report. Classify the kernels according to (6)a.
  3. Report-- Report percent of decomposed kernels by weight for each category. Also report the total percent of decomposed kernels for each subsample. Describe principal type(s) of damage found.

(7) Procedure: Determination of Rodent Excreta in Corn

  1. Sample Preparation-- The sample consists of a representative number of subsamples taken from a lot. Determine the volume in pints to the nearest tenth of a pint for each subsample.
  2. Visual Examination-- Examine as in (4)b.
  3. Report-- Calculate the number of rodent excreta pellets per pint for each subsample. Also report the average number of pellets per pint for the entire sample.

(8) Procedure: Determination of Insect-Damaged Corn Kernels

  1. Sample Preparation -- Prepare sample as in (5)a, except reduce each subsample to slightly more than 250 g.
  2. Visual Examination and Report -- Follow (5)b and c.

(9) Procedures for Other Grains

Select procedures as appropriate from (4) through (8).


(1) Anonymous, Grain Inspection Handbook, U.S.D.A., Federal Grain Inspection Service, 1979.

(2) Christenson, C. M., and H. H. Kaufman, Storage of Cereal Grains and their Products. American Association of Cereal Chemists, St. Paul, MN, 1982.

(3) Matz, S. A., The Chemistry and Technology of Cereals as Food and Feed, AVI Publishing Co., Inc., Westport, CT, 1959.

(4) "Wheat - Internal Insect Infestations," JAOAC 65: 499-500, Method 44.C01-44.C03, 1982.

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