The value and importance of insect control in the milling process.
Wheat is considered infested if the representative sample (other than ship lots) contains two or more live weevils (Sitophilus spp.), one live weevil and one or more other live insects injurious to stored grain, or two or more live insects injurious to stored grain.
A stored grain insect, such as a lesser grain borer, is injurious to stored grain because they consume grain and generate by-products, including carbon dioxide, water vapor organic acids, and digestive waste or frass. Water vapor may support microbial growth of yeast and molds when condensed to a liquid during long term bulk storage.
Additionally, insect-damaged kernels retain insect growth by-products and are a source of dust and fines generation during handling. The U.S. Food and Drug Administration has established a defect action level of 75 insect fragments per 50g of flour. Stored grain insects and insect fragments are well known aesthetic issues, but stored grain insects are also a mill economics concern.
Insect Infestation Testing
In ongoing work, clean and dry organic wheat of seed-grade quality at 11.25% moisture was weighed out in 1,000g samples. Three sample sets of 12 were made; one set was not infested with lesser grain borer adults, and the remaining two sets were infested at a low- and high-level starting population.
The samples were held for a nine-week (63 days) period at approximately 27 degrees C (80.6 degrees F) and 60% relative humidity. After 63 days, the samples were frozen to terminate insect growth, and the number of adult insect counts were performed on all samples and prepared for milling in order of increasing adult insect count.
In preparation for milling, samples were passed through a first cleaning of dry wheat consisting of a Carter Day dockage tester set up according to U.S. Grain Standards testing procedure for wheat (Feed Rate-6, Air-4, #2 Riddle and 2-#2 screens [Round-0.078 inch, 1.9812mm diameter]).
Materials over the Riddle and that lifted by aspiration were considered cleaning loss. Sound wheat over the first #2 screen, second #2 screen, and throughs from the second #2 screen were combined and passed through a laboratory entoleter operating at 1,800 rpm at a rate of 500g per minute.
After the laboratory entoleter, the wheat was passed through the Carter Day dockage tester and again, materials over the Riddle and that lifted by aspiration were considered cleaning loss. Sound wheat over the first #2 screen, second #2 screen, and through the second #2 screen were combined for tempering.
Sufficient water to bring the clean dry wheat to 15% moisture was added to the wheat in a container which was tumbled for 30 minutes and held for approximately 24 hours prior to a second cleaning operation and milling. Tempered wheat samples, passed through a second cleaning prior to milling.
The second cleaning operation involved feeding the tempered wheat at approximately 500g per minute through the laboratory entoleter operating at 1,800 rpm. Wheat was subsequently passed through the Carter Day dockage tester with materials over the Riddle and that lifted by aspiration considered cleaning loss.
Samples were milled on a Brabender Quadrumat Senior laboratory mill following a warm-up period and milling of a control warm-up sample. At the end of each day, an ending sample was run to flush out the system in preparation for the next milling.
Losses Identified
Quadrumat Senior laboratory mill flour yield identified on various basis are provided in Table 1. The control sample with no infestation provided for the highest yield on all bases. Increasing insect counts result in progressively lower flour yields as one might expect.
Dry matter losses through the processing system are shown in Table 2 above. A loss of dirty wheat and corresponding dry matter in storage was observed for infested wheat and increased with elevated insect population.
A measurable dry matter loss was observed during storage, with 0.22% of dry matter loss in the low-insect-count group and 0.5% dry matter in the high-insect-count group. Additionally, a slight increase in moisture content from 11.25% to 11.37% for the control sample was observed, perhaps due to the elevated relative humidity. The increase, however, was greater in the low final insect population from 11.25% to 11.39% and for the high final insect population 11.25% to 11.46%. Infestation resulted in both dry matter loss and perhaps increased moisture content.
All fractions from the first and second cleaning of the non-infested sample were of sound quality suitable for milling. Material passing through the second #2 screen in all cleaning steps contained a high level of dead insects and insect parts that were not removed via aspiration or concentrated by screening by the dockage tester with settings for grain grading purposes.
The combination of feed rate and aspiration setting of the dockage tester handling the level of infestation was not adequate to produce a clean wheat fraction suitable for milling. The throughs of the second #2 screen in both final insect populations were collected for milling as carried out for the control sample with zero infestation.
Like in Table 2, Table 3 below reports various basis as well as dry matter losses through the processing system with the exclusion of material passing through the second #2 screen in all cleaning steps in preparation for milling. Mass flow was rebalanced to account for changes in materials to tempering, second cleaning, and milling. The goal is to estimate the extreme impact due to infestation with the understanding that with cleaning system operating parameters such as feed rate, aspiration setting, and screen size for the dockage tester, as well as feed rate and rotor speed for the laboratory entoleter.
Financial Impact
The impact of infestation level on dry matter losses in the cleaning systems suggests a need for process flow modification and/or adjustments of significance.
Process flow modifications may be expensive, and adjustments may not be effective in correcting the impact of infestation during operation. The financial impact on mill economics is not immediately evident using laboratory milling data. Figure 1 below reports yield on a dry matter basis and target moisture level for flour and feed combined with screenings. The dirty wheat yield used was 2.16 bushels per cwt. flour, and the screenings estimate was based on publicly available reporting for dockage and defects for hard red winter wheat.
Dirty wheat and screenings moisture reflects those observed in the process reported. The analysis identified the required amount of clean dry wheat dry matter need to produce 100 pounds of flour at the target moisture content of 14%. A table was created assigning all dry matter losses to screenings value for assessing loss and permitting impact on dirty wheat yield per unit of flour produced.
The economic impact of infestation based on a commercial size mill with commercial level of flour yield is provided in Figure 2 below. The input values are based on recent spot market prices and may not reflect your plant economic situation.
The financial impact of infestation varies with the current dirty wheat yield in your facility and will be greater with increased dirty wheat requirements. The low final insect population annual gross profit loss estimate is approximately $77,000 retaining the stock through the second #2 screen in both cleaning operations to approximately $803,000 or less depending on the facility’s ability to remove dead insects in cleaning. The high final insect population annual gross profit loss estimate is approximately $134,000 while retaining the stock through the second #2 screen in both cleaning operations to approximately $814,000 or less depending on the facility’s ability to remove dead insects in cleaning.
Even if successful in complete dead insect removal, the insect-damaged kernels are likely to impact material handling in both the tempering and milling process. It follows that excessive fragment counts as well as negatively impacted flour performance, and compromised aesthetics will result.
Conclusion
Every milling operation is different in terms of raw material quality issues arriving at the mill site, storage time and conditions as well as cleaning system design and operation.
A similar analysis for grain held for more than nine weeks (63 days) may reveal even greater losses. In either instance, the impact on flour quality attributes or aesthetics has yet to be identified; however, they are undoubtedly compromised.
Work explores a wider range of variables and the impact of flour quality. Stay alert for coming issues of Milling Journal as the work continues. Until then, keep those insects in check!
This research partially funded by the U.S. Department of Agriculture (USDA) and Central Life Sciences was conducted by the Stored Product Insect and Engineering Research Unit and Grain Quality and Structure Research Unit at the USDA-ARS-CGAHR Laboratory, Manhattan, KS.
Dr. Jeff Gwirtz is CEO of JAG Services, Inc., an international consulting company in Lawrence, KS; 785-341-2371; jeff@jagsi.com. He also is adjunct professor in the Department of Grain Science and Industry at Kansas State University, Manhattan.