Milling for quality evaluation of new wheat lines

Marta Izydorczyk and Jerry Kletke, Grain Research Laboratory

Buhler MLU 202 Laboratory Mill
Photo 1. The Buhler MLU 202 Laboratory Mill for milling new wheat lines at the Grain Research Laboratory.

The Grain Research Laboratory uses three Buhler MLU 202 Laboratory Mills for milling new wheat lines tested in the Canadian wheat breeding programs. Two mills are used for hard wheat and one mill is used for soft wheat. The Buhler mill is a batch mill with stock being conveyed pneumatically (photo 1). The milling flow consists of three corrugated roll break (B) passages, three smooth roll reduction (C) passages, and one bran finisher passage. A rotary sifter is used for final sieving of the flour and to separate the bran flour from the residual bran.

For the evaluation of milling and functional qualities of late generation lines in the Western Canadian wheat breeding programs, an extraction rate (flour yield) of 74% is kept constant for all lines. The constant extraction rate is used so that samples can be compared on an equivalent basis, in particular for ash content. A sample with lower ash content than the check varieties, prepared at the same extraction rate, indicates that a miller could extract more flour compared with the current varieties while meeting customer specifications for ash. In contrast, if a sample has higher ash content than the check varieties at equivalent extraction rate then it would have to be milled to a lower extraction rate in order to produce flour with equivalent ash content.

Handling and tempering wheat before milling

  1. Wheat samples are cleaned using a Carter dockage tester to remove impurities. After cleaning, a test weight is performed, and the grain is cleaned again using a Forster Cyclone Grain scourer to further remove any dust and dirt adhering to the grain kernels.
  2. Wheat moisture is determined using the Brabender MT-C moisture tester. 10 grams of grain is ground with a Brabender SM-3 Break mill and placed in the moisture tester for one hour at 130°C.
  3. The samples are tempered to different moisture levels depending on the wheat class:
    • Canada Western Red Spring wheat is tempered to 16.4% moisture
    • Canada Western Hard White wheat is tempered to 16.4% moisture
    • Canada Western Prairie Spring wheat is tempered to 16.0% moisture
    • Canada Western Amber Durum wheat is tempered to 16.0% moisture
    • Canada Western Red Winter wheat is tempered to 15.5% moisture
    • Canada Western Soft White wheat is tempered to 14.5% moisture
    • Canada Northern Hard Red wheat is tempered to 16.4% moisture
  4. When the wheat moisture content is above 10%, an appropriate amount of water is added to achieve the desired tempered milling moisture. If the wheat moisture content is below 10%, the wheat is tempered in two stages: initially to approximately 13% and then to the desired level. Wheat samples are placed in 6 litre plastic containers with airtight lids. The samples are shaken by hand after the initial addition of water and then inverted intermittently by a mechanical mixer for 6 hours. Generally wheat samples are tempered overnight for at least 18 hours. The milling performance does not change if the total tempering time ranges from 12 to 48 hours. Therefore the samples can be milled over several hours without fear that the milling results will change from the first to the last sample.
  5. After tempering, the moisture content of the wheat samples is determined again to confirm the desired level.

Milling

Flour tumbler for mixing flour streams
Photo 2. Flours streams are combined in a flour tumbler and mixed for 10 minutes.
  1. Milling is performed using the Buhler MLU 202 Laboratory Mill in a temperature and humidity-controlled room: 21°C and 60% humidity to assure repeatable results, regardless of the ambient conditions.
  2. The routine Grain Research Laboratory hard wheat mill flow is illustrated in figure 1. The feed rate for hard wheat is 100 grams per minute.
  3. The routine Grain Research Laboratory soft wheat mill flow is illustrated in figure 2. The feed rate for soft wheat is 75 grams per minute.
  4. The smooth rolls (C1-3) are warmed up to 30°C (overnight using electric heaters) to avoid any mill warm up time and to ensure constant temperature of the rolls during milling. Variations in grinding roll temperature will influence milling results, since rolls expand as they warm up, decreasing space between rolls. Decrease in roll spacing increases grinding action and may affect the bran and flour yield and the level of starch damage.
  5. After verifying the room temperature and humidity, roll temperature, roll gaps, and the feed settings, the mill is ready for operation. The tempered wheat is weighed to the appropriate amount on a 14% moisture basis (for example, 1.029 kilograms at 16.4% moisture equals 1 kilogram at 14.0% moisture).
  6. A 2 kilogram check sample is milled on a daily basis at various times of the day. The following parameters for the mill check are determined and documented to ensure proper mill performance and reproducibility of results:
    • flour yield
    • flour moisture
    • farinograph absorption
    • flour ash
  7. Wheat lines tested in the 1st year breeding trials are milled once (3 kilogram portion).
    Wheat lines in the 2nd and 3rd year trials are milled in duplicate (2 x 2 kilogram portion).
  8. Milling generates several flour streams. The moisture content of flour streams is determined using the Brabender moisture oven.
  9. The exact extraction rate (flour yield) is kept constant for all lines within a particular wheat trial. The streams are combined from the lowest to the highest ash content (usually the B1-B3, C1, C2 flour streams are combined immediately and kept separate from the C3 and Bran flour streams).
  10. Taking into consideration the ash and moisture content of the flour streams, the desired extraction rate is reached by blending the streams in appropriate proportions. Flour streams are combined in a tumbler and mixed for 10 minutes (photo 2).
  11. Flour yield (extraction rate) is expressed as percentage of clean wheat on a constant moisture basis (14%).

Figure 1. Mill flow for hard wheat using the Buhler MLU 202 Laboratory Mill

Buhler MLU 202 Laboratory for hard wheat (diagram)
B=break rolls
C=smooth reduction rolls
W=wire mesh sieve
N=nylon sieve (number indicates aperture in µm)

Figure 2. Mill flow for soft wheat using the Buhler MLU 202 Laboratory Mill

Buhler MLU 202 Laboratory for soft wheat (diagram)
B=break rolls
C=smooth reduction rolls
W=wire mesh sieve
N=nylon sieve (number indicates aperture in µm)

Reliability of laboratory milling of common wheat

Target values and acceptable tolerances for Canada Western Red Spring 'mill checks'
Flour moisture 14.5% plus or minus 0.3%
Flour yield 75.5% plus or minus 1.0%
Farinogragh absorption 63.8% plus or minus 0.5%
Flour ash 0.49% plus or minus 0.03%

To maintain a high degree of accuracy and precision of milling and test results, reliable laboratory milling procedures and tolerances for control samples have been established at the Grain Research Laboratory to produce milled products that are comparable and repeatable not only day-to-day but year-to-year and longer. Standard control samples ('mill check'), included with the test samples every day, provide constant verification of the mill performance and related test procedures. The 'mill check' samples are prepared in large quantities by blending No. 1 Canada Western Red Spring wheat from two crop years; the ‘mill checks’ are stored less than 5°C.

Experimental wheat samples are thoroughly cleaned, blended, and sub-sampled. Tempering moisture can strongly influence wheat milling properties, and improper wheat conditioning moisture will affect the flour yield, flour moisture, and ash content. If the tempered wheat is too dry, more of the bran will be ground finer, causing flour yield and ash to increase. If the tempered wheat is too wet then more of the endosperm will adhere to the bran, causing a flour yield decrease. Proper tempering procedures, including determination of moisture in wheat before and after tempering, determination of moisture content in flour, and temper time and temperature, must be ensured. Variations in ambient temperature and relative humidity in the mill room can have marked effects on experimental milling results. Milling at the Grain Research Laboratory is performed in a temperature and humidity-controlled mill room.

Standard procedures involved in preparation of the mill include pre-warming mill rolls overnight, checking the mill roll temperature, checking the proper roll gap setting and the feed rate settings. These practices guarantee the correct mill performance in achieving flour yields and flour farinograph absorption within the set tolerances. Proper maintenance is performed to prevent leakage, tears, and clogging of the mill sifters. Thorough cleaning of rolls and sifters during the milling process will ensure correct flour yields and recovery.

Durum wheat milling

Allis-Chalmers Laboratory Mill for durum wheat
Photo 3. General view of the Allis-Chalmers Laboratory Mill for durum wheat
Laboratory purifier for durum semolina.
Photo 4. General view of the purifier for durum semolina

The experimental durum wheat lines tested in the Western Canadian wheat breeding programs are milled using the Allis-Chalmers Laboratory Mill (photo 3) in conjunction with a laboratory purifier (photo 4).

The procedures for handling and tempering of durum wheat are similar to those used for common wheat. Durum wheat is tempered to 16% moisture before milling. Samples are milled according to the flow illustrated in Figure 3. The flow consists of four corrugated break roll passages, five corrugated sizing roll passages and ten purification steps. Roll gaps on the first three breaks are kept relatively wide to produce semolina with coarse granulation. The purification steps in durum milling ensure better particle size separation and bran removal. Semolina streams are collected after each purification step, whereas the coarser material retained on a 630 µm screen is passed through the sizing rolls and sifted on a box sifter. After sizing passages, streams with fine particles passing through the 180 µm sieve are collected as flour, whereas the coarser streams are passed through the purifier. Our definition of durum semolina is any particles greater than 180 µm and less than 600 µm. The distribution of particle size in semolina produced with our mill is shown in Figure 4.

Durum wheat lines being tested in the Western Canadian wheat breeding programs are milled in duplicate (2 x 2.3 kilogram portions) to a constant extraction rate of 70% semolina and 75% total yield (semolina and flour).

Reliability of laboratory milling of durum wheat

Target values and acceptable tolerances for durum wheat 'mill checks'
Tempered wheat moisture 16% plus or minus 0.2%
Semolina moisture 15% plus or minus 0.3%
Semolina ash 0.66% plus or minus 0.02%
Semolina yield 66.7% plus or minus 1.0%
Total yield 75.0% plus or minus 1.0%

Major sources of error in experimental milling of durum are similar to those involved in milling of common wheat and include: improper sampling, incomplete removal of foreign material, inappropriate tempering, variations in ambient conditions, poor conditions of milling equipment, and improper mill settings. Durum wheat semolina milling is particularly sensitive to fluctuation in relative humidity of the mill room because stocks are exposed to air during purification passages. Milling of durum wheat occurs in a humidity and temperature-controlled room (21°C and 60% humidity). The mill performance is verified by milling the standard ‘mill check’ sample and ensuring that the yield and the results for moisture, yield, and ash fall within the defined tolerances.

Figure 3. The Allis-Chalmers laboratory mill flow for durum wheat semolina incorporating modified Grain Research Laboratory sifter unit (70% extraction rate)

Allis-Chalmers laboratory mill flow for durum wheat semolina (diagram)
B=break rolls, P=purifier, S=sizing rolls, semo=semolina, REB=stock to rebolted

Figure 4. Distribution of particle size in durum semolina

Particle size distribution by volume in durum semolina (chart)
Distribution of particle size by volume % for durum semolina milled from Canada Western Amber Durum wheat varieties Brigade and Strongfield
Figure 4. Distribution of particle size in durum semolina
Brigade Strongfield
Particle size (µm) Volume (%)
0 to 149 6.7 6.5
149 to 180 4.7 4.8
180 to 250 14.8 15.3
250 to 350 23.8 24.3
350 to 450 19.8 19.8
450 to 550 13.7 13.3
550 to 650 8.5 8.2
650 to 750 4.8 4.5
750 to 850 2.5 2.3
850 to 1000 1.0 0.8
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