By Charles Stark, Julie Kalivoda and Chad Paulk, Kansas State University
The particle size of ground grain influences feed digestibility, feed efficiency, mixing performance and pelleting. Therefore, periodic particle size evaluation is a necessary component of a feed-manufacturing quality-assurance program and is recommended by nutritionists.
The standard for particle size analysis by sieving is published by the American Society of Agricultural and Biological Engineers. As stated in their publication, Method of Determining and Expressing Fineness of Feed Materials by Sieving (ANSI/ASAE S319.4 FEB 2008 R2012), “The purpose of this standard is to define a test procedure to determine the fineness of feed ingredients and to define a method of expressing the particle size of the material.”
The standard allows several variations for this testing procedure. Specifically, it allows the use of different sieve shakers, such as a Tyler Ro-Tap, Retsch, or equivalent unit. It also allows optional use of sieve agitators, such as small rubber balls and bristle sieve cleaners to help move particles around on finer sieves. Another option is whether to use a flow agent, also referred to as a dispersing or sieving agent, to help material move through the sieves. Finally, the time of sieving can range from 10 to 15 minutes in the official procedure.
Laboratories that test particle size may obtain differing results because they use different procedures. For quality control, it is important to know the procedure used by the testing laboratory and how it relates to your particle size goals. Therefore, an experiment was conducted to: 1) determine which method of particle size analysis best estimates the particle size of various cereal grains, and 2) assess analytical variation within each method.
Grain samples were ground and analyzed using different variations of the standard particle size analysis method. Treatments were arranged in a 5 × 3 factorial design with five sieving methods: 1) 10-minute shake time with sieve agitators and no flow agent; 2) 10-minute shake time with sieve agitators and flow agent; 3) 15-minute shake time with no sieve agitators or flow agent; 4) 15-minute shake time with sieve agitators and no flow agent; or 5) 15-minute shake time with sieve agitators and flow agent conducted in three grains — corn, sorghum or wheat.
Results for mean particle size and standard deviation were calculated according to both standard methods S319.2 and S319.4. The analytical method that resulted in the finest mean particle size and greatest standard deviation was considered desirable because it is presumably representative of the largest quantity of particles moved through the appropriate sieve.
There was no analytical method × grain type interaction for mean particle size. Analytical method affected (P < 0.0001) mean particle size and standard deviation measured by both standards. Inclusion of sieve agitators and flow agent resulted in the finest mean particle size, regardless of sieving time. Inclusion of flow agent reduced (P < 0.05) the mean particle size by 32 or 36 micrometers when shaken for 10 or 15 minutes, respectively, compared to the same sample analyzed without flow agent. Flow agent was also an important factor to alter standard deviation (Figure 1).
Because the flow agent increased the quantity of very fine particles collected in the pan, standard deviation was substantially greater (P < 0.05) when flow agent was included in the method. Shaking samples for 10 or 15 minutes did not affect the measurements when including sieve agitators and flow agent. Particle size of corn and sorghum ground using the same mill parameters was similar (P > 0.05), but wheat ground using the same mill parameters was 120 to 104 micrometers larger (P < 0.05) compared to corn or sorghum, respectively.
In conclusion, both sieve agitators and flow agent should be included when conducting particle size analysis, but only 10 minutes of shake time is required. Kansas State University has published new standardized guidelines for particle size analysis (C. Stark and J. Kalivoda, 2016. MF3342) and can be found at KSUswine.org. A fact sheet is also available online.