Ensuring Optimum Mixability In Feed Manufacturing: Part 4 - Mixer Testing

Mixer Testing

Routine mixer testing should be an integral part of the quality assurance program and should be conducted quarterly. Procedures for mixer testing are relatively simple and involve taking samples at specific time intervals. The assay used and statistical treatment are relatively straightforward.

Sampling
Good sampling is essential for a mixability study to be worthwhile. An analysis is only as good as the sample. The intent of good sampling is to obtain a small portion of a feed that is representative of the whole. One cannot take only the fines or only large particles and expect to obtain an accurate analysis. The eight factors summarized here are important in order to obtain good samples. These apply to all samples and not just to those taken for mixability studies.

Important factors to obtaining good samples
1. Planning
2. Location
3. Quantity
4. Timing
5. Tools used for sampling
6. Containers
7. Proper labeling
8. Sampling preservative

The number of samples to be taken depends on the accuracy of the results desired. Herrman and Behnke (1994) suggested that 10 samples per batch per mixing time would yield sufficient satisfactory coefficient of variance. Eisenberg and Eisenberg (1992) indicate that the number of samples assayed depends mainly upon the laboratory time and costs.

Ideally, samples should be taken within the mixer either at spaced intervals during the mix or on completion of the mix. Sampling within a mixer may be particularly desirable under these conditions:
1. When a mixer design is being studied, an attempt is made to determine where certain ingredients may concentrate.
2. When the effect of time of mixing is being studied, it is not desirable to discharge the mixer frequently or before mixing is complete.

If one cannot take samples from the mixer, then take them as near the mixer in the production system as possible. Frequently if a mixer discharge is being sampled, it is necessary to sample as rapidly as possible at almost uniform time intervals, which may mean taking samples at 5 to 10 second intervals. If poor mixer or segregation is indicated after preliminary trials, it may be desirable to make a special effort to sample from particular locations or at particular times to locate trouble spots.

Recommended Procedure For Sampling Feeds From The Mixer
1. Decide and make arrangements for the analytical work. Obtain equipment and containers for sampling. The suggested size is 100 to 200 gram and sufficient enough for the planned analyses. Mark sample containers for sample identification.
2. Select a suitable location for taking samples preferably as close to the mixer discharge as possible. The site and sampling procedure should not pose a safety hazard to the person involved in taking the samples.
3. Timing the mixer discharge at 8 to 10 samples per batch are recommended, beginning with initial discharge and ending with samples of the tailings at the final discharge. In between samples will be taken at evenly spaced time intervals between the initial and final discharge.
4. Begin sampling sequence when the mixer is ready to discharge. Record the mixing time; mixing time begins when the last ingredient is added to the mixer and ends when the mixer begins to discharge.

Assay Selection
Numerous assay methods have been used for mixer evaluation. However, there is no "perfect" procedure available. The criteria for the assay selection should be as follows:
1. The assay principals should be based upon a common ingredient, nutrient, or chemical that comes from a single source. Salt, therefore, is a good selection while protein or nitrogen would be a poor selection.
2. The cost of the assay in terms of labor, chemicals, and time should be minimal.
3. The assay procedure should be relatively simple, fast, accurate, precise and should be able to perform in the mill or laboratory, and not require expensive equipment or highly qualified personnel.
4. The assay principle should be supplied from a single source.
5. The sample size required should be reasonable but large enough to reduce or eliminate sampling error.
6. The target mix uniformity (CV values of the tracer) should be about 2 times the proven analytical variation for the assay selected but in no case should it exceed 10%.

Common Mixer Tests
Salt (NaCl) is a common component of most livestock and poultry rations. Therefore, sodium (Na) or chlorine (Cl-) ions are often used as mixer test markers. Assaying samples for salt content may be performed using several techniques.
1. Omnion Sodium Analysis (Omnion, Inc., Rockland, Massachusetts): Use a sodium ion electrode to determine the concentration of Na+ in the samples. The percentage of salt can be calculated from these values. The technique appears to be quite accurate and reliable.
2. Quantab® (Environmental Test Systems, Elkart, Indiana): This method determines the chloride ion concentration of a solution. Salt from the feed samples is extracted in hot water. The titrators consist of a thin strip laminated with a capillary column, impregnated with silver dichromate. The column is a reddish-brown color. When the strips are placed in an aqueous salt solution, the fluid will rise in the column. The indicator across the top turns blue, the reaction is completed. Chloride ion concentration is calculated and variation from the expected concentration is used to determine mixer performance.