Make the most of mixing

by Suzi Fraser
Share This:

By Warren G. Dominy and Suzi Fraser Dominy

The mixer, the heart of a feed mill, deserves careful selection and routine testing to ensure optimal operating efficiency.

As ingredients and their byproducts were combined to produce compounded diets for animal feeds, accurate proportioning and uniformity of mix were required to blend the different ingredients in order to meet the nutritional requirements of the animal. Today, the increased use of medication, the availability of novel feed ingredients and new feed manufacturing methods have forced the feed industry to work with diets of increasing complexity to meet nutrient requirements and to comply with Good Manufacturing Practices. These requirements amplify the need that all ingredients and medications be proportioned accurately and blended homogeneously with minimum time and expense.

Mixing is one of the most important operations in the manufacture of feeds, but mixers are frequently given little consideration and are expected to work efficiently with minimum maintenance or calibration. Many thousands of dollars are spent on equipment to gather, process and store ingredients in semi-automated or fully automated proportioning systems that deliver precise amounts of ingredients to the scale. However, if these varied ingredients are not uniformly mixed, the proportioning control system prior to mixing loses its effectiveness.

The mixer is the heart of a compound feed plant. Before selecting a mixer, the following factors should be considered:

•Sizing the mixer to the production capacity of the plant.

•Bulk density of the ingredients to be mixed in the feed.

•Amount and type of liquid(s) to be added.

•Location and space restrictions in the mill.

•Degree of clean-out for proper operation.

•Sanitation requirements.

•Desired standard mixer performance (batch mixing cycle, speed of mix, uniformity of mix).


To determine the type of mixer that is suitable for your feed operation, it is important to test your compounded feed with the different types of mixers on the market. Ideally, testing should be performed before purchase. At the least, performance standards should be included in the specifications and should be validated before the mixer is accepted and paid for.

During the manufacture of animal feeds, many factors may create or contribute to incomplete mixing. Some of these include the type of mixer selected and how it is designed and the physical properties of the ingredients used in the formulation (particle size, shape, ingredient density, static charge, hygroscopicity and adhesiveness).

The first three ingredient characteristics are the most important. Large and small particles do not mix well, and a better mix can be achieved if the range of particle sizes is small. High-density particles, such as minerals, tend to segregate to the bottom of the mixer or bin.

Operational mixer parameters can influence the uniformity of a mix. They are cleanliness of the mixer; the sequence of ingredient addition; ingredient loading point; the amount and types of liquid addition; overfilling or underfilling the mixer; mix times; mixer speeds; and worn, altered or broken mixer parts.


Mixer testing determines the amount of time needed to obtain a satisfactory uniformity of mix of feed ingredients and medication. There are several accepted mixer test methods. One or more should be performed at the time of installation and routinely (about every three months) thereafter.

Numerous assay methods have been used for mixer evaluation. One criterion for selection should be to assay for an ingredient, nutrient or chemical that comes from a single source. Using nutrients such as crude protein, crude fat, ash and crude fiber can compromise the uniformity of mix analysis because the nutrients can be contributed by more than one ingredient in the feed mix.

Salt has been a good selection for most plant protein-based animal diets (corn/soy). However, many marine-derived ingredients contain salt, such as fishmeal, fish byproducts, squid meal, shrimp meal or crab meal, which makes salt a poor ingredient for validating a feed mix.

The assay should be relatively simple and should be able to be ideally performed in the mill or quality control laboratory without requiring expensive equipment or highly trained technical personnel. The cost of the assay in terms of labor, chemicals and time is critical. A mixer study can include 20 or more sample assays, making the cost of each test and the time involved a significant factor if accuracy of the results is desired.

A successful mix time is achieved when the coefficient of variation (CV) from the samples is at or near the CV for the assay procedure. Therefore, it is desirable to select a procedure that has good analytical reproducibility (low CV).

The target mixer uniformity (CV values) should be at or below 10% by convention. The percent coefficient of variation (% CV) is calculated by dividing the standard deviation (sd) by the mean (m) concentration of the assayed marker or the %CV is equal to (sd/m) (100). In very simple terms, CVs help measure the distribution of values and express them as one number.

Sampling is also very important to any mixer evaluation. Sample number may also depend on the time and cost of the analysis. However, it is recommended that at least 10 samples should be taken either from several locations in the mixer or at timed intervals given the time it takes for the mixer to discharge.

If the mixer takes 60 seconds to discharge, then every 5 to 6 seconds would be an adequate sampling time interval. Enough samples should be taken each time (approximately 100 to 200 gms) to allow for a good analysis and repeat analysis, if necessary.

There are three types of commonly accepted mixer tests used in the feed industry: chemical assay, the use of color-coded iron filings and the salt (NaCl) test.

Chemical assay for pharmaceutical drugs, vitamins (ascorbic acid), amino acids (lysine, hydroxy-methionine) and trace minerals ingredients can be quantitatively determined and are usually very accurate but quite expensive to run. Chemical assays can be done by biological tests that take a long time, but the delay in obtaining results reduces their value and is not practical for a routine mixer test. However, chemical assays can be used to confirm mixing problems, or to confirm the presence or absence of the product added.

Color-coded dyed iron filings consist of iron grits (95% passing through 35 mesh and 95% retained on a 120 mesh) coated with food colors stabilized with sodium carbonate. Colors include blue, red, orange, green and various combinations that allow for several iron tracers (different colors) to be added at different times and at different locations. In this way, several sets of data can be obtained and tracked in a single mixer test.

Recovery of the colored iron tracers is done with a rotary detector that separates the iron filings from the feed by a magnet. The filings are then placed on a large filter paper or paper towel pre-wetted with 50% ethanol. The dyed iron filings will color the paper with the different colored spots, which may then be counted. Variation from the expected inherent Poisson Distribution (the standard deviation for a series of counts from a complete mix should equal the square root of the mean count) is calculated to determine mixer performance.

This method of iron tracers may be compromised by magnets in the mixing system, feed mixes containing high concentrations of propylene glycol, choline chloride or added water, which will cause the iron filings to leach their dyes. According to the manufacturer, modifications to the standard iron tracers and the test methods can eliminate these problems.

Assaying for salt (NaCl) content in a feed mix is another way of measuring the uniformity of mix by analyzing for the sodium (Na+) or chloride (Cl-) ions. Listed are two assaying methods, sodium analysis, used for assaying the sodium ion (Na+), and the chloride ion (Cl-) analysis. Salt found in most marine products and by-product meals will compromise the results of these tests.

The Omnion Sodium Analysis is a method of determining the percentage of salt in a feed. A feed sample of 1 to 2 grams is homogenized and 90 ml of a sodium reagent is added to the mix. The Omnion meter with a sodium ion electrode is calibrated then used to determine the sodium (Na+) ion concentration in the samples in about one to two minutes.

The Quantab test method is used to determine the chloride ion. First salt is extracted from the feed samples in hot water. The titrators consist of a thin plastic strip, laminated with a capillary column and impregnated with silver dichromate.

The column is a reddish-brown color. When the strips are placed in an aqueous salt solution, fluid will rise in the column. The chloride ion reacts with the silver dichromate to produce a color change in the column. When the indicator across the top turns blue, the reaction is complete.

Chloride ion concentration is calculated and variation from the expected concentration is used to determine mixer performance. This method is relatively fast (10 to 15 minutes), can be done in the plant and does not require elaborate equipment. One must have hot water, filter paper, a graduated cylinder and paper cups. This is a simple and cost-effective procedure.


Maintaining the uniformity of a feed mix post-mixer is also a challenge as segregation can occur as the feed mix is transferred from the mixer by conveying systems to processing bins during the manufacturing process or to the load-out or packaging bins. One may also use these mixer test procedures to isolate points of segregation in the feed mill system and correct them.

If a batch of feed leaving the mixer has a CV of 7% but a CV of 18% is measured in a mash load-out bin, there is a problem between the mixer and that load-out bin. This ingredient segregation can occur as an effect of conveying the completed feed mix from the mixer to packaging, bulk load-out bins or to an agglomeration system (pellet mill, extruder and expander).

Not only a mixer should be considered in testing the uniformity of mix, but also the total mixing system, which impacts the uniformity of a feed mix. This includes particle size reduction, which keeps ingredient size and shape uniform, to testing the uniformity of the finished mash feed or an agglomerated feed. If the finished product is not uniformly mixed, it may be necessary to start checking where segregation is occurring in the plant.

In summary, the objective in mixing is to create a completely homogeneous blend. Intuitively, the concept of feed uniformity is important. Those associated with animal feed production realize that if feed ingredients, particularly micro-ingredients such as vitamins, amino acids, trace elements and drugs, are not properly blended, overall animal performance will be reduced and wide variation within the group of target animals will exist. This is especially true for the young or small animals (lab animals, pets, quail, shrimp, etc.) and for those animals with short digestive tracts (monogastrics).

The incomplete mixing of a feed can create a toxic situation if some ingredients are not properly mixed. Most feed additives, such as fat-soluble vitamins, trace minerals, antibiotics and growth promoters, will not perform their intended function if they are not properly blended in the feed.

Regardless of the target animal, good manufacturing practices dictate the production of feeds that are as uniform as possible. Equipment selection should be based on known compatibility. Operational protocols should be set to ensure that maximum uniformity is obtained. Personnel should be educated on the concept of uniformity, and appropriate testing should be conducted to ensure that these objectives are met.

A great deal of emphasis is placed on uniformity because of regulatory compliance, animal performance and the satisfaction obtained in producing high quality feeds. It is surprising how few mixers are actually tested at all, let alone on a routine basis.

Ideally, mixer testing will become more commonplace and will be a part of the quality control procedures of every feed producer.