Mixing-a critical function in Feed manufacturing

The daily ration of nutrients that an animal receives from a feed may vary from time to time due to a number of reasons. The sources of variation will probably cause variation in the day-to-day level of nutrition received by an individual animal. The nutrient variation in feeds is most likely to occur for the following reasons (Wilcox and Balding, 1976):
a. Variation in the composition or quality of ingredients from batch to batch or from time to time
b. Poor mixing or segregation after mixing
c. Errors during weighing or proportioning
aIn most cases, a sound quality control program can insure optimum feed preparation. Routine inspection of the mixer, proper mixer “tuning” , maintenance of all liquid systems and close attention to ingredient inventories will go a long way to ensure that the nutrient specifications prescribed by the nutritionist, actually reach the bird. The major disadvantage of variation is normally the effect on animal performance.
Mixing is one of the most essential and critical operations in the process of feed manufacturing, yet it is frequently given little consideration. The objective in mixing is to create a completely homogeneous blend. In other words, every sample taken should be identical in nutrient content.
Importance of uniform mixing for animal performance
In order for birds to reach their genetic potential for growth and meat yield, levels of protein, energy vitamins and minerals must be provided in their proper ratio. Duncan (1989) reported that as protein variation increased in feeds, growth rate and feed conversion were depressed (Table 1). A 10% variation in the feed quality significantly reduced both weight gain and increased feed conversion. When the coefficient of variation (CV) of the feed was increased to 20%, another significant increase was observed in feed/gain (F/G ).
A recent study on the effect of mixing uniformity on day one old broilers was conducted by McCoy et al. (1994). Feed was formulated to meet or exceed NRC requirements for all nutrients for broiler chicks from 0 to 3 week of age. However, in an experiment 2, feeds were formulated to 80% of NRC recommendations for crude protein (CP), lysine, methionine, Ca, and P. The purpose of using deficient diet in this study was to accentuate any difference in growth performance that might result from diet non-uniformity.
peIn experiment 1, feeds were collected from mixer after 20, 40 and 80 revolutions of mixing (20 = highly non-uniformity mixing, 40 = moderate non-uniformity mixing and 80 = uniform mixing). Variability of feed decreased sharply between 20 and 40 revolutions and no further reduction occurred between 40 and 80 revolutions (Table 2). The CV values from analyses of salt concentrations were 43, 11 and 13% for 20, 40 and 80 revolutions, respectively. No difference occurred among treatment for average daily gain (ADG), average daily feed intake (ADFI), bone strength, bone ash, carcass crude protein, carcass fat, or carcass ash. However, there was a trend for a linear increase in gain:feed (G/F) ratio when mixer revolutions were increased.
In experiment 2, feeds were collected after 5, 20, and 80 revolutions. The salt test CV % decreased from 40.5% to 12.1% when mixing was increased from 5 to 20 revolutions, but there was no further reduction of CV % from 20 to 80 revolutions (Table 3). ADG, ADFI and G/F improved when CV % decreased from 40.5 to 12.1%. However, mortality was not affected by treatment.
Factors affecting mixer performance
Although insufficient mixing time and filling the mixer beyond the rated capacity are often implicated as common sources of variation in finish feed. Other factors such as particle size and shape of the ingredients, ingredient density, static charge, sequence of ingredient addition, worn, altered, or broken equipment, improper mixer adjustment, poor mixer designed, and cleanliness can affect the mixer performance (Wilcox and Balding, 1986; Wicker and Poole, 1991).
The mixing time necessary to produce a homogenous distribution of feed ingredients should be measured for each mixer. Each mixer should be “tuned” to its proper Revolutions Per Minute (RPM) for optimum ingredient dispersion. Different types of ingredients may have a different flow pattern within a mixer at similar RPM. Generally, (Wilcox and Unruh, 1986), the higher the RPM, the faster the more efficient the pattern of dispersion. The size uniformity of the various ingredients that comprise the finished feed can directly impact final ingredient dispersion (Herrman and Behnke, 1994).
The sequence of ingredient addition also determines ingredient dispersion in the mixing process (Herrman and Behnke, 1994). Mixers may have dead spots, where small amounts of ingredients may not be readily incorporated into the feed. This situation is exasperated when mixing ribbons, augers, or paddles become worn. Ground grain or soybean meal should be the first ingredient added into a horizontal mixer. It has been determined that for the quickest distribution of the micro-ingredients within the mass of major ingredients, the micro-ingredients should enter the horizontal mixer early in the dumping order, no later than 10 seconds after the first of the major ingredients begins its entry (Lanz, 1992).
Overfilling or under-filling a mixer can lead to inadequate mixing (Wilcox and Balding, 1976). Overfilling a mixer can inhibit the mixing action of ingredients in horizontal mixers at the top of the mixer. Filling a mixer below 50% of its rated capacity may reduce mixing action and is not recommended.
The incorporation of liquid ingredients (fats, oils, molasses, liquid chlorine chloride, Alimet and other liquids) into the mixer is a common practice in many milling operations. The best way to introduce liquid ingredients are through a spray bar installed at the top of the mixer. Dry ingredients should be adequately mixed prior to the introduction of liquids into the system. Premature liquid addition tends to impede the transport of micronutrients and may even agglomerate the fine particles into “snowballs” .
Most engineers agree that multiple points of application (4-8) are necessary to insure adequate dispersion (Lanz, 1992). The preferred location is such that the manifolds are parallel and located on the “up-turning” side of the rotor. Pressure-loaded check valves and air-purged manifolds help minimize the post-spray dripping that can foul the mixers’ rotor.
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.
Feed costs comprise the single most expensive component in producing poultry or other types of meat animals. As a result, effort to reduce nutrient variability within feeds will yield a significant return to commercial operations. Proper ingredient processing and storage, adequate maintenance of mill equipment and routine testing of the final feed are essential to insure optimum animal response to feed nutrients, while controlling feed costs. Nutritionists and feedmill operators should work together to closely monitor feed preparation, and final feed specifications. The bottom-line result will be a reduction in the production cost of chicken or eggs.
by Chin Sou Fei, Novus International

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