By Hans-Walter Lucht and Manfred Peisker
The list of micro-ingredient feed additives is growing, presenting opportunities for the formulator and challenges for the processor
Despite recent regulatory efforts to ban antibiotic growth promoters from use in compound feed, the number of micro-ingredients (MIs) available to feed manufacturers is increasing. The list, though far from complete, goes from direct-fed microbials, probiotics, enzymes, vitamins, amino acids, organic and inorganic acids, feed aromas, macro- and trace- minerals to phytogenic additives. The substances are available in dry, gel or liquid forms. Many micro-ingredient manufacturers offer both dry and liquid forms simultaneously.
The challenge for feed plant managers is to find the appropriate physical form and the correct site of addition for the substances that the feed formulator wants in the diet.
Generally speaking, MIs should be added early in the process (in the batch mixer), provided that the MI is thermo-stable enough to withstand the heat stress in the processing line (conditioner, pellet press, expander, extruder, drier) and that there is no risk of the MI contaminating the processing line, which would create residue problems and require laborious cleaning between batches.
Adding the MIs early assures good distribution into the feed mash and does not require extra investment in a post-pelleting application unit (PPA). Physical form is of lesser importance in this case, since an in-house or premix manufacturer’s micro-dosing unit can be used for MIs in dry form, while for liquid forms of heat-stable MIs, such as amino acids, a simple one-way, nozzle-type-spraying unit does the job satisfactorily. Another advantage of pre-pelleting application is the avoidance of MI interactions, either chemical or physical. This can largely be achieved by making full use of the mixing cycle time when adding the MI.
POST-PELLETING ADDITION (PPA)
The driving force behind a large number of PPA installations was the introduction of feed enzymes onto the market in the beginning of the 1990s. Enzymes are heat labile, and that quickly triggered the development of liquid forms and technical systems for PPA.
Access to reliable technical systems, in turn, sparked development of liquid forms of other MIs, including thixotropic gel premixes for the manufacturing of medicated feeds. However, post-pellet addition of liquid MIs faces several challenges that must be addressed in order to optimize its benefits:
a) Tiny amounts must be distributed on large surfaces
Due to low liquid MI dosage, as for example, with enzymes, many of the dosing systems require a carrier fluid. In most cases, water is used for that purpose. But one has to be aware of possible reactions between the carrier fluid and the MI. In addition, the MI and the carrier liquid must be mixed just prior to pelleting.
A spraying system that operates without a carrier is preferable. The pump can then be connected directly to the supplier’s container, which also virtually eliminates the risk to personnel of exposure to the MI.
b) Compatibility of liquids
One must be aware of reactions between MIs themselves, such as between enzymes and acids. PPA systems that allow separate but simultaneous dosage of incompatible liquids are preferable to systems that require blending of the MIs before dosing.
c) Dosing accuracy
Dosing accuracy is an important consideration, since MIs are expensive ingredients in a feed formulation. Waste and over-dosing cannot be tolerated. The recovery should be 95% minimum and the coefficient of variation (CV) for the distribution of the MI in the feed should be less than 10%, and ideally only 5%.
d) Physical form and quality of feeds
PPA systems usually work better with good quality feed pellets. Dust and fines tend to absorb liquids faster and thus lead to uneven distribution in the feed. Apart from that, liquid-loaded dust settles in pipes, tubes, valves and other difficult-to-clean parts of the feed transportation chain. This leads to lack of recovery and unwanted carryover effects.
The PPA system itself should exert as little physical stress to the feed as possible. Systems requiring mechanical mixing support for even distribution of the liquid MIs, such as batch mixers, should be rejected because they spoil pellet (granules, extrudate) quality and lead to dust formation.
e) Physical stress in the dosing process
Some MIs, such as direct-fed microbials (probiotics, yeast), cannot be subjected to pressure or sprayed with pressurized systems, since that would lead to the destruction of the biological integrity of the microbial cells, rendering them useless. Appropriate care has to be administered in such cases, and pressureless systems should be selected.
f) Batch versus continuous systems
Batch systems for PPA might feature some advantage in recovery and dosing accuracy. However, in high-performance pelleting lines, it appears to be much more economical to work with continuous systems. The feed to MI ratio should be allowed the utmost flexibility, say from 50 ppm (50g/t) to 10 ppm (10 kg/t), without the need to change the spraying nozzle. Only very few continuously operating systems fulfill this criterion.
g) Workplace safety
Some MIs can be harmful to human health. Specific measures must therefore be taken to protect workers from inhaling substances or having skin contact. Despite all personal protection equipment, it is certainly best to prevent MIs from escaping into the workplace environment.
Tightly sealed charging and discharging fittings are crucial. The use of two-way nozzles, operating with pressurized air support, should always be avoided. The best indicator of how well a system works is the percentage of recovery of the MI in the final product, since anything not recovered must be assumed to have escaped into the environment.
One will rarely find a system that meets all challenges in a 100% satisfactory manner. However, systems that work with rotating nozzles come very close to it.
Such systems feature a wide and flexible proportioning range with a uniform spraying pattern without the need for nozzle exchange. The "nozzle" is actually a rotary disc, spinning at high speed, thereby atomizing any liquid into millions of tiny droplets without a carrier. The feeding of this disc-nozzle can be pressureless if needed, or requires only very low pressure at the supply pump (0.2 bar). Thus, delicate MIs such as live lactobacilli or yeast can be sprayed.
Several different liquids, regardless of compatibility and their physical properties (low or high viscosity), can be sprayed simultaneously. Provided the systems are designed as a closed and sealed device, pollution-free operation is possible, since it neither requires high pressure for the liquid nor any pressurized air. Such systems should be able to work with pellets, crumbles, mash or distinct extrudate shapes.
The rate of the dry feed flow can be monitored volumetrically with a proportioning screw (CV 5%) or gravimetrically with a continuous weigher (CV 2%). The selection of the proportioning device depends on economical considerations. For mass production, adding common MIs with common accuracy, a volumetric dosing system will certainly do the job. High-end products like pet food extrudates might require the more expensive and more accurate gravimetric dosing device.
On the liquid dosing side, a piston pump offers a CV of 3%, and the installation of a gravimetric mass flow meter brings the CV down to 1%. Basically, dosing accuracy (CV 5% to 7%) is determined by the CVs of the dry and liquid feeding devices. Now, systems with rotating nozzles are available in different sizes, accommodating the needs for pelleting lines (20 to 40 tph) and load-out plants with 80 to 100 tph throughput.
Flexible formulation management requires the use of a seemingly ever-increasing range of liquid feed additives. Addition of these substances to feeds may pose some challenges. However, there are now PPA systems available that address most of the issues.
There are no easy answers to the frequently asked questions: "How many liquid feed additives will there be in the future?" and "How much liquid can a common feed contain before it suffers disintegration?" Certainly, there are limits to how much liquid a feed can absorb, depending on feed composition and treatment. On the other hand, the feed additive industry is constantly trying to make its additives more stable to withstand the stress of processing.
In the future, we might see a balanced portfolio of dry thermo-stable and liquid feed additives at the disposal of the feed manufacturer. Dry or liquid, pre- or post- pelleting addition of micro-ingredients has not only technical significance but also major economic implications.