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Chapter 14. Stability of Micro-Ingredients in Fish Feed


G. L. Rumsey
Tunison Laboratory of Fish Nutrition
Cortland, New York


Diets for various fish species have undergone rapid development in the last decade (most commercial fish feed is in pelleted forms). It is, therefore, only natural that the effects of mixing and pelleting on the more unstable constituents of dry-type feeds are getting increasing attention.

It is the general practice of fish feed manufacturers to fortify feeds with vitamins, minerals, preservatives, and sometimes pigmenters and antibiotics. These additives are used in such small quantities that they are generally termed micro-ingredients. The use of micro-ingredients in manufactured feed represents an increase in the feed manufacturers responsibilities regarding premixing, product stability, quality control, etc. Vitamin B12, for example, might be added at as low a level as 5 mg representing a dilution of 5 ppb. If such micro-ingredients were added directly to feed, uniform distribution would be impossible in commercial feed mill practice. The problem becomes even greater when ten or even twenty of these ingredients are added to an individual feed. On the other hand, it also portrays the increased opportunities to the feed manufacturer to render additional service to the fish raiser. Today's typical fish food formulae may contain from 20-30 different micro-ingredients.


2.1 Particle Size
2.2 Flow Characteristics

2.1 Particle Size

In selecting a commercial source of a micro-ingredient, one must consider carefully the question of particle size. At the usual low levels of supplementation, there are very definite upper limits on particle size if one is to ensure adequate distribution in the finished feed. Minute intakes of very small fish also demand uniform dispersion of all ingredients. However, a correlation exists between stability and exposed surface per unit weight. A balance exists between distribution requirements and ingredient sensitivity which contra-indicates pulverizing. When comparing sources of micro-ingredients one should ensure that the average particle size of the active micro-ingredient is optimum in relation to the level of incorporation in the feed and its stability.

2.2 Flow Characteristics

Active ingredients should be free-flowing. In some cases a fine particle size is essential and extra-blending is mandatory, followed by light milling to remove balls and lumps. Particle shape should correspond to that of the diluents. Micro-ingredients which acquire static electric charges require extra care in handling. This can often be minimized or eliminated by using different forms of the ingredients where similar choice can be exercised with normally hygroscopic micro-ingredients to avoid caking. Often such tendencies toward moisture pick-up will be satisfactorily reduced in a finished premix, particularly when a suitable conditioning agent is added.


3.1 Considerations of Stability
3.2 Pelleting of Feeds and Micro-Ingredient Stability

New problems have arisen with the switch to pelleted feeds for fish. The physical process of pelleting a composite of ingredients has raised many questions concerning the stability of the more labile ingredients and/or nutrients. This concern is increasing as techniques employing more steam, thicker dies, and higher pellet temperatures are attempted.

The term "stability" has been widely used and is sometimes abused. Often it is defined in terms of analogies. One of the most useful states: "Stability is a state of condition of matter which resists change". In applying this definition to the subject of interest, it appears that interest lies in any force or stress, or other action which could conceivably change the nutrient level and/or biological activity in finished feeds.

Before going into the myriad of factors affecting micro-ingredient stability, it will be necessary to distinguish between two types of instability.

It is often concluded that an active ingredient has low chemical stability when, in fact, faulty manufacturing and assay procedures result in apparently low activities in the sample. The number of factors or combination of factors which might cause such mechanical or physical instability are almost limitless. Some of these factors are discussed below.

3.1 Considerations of Stability

3.1.1 Separation

Any finished feed which can separate into two or more of its component parts through the action of such stresses as dusting, vibration, screening, and generation of static electrical charges is said to be physically unstable. If this property is not recognized, analytical procedures will indicate irregular and, almost invariably, low assays for one or more active ingredients.

3.1.2 Under-mixing or over-mixing

If too much time is omitted from the mixing cycle, in homogeneity in the final mix will produce irregular assay results. Over-mixing, on the other hand, leads to actual destruction of sensitive micro-ingredients.

3.1.3 Improper analytical procedures

Occasionally, a physically stable and well-mixed feed or concentrate will be submitted to a laboratory for one or more active-ingredient assays. Abnormal values are sometimes reported because old or improper reagents were used, the approved procedure was not followed, or mistakes were made in calculations. This type of error is difficult to detect except through repetition of the assay, change in analytical personnel, and/or change of laboratory conducting the assays.

In each of these aforementioned cases, the first reaction would be to condemn the active ingredient as being unstable. Therefore, it is absolutely essential that all factors of physical or mechanical instability be carefully eliminated before charging such deficiency to chemical instability or biological inefficiency, or both.

3.2 Pelleting of Feeds and Micro-Ingredient Stability

The very nature of feed pelleting seems to render it almost immune to precision and standardization. The operation itself might be considered as variable since it is generally very difficult to duplicate conditions and results from day to day, and sometimes even from run to run.

Major variables of feed pelleting include:

3.2.1 Feed uniformity

Non-uniformity of pellets and crumbles may be very critical for trout having low feed intakes. Appropriate milling of bulk ingredients and adequate premixing of micro-ingredients are generally helpful in eliminating particle separation. The use of ingredients, such as fish solubles or oils, helps retard particle separation.

3.2.2 Mash conditioning

Moisture (steam or water) is added in varying amounts to most feeds before pelleting. It is apparent that increasing the moisture content of feed products by steam or water, will accelerate the decomposition of sensitive micro-ingredients. Since this, apparently, is an irreversible process, the use of hygroscopic forms of additives should be avoided as much as possible.

3.2.3 Pelleting temperatures

Temperatures inside the die significantly exceed those in the hot pellets as they leave the die. Die temperature often exceeds 120°C and extruded pellets may reach 92°C. Elevated temperatures accelerate decomposition of micro-ingredients by increasing,- chemical reactivity.

Animal fats, wax coatings, and several vitamins melt under these elevated temperatures. Many substances are known to be most unstable near their melting point (e.g. vitamin C).

Some factors that influence pelleting temperatures include:

(a) mash composition, fat and mineral levels;
(b) die size, thickness, age, polish;
(c) temperature and quality of steam added;
(d) production rate;
(e) time feed is in the die; and
(f) hardness of the pellets produced.

3.2.4 Pellet pressures

Pelleting pressures are thought to run between 5 000 and 10 000 psi (300-600 kg/cm2). The use of such pressure can cause deformation and crushing of certain crystalline micro-ingredients. Wax coatings and stabilized fat beadlets can be obliterated. Reduced particle size increases the exposed surface area of ingredients with resultant greater susceptibility to decomposition. Furthermore, the compression simultaneously places sensitive additives in more intimate contact with other incompatible ingredients.

3.2.5 Feed time in die

The next pertinent question is how long the feed is actually in the die and subjected to these severe conditions. Data indicate that with good production rates, the holding time in the die is a fraction of a second.


After deciding on a source of each of the needed micro-ingredients, attention must be directed to diluents. Any diluent used should maintain or improve the stability and physical properties of the active components (micro-ingredients). Most feed formulators attempt to match the micro-ingredient densities to those of the diluents whenever possible. This is very desirable, although not a completely reliable procedure to eliminate separation. The shape and surface texture of the micro-ingredient and diluent particles can often be most important. Better premixes can often be prepared by employing a blend of several diluents rather than with just one. It is a rare, but happy, coincidence when only one diluent is required. In most cases, it is necessary to employ a predetermined ratio of three or more diluents for best results. One or more diluents may require pre-milling to obtain a desirable particle size range. The micro-ingredient, depending on its own particle size and density, may be blended before or after diluent milling. Finally, it is generally desirable to employ diluents having definite nutritive value (e.g., calcium carbonate, soybean meal, etc.).

A number of additional factors influence the usefulness of diluents. Most micro-ingredients exhibit optimum stability in the pH range of about 5.5-7.5. The pH value becomes more important with increasing moisture levels in blends. Proposed formulations should be carefully checked for this property. The presence of natural or added fats may create storage problems if rancidity develops due to inadequate stabilization. The off-flavours and odours resulting may affect palatability, and peroxides and carbonyls can reduce vitamin potencies as well as destroy pigmenters (e.g. oxy- and keto-carotenoids). Trace minerals can be troublesome, especially in the presence of high moisture levels, to many of the water and fat soluble vitamins.

Unprotected fats rapidly oxidize in the presence of trace minerals and moisture to form peroxides which can, in turn, decompose fat-soluble vitamins. It is essential to employ diluents which have low moisture equilibrium and that are relatively non-hygroscopic. If carriers have pronounced hygroscopic tendencies, there may be caking, potency loss, increased degradation due to trace metals, and possible mould growth with attendant overheating during summer storage. Inorganic diluents are generally ideal from the moisture equilibrium standpoint, but they seldom can be used exclusively because of high density. Commonly used organic carriers, such as soybean meal or distillers grains, are often pre-dried. The finished supplements are generally packaged in multiwall paper bags having a vapour and water-proof laminated layer.


A great deal of thought should be devoted to problems involved in handling micro-ingredients (Table 1). A case in point concerns the potency at which these are purchased. Some of the common B-vitamins can be purchased in almost totally pure forms, for example, niacin. Others are purchased in more dilute forms; for example, vitamin B12 Still others are purchased in protected (stabilized) forms to preserve biological activity; for example, vitamin A.

In general, the question of whether to buy ready-to-use premixes, or concentrated ingredients to make one's own premix, is usually resolved on the basis of experience, safety, and economics. If it is decided to purchase the required premixes, these can usually be custom-made so that they can be utilized without further premixing. Generally, the premix should be designed to be used at a rate of five pounds or more per ton of finished feed to avoid mixing problems. In the final analysis, the amount of premix that should be used depends upon the capabilities of the equipment available and the operating procedures employed in a specific feed mixing operation. It is foolhardy to assume that an operation which is successful in one mill will be equally successful in another.

Table 1 Vitamin Loss Due to Processing. Storage, and Water Leaching 1/

Ascorbic acid, %

Pantothenic acid, %

Folic acid, %

Thiamine, %

Pyridoxine, %

Processing loss 2/






Processing and storage loss (6 months)






Processing, storage and water leaching (10 sees) loss






Total (%. of original level)






1/ Practical-type diet using conventional feedstuffs
2/ Immediately after processing

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