Methods of Fish feed formulation
Pearson square method:-
In most fish diets, protein is the most expensive portion and is usually the first nutrient that is computed in diet formulation. The energy level of the diet is then adjusted to the desired level by addition of high energy supplements which are less expensive than protein supplements.
The Pearson square method is an easy way to determine the probable ratios of mixing different protein ingredient and to formulate a feed with desired dietary protein level.
For example, consider rice bran (with 8.2% protein) and soybean meal (with 44% protein) were available as feed stuffs to prepare a diet for carp with 27% protein, a square is constructed and the two feed stuffs with their protein values are put on the two left corners along with the protein content of each ingredient.
The desired protein level (27%) of the feed is placed in the middle of the square. Next, the protein level of the feed is subtracted from that of the feed stuffs, placing the answer in the opposite corner from the feed stuff. Ignore positive or negative signs.
Thus to make the 27 percent crude protein of carp feed, we must mix 17/35.8 of rice bran and 18.8/35.8 soybean meal.
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| Fish Feed Formulation: How to make your own fish feed |
Now, we have to calculate the proportion of mixing the ingredients by weight in relation to their protein content. This could be elucidated as given below.
That is, for the preparation of 100 kg of fish feed with 27 percent protein, 47.50 kg of rice bran and 52.50 kg of soybean meal are to be mixed.
If more than two feed stuffs are used in a feed, they may be grouped into basal feeds (CP < 20 percent) and protein supplements (CP > 20 percent), averaged within each group, and plugged into the square method. For example, suppose shrimp meal and corn were also available for the carp feed mentioned above. The crude protein levels of the shrimp meal (52.7 percent) and of corn (10.2 percent) are averaged with soybean meal and rice bran, respectively.
Basal feed = (21.35/39.15) x 100 = 54.53%
Protein supplement = (17.8/39.15) x 100 = 45.47%
Thus, to make 100 kg of this feed one would mix the following:
Rice bran 27.265 kg
Corn 27.265 kg
Soybean meal 22.735 kg
Shrimp meal 22.735 kg
The square method is helpful to novice feed formulators because it can get them started in diet formulation without the need to resort to trial and error. The square method can also be used to calculate the proportion of feed ingredients to mix together to achieve a desired dietary energy level as well as a crude protein level.
Linear Programming:-
The mathematical technique available to nutritionists for selecting the best combination of feed ingredients, to formulate diets at the least possible cost is linear programming. The following information is necessary for feed formulation using linear programming
Nutrient content and DE or ME of ingredients;
Unit price of feed stuffs including vitamin and mineral mixtures;
Any other additives to be used in the feed
Minimum and maximum restriction on the amounts of each ingredient in the feed
Least-cost linear programming software for diet formulation is readily available and the cost varies with the sophistication. A simple Lotus 1-2-3 spread sheet can also be utilized for formulating feeds, incorporating a smaller number of variables. It should be noted that least-cost feed formulation is not always practical for small scale aquaculturists using on-farm feed manufacture facilities where the choice of ingredients available is limited.
Quadratic programming:
Nutrient requirements used in linear programming feed formulation are usually fixed for maximum rate of growth. This may not be the best decision from economic point of view. Nutrient constraints may be relaxed to bring down feed cost while still achieving acceptable lower growth.
Quadratic programming takes into account the growth response within a range of nutrient constraint. Therefore, good understandings of biological response functions from actual feeding trials are essential in the use of quadratic programming. For example, inclusion level of arginine could be reduced by 20% with only a 5% likely reduction of growth of Nile tilapia. Thus high cost towards arginine supplement can brought down by hardly loosing likely growth of 5% in the fish.
Feed Manufacturing:-
The technology of feed processing has undergone substantial improvement in recent years. It was only sixty years ago that feed stuffs were mixed on the warehouse floor by the use of a shovel. Feed processing has progressed from the simple mixing of several ingredients by hand to mechanical mixing, to continuous mixing, and now to computer controlled mixing and pelleting. However, the basic concept of mixing ingredients together to result in a nutritionally balanced feed, has remained unchanged.
To accomplish the mixing of different ingredients, grinding these ingredients to similar particle sizes, and then putting them together in a single unit, requires a considerable amount of specialized equipment and technical expertise. Some feed plants are designed for specific functions, such as making poultry feeds exclusively; others are designed for producing a variety of feeds.
Methods of feed formulation and manufacturing
Receiving:-
The first operation in the feed processing plant involves the receiving of raw materials into the plant premises. Feed ingredients arrive in sacks, or other small containers, and in bulk.
Sacked ingredients are checked for identification and condition. They are then logged in after segregation of drugs and medications. Sacked ingredients must then be stored in a dry location with proper protection from rodent and insect infestation. Sacked stocks are then rotated to minimize staleness, product degradation, and insect infestation.
Bulk ingredients are handled according to their physical form. Liquid ingredients, such as oils and molasses, are generally stored in bulk tanks. Proper storage temperature is maintained and the filter screens are checked periodically. Solid bulk ingredients such as grains, oil meals, etc., are cleaned with a scalper to remove foreign material prior to storage in bins. Bin temperatures are monitored to prevent heating due to grinning respiration.
Methods of feed formulation and manufacturing
Processing:-
Material flow during processing includes:
- Particle size reduction,
- Premixing,
- Mixing,
- Pelleting, and
- Sacking.
Coarse ingredients pass over a permanent magnet which removes tramp metal and then through a hammer mill which reduces particle size to the desired screen analysis. Ground material is monitored periodically to ensure size uniformity and to help detect wear of hammermill screen and hammers. The ground material is then routed to ingredient holding bins.
There are two mixing operations in feed milling. One is for the mixing of micro-nutrients; the operation is generally termed pre-mixing. The other mixing operation involves the actual blending of all components of the diet.
Micro-nutrients, such as vitamins and trace minerals, are accurately weighed with carrier material which has a density approximating that of the predominant micro-ingredient. The materials are then mixed in a batch mixer for a period of time specified by the equipment manufacturer to ensure homogeneity. The premix is finally routed to the premix holding bin.
Diet mixing begins when augers are set in motion to deliver; the correct amounts of each ingredient including the premix, according to the formula, into the mixer. Where manual changing of the mixer is done, ingredients are weighed out in sacks or hopper carts. The mixing period is according to the equipment manufacturer’s specifications, but final mix is checked periodically with a tracer to ensure homogeneity of the mix. If the mixed diet is to undergo pelleting, it is routed to the pelleting bin.
Mixed feed mash for pelleting is first conditioned with steam in the steam conditioner section of the pellet mill, after which it enters the die where it is finally extruded. Freshly extruded pellets are hot and contain excess moisture which is removed during passage through the cooler. Fines are then screened from the cooled pelleted feed and returned for repeating. Fish oil, if added, is now applied prior to the routing of the finished pellets into the packer bins.
Processes:-
Feed manufacturing and the associated quality control program are keys to successful fish culture. Unless the fisheries biologist understands and specifies the activities of the feed mill and its laboratory, profitable fish farming will be a matter of chance.
Dry feeds may be ground, sifted, screened, mixed, compressed, expanded, textured, colored and flavored. By one or more of these processes, a wide variety of ingredients can be prepared into a standardized product. Since most fish have size and texture preferences and often react to color, odor, and flavor, processing research is an integral part of fish culture.
Grinding:-
- Hammer mills
- Attrition Mills
- Roller Mills
- Cutters
- Screening
Grinding or particle-size reduction is a major function of feed manufacturing. Many feed mills pass all incoming ingredients through a grinder for several reasons:
clumps and large fragments are reduced in size, some moisture is removed due to aeration, and additives such as antioxidants may be blended.
All of these improve the ease of handling ingredients and their storage.
There are other reasons for grinding and the associated sieving of ingredients in formula feeds before further processing. Small fish and fry require plankton-size feeds available in dry form as a meal or granule. Extremes in particle sizes are wasteful and often dangerous. Fry have been killed because of their inability to pass through the digestive system large pieces of connective tissue and bone present in dry animal byproducts, or hull fragments found in cottonseed meal and rice bran. On the other hand, dust or “fines” may become colloidal suspensions in water, so dilute that several mouthfuls carry little nutritive value.
The grinding of ingredients generally improves feed digestibility, acceptability, mixing properties, pellet-ability, and increases the bulk density of some ingredients. It is accomplished by many types of manual and mechanical operations involving impact, attrition, and cutting.
Hammer mills:-
Hammer mills are mostly impact grinders with swinging or stationary steel bars forcing ingredients against a circular screen or solid serrated section designated as a striking plate (Figure 1). Material is held in the grinding chamber until it is reduced to the size of the openings in the screen. The number of hammers on a rotating shaft, their size, arrangement, sharpness, the speed of rotation, wear patterns, and clearance at the tip relative to the screen or striking plate are important variables in grinding capacity and the appearance of the product. Heat imparted to the material, due to the work of grinding, is related to the time it is held within the chamber and the air flow characteristics. Impact grinding is most efficient with dry, low-fat ingredients, although many other materials may be reduced in size by proper screen selection and regulated intake.
Most hammer mills have a horizontal drive shaft which suspends vertical hammers but for some ingredients, such as dried animal byproducts, a “vertical” hammer mill is more efficient. In this mill, the drive shaft is positioned vertically and screens and hammers are positioned horizontally. Material successfully reduced in size to the diameter of screen holes or smaller, are carried by gravity outside the mill and thence by air or conveyor to storage in “make-up” bins. Over-size particles, not easily broken, drop through the mill and may be re-cycled or discarded. Thus foreign materials, such as metal and stones, are discharged before they are forced through the screen causing damage.
Attrition Mills:-
Attrition mills use the hammer mill principle to a certain extent; i.e., shattering by/impact. However, they also impart a shearing and cutting action. Grinding is done between two discs equipped with replaceable wearing surfaces. One or both of these discs is rotated; if both, they rotate in opposite directions. When one disc is rotated, and the other stationary, the assembly is used for shredding and deferring. Often materials which have been coarsely ground by other mills, are passed through an attrition mill for blending or smoothing out an ingredient or mixture containing liquids which may have clumps. The discs of an attrition mill are generally in a vertical position so that materials not capable of reduction can pass by gravity out of the grinding area.
Roller Mills:-
A combination of cutting, attrition, and crushing occurs in roller mills. These are smooth or corrugated rolls rotating at the same speed set at a pre-determined distance apart with material passing between the two. A tearing action may be added by operating the rolls at different speeds and by corrugations which are different for each roll; i.e., the top roll may have off-radial spiral corrugations and the bottom roll lateral corrugations. This last type, called a “Le Page cut” is used in making granules from hard pellets, as it provides a breaking surface without much impact to cause dust. Roll grinding is economical but limited to materials which are fairly dry and low in fat.
Cutters:-
Rotary cutters are a type of grinder which reduces dry particle solids mainly by shearing with knife edges against a striking plate. The mill also includes the processes of attrition and impact, although these actions are limited if the material is easily reduced by cutting and the screen limiting discharge has large perforations. The mill consists of a rotating shaft with four attached parallel knives and a screen occupying one fourth of the 360 degree rotation. The mill is best used to crack whole grains with a minimum of “fines”. It is not used as a final process for reducing the size of ingredients used in fish feeds.
Screening:-
Associated with grinding feeds for fish fry, a sieving system is required which classifies materials to any desired particle size. The “overs” in this system may be re-ground or rejected. The “through” may be selected to comply with fish preferences for size and mixed according to formula specifications. Feeds sifted through a 177-micron opening (a U.S. No. 100 sieve) have been successfully used for increasing survival and growth of minnows and catfish fry. Hammer mill or impact grinding of dry feeds, especially cereal grains, creates particles within the range called “dust”, and a dust-collecting system may be necessary to remove this. An excess of dust in the feed may lead to gill disease, a situation where organic matter adhering to gills becomes a nutrient for bacteria or parasites.
The problem of excess dust formed by grinding feeds may be partly alleviated by adding a spray of oil or a semi-moist ingredient, such as condensed fish solubles or fermentation solubles, on feeds entering the grinder. Dehydrated alfalfa is prepared as a dust-free meal, similar in texture to a sifted crumbled pellet, by spraying mineral oil into a hammer mill chamber during grinding.
Mixing:-
The objective of feed mixing is to start with a certain assortment of ingredients called a “formula”, totalling some definite weight. This is processed so that each small unit of the whole, either a mouthful or a day’s feeding, is the same proportion as the original formula. Mixing is recognized as an empirical unit operation, which means that it is more of an art than a science and must be learned by experience.
Feed mixing may include all possible combinations of solids and liquids. Within each ingredient are differences in physical properties. For solids there are differences in particle size, shape, density, electrostatic charge, coefficient of friction as represented by the angle of repose, elasticity or resilience and, of course, color, odor, and taste. For liquids there are differences in viscosity and density.
The term “mixed” can mean either blended, implying uniformity, or made up of dissimilar parts, implying scattering. As applied to formula feeds, the objective of mixing combines each of these definitions; i.e., the scattering of dissimilar parts into a blend. However, it is improbable that uniformity is attained with particles within a, sample arranged in some order of position or concentration. That is only a quality control; goal. It has been suggested that a proper title for a discussion of mixing should be “mixing and un mixing”, for during the operation there is a constant tendency of particles which have been mixed to become separated. Three mechanisms are involved in the mixing process:
(a) the transfer of groups of adjacent particles from one location in the mass to another,
(b) diffusion distribution of particles over a freshly developed surface,
(c) shear slipping of particles between others in the mass.
In general, the smaller and the more uni-formally sized the ingredients are prepared, the more nearly they will approach random distribution during mixing.
In many formulae, a decrease in particle size is necessary to attain a sufficient number of particles of an essential additive (vitamin, mineral, medication) for dispersion in each daily feed unit. This may require the particle size to be the diameter of dust, 10 to 50 microns. Certain ingredients are unstable in finely divided form and likely to acquire an electrostatic charge. Concentration of particles on a charged surface, roughness of the mixed and stickiness of oily and wet ingredients are factors in causing segregation when very small particles are mixed and when these are much smaller than the bulk of other ingredients.
Mixing may be either a batch or a continuous process. Batch mixing can be done on an open flat surface with shovels or in containers shaped as cylinders, half-cylinders, cones or twin-cones with fixed baffles or moving augers, spirals, or paddles. Continuous mixing proportions by weight or volume, is a technique best suited for formula feeds with few ingredients and minimal changes.
Horizontal Mixers:-
Continuous ribbon mixers:
The continuous or “twin-spiral” mixer consists of a horizontal, stationary, half-cylinder with revolving helical ribbons placed on a central shaft so as to move materials from one end to the other as the shaft and ribbon rotate inside. Capacity can be from a few litres to several cubic metres. The speed of shaft rotation will vary inversely as the circumference of the outer ribbon; usually optimum between 75-100 metres per minute. Since material travel is from one end to the other, either end may be used for discharge. These mixers may be inverted for cleaning.
Non-continuous ribbon mixers:
Non-continuous or interrupted ribbons are similar to the continuous ribbon mixers except that short sections called “paddles” or “ploughs” are spaced in a spiral round the mixer shaft. Action is different from that of continuous ribbon mixers, and may be more satisfactory for mixing liquids with dry solids. These mixers are made in a wide variety of sizes with travel of the outer diameter of paddles from 100 to 120 metres per minute.
Vertical Mixer:-
Vertical mixers may consist of a cylinder, cone, or hopper-shaped container, with a single or double screw (auger) located vertically through the center. The screw operates at speeds of 100 to 200 rpm and vertically conveys incoming materials from the bottom (generally the intake) end, like a screw conveyor, to the top where they are scattered and fall by gravity. This sequence is repeated several times until a blend is attained (usually from 10 to 12 minutes). These mixers may also be loaded from the top. Results show that vertical mixers are not efficient for uniform mixing of solids and liquids or for materials of quite different particle size or density. This unit is difficult to clean and there may be inter-batch contamination.
Pelleting:-
A control valve introduces dry steam into a header from which, through several port-holes, steam enters the conditioning chamber in contact with dry feed. Between this valve and the steam generator or boiler are a strainer and trap to remove condensate, providing only dry steam in the mill. At the discharge end of the conditioning chamber is a gate to restrict feed from immediately leaving and allows more time for moisture to be absorbed into the feed. A chute or funnel usually guides moisture-conditioned feed into the pellet chamber where compression and extrusion occur. If the ammeter goes much above the optimum reading, this chute may be quickly raised to prevent a choke-up of feed in the die holes. From the pellet chamber, feed may be directed by means of a butterfly valve to the cooler or on the floor for inspection.
Another variable that may be introduced into the operation of a pellet mill is the rotational speed of the die. For the production of small diameter pellets (i.e., 3 mm or less) high rotation speeds are used. This results in a thinner layer of soft feed inside the die ring ahead of the rolls, and for a given volume of feed the efficiency of pelleting and pellet hardness are improved. Die speeds may be changed by replacing the pulley on the main motor shaft of the pellet mill. Speeds generally range from 130-400 rpm. Feeds-of low bulk density are formed best in dies rotating at higher speeds.
Cooling and Drying:-
The temperature imparted to pellets in the process of their manufacture assists the removal of moisture by the air-drying process. Generally, within ten minutes after extrusion, hard pellets are cooled to ambient temperature and brought to a moisture content slightly above that of the entering soft feed. This may be done by spreading pellets in a thin layer on the floor and blowing air over them. Commercially, it is done by passing the hot pellets through a vertical or a horizontal chamber designed to bring air at ambient temperature into intimate contact with the outer surface of the pellets.
Crumbles:-
Cooled pellets may be ground on corrugated rolls and the resulting product sifted into various sizes of granules or crumbles. For small fish, the physical properties of crumbles are often more desired than a meal ration and easier to manufacture than a small pellet. Crumbles provide a multi-faceted surface to reflect light, this being a lure for sight feeders. They ensure that all the ingredients of a formula will be ingested, whereas components of meal feeds separate on entering water, allowing selection of certain
Screening or Grading:-
Some sifting is necessary in the production of pellets and crumbles. Small fragments (fines) are produced as hot, moist pellets are cut off from the die inside the pellet chamber, and as pellets pass through the cooling and conveying equipment. Fines may be returned to the pellet mill for reprocessing or used as feed for fry.
There are many types of sifting and grading systems both manual and mechanized. Most shake or rotate from side to side with material passing over screens of specified openings and covered to confine dust to the equipment. Sifting is the last process in manufacture of pellets and crumbles and this equipment should be located just above the bagging or final discharge bin.
Extrusion Pelleting:-
Steam pelleting has advantages over mash feeds. Extrusion pelleting has several advantages over steam pelleting
Extruder:-
The extruder can be defined as a bioreactor where the raw materials are subjected to high temperature and pressure over a relatively short time.
Feed ingredients are propelled along the barrel of the extruder by one or two screws.
The mechanical screw action and the friction it creates, blends, shears, and cooks the material. (Held at 120-175ºC for about 30 seconds).
At the end of the barrel, the mixture is forced through a die at high pressure; because of the difference the between pressure inside and outside of the barrel, the material expands.
This sudden drop in pressure causes the water in liquid phase to change to the gaseous state producing what is known as expansion.
As the steam escapes small voids are created which determine the cellular structure of the extrudate.
The more cells the final product has, the less dense it becomes, and hence becomes a floating feed.
If the cellular structure is more compact the final product is denser; therefore, heavier and can be considered a sinking feed.
Floating feeds have a density that is usually below 450 g/l, while sinking feeds have densities above 550 g/l.
Benefits of extrusion:-
The heat and pressure created in the barrel can destroy harmful organisms such as salmonella
The raw material is expanded; Starch is gelatinised and oil cells are ruptured (improved digestibility in young animals).
The heat and pressure deactivate destructive enzymes such as those that cause rancidity.
Increase availability of carbohydrates.
Neutralises growth inhibitors.
Increase availability of sulphur amino acids.Improves palatability.
Advantages Of Extrusion Cooking:-
Extrusion cooking has several advantages over pelleting.
Versatility:
Any type of feed can be prepared.
Gelatinized Starch:
Starch is the primary binder used in shrimp feeds.
In order to activate binding functionality, starch must be cooked.
In the presence of water and heat, starch granules swell, lose their crystalline structure, and become hydrated in a process known as “gelatinization”.
In pelletiser complete gelatinization does not occur within the temperature range of 55-85 C.. In extruder, because of higher temperature, pressure and shear, complete gelatinization occur.
Water stability:
Water stability of feed is very important to the shrimp farmer, as shrimps, are slow scavenger type bottom feeders.
The longer the feed holds together the greater the chance it will be consumed.
The average accepted stability is approximately 2 to 4 hr, however, extruded feeds have been produced with water stability of 12 hr without binders and 24 hr and longer with binders.
Sterilization:
An extrusion cooker eliminates bacteria in the raw materials.
A total bacteria plate count on an unprocessed shrimp feed formula had 1,200,000 microbes/g. A subsequent plate count of the extruded diet yielded 1,000 microbes/g. A general guideline is that feed mill products with a plate count of 10,000 microbes/g are considered pasteurized.
The animal and fish meals used in the formulations are generally the ingredients containing microbes. Formulations that are high in fish meal and meat & bone meal benefit greatly from extrusion cooking due to the reduction in the total bacterial count.
This helps maintain stable feed quality and may reduce disease occurrence in the cultured animal.
Density control:
Control of density of the finished extruded product allows the feed miller to produce products that float, sink or sink slowly so that the optimum feed can be produced for the intended species.
This is controlled by changing the expansion rate and pressure of the extrudate.
Mechanical durability:
Mechanical durability is important as any breakage or degradation of the product before use results in fines. This fine affects water quality and reduces feed conversion.
Extruded products have an internal matrix system, which tends to increase resistance to mechanical handling in screw conveyors, automatic feeders or simply bag handling.
Pelleted feeds generate 2% – 5% fines in the handling of the feeds in bulk or in bags. Extrusion cooked feeds produce approximately 1% – 2% fines reducing the amount of waste material that enters the water.
Fines are wastes because these particles are rarely eaten in a production pond. Decreasing the fines will increase the productivity and efficiency of the feed.
Storage Life:
Extrusion cooked feeds have a prolonged storage time. The reason is that extrusion cooking tends to stabilize the raw materials by reducing bacteria, reducing oxidation, oils do not go rancid and vitamins in extruded feeds are more stable than in pelleted feeds.
It is noted that overdosing of vitamins in an extruded diet is required due to heat liability of vitamins.
However, the advantage of the vitamin stability partially overcomes this disadvantage, as pelleted feeds also required overdosing to get acceptable shelf lives.