F. Majoros
Fish Culture Research Institute
Szarvas, Hungary
1. INTRODUCTION
2. CEREALS
3. TYPES OF DRIERS AND SOME DRYING TECHNOLOGIES
4. STORAGE OF FEEDS
5. MACHINERY FOR PREPARATION OF COMPOUND FEED
6. IMPORTANT POINTS IN THE PREPARATION OF MEALY MATERIALS
7. TYPES OF MILL CONSTRUCTION
8. AUXILIARY PARTS OF THE FEED MILL
9. MACHINERY FOR TRANSPORTATION
10. REFERENCES
Fish feed - particularly in extensive pond fish culturing practices - is of plant origin, of which cereal grains form 60-100 percent.
2.1 Cereal Harvesting and Treatment
2.2 Moisture Content of Cereals
2.3 Storage of Cereals
2.4 Chemical Treatment
In modern agriculture cereals are harvested mostly with combine harvesters, which reap, thresh and clean the grain. However, during the harvesting of cereals such as maize the grain often has a high moisture content which requires further treatment and conservation, so as to store it without any loss.
The most frequent treatment is drying, during which a physical change takes place, i.e., the decrease of moisture content, and biochemical processes occurring within the cells of the grains.
For optimal Storage the moisture content of cereals should not exceed 12-13%. A significant respiration starts above 15% moisture content, which impairs the quality of the grains (due to biochemical processes). The deterioration can be as high as 20-30%.
Respiration is markedly influenced by temperature. The higher the temperature the more intensive respiration becomes, and the greater the carbon dioxide generated.
This is illustrated as follows:
|
Cereal Moisture Content |
Aerial Temperature |
Carbon dioxide generated |
|
|
18 |
1.4 |
|
14-15 |
30 40 |
7.5 20.0 |
|
|
52 |
248.0 |
If the humidity of air reaches or exceeds 74%, the maize starts to develop mould. An ideal condition of storing is a moisture content under 14% and a relative humidity of air under 70%.
To approach this ideal condition, the following should be noted:
(i) Only dry cereals should be harvested, if possible.(ii) The following temperature limits should be considered when using hot-air drying:
|
- feed wheat |
82°C |
|
- bread wheat |
65°C |
|
- malting barley, seed grain |
43-48°C |
Air at ambient temperature may also be used for drying.(iii) Chemical conservation is applied.
2.3.1 Vacuum storage
2.3.2 Storage with ventilation
2.3.3 Cold storage
During vacuum storage microbiological processes are inhibited, even at a moisture content of 17-22%. However, certain anaerobic bacteria and yeasts are still able to survive and multiply under these conditions. This method of storage is generally employed in silo systems.
Air is pumped through air tubes into the heap of cereal which results in water and heat loss, and as a result temperature decreases.
Storage time can be lengthened, in this way drying can be replaced.
2.4.1 The freshly harvested grains are treated and conserved with propionic and formic acid and then hermetically covered. pH value is 2-3, which inhibits any microbiological process.
2.4.2 The cereals can be conserved after preliminary drying. A 80% dry matter content can be reached with good specific energy utilization.
If any material with 20% moisture content is conserved with organic acids, we can save energy and obtain a relatively 'dry' good feed.
Convection drying is carried out with hot-air ventilation. If the ventilated air contains some smoke or gases it may impart an off-flavour to the grain.
There are driers where the heat is transmitted from incandescent wires, hot tubes or from the hot walls of the drier.
In contact driers the cereal is placed on the heated floor.
Cereals can be dried with heat generated directly into the material via an electric current, electric field, induced whirling current or chemical reaction.
Driers can be designed for periodic or continuous operation, according to their requirements.
Bulk feed cereals, leguminous crops, coarse meals or green plant meals can be stored.
These feeds can be stored in buildings and barns or in the open air, covered with canvas or plastic foil. Apart from manual collection, a shovelling machine or screw feeder can be used. The latter is of a higher capacity, being 40-60 t/h.
Bulk raw materials or ready-made feed can be stored in silo systems made of concrete or insulated metal plate. Polyurethane foam is inserted between two metal plates, or a prefabricated 'sandwich' panel is used.
Blowing ventilators are used for the ventilation of modern storing silos, which- not only cools the stored material but also decrease the moisture content.
Cereals can be stored at 20°C for 2-3 weeks; 15°C for 4-5 weeks; and at 10°C for 10 weeks without any deterioration in quality.
The bottom of these silos can he flat, or of conical form. Emptying of smaller, 20-80 m3 silos with conic bottoms is achieved by gravitation, while for larger silos by using built-in pulleys.
Other equipment which may be necessary for the storage of cereals include: loading hoppers, conveyor belts, trucks or dump trucks, and dumpers.
Compound feeds are complete animal nutriments which contain different basic animal or plant materials, mineral and synthetic ingredients.
Preparation of compound feeds starts with the treatment of materials:
(i) Cleaning of grain - sifters are used only if the harvest is not modern or the corn contains some impurities of other origin.(ii) Extracted meals - oil-seed cases are very often utilized.
In these processes the cakes or pellets are crushed with cogged-wheels, or spiky iron cones.The material is then conveyed on to a sifter which saves energy since the grinding of the tiny parts is unnecessary.
(iii) There are several ways of preparing mealy materials:
- the material is conveyed into the silo by pneumatic pipes;- a precompound material can also be prepared from mealy substances, e.g., bran and fish meal is mixed 1:1, resulting in a less greasy and more easy to grind material;
- the mealy substance can be poured directly into the mixer (mostly premixes, concentrates or complementary feed).
In the loading hopper there should be a protective screen of 20 × 20 mm mesh size which is necessary not only to avoid accidents but also to prevent paper, string, rubber, etc., from getting into the system.
Next to the hopper a magnet should be built in. Iron pieces mixed into the substance can cause serious problems and loss in time and production.
From the hopper the material is conveyed to a control sifter. The control sifter can be flat, which is more durable, or cylindrical, which needs smaller space, but damage is more frequent.
The materials from the sifter are transported to the storage silo by a pneumatic conveying system.
The coarse particles are usually ground again or collected separately and used. One of the most important machines used in compound feed preparation is the mill.
The mixing homogeneity, the possibility to granulate, and the quality and digestibility are largely influenced by the size of the substances.
Grinding can be performed:
(i) with two solid surfaces
(ii) by pumping the material onto a solid surface
(iii) by whirling without any solid surface.
7.1 Types of Feed Mixing Apparatus
7.2 Preparation of Pellets
7.3 Types of Driers
Stone mills - the most ancient type of mill, needing constant maintenance and a lot of space. Their importance is decreasing.
Roller mills - originally used by the mill industry. They produce a lot of coarse, but little fine meal. They are not used in preparing compound food.
Combined mills - they are used mostly in Western Europe. They subject the substance to a pressing effect.
Plate grinders - they are used mostly in the U.S.A. The substance proceeds in a zig-zag way to the centre of the plates, being crushed in the process.
Pinned grinders - here the substance bumps into rotating pins - use not widespread.
Beating cross grinders - are similar to hammer mills, but the hitting parts are stable. The advantage is that the parts can be replaced easily. In the filling tube there is a magnet.
Swing - hammer mill - is the most widespread in the feed mill industry. It is very simple, the hammers being fixed on a rotor, and screens fixed. It is very practical as the parts can be easily replaced. Driving is with a V-shaped belt or with shaft coupling. Hammers can be easily changed and the mill can be connected to a pneumatic grain transporting system.
The generally used mesh size of the screen is 3.1-5.5 mm. In fish feed milling it is 1-1.5 mm.
Based on tests in grinding three types of substance, the following is known:
- grains - easy to grind
- resilient material - mostly their peel is difficult to grind, e.g., lupine
- greasy, smearing substances, e.g., fish meal, cannot be ground at all in themselves.
Pre-smashed, ground and sifted substances are conveyed to individual storing silos prior to formulae mixing. Mixing is very important, since even the smallest amount of a compound feed should reflect the proportions of the formula. Accurate measuring of the components has an equally great role.
Measurements can be periodic or continuous, and controlled manually or automatically.
The most modern is the punch-card system controlled machinery, furnished with a safety instrument, which immediately stops the operation if the automatic scale makes a mistake, or if the manual portioning (e.g., in the cases of premix or medicated pellets) does not comply with the designated formula.
From the storing silos the material is transported via conic pulleys with different speed rates.
These parts are also essential for accurate measurements. In the meantime the mixing of components starts in the balance tank.
Requirements:
(i) the balance tank should be easy to empty, and provided with a vibrator or automated closing valve(ii) measurements should be accurate
(iii) the programme should be easily adjustable
The preweighed materials enter into the mixing tank through a flapping valve. The homogeneity of mixing depends on:
grain size
measurements
type of grinder
duration of mixing
Diffuse mixers - are mixing drums where the vessel rotates and the centrifugal forces are used for mixing. These are generally continuous mixers. In their improved versions baffles are mounted on the shaft.
Gravitation mixers - are used in the mill industry and in air-current vertical mixers. The most widely used mechanical mixers are:
- Vertical mixers: made up of a tank and a vertically located pulley within. The pulley can be stable or mobile.- Horizontal mixers: mixing is performed with the mixing parts. They can be paddle or belt pulley types or a combination of the two.
These counter-current mixers work with an accuracy of 1:100 000. The 'Lödige' type mixers should be mentioned here, where even mixing is ensured by plough-iron elements and rotating knives.
These types are suitable for mixing liquids, meat-paste, oils, grease, and are an ideal mixer in fish feed preparation.
Fluidizing mixers - besides the effect of air-current and floating, the grains are also affected mechanically.
During mixing not only dry components are used, but liquids, e.g., fat and molasses. These are to improve the energy content, taste and durability of the pellet. They are generally stored in heatable, insulated tanks with a double wall. The heating is solved with a spiral pipeline with steam or hot water. Duration of mixing is 3-5 minutes.
The most important feature of the pellet is durability, crushing strength and water-resistance.
The following factors ensure the stability of a grain:
- solid adhesive medium, e.g., powdered milk, sugar, or carbamide, developed by steaming or heating of starch- connection of interface strengths of liquid between the grains
- adhesion - capillary suction effect between two grains.
There is a buffer tank between the pelleting machine and the mixer. From here the feed goes into the conditioning block of the mill by gravitation.
This conditioning block works as a continuous mixer suitable for feeding both liquids or steam. The quantity of steam or liquid material is controlled manually. The most frequent pelleting method is "thread pelleting", where the material is pressed through a die. In modern mills the presser-rollers are stable and the die revolves; the gap between them being adjustable. Pelleting machines with high (10-25 t/h) capacity have three rollers. To change the dies within these types of mills is difficult and their life-time is short. The mill is prevented from overloading by a shear pin.
At the adjusted length, the pellet is cut by knives. The warm, wet and soft pellet from the mill should be dried and cooled. In the cooling system, the air stream alters the steam pressure, and results in water withdrawal and cooling of the pellet. The cooling process should last until the temperature of the pellet and its surroundings is the same.
|
Vertical: |
their capacity is high and they need relatively small space. |
|
Horizontal: |
the advantage of these driers is that the belt moves and not the grain, which does not crumble. If the pellet layer on the belt is 15-20 cm thick, it needs 16-20 min to dry. A 4-6 cm thick layer of pellets generally needs 5 min to dry. These are then sifted to select the broken pellets, which go back to the pellet press. The intact pellets are packed in sacks or shared in silos. |
If fry feed is manufactured, and the pellet emerging from a 3-4-5 mm die is still too large and has to be crumbled with rollers and then sifted to the appropriate size range.
- rotating portioning chambers which are to prevent the accumulation of material -they generally have six blades;- portioning screws - with adjustable speed and lead;
- dust separators;
- dust sucking devices are to prevent losses from crumbling.
- gravitation runways;
- scraping conveyor;
- bucket conveyor;
- pneumatic conveyors with sucking, pressing or combined types.
The pneumatic transportation is influenced by the specific weight of the material, the size of the grains and by the flotation speed limit of the material.
ADCP 1980, (Aquaculture Development and Coordination Programme), Fish feed technology. Lectures presented at the FAO/UNDP Training Course in Fish Feed Technology, held at the College of Fisheries, University of Washington, Seattle, Washington, 9 October-15 December 1978. Rome, FAO/UNDP, ADCP/REP/80/l1:395 p.
Csávás, I., F. Majoros and L. Váradi, 1979, Technology of pellet feed manufacturing for warm-water fishes in the experimental fish feed mill of the Fish Culture Research Institute, Szarvas, Hungary. Schr. Bundesforschungsanst. Fisch., Hamb., (14/l5), Vol. 2 :75-86
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