1. Extruded puffed fish material
Extrusion ripening is a combination of water, pressure, temperature and the role of mechanical shear to complete. Extrusion ripening, the temperature in the machine boring up to 90 ~ 200 ℃, extrusion duration in the range of 2 ~ 30 seconds extrusion products will undergo a series of physical and chemical changes, such as starch pasting, protein denaturation, enzymes, toxic components and microbial inactivation.
Processing of puffed fish feed requires the injection of steam and water, the material should contain at least 20% starch, starch mainly plays the role of binder and energy material. Extrusion products temperature before the ring die should be 125 ~ 138 ℃, 34 ~ 37 atmospheres (33 ~ 36kg / cm2), 25 ~ 27% water; extrusion, the weight should be 320 ~ 400g / L, moisture 21 ~ 24. extrusion out of the die when the loss of 3 ~ 4 percentage points of water, on the moment of extrusion out of the die, the sudden release of pressure so that the superheated water into steam and extrusion The water content is reduced and many small bubbles are generated to reduce the density, so that the extruded feed can float. Further drying of the fish feed (usually below 10% moisture) also improves floatability. The capacity of 480g/L is identified as the turning point of pellet feed floating and sinking, below this capacity is floating, above this capacity is sinking.
2. Extruder type and structure
There are many types of extruders on the market, according to the screw composition can be basically divided into single-screw extruders and twin-screw extruders. Single-screw extruder can be divided into wet-ripening extruder, dry-ripening extruder and cold-forming extruder. Extruder generally includes four major parts, including feeding system, modulator, extruder boring and cutting device. The feed system has a variable speed feed screw that feeds the raw material into the pre-moderator, where the operator can monitor the feed through high and low level indicators and a visual glass window. The modulator is filled with water and heat in the form of steam, and the entire modulation process lasts 5 minutes and can complete 40 to 50% maturation. The extruder boring includes the head and screw, the role of the screw is to push the material and control the ripening process in the extruder. The feed section of the screw to advance the material, usually deeper screw blade; the middle section of the screw mixing and kneading materials; the end of the screw gradually become thin, so that the material out of the mold to make the final product.
3. Making sinking puffed feed
When making sinking puffed feed, inject water into the modulator (no injection or less steam injection), the ring die should be 26-30 atmospheres (25-29kg/cm2), extruded material should contain 28-30% water, extrusion capacity is 450-550g / L, temperature 120 ℃, moisture 26%. The warm maturation extruder uses the die head with air release port, and the dry extruder uses the secondary extrusion die head, which can reduce the temperature, moisture and expansion rate of the extruded product, and smoothly make sinkable feed. The die head with air release port is close to the die plate, so it can also be used when vitamins, colors and flavor enhancers need to be added to the extruded material to avoid excessive dunning. The sinkable fish feed should contain 10% starch and 12% fat, and the final extruded product should be dried to 10%-12% water; excessive drying will cause the sinkable feed to float.
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