In-depth analysis of twin-screw pet feed extruder

in the grains will be vaporized in an instant and a strong explosion will occur, and the water molecules can expand about 2000 times. The huge expansion pressure not only destroys the external shape of the grain, but also breaks the internal molecular structure of the grain, and shortens the insoluble long-chain starch into water-soluble short-chain starch, dextrin and sugar.

As a result, the insoluble substances in the puffed food are reduced, and the water-soluble substances are increased.

2. Equipment

According to the number of screws, it can be divided into: single-screw extruder (relying on the friction between the material and the barrel wall to push the material), twin-screw extruder (two screws are arranged side by side).

According to the processing steps, it can be divided into: dry puffing machine (one-step puffing), mixed puffing machine (two-step puffing, the raw materials are pre-heated and pre-cured)

3. Factors affecting the puffing effect

Raw materials: type, moisture, physical state, chemical composition (quantity and type of starch, protein, fat, sugar), pH.

The type and physical parameters of the extruder: the number of screws, the geometric size and speed, the inner diameter and length of the cylinder, the compression ratio, and the helix angle.

Extrusion process: time, pressure, temperature, die diameter, shear force.

4. Advantages and disadvantages and principles of puffing treatment on feed

Pros: Proper puffing can reduce anti-nutritional factors and improve the digestibility and utilization of feed protein, oil, carbohydrate and fiber.

Disadvantages: Excessive expansion leads to the loss of nutrients (amino acids, vitamins, etc.).

Principle: maintain a balance between reducing anti-nutritional factors, improving nutrient digestibility, and reducing nutrient loss.

5. Changes in the physical and chemical properties of raw material nutrients during the puffing process

Starch raw materials

Appropriate processing of starchy raw materials can improve the digestibility of pet food. The ease of processing varies with the varietal source of starch or starchy grains.

Corn and tubers tend to expand easily, and when used as the main source of starch, the test weight of their pet food products is generally low. In contrast, wheat and rice or flour require more power to achieve similar test weights and yields. Even within the same grain variety, there are differences in its machinability. Soft wheat and its by-products puff more easily than hard wheat, which contains more protein.

protein raw material

Uncured canola protein is one of the main ingredients in most pet foods and is partially soluble in water. Such proteins will have ductility after being subjected to conditions such as temperature, moisture and residence time in the extrusion system, forming a good envelope layer outside their cell structure, and at the same time losing water solubility due to denaturation. Once a protein loses water solubility, it loses many functional properties and even becomes inactive, ie it is difficult to reproduce its ductility. Therefore, many available animal protein raw materials have little effect on product expansion due to their low protein solubility, and may even hinder product expansion.

Many plant-based proteins are highly water-soluble and can be processed into textured protein products by extrusion curing. The bulk density of this product is controlled by the extrusion process, which is very similar to the bulk density of starch-based materials. Due to the different processability of feed ingredients, mixing starch-based ingredients with protein-based ingredients can produce a variety of quality products.

During the extrusion and puffing period, if the content of starch raw materials in the product is high, it is most conducive to the puffing of the product, followed by vegetable protein, while animal protein does not promote puffing, and even hinders it. Therefore, in those formulations that limit the amount of starch due to high protein content, the starch-based material must be pure starch or a strictly selected grain to ensure optimal expansion. If the starch content cannot be brought to the level required for proper puffing, a vegetable protein such as wheat gluten or soy flour should be added to promote puffing.

Fat or lipid material

In addition to enhancing the chewiness and toughness of feeds similar to proteins, fats or lipids also have lubricity and elasticity. When the total fat content is less than 8%, the puffing performance of pet food can be enhanced, but when the fat content is higher than 8%, the dough strength and puffing performance will be weakened, and the texture of the product will become soft. When producing high-energy pet food with a processed fat content of 17% or more, it is recommended that the fat be added to the product by spraying from the outside after the product has dried and cooled.

The source of the fat also has an effect on the puffing rate, with fats provided in the form of specific raw materials having less effect on puffing than fats added in the form of pure or almost pure fats, such as the effect of providing fat in full-fat rapeseed on puffing less than pure rapeseed oil.

Adding 0.5% to 1% monoglycerides to pet food formulations can eliminate rough surfaces, reduce fine dust, and give products a more pronounced shape. In addition, monoglycerides can form complexes with amylose and proteins, thereby reducing the viscosity of many extruded products.

Crude fiber raw material

Substances such as cellulose and hemicellulose in crude fiber are relatively less active components in the feed. When their content accounts for more than 15% of the total weight of the mixture, the puffing performance of the product will be weakened.

Ash raw material

Low ash feedstock improves product puffing properties and reduces metal wear on extruded parts. From a nutritional and health perspective, the selection of low-ash raw materials is particularly important for the production of cat food.

Most pet food formulas contain cellulose, minerals, salt, vitamins, colorants, emulsifiers, etc. The increase in the content of these ingredients or raw materials will reduce the puffing performance of the product.

Vitamins - the loss rate is 15-100%, which can be supplemented by using high temperature resistant raw materials or puffing

Fat-soluble vitamins: Vitamin D and K have better stability to heat and oxygen (expansion) than A and E.

Vitamin B1: High temperature, high screw speed and high pH can reduce the retention rate of material B1, and high moisture and high feeding speed can improve the retention rate of material B1.

Vitamin B2: High temperature and high moisture have no effect on the stability of B2, and high screw speed reduces the retention rate of material B2.

Minerals - improve the utilization of plant phosphorus, but no benefit to other mineral elements.

Moisture - reduce

So far, 70% of aquafeed production in the United States and 100% of aquafeed in South Korea have used extrusion processing technology. In my country, more and more feed processing enterprises are gradually replacing traditional pellet mills with extruders to produce aquatic feed and other special feeds. In the current Chinese aquatic feed market, because most aquatic feed production enterprises are limited by funds, they usually choose domestic single-screw extruders with lower prices. In aquatic feed processing, it is used to produce lower-grade freshwater fish floating feed, and it is difficult to produce sinking feed and other aquatic feed products. Although some imported single-screw models can meet the production of sinking feed products, their prices are too high, and they have no good cost performance. When we choose extrusion equipment, we should not only see that the price of twin-screw extruder is higher than that of single-screw extruder, but should also consider the comprehensive cost performance and product production cost. The technical performance of the domestic twin-screw extruder has reached a certain advanced level. Although the price is higher than that of the domestic single-screw extruder, it is far lower than the price of the imported single-screw extruder, and its production performance has surpassed that of the imported one. The single-screw model can completely replace the imported twin-screw model, which can not only meet the needs of aquaculture feed in the aquaculture industry, but also achieve high cost performance. It should be a good choice for the current aquaculture feed processing industry. With the emergence of large-scale, large-capacity and special-purpose twin-screw models, the use of twin-screw extruder extruders can achieve greater benefits, and can give full play to the wide range of raw materials used in extrusion and puffing processing technology. High degree of gelatinization, water stability is much higher than ordinary pellet feed, product density is adjustable, can process buoyant, slow-sinking, fast-sinking feed and high-moisture, high-fat feed, loose structure of pellets is conducive to post-spraying It has unique advantages such as avoiding the loss of heat-sensitive raw materials, preventing environmental pollution and preventing and resisting diseases, improving the nutritional value of feed, and reducing production costs.