Changes of material composition during extrusion
starch
With the increase of extrusion strength, the gelatinization degree of starch will also increase. The degree of degradation of these macromolecules is also affected by extrusion factors such as temperature, water content and screw speed. These extrusion factors lead to a series of physical and chemical changes in the final product, and also lead to changes in its digestibility. Starch can be divided into amylose and amylopectin, and they show different characteristics during extrusion. The ratio of amylose to amylopectin in starch affects the microstructure of extruded products. Amylopectin can promote puffing, making the product light and crisp; On the contrary, the products made of starch with high amylose content or starch from tuber plants are hard in texture and less puffed. The higher the amylose content in starch, the smaller the expansion index of the puffed product.
protein
Under general extrusion conditions, it means low temperature, high water content and low screw speed, the nutritional value of plant protein usually increases, which is mainly due to the structural modification of the first and second grade structures of protein and the denaturation and inactivation of protease inhibitors originally existing in plant foods. Under severe extrusion conditions, namely high temperature, low water content and high screw speed, the digestibility of protein and the utilization rate of amino acids will decrease. One of the main reasons is that Maillard reaction leads to the decrease of amino acid utilization. Lysine is a restricted amino acid in grains, and the decrease of its utilization rate will immediately lead to the decrease of the nutritional value of protein.
fat
Extrusion may reduce the nutritional value of fat, and its mechanisms include oxidation, hydrogenation and cis-trans isomerization. After extrusion, the fat content will decrease with the formation of amylose-fat complex. The ratio of unsaturated fatty acids to saturated fatty acids will decrease and trans fatty acids will increase. But this change is too small to have a significant impact on nutritional value.
Other ingredients
With regard to the changes of cellulose during extrusion, the consensus is that extrusion can significantly increase soluble dietary fiber, improve its physical and chemical properties and storage properties, and produce a micronization effect. This is mainly due to the chemical bonds between cellulose and lignin molecules cracking after extrusion under high temperature, high pressure and high shear force, and the polarity, chemical characteristics and biochemical characteristics of the molecules are changed.
In the process of extrusion, sugar is in a melting state. If the temperature is too high, it will easily lead to caramelization, which will affect the sensory quality of the product, and even lead to machine blockage in severe cases. During extrusion, the loss rate of vitamins in food increased with the increase of sleeve temperature, the decrease of material moisture content and the residence time in sleeve.
Application in food industry
The application of extrusion technology in food processing can be divided into two categories: one is to use sweet potatoes, rice and other potatoes and grains as main materials, which are puffed to form loose and porous products, and then after dehydration and frying, various delicious seasonings are added on the surface to make puffed snack foods suitable for all ages, such as glutinous rice fruit and puffed shrimp strips. The other kind is sandwich puffed snack food, and its production method is: when the puffed product is extruded, fillings such as chocolate, cheese, sugar and the like are injected into the tube previously formed by the corresponding mold, so as to produce puffed sandwich food. This kind of sandwich puffed snack is not only fluffy and crisp, but also can be infused with various kinds of flame materials to give it the flavor of chocolate, fruit, ice cream, etc. In addition, extrusion technology is also used in food such as dietary cereal breakfast, infant food and flavoring agent.
Extrusion technology has been successfully applied in vinegar, soy sauce, rice wine, beer and other fermentation industries. The use of extruded cereal materials is very beneficial to the fermentation process. Macromolecules such as protein and starch are degraded into small molecules such as soluble sugar and amino acids, which are exactly what yeast needs in the initial stage of fermentation. At the same time, after extrusion, the structure of raw materials is changed, the contact range with enzymes and yeasts is greatly increased, the enzymatic reaction is accelerated, and the dosage of yeasts and enzymes is reduced, thus shortening the fermentation period.
Because of the edible oil produced by extrusion, expansion and leaching. It has the advantages of high oil quality, fast oil leaching speed and low energy consumption. At present, most oil production factories have used extrusion pretreatment technology, and related enterprises are considering whether to popularize extrusion pretreatment technology as a standard production technology.
Application in feed industry
Extrusion technology is applied to feed industry, mainly used to produce urea feed for processing pet food and ruminant protein supplement. It has incomparable advantages over traditional processing methods in processing special animal feed, aquatic feed, early weaned piglet feed and development of feed resources. The application of extrusion technology in feed industry has greatly increased the utilization rate of resources, high production efficiency and reduced cost. The obtained feed has comprehensive nutrition and good product shape.