In recent years, plant extracts, as a green, pollution-free and residue-free feed additive, have received widespread attention due to their certain antioxidant, immunomodulatory and growth-promoting effects. Studies have shown that plant extracts not only can selectively inhibit the growth of exogenous harmful bacteria, but also can ultimately achieve the purpose of improving intestinal function and enhancing host immunity by regulating the composition of intestinal microbial flora.
The extraction process of plant extracts: By using physical extraction or extraction methods, chemical reagent soaking method, and biological fermentation method to treat the entire plant or part of the plant material, resulting in liquid or solid substances with the effective components' structure unchanged are called plant extracts.
In recent years, plant extracts have been used as animal feed additives due to their advantages such as stable content, safety, high efficiency, no pollution, no residues, and no development of drug resistance. The sources of the studied plant extracts mainly include medicinal plants including Chinese herbal medicines, fruits, and vegetables. Commonly used plant extraction methods include traditional water extraction method, alcohol extraction method, as well as microwave-assisted method, ultrasonic-assisted method, acidification method, enzymatic hydrolysis method, and supercritical carbon dioxide (CO2) extraction method. These methods have both advantages and disadvantages. The traditional method has the common advantages of simple operation and low cost, and for the water extraction method, it also has the characteristics of safety, environmental protection, and maximum protection of the structure of polysaccharides. For the alcohol extraction method, it is easier to achieve industrial production, but both of these methods have the disadvantages of long extraction time, large solvent dosage, low extraction rate, and high impurity content in the extract. Other more efficient extraction methods also have their own advantages. Steam distillation method, in addition to having the advantages of the water extraction method, also has the advantages of easier access to distillation equipment and easier separation of components from water. However, during the extraction of active components, heat-labile components such as polysaccharides are easily destroyed, and volatile oils are prone to hydration reactions with water, resulting in a change in flavor. In addition, the water separation instrument is expensive, and it generates high biological oxygen demand wastewater. Microwave-assisted method has the advantages of short extraction time, high solvent utilization rate, and high extraction efficiency, but it causes greater damage to the cell structure, leading to solvent residue and changes in the structure of polysaccharides. Moreover, the microwave equipment is expensive, and it is currently limited to laboratory research. Ultrasonic-assisted method retains the advantages of the microwave method and does not damage the structure of active components, and it has a low extraction temperature and low energy consumption, making it suitable for extracting polar and heat-unstable components. However, the current ultrasonic equipment capacity cannot achieve industrial production. Supercritical CO2 extraction method has the advantages of both the microwave method and the ultrasonic method, effectively preventing the oxidation and escape of heat-sensitive substances, and because there is no solvent residue, the extracted raw materials can be used as feed or for extracting other components. However, this method will cause loss of physical properties due to high pressure and has extremely high investment costs and high safety requirements. Although acidification method results are reliable, the large amount of extract liquid makes concentration difficult, and the water-soluble impurities are numerous, requiring purification and enrichment. Enzymatic hydrolysis is another method with a low extraction temperature and the mildest extraction conditions, fast reaction speed, and easy removal of impurities. However, this method is costly, has high equipment and technical requirements, and has significant limitations. The main bioactive components in the plant extracts obtained by the above methods include volatile oils, polysaccharides, flavonoids, phenols, alkaloids, triterpenoids, organic acids, saponins, and plant tannins, which are mostly relatively stable secondary metabolites. Their chemical structures often contain groups such as phenols, ethers, terpenes, and ketones. Although these organic functional groups have different chemical properties, they can interact to maintain the health of the organism. The sources and extraction processes of active components in plant extracts reported by different scholars in recent years are shown in the table
