Stevia rebaudiana is a perennial, short-lived herb of Compositae family, which originated in the Amambai Mountains in the northeast of Paraguay, bordering Brazil. Local residents often use it to make healthy stevia plant drinks. China began to introduce and cultivate Stevia rebaudiana in the 1970s. The production areas are mainly concentrated in Shandong, Xinjiang, Jiangsu, Anhui, Fujian and other provinces, and the planting area is increasing year by year. At present, the average annual production of Stevia rebaudiana in China is about 40000 tons, which has become the largest producer and exporter of Stevia rebaudiana.
Stevia rebaudiana contains a variety of primary and secondary metabolites, such as protein, lipids, vitamins, mineral elements, steviol glycosides, chlorogenic acids, flavonoids, auzelanin, pyrogallols and other substances [2]. It not only has high nutritional value, but also has good biological activity and medicinal value. It is not only a high-quality dual-use resource for medicine and food, but also a natural extraction source of a variety of bioactive substances.
1. Stevia glycoside
Stevioside compounds are the most abundant functional substances in Stevia rebaudiana, accounting for 4-20% of the dry weight of Stevia rebaudiana leaves. Stevioside is a tetracyclic diterpenoid compound. It has been found that there are more than 40 different steviosides in Stevia rebaudiana, mainly including Stevioside, Steviol, Steviol disaccharide, ABCDEF, Stevioside, dulk glycoside, raspberry glycoside, etc. Figure 1 shows the molecular structure of Stevioside and rebaudioside A. Stevia glycosides are non nutritive natural sweeteners that are 200 to 300 times sweeter than sucrose. They do not cause elevated blood sugar levels when consumed, and have advantages such as low calorie content, non enzymatic hydrolysis, and good thermal stability. They can be used as a natural substitute for sucrose.
As chronic diseases such as obesity and diabetes caused by excessive sugar intake continue to increase, people's demand for a variety of sugar reduced health food continues to increase. In this context, as a high-quality "sugar substitute", Stevioside has considerable application value in the development of new sweeteners and sugar free and low sugar foods. Compared with natural sweeteners with extremely low content in plants such as Xylitol, glycyrrhizin, monelin, Curculin, Neohesperidin, or with the poor taste of artificial sweeteners such as saccharin, Aspartame, Sucralose, which have a metallic aftertaste, Stevioside is cheap and easy to obtain, with simple extraction process and wide application scope, and has gradually become a new favorite in the sweetener market.
In addition to being a natural sweetener, the pharmacological activities of various stevia glycosides have gradually been discovered. The study found that Stevioside and rebaudioside A have the effects of reducing blood sugar, promoting the secretion of insulin and Glucagon, reducing blood pressure and improving immunity in diabetes patients.
Stevioside is easily soluble in water and alcohol solvents, so water or ethanol is generally selected as the solvent to extract stevioside from Stevia rebaudiana. At present, in addition to the traditional methods such as cooking, fermentation, cold water immersion, there are also more emerging methods such as microwave assisted extraction, ultrasonic assisted extraction, high-pressure assisted extraction, Supercritical fluid extraction, etc. Research has found that among traditional methods, steaming has the highest extraction rate of stevia glycosides, followed by cold water immersion and fermentation. Compared with traditional methods, emerging methods have varying degrees of improvement in the extraction rate of stevia glycosides. For example, the extraction rate of stevia glycosides using ultrasonic assisted extraction method is 12.78%, which is 10.27% higher than traditional steaming method.
2. Chlorogenic Acids
Chlorogenic acid is a natural component of phenolic acid, which is formed by dehydration and condensation of Quinic acid/quinic acid and trans Cinnamic acid (Caffeic acid, P-coumaric acid, Ferulic acid, etc.) through enzymatic esterification reaction. It is another major class of bioactive substances in Stevia rebaudiana. Chlorogenic acid is distributed in the whole plant of Stevia rebaudiana, and the content in the leaves is the highest. 24 Hydroxycinnamic acid derivatives of Quinic acid and Shikimic acid have been detected in stevia rebaudiana. Animal experiments have found that chlorogenic acids have a variety of activities such as bacteriostasis, anti-oxidation, anti-virus, blood pressure reduction, and are known as "plant gold". Among various beneficial effects, antioxidant and anti-inflammatory are one of the most important roles of Chlorogenic acid, which is closely related to the phenolic hydroxyl structure in its molecule. The results showed that the scavenging capacity of Chlorogenic acid was higher than that of vitamin E; It is also reported that Chlorogenic acid can improve alcohol induced liver injury by scavenging reactive oxygen species.
3. Flavonoids
Stevia rebaudiana leaves also contain a variety of flavonoids, mainly rutin, flavonol and its derivatives, Quercetin and its derivatives, kaempferol and its derivatives, Apigenin and its derivatives. At present, more than ten Flavonoid have been isolated from Stevia rebaudiana, and different Flavonoid have different biological activities, generally manifested as antioxidant, antibacterial, anti-virus, anti radiation, anti-tumor, improving the body immunity and other pharmacological activities. Research has found that the total stevia flavonoids extracted by microwave have a scavenging rate of 91.39% on DPPH free radicals; Another study found a significant dose-effect of total stevia flavonoids on DPPH free radical scavenging ability.
4. Stevia leaf protein
Leaf protein is a kind of high molecular material with 17-18 amino acids, of which 8 are Essential amino acid. Stevia rebaudiana contains about 13% crude protein. The early use of Stevia rebaudiana is mainly to extract Stevioside and other active substances, and the use of leaf protein is very little. Leaf protein is a high-quality protein with high lysine content, which can effectively supplement the lysine intake restriction that occurs when Chinese people rely on grains as their staple food. Stevia rebaudiana leaf protein extract does not contain cholesterol and can be used as a protein supplement for patients with fatty liver, cardiovascular disease and other diseases. However, due to the fact that plant leaf proteins are often green or grayish white in color and have a certain grassy odor, it is necessary to improve the extraction process to improve their appearance and taste, making them more economically valuable.
Application situation
Stevia extract is widely used in food, beverage, chemical, pharmaceutical and other industries due to its good biological activity and medicinal value. For example, stevia sugar itself has been added to beverages, candies, desserts, and other products as a sweetener instead of sucrose. The glucose based steviol glycoside obtained by glycosylation of steviol glycoside has better sweetness than natural steviol glycoside, and there is no problem with the post bitterness in natural steviol glycoside; The antioxidant activity of Chlorogenic acid can be used in cosmetics formulations. Chlorogenic acid has been added to many products in terms of urease resistant cosmetics, anti-aging skin care products, ultraviolet protection and hair dyes. Chlorogenic acid with antibacterial activity can also be used as a natural preservative; The various beneficial effects of Flavonoid have become the darling of the health product industry, and are widely used to enhance the body's resistance to oxidation. The development and application of stevia leaf protein is still relatively small, but its potential as a dietary supplement and nutrition enhancer is very significant.
In addition to the above main bioactive substances, the by-products such as filter residue and filtrate produced by Stevia rebaudiana extraction
There is a large amount of cellulose, vitamins, natural pigments, and alkaloids, which also have high utilization value. If this part of resources can be comprehensively utilized, it can not only reduce the pressure of waste on the environment, but also improve the added value of products, so that the Stevia rebaudiana industry chain is more complete and has better social, economic and environmental benefits.
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