Stevia Is What?

Stevia rebaudiana, also known as stevia or sugar grass, is a species of the Compositae family native to the subtropical regions of South America. It was successfully introduced for cultivation in China in 1977. Stevia contains 14 trace elements and 32 nutrients, making it an excellent source of both sugar and nutrition. Stevia rebaudiana's roots, stems and leaves contain steviol glycosides, which have the characteristics of high sweetness (200-300 times that of sucrose) and low calories (only 1/300 of sucrose)[1]. As a result, it has attracted increasing attention and is regarded as the most promising new sugar source[2,3].

China is a country with a shortage of sugar. At present, annual per capita consumption of sugar is less than 7kg, which is a significant difference compared to European and American countries, where annual per capita consumption is 40-60kg[4]. Due to China's national conditions of a large population and scarce land, and with the adjustment of the industrial structure, the acreage of sugar crops is decreasing year by year[5]. Steviol glycosides alleviate the problem of sugar shortage in China with their high sweetness. The sweetness of the sugar produced from 1hm2 of stevia is equivalent to that produced from 6–7hm2 of sugarcane or 11–12hm2 of sugar beet. Moreover, stevia requires less land, which can greatly save arable land. Stevia can also be used in medicines, fertilizers, feed, etc., and is a crop with high economic value that is comprehensively developed and utilized.

1 Stevioside

1.1 Types and characteristics of the sweetening components of stevia

The main component of dried stevia leaves is stevioside [4], which accounts for about 10% of the content. Stevioside is a kind of sweet-tasting terpene glycoside extracted from dried stevia leaves. It is a white powder; its molecular is C38H60O18 and its molecular weight is 803. Stevioside is easily soluble in water, but not in propylene glycol or ethylene glycol. It is hygroscopic in the air and loses 1.5% to 4.0% of its weight when dried. It has a significant synergistic effect when mixed with sucrose. Steviol glycosides also have good heat resistance and are not easily decomposed by light. The sweetness remains unchanged after heating at 95°C for 2 hours, and even after heating for 8 hours, the sweetness loss is minimal. Steviol glycosides are stable in a pH range of 3 to 9, and remain unchanged after heat treatment at 100°C for 1 hour. It has good salt tolerance, no Maillard browning, and is not assimilated or fermented by microorganisms, so the shelf life of stevia products can be extended and they are easy to store. It has very low calorific value, and its caloric value is basically close to zero.

So far, eight glycosides with different sweetness have been isolated from Stevia [6]: (1) stevioside (St), (2) stebioside (SBio), (3) rebaudioside A (RA), (4) rebaudioside B (RB), (5) rebaudioside C (RC), (6) rebaudioside D (RD), (7) rebaudioside E (RE). (8) Dulcoside A (DA). Among these glycosides, the ones with high content and economic value are St, RA, RC, and DA. In particular, RA is well received both in terms of sweetness and taste quality. The content of sweet-tasting glycosides is generally expressed in terms of the amount of total stevioside. Stevioside is the general term for the various sweet glycosides contained in stevia, i.e., total glycosides. The stevioside currently extracted through industrial production in China is a mixture of multiple glycosides, of which the glycoside components can be roughly divided into the following three categories [7]:

(1) Stevioside: Colorless crystals, 270 to 280 times sweeter than sucrose, the main component of stevia products, accounting for 50% to 70% of the mixture. It has a good taste quality with a long, slightly bitter aftertaste.

(2)Rebaudioside. This glycoside can be divided into RA, RC, RD, RE and other four types, among which RA is a colorless crystal with a sweetness 450 times that of sucrose. It has a pure sweetness similar to sucrose and is the sweetest part of the stevia test product. RC is a colorless crystal with a sweetness less than 50 times that of sucrose and a bitter taste. RD is a colorless crystal with a sweetness 150 times that of sucrose and a bitter taste. RE is 100–150 times as sweet as sucrose. RA accounts for about 15–20% of the total glycoside in the dried leaves, RC accounts for about 10–15%, and RD and RE account for a smaller proportion.

(2) 

(3) Dulcoside: This glycoside can be divided into DA and DB. DB and RC are the same substance, and DA also accounts for a relatively small proportion of the total glycoside in the dried leaves. Its sweetness is about 150 times that of sucrose.

In general, steviol glycosides are the main component of stevia products, followed by RA. However, the higher the RA content in a product, the purer the sweetness and the more popular it will be with consumers. Therefore, if we want to improve the flavor of existing stevia products, we must find a way to increase the RA content in stevia products.

Currently, there are three basic forms of industrially produced stevioside [8]: (1) a light brown crude extract that is 50% pure; (2) a yellowish brown, high-grade product that is 80% to 90% pure; and (3) a white powdery, high-grade product that is more than 90% pure.

1.2 Stevioside extraction and separation process

Steviol glycoside extraction methods [9] usually include alcohol extraction, adsorption, maceration, resin, and molecular sieve methods. The most widely used method is the resin process. Although this method has a certain processing capacity, it is still a traditional method. The total yield of glycosides can reach 85%, and the product content is 90%. However, the production cost is relatively high, so reducing production costs and improving product quality are urgent problems that need to be solved in the steviol glycoside extraction process.

The crude stevioside obtained by extraction needs to be further concentrated and purified. There are currently two relatively advanced processes: one is the complete reverse osmosis method, and the other is the tandem concentration method of a reverse osmosis device and a heat evaporator. In terms of concentration time, the two are basically the same. However, after the process is improved, it is easier to operate, the quality of the product is improved, it is not prone to moisture absorption, and the phenomenon of caramelized sticky walls rarely occurs. The trial production of these two methods has achieved good results, fully meeting production requirements, and the product quality is relatively high.

The product obtained by the above method is a mixture of various glycoside components, and further separation of glycoside components can be carried out. The currently reported separation techniques are: (1) high performance liquid chromatography (HPLC) [10,11]; (2) droplet countercurrent chromatography (DCCC) [12]; (3) thin layer chromatography (TLC) [1 3,14]; (4) recrystallization method [15]; (5) supercritical extraction method [16, 17]; (6) capillary electrophoresis method [18]; (7) resin adsorption separation and extraction method, etc.

The use of resin adsorption separation [6] to extract and purify stevia has the advantages of being fast, economical, stable and long-lasting, and is currently an indispensable process in the purification of stevia. The adsorption of stevia extract components by macroporous adsorption resin has a certain degree of selectivity. In recent years, the Institute of Polymer Chemistry at Nankai University has been working on the design and synthesis of a macroporous affinity adsorption resin with high adsorption selectivity for stevioside, and studying its separation effect on the eight glycosides of stevia, in an attempt to use the selective adsorption of the resin to extract and separate stevia products with high Rebaudioside A content.

1.3 Flavor improvement of stevioside

Since stevia leaf extract is a mixture of eight types of diastereoisomers, its taste and sweetness vary. Stevioside and rebaudioside C have a certain aftertaste, which seriously affects the flavor of stevia. For this reason, scholars at home and abroad have conducted a lot of research and improvement work.

Stevioside, rebaudioside A and rebaudioside C are the main components of stevia, accounting for more than 90% of the total. Rebaudioside A has a high sweetness and a taste close to that of sucrose, making it an ideal sweetener. Stevioside and rebaudioside C have a strong aftertaste, which greatly affects the taste of stevia and limits its use. Therefore, steviol glycosides, rebaudiosides and steviosides, which have a strong aftertaste, should be modified and converted to improve their taste quality. In recent years, many studies have reported the use of enzymatic transglycosylation to convert steviol glycosides and rebaudioside C into oligoglucose ligands to improve their taste quality. There have also been studies on improving the taste using esterification, bacterial microorganism and blending methods, which have effectively improved the taste of stevioside.

2 Medicinal effects

Stevia has certain pharmacological effects, including controlling blood sugar, lowering blood pressure, promoting metabolism, treating diabetes, obesity, regulating gastric acid, and relieving nerve fatigue [19]. Stevia stems are extracted with activated water at 45–60°C, and concentrated at this temperature to prepare healthy drinks and breast lotions for livestock to treat mastitis, promote health, and treat stress-induced diseases, chronic bronchitis, pneumonia, liver dysfunction, hepatitis, and ovarian dysfunction [20]. Stevia can increase the appetite of livestock, racehorses and pets and treat their chronic diseases and cattle infertility, which has attracted the attention of foreign animal husbandry and feed workers[21].

3 Used in fertilizers and animal feed

In addition to stevioside, dried stevia leaves also contain a large amount of non-sugar components such as protein, fat, cellulose, ash and nitrogen-free extractives.

Stevia leaf residue is an industrial waste[4], but it is a very good organic fertilizer. It has a very high organic matter content and contains Ca2+ and Fe2+, which can improve soil fertility. Second, when well-rotted stevia leaf residue is mixed with a basic substrate, it makes the best vegetable seedling soil for melons, watermelons, citrus fruits and tomatoes. It can promote seedling growth and development, increase the dry matter weight of seedlings, promote early maturity and increase sweetness.

It is a very good seedling substrate and is also low-cost. Third, adding stevia leaf residue to the cultivation substrate for fungi can meet the nutrient needs of edible fungi as well as their requirements for various trace elements, vitamins and air permeability. As a result, the fungi grow quickly, mature early, have good texture and high yield. In particular, white fungus grows big and white; cultivated enoki mushrooms have a slightly sweet and unique flavor. Fourth, stevia leaf residue can be used as poultry feed at a rate of 5%, which can prevent diarrhea in poultry, regulate their digestive function, and increase egg production. Fifth, stevia leaf residue can be mixed into feed and used to feed dairy cows and goats, which can increase the sweetness of the milk, improve the quality of the milk and the content of trace elements and amino acids in the milk, and have a certain effect on milk production.

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