Botanic and sweet Components
Stevia rebaudiana Bertoni (Bertoni) is a perennial herbaceous plant and is part of the Asteraceae family. This family includes well known plants such as dandelion, sunflower and chicory. The plant was first described botanically by Dr. M.S. Bertoni in 1899. The lanceolate leaves are about 5 cm long and 2 cm wide and are planted crosswise, facing each other. In the wild, the height of the plant varies from 40 to 80 cm but when cultivated, the Stevia can become 1 metre high. Stevia can be grown on relatively poor soil. The plants can be used for commercial production for 6 years, during which five times a year a harvest takes place of the part of the plant that is above the ground (in Paraguay and Brazil). The roots remain in place and so the plant regenerates again. Plants which are 1 metre high have a dry weight of 70 g on average. The dry weight of the leaves can vary from 15 to 35 g per plant.
Origin and History
The history of the culture of Stevia mainly stems from Paraguay and Brazil. Originally Stevia only grew in the northern regions of South America. The plant has been known for centuries by the native Guaranay-Indians for the sweet taste of its leaves. They use it, amongst other things, to make "mate" herbal tea. Stevia is often described as "sweet herb of Paraguay" and is referred to as the "sweetest plant of the world". Such terms show the amazing power of this herb.
Europe first came in contact with the herb when, in the 16th century, the Spanish rulers learned of the "sweet honey herb" used by the natives of South America. In spite of the description of the plant by the Swiss botanist M.S. Bertoni in 1899, the research and commercial use of the plant had a slow start.
Around 1908 the presence of several sweeteners in Stevia was reported but it only became possible to crystallise stevioside in 1931. During World War II, the allies considered extracting stevioside commercially as an alternative for sugar supplies which were running out. Unfortunately, at that time the technology needed for industrial production was lacking. Because of the restriction on the use of artificial sweeteners, imposed around 1970 in Japan, the research in Japan for the commercialisation and utilisation of stevioside made quick progress. For over twenty five years now, Japanese consumers have been using the extract from the plant as a safe, natural, non-calorific sweetener. It is currently the most used sweetener on the Japanese and Korean market. The commercial production takes place mainly in Paraguay, Uruguay, Central America, The United States, Israel, Thailand and China
The sweet Components
Stevia rebaudiana (Bertoni) Bertoni contains a mixture of sweeteners in the leaves of which stevioside is occurring mostly in the highest concentration.
Fig. 1. Structures of stevioside and related compounds.
In rebaudioside D and E R1 is composed of 2 b-Glc-b-Glc(2-1). In rebaudioside A, B, C, D, E and F in group R2 an additional sugar moiety is added on carbon 3 of the first b-Glc. In rebaudioside F one b-Glc is substituted for by -b-Xyl. The sweetener content may vary between 4 and 20 % of the leaf dry weight depending on variety and growth conditions, but is around 10 % in most crops grown in the field. Stevioside is a steviol glycoside that is about 300 x sweeter than sucrose (0.4 % solution). It is a 100 % natural substance without calories. The biosynthesis of steviol, the aglycone of stevioside, follows the recently unraveled 2-C-Methyl-D-erythritol-4-phosphate pathway (MEP; Totté et al., 2000), as do all terpenoids formed in plant plastids. Up to the formation of ent-kaurenate the steviol biosynthesis is similar to that of the gibberellins, a group of plant hormones. In Stevia a 13-hydroxylase is present that hydroxylates ent-kaurenate at its 13-position to form steviol (Fig. 2). This is a unique hydroxylation and is a diversion from gibberellin biosynthesis in which ent-kaurenate is first hydroxylated at the 7a-position.
2: Synthesis of steviol from ent-kaurenate by 13-ent-kaurenate hydroxylase.
The steviol formed is immediately transglycosylated to form steviolmonoside, steviolbioside, stevioside and rebaudioside A (Fig. 3).
3: Transglycosidation of steviol to form steviolmonoside, steviolbioside, stevioside and rebaudioside A.
The advantages of steviol glycosides are manifold: it is non-toxic, without calories, heat stable, not fermentative, flavour enhancing, 100% natural, no aftertaste or bitterness, no cariogenic effects, non-addictive sweetener for children, leaves and steviol glycosides may be cooked, only small quantities are needed.