HERBS: Stevia (rebaudiana Bertoni)

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Stevia, Nature's Natural Low Calorie Sweetener

 

Stevia is a plant that produces a variety of high-potency low-calorie

sweeteners in its' leaf tissue. Stevia sweeteners are in used in food

products a number of countries including Japan, Brazil and China. Here

in North America, the movement of society towards more natural foods has

created a great deal of public interest in stevia.

 

Stevia (rebaudiana Bertoni) is one of 154 members of the genus Stevia.

It is a member of the Compositae family and as such is related to

sunflower, marigolds etc. Stevia has an alternate leaf arrangement and

herbacious growth habit with flowers arranged in indeterminate heads.

The flowers are small and white with a pale purple throat. The pollen

can be highly allergenic. Stevia is self-incompatible and probably

insect pollinated, the seeds are small, have very little endosperm and

are dispersed in the wind via a hairy pappus.

 

Stevia, as it is commonly known, is native to valley of the Rio Monday

in North Eastern Paraguay and is found on the edges of marshland on acid

infertile sand or muck soils. The conquistadors had reported stevia to

the Spanish during the 16th century but it remained in obscurity until

it was again brought to the attention of Europeans in 1888 by M.S.

Bertoni. Prior to any European discovery, it had had long to been known

to the indigenous Guarani peoples native to that region. The Guarani

called it caá-êhê, meaning sweet herb and used it to sweeten bitter

drinks such as mate.

 

The sweet compounds found in stevia leaves are diterpene glycosides

(steviol glycosides) and are synthesized, at least in the initial

stages, using much the same pathway as gibberellic acid, an important

plant hormone. The steviol glycoside and gibberellin pathways diverge at

kaurene. In stevia, laurene is converted to steviol, the " backbone " of

the sweet glycosides, then glucosylated or rhaminosylated to form the

principle sweeteners. The precursor compounds are synthesized in the

chloroplast, then from there are transported to the endoplasmic

reticulum, Golgi apparatus and then vacuolated.

 

The purpose of these compounds in the stevia plant is not yet clear, but

their high concentration in the leaf and the conservation of the pathway

within the species would indicate that, at some point in evoluntionary

time, their presence conferred significant advantage upon those

individuals that possessed them. Some researchers feel that they act to

repel certain insects and others speculate that it is an elaborate means

of controlling levels of gibberellic acid.

 

The four major steviol glycosides are:

stevioside

rebaudioside A

rebaudioside C

dulcoside A

 

Two other glycosides that may be present in plant tissue are

rebaudioside D and E; rebaudioside B has been detected but is probably

an artifact formed during isolation. The normal proportions (w/w) of the

four major glycosides are: stevioside 5-10%, rebaudioside A 2-4%,

rebaudioside C 1-2% and dulcoside A 0.5-1%. They range in sweetness from

40 to 250 times sweeter than sugar. A number of stevia genotypes with

anomalous glycoside proportions have been reported in the Korean and

Japanese scientific and patent literature. It has long been known that

rebaudioside A has the best sensory properties (most sweet, least

bitter) of the four major steviol glycosides. Steviol glycosides are

heat and pH stable, non-fermentable and do not darken upon cooking and

therefore have a wide range of applications in food products.

 

On the whole plant level, steviol glycosides tend to accumulate in

tissues as they age, so that older lower leaves have more sweetener that

younger upper leaves. Since chloroplasts are important in precursor

synthesis, those tissues devoid of chlorophyll, like roots and lower

stems contain no or trace amounts of glycosides. Once flowering is

initiated glycoside concentrations in the leaves begin to decline.

 

Biosynthetic Pathway for Synthesis of steviol glycosides

 

The steviol glycosides are synthesiszed via melvalonic acid in the same

way as all isoprenoid compounds. The steps up to the formation of

ent-kaurenoic acid are identical to those used in the synthesis of the

plant hormone gibberellic acid. We have cloned and sequenced the copalyl

pyrophosphate synthase gene from stevia that is responsible for the

conversion of GGPP to CPP. The hyroxylation of ent-kaurenoic acid at

the the C13 position to yield steviol is the point of divergence.

Following the formation of steviol glycan side chains containing glucose

and/or rhamnose are added to the C13 alcohol and C19 carboxylate groups

to form the various steviol glycosides.

 

Stevia has a basic chromosome number of n=11. We have constructed a

genetic linkage map of the genome using 183 RAPD markers that has

resolved the stevia genome into 21 linkage groups covering a total

distance of 1389cM.

 

 

If you need further in-depth information, please read the following

recent articles:

 

1. Stevia rebaudiana: Its biological, chemical and agricultural

properties by J.E. Brandle, A.N. Starratt and M. Gijzen

 

2. Heritability for Yield, Leaf:stem Ratio and Stevioside Content

Estimated from a Landrace Cultivar of Stevia rebaudiana by J.E. Brandle

and N. Rosa

 

 

 

Contact : Dr. Jim Brandle

FAQ Menu URL: http://res2.agr.ca/london/pmrc/english/faq/menu.html

 

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