The plant Stevia Rebaudinia is the source of commercial Stevia products, embraced as a ‘sugar-free’ and ‘all-natural’ sweetener.
The plant’s major ‘sweeteners’ consist of two similar molecules: Stevioside and Rebaudioside.
How do plants produce these molecules – and when labs extract stevioside and rebaudioside with the use of dangerous solvents, are these ‘natural’ sweeteners still safe for human consumption?
Stevioside and Rebaudioside
Both molecules are in high concentrations in the leaves of the Stevia plant.
A primary difference between the two ‘molecules’ is in the substitution patterns of glucose groups at the top of the molecule.
Stevioside employs a glucose group and a hydrogen while Rebaudioside employs two glucose molecules.
So how does the Stevia plant synthesize the Stevioside molecule? A distinguishing feature of the molecule is the di-terpene skeleton in the ‘middle’ of the molecule.The di-terpene, ent-kaurene, is synthesized via the mevalonic acid cascade.
What is a Terpene?
The term terpene comes from the fact that the individual units, the terpene molecules, have a strong odor – the word itself comes from ‘turpentine.’ (If you’ve never smelled turpentine, don’t start now.)
Terpenes are a fundamental unit in all plants. As a class of compounds, terpenes come in many varieties: from beta-carotene and camphor to plant-based cholesterol. In fact there are, perhaps, 40000 different terpene-derived compounds.
Biosynthesis of Stevia
When chemists and biologists started to work with plants, they couldn’t deduce the inner workings as readily as they did with animals, so the exact steps of biosynthesis are something of a mystery.
In the case of the Stevia plant, what we do know is that the plant must first synthesize a di-terpene prior to the attachment of the glucose molecules. This is a fundamental principle that occurs in the plant kingdom—otherwise known by the term compartmentalization.
The bio-synthesis of Stevioside may start with the biosynthesis of the molecule known as ent-kaurene.
Presently, there are two different proposed pathways for terpene biosynthesis— the mevalonic acid pathway and a cellulose sugar pathway.
Mevalonic Acid Terpene Biosynthesis
In the first steps of the Mevalonic acid pathway for terpene biosynthesis, three molecules of acetyl coenzyme A condense to form (R)-Mevalonic acid.
After the acid forms, it further reacts to give mono- and di-phosphate of the pentanoic acid.
The resulting acid phosphate further condenses to (in chemical steps that are somewhat vague) to give the di-terpene.
As noted, the mechanistic details for the biosynthesis seems sketchy at this juncture.
However, a group of workers from the Himalayan Bioresource Center, India have seemingly found a ‘genetic’ pathway for the terpene biosynthesis. Through a technique known as functional genetic analysis, the workers studied the bifurcation pathway in the biosynthetic scheme.
Gibberellin Pathway or Stevioside Sugar?
After the plant synthesizes the ent-kaurene, there are two possible pathways that it may follow: (1) further reactions in Gibberellin pathway or (2) to Stevioside sugar pathway. The Gibberellin pathway fortifies plant reproduction through the production of healthier plants. A typical gibberellin is shown below: (note the similarity to ent-kaurene above).
The ground-breaking work by the group in India suppressed the synthesis of Gibberellins by silencing the genetic pathway responsible for Gibberellin biosynthesis.
The process may be summarized as follows: the molecular mechanism of the specific genes that are responsible for Stevioside synthesis are “amplified” by the silencing (or blocking) the pathway responsible for the Gibberellin synthesis.
Extracting the ‘Sweet’ From the Stevia: Dangerous?
Table on left is adapted from US patent disclosure; 20070292582A1 lists unique solvents added to an Ethanol/water solution for purification and drying of stevia for use in commercial products.
Manufacturers extract the sweetening components from the leaves with common laboratory solvents and water.
You may have read something on the Internet worrying about the use of these solvents, but concern is not well-founded upon the facts; especially in light of the nature of chemicals employed, the procedures used to purify the stevia crystals, and the fact that the solvents evaporate during the process, leaving behind no residue.
Of the chemicals listed in as the choices for extraction, the most noxious are the isomeric butanols, acetonitrile and ethyl acetate.
Given that 1- and 2-butanol is used in everyday cleaning agents and cosmetics, one is apt to believe that the call for a ban on the use of commercial Stevia to be a ‘red herring.’
The other four solvents; ethanol, methanol, acetonitrile and ethyl acetate would be of concern if they were found within the stevia crystal lattice, but alas, the solvents are not found within the crystal lattice by the very fact that the solvents evaporate.
In fact, under the listed conditions of purification, all of the solvents evaporate within the alotted time of purification. The listed purity of 95% percent and greater is the result of traces of other components from the plant’s leaves and not the solvent.
Cancer With Your Stevia?
But some of the chemicals manufacturers use to purify Stevia are known carcinogens – doesn’t that mean we should worry about using commercial Stevia products? No, the exposure is non-existent to the consumer. The purification processes allow for complete evaporation of any noxious material that may be present.