Stevia, a natural sweetener, originates from the Stevia rebaudiana plant, a perennial herb native to South America. Its leaves contain sweet compounds, making it cultivated worldwide. Understanding these molecules is key to appreciating stevia’s role as a sugar alternative. This article explores the molecular basis of stevia’s sweetness, how these molecules are processed, and their journey through the human body.
The Sweetness Source: Steviol Glycosides
The primary molecules responsible for stevia’s sweetness are a group of compounds called steviol glycosides. The leaves of Stevia rebaudiana contain over 30 different steviol glycosides, with stevioside and rebaudioside A (Reb A) being the most abundant and well-studied. These compounds are glycosides, meaning they consist of a non-sugar compound called steviol, to which various sugar molecules are attached.
Stevioside and Reb A differ in their chemical structures, particularly in the number and arrangement of glucose molecules attached to the steviol backbone. Reb A, for instance, has four glucose molecules, while stevioside has three. These structural variations influence their sweetness intensity and taste profiles. Reb A is often preferred in commercial products because it offers a cleaner sweetness with less of the bitter aftertaste sometimes associated with stevioside.
Steviol glycosides are significantly sweeter than sucrose, with sweetness levels ranging from 30 to 450 times that of sugar. Reb A can be 200 to 400 times sweeter than sugar, while stevioside ranges from 110 to 270 times. They are heat-stable, pH-stable, and do not ferment, making them suitable for various food and beverage applications.
How Stevia Molecules Deliver Sweetness
Steviol glycosides produce the sensation of sweetness by interacting with specific sweet taste receptors on the tongue. The human sweet taste receptor is a heterodimer composed of two subunits, T1R2 and T1R3. These molecules bind to active sites on both subunits, triggering a signal that travels from the taste buds to the brain, where it is interpreted as sweetness.
The interaction of steviol glycosides with these receptors involves the glucose moieties on the steviol core. This binding mechanism differs from sugar molecules, which are metabolized for energy. Stevia molecules are not metabolized for energy, which is why stevia provides sweetness without calories.
Despite their intense sweetness, some steviol glycosides, particularly stevioside, can also activate certain bitter taste receptors, contributing to an occasional lingering or metallic aftertaste. The structural differences among steviol glycosides influence their binding to these receptors, explaining why compounds like Rebaudioside D and M are perceived as having a more sugar-like taste with less bitterness. This complex interaction with both sweet and bitter receptors shapes the overall taste profile of stevia-based sweeteners.
From Plant to Product: Molecular Extraction and Refinement
The journey of stevia molecules from the plant to a commercial sweetener involves a series of extraction and refinement steps. The process begins with harvesting and drying the leaves of the Stevia rebaudiana plant. Once dried, the leaves are often ground into a fine powder.
Steviol glycosides are then extracted by steeping the dried leaves in hot water, similar to brewing tea. This aqueous extract undergoes several purification steps, including filtration and centrifugation to remove insoluble materials.
Further purification often involves techniques like ion exchange chromatography, where steviol glycosides are selectively captured based on their electrical charge, allowing other impurities to be washed away. The concentrated steviol glycosides are then subjected to crystallization, often using food-grade alcohol like ethanol. Repeated recrystallization helps to increase the purity of specific steviol glycosides by separating them from other glycosides, resulting in a highly concentrated sweetener.
Stevia Molecules in the Body: Metabolism and Safety
Once consumed, steviol glycosides undergo a specific metabolic pathway. They are not significantly absorbed in the upper gastrointestinal tract. Instead, these molecules pass largely unchanged into the colon.
In the colon, steviol glycosides encounter the gut microbiota. These bacteria hydrolyze the glycosidic bonds, releasing sugar molecules and the non-sugar core, steviol. This steviol is then absorbed into the bloodstream.
Upon absorption, steviol is further processed by the liver, where it undergoes glucuronidation, forming steviol glucuronide. This conjugated form is water-soluble and efficiently excreted from the body, primarily via urine. This rapid processing and excretion mean that steviol glycosides and their metabolites do not accumulate in the body. Based on extensive research, major regulatory bodies, including the U.S. Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA), have approved high-purity steviol glycoside extracts for use as sweeteners, designating them as “Generally Recognized As Safe” (GRAS).