Monellin is an intensely sweet protein derived from the serendipity berry (Dioscoreophyllum volkensii), a plant native to West Africa. Its sweetness can be thousands of times greater than sucrose (table sugar) on a weight basis, making it a subject of scientific and commercial interest.
Unlike sweeteners that are carbohydrates or artificial small molecules, monellin is a protein. Discovered in 1969, it was named in 1972 after the Monell Chemical Senses Center where it was first characterized. This protein-based sweetener offers a different metabolic pathway compared to sugars, presenting a valuable option for food science and nutrition.
The Scientific Profile of Monellin
Monellin’s sweet taste is a direct result of its three-dimensional structure, which allows it to interact with the T1R2-T1R3 sweet taste receptors on the human tongue. This interaction sends a signal to the brain that is perceived as sweetness. The protein itself is relatively small, with a molecular weight of about 10.7 kilodaltons.
The structure is a heterodimer, composed of two separate polypeptide chains: the A chain (44 amino acids) and the B chain (50 amino acids). These chains are not connected by strong covalent bonds but are held together by weaker, non-covalent interactions. This association forms a globular shape with a five-strand beta-sheet partially wrapped around an alpha-helix.
The specific arrangement of amino acids on the protein’s surface enables its binding to taste receptors. Any disruption to this structure can eliminate its sweetness, which presents challenges for its use in food products.
Limitations in Food Production
The primary obstacle to monellin’s widespread use is its structural instability. The weak, non-covalent bonds holding the A and B chains together are susceptible to disruption by heat and changes in pH. This denaturation causes the protein to lose its three-dimensional shape and, consequently, its sweetness.
This instability limits its application in the food industry. Monellin loses its sweetness when heated above 50°C (122°F), especially in acidic conditions, making it unsuitable for products requiring pasteurization, such as many beverages and dairy products, or baking. The protein’s functionality is maintained only within a pH range of approximately pH 2 to 9.
Its sweetness can also be diminished in beverages like carbonated drinks, which have an acidic pH. The combination of low pH and processing temperatures can lead to the denaturation of the protein, rendering it ineffective as a sweetener.
Engineering a Better Sweetener
To overcome stability issues, scientists used genetic engineering to create a more robust “single-chain monellin.” This was done by linking the A and B chains into one continuous polypeptide with a short linker, creating a more resilient molecule.
This engineered version is significantly more resistant to heat and pH changes. For example, a single-chain variant known as MNEI has a melting temperature over 20°C higher than its natural counterpart. This enhanced stability opens the door for its use in a wider array of food and beverage applications, including those requiring heat treatment.
Single-chain monellin can be produced scalably in microorganisms like E. coli, avoiding the need for extraction from the serendipity berry. As a protein, engineered monellin is digestible and does not trigger an insulin release. Its use in consumer products is still subject to regulatory approval processes in various regions.