Monellin is a sweet protein which was discovered in 1969 in the fruit of the West African shrub known as serendipity berry (Dioscoreophyllum cumminsii), it was first reported as a carbohydrate. The protein was named in 1972 after the Monell Chemical Senses Center in Philadelphia, U.S.A., where the it was isolated and characterized.
So far, five high intensity sweet proteins have been reported: monellin (1969), thaumatin
(1983) and brazzein
Monellin's molecular weight is 10.7 kDa
. Monellin is a two noncovalently associated polypeptide chains: an A chain sequence with 44 amino acid residues, and a B chain with 50 residues.
Monellin chain A (44 AA):
REIKGYEYQL YVYASDKLFR ADISEDYKTR GRKLLRFNGP VPPP
Monellin chain B (50 AA):
GEWEIIDIGP FTQNLGKFAV DEENKIGQYG RLTFNKVIRP CMKKTIYEEN
Amino acid sequence of the sweet protein monellin adapted from Swiss-Prot biological database of protein.
The A and B chains of monellin are linked with a Gly-Phe dipeptide bond
into the single chain sweet protein MNEI, an artificial engineered protein.
The relative sweetness of monellin varies from 800 to 2000 times sweeter than sucrose, depending on the sweet reference it is assessed against. It is reported to be 1500-2000 times sweeter than a 7% sucrose solution on a weight basis and 800 times sweeter than sucrose when compared with a 5% sucrose solution on a weight basis.
Monellin has a slow onset of sweetness and lingering aftertaste. Like miraculin, monellin's sweetness is pH dependent; the protein is tasteless below pH 2 and above pH 9.
Blending the sweet protein with bulk and/or intense sweeteners reduces the persistent sweetness and shows a synergistic sweet effect.
Heat over 50°C at low pH denatures monellin proteins causing a loss of the sweetness.
As a sweetener
Monellin can be useful for sweetening
some foods and drinks as it is a protein readily soluble in water. However it may have limited application because it denatures under high temperature conditions which makes it unsuitable for processed food. It may be relevant as tabletop sweetener.
In addition, monellin is costly to extract from the fruit and the plant is difficult to grow. Alternative production such as chemical synthesis and expression in micro-organisms
are being investigated. For instance, monellin has been expressed successfully in yeast
) and synthesised by solid-phase method. The synthetic monellin produce by yeast was found to be 4000 times sweeter than sucrose when compared to 0.6% sugar solution.
Legal issues are the main barrier in widespread use as a sweetener as Monellin has no legal status in the European Union
or the United States
. However, it is approved in Japan
as a harmless additive
, according to the List of Existing Food Additives issued by the Ministry of Health and Welfare (published in English by JETRO