Monellin

Monellin is a sweet protein which has been discovered in 1969 from the fruit of the West African shrub known as serendipity berry (Dioscoreophyllum cumminsii), however firstly reported to be a carbohydrate. The protein was named in 1972 after the Monell Chemical Senses Center in Philadelphia, U.S.A., where the proteins was isolated and characterized.

Protein composition
So fare, five high intensity sweet proteins has been reported: monellin (1969), thaumatin (1972), pentadin (1989), mabinlin (1983) and brazzein (1994).

Monellin molecular weight is 10.7 kDa. Monellin is a two noncovalently associated polypeptide chains, A chain is sequenced with 44 amino acid residues and 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. ''

Sweetness properties
The relative sweetness of monellin, vary from 800 to 2000 time sweeter than sucrose depending of the sweet reference against it is assessed. It is reported to be 1500-2000 times sweeter than 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 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 reduced the persistent sweetness and showed synergistic sweet effect.

Heat denature monellin proteins which loose its sweetness property when heated over 50ºC at low pH.

As a sweetener
Monellin can be useful for sweetening some foods and drinks has 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 investigated. For instance, monellin has been expressed successfully in yeast (Candida utilis) 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.

The main issue to use it as sweetener is the legal side as Monellin has no legal status in European Union and 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).