Miraculin



Miraculin is a glycoprotein extracted from the miracle fruit plant, a shrub native to West Africa (Synsepalum dulcificum or Richadella dulcifica).

Local names for the plant include taami, asaa, and ledidi. Miraculin itself is not sweet, but the human tongue, once exposed to miraculin, perceives ordinarily sour foods, such as citrus, as sweet for up to an hour afterwards. This small red berry has been used in West Africa to improve the taste of acidic foods. Because the miracle fruit itself has no distinct taste, this taste-modifying function of the fruit had been regarded as a miracle.

The active substance, isolated by Prof. Kenzo Kurihara, a Japanese scientist, was named miraculin after the miracle fruit when he published his work in Science in 1968.

Glycoprotein structure
Miraculin was first sequenced in 1989 and was found to be a glycoprotein consisting of 191 amino acids and some carbohydrate chains.

Miraculin occurs as a tetramer (98.4 kDa), a combination of 4 monomeres group by dimere. Within each dimere 2 miraculin glycoproteins are linked by a disulphide bridge.

The molecular weight of the glycoprotein is 24.6 kDa including 3.4 kDa (13.9% of the weight) of sugar constituted (on molar ratio) of glucosamine (31%), mannose (30%), fucose (22%), xylose (10%) and galactose (7%).

Sweetness properties
Miraculin, unlike curculin an other taste-modifing agent, is not sweet by itself, however it can change a sour beverage into a sweet tasting beverage, even for a long period after consumption. The anti-sweet compound, Gymnemic acid suppresses sweet taste of miraculin, like it does for sucrose.

The duration and intensity of the taste-modifying phenomena depends on various factors; miraculin concentration, time of contact of the miraculin with the tongue and acid concentration. Maximum sweet-induced response has been shown to be equivalent to the sweetness of 17% sucrose solution.

Glycoprotein is sensitive to heat, when heated over 100ºC miraculin looses its taste-modifying property. Miraculin activity is inactivated at pH below 3 and pH above 12 at room temperature. The sweet modifying effect stays unchanged at pH 4 (in acetate buffer), for 6 months at 5ºC.

The detailed mechanism of the taste-inducing behaviour is still unknown. It has been suggested that the miraculin molecule can change the structure of taste cells on the tongue. As a result, the sweet receptors are activated by acids, which are sour in general. This effect remains until the taste buds return to normal. It is believed that miraculin has 2 actives sites. One site maintains the sweet protein attached to the membranes while the other (Xylose or Arabinose) active the sweet receptor membrane in acid solution. The presence of positively charged ions (Ca2+ and/or Mg2+) interferes with the binding of the active sugar of miraculin to the sweet receptor and therefore inactive sweet taste.

As a sweetener
As miraculin is a readily soluble protein and heat stable, it is a potential sweetener in acidic food (e.g. soft drinks). Japanese researchers attempts to mass produce it, are focussed on recombinant technology with more or less success. While attempts to express it in E. coli bacteria have failed, Japanese researchers have succeeded in preparing genetically modified plants, such as lettuce, that express miraculin.

As all new sweetener the main issue is food regulation. For instance, Miraculin was denied approval for this purpose by the Food and Drug Administration (FDA).

Miraculin has also no legal status in European Union. However it is approved in Japan as a harmless additive, according to the List of Existing Food Additives published by the Ministry of Health and Welfare (published by JETRO).

Links

 * Miracle Mystery Fruit Turns Sourness Sweet on NPR
 * Miracle berry lets Japanese dieters get sweet from sour
 * Documented tasting experiment
 * Photo of miracule fruit

Miraculin ミラクリン Mirakulina