Facts about miracle fruit (miraculin revisited - part 2:2)

Short introduction in Norwegian: I anledning at jeg deltok i en episode om mirakelfrukt på Schrödingers katt på NRK (og YouTube) publiserer jeg to blogginnlegg om temaet. Det første innlegget handler om smakstesting av mirakelfrukt. Innlegget nedenfor er del 2 av 2 og er en samling fakta om mirakelfrukt med referanser til forskningslitteratur. Siden denne bloggen normalt er på engelsk fortsetter jeg herved på engelsk.

On the occasion of me attending an episode of the Norwegian popsci TV series "Schrödingers katt" (and YouTube) about miracle fruit I post two entries on miracle fruit and its key constituent miraculin. The first post describes a tasting of miracle fruit with a number of sour foods. The second post below is a collection of facts about miracle fruit based on research literature. Part 2:2 below is divided into the following main topics:

• Miracle fruit/berry - properties
• Traditional/historic use of miracle fruit
• Taste sensation when using miracle fruit/miraculin
• Cultivation and production
• Use in commercial products, potential applications
• Health risk and governmental/public approval
• Elimination of the sweet-inducing effect
• Mechanism of taste-modifying activity
• Some other sweet and taste-modifying substances
• References

It is important to note that scientific results are not necessarily equal to "facts" or "absolute truth" but rather the best knowledge we have thus far. This tentative nature is inherent in science. When I have used the word “facts” it should hence be interpreted as published scientific results. You might observe that some statements below seem to contradict each other. The reason for this may be that more recent research has corrected older incomplete results, or that two studies might come to different conclusions. These things occasionally happen in science and are perfectly natural parts of science in general. The information was collected from scientific papers during 2011, so it might (most likely) exist research published late 2011 or in 2012 which has escaped me.

The first paper published on miracle fruit was by Inglett and coworkers in Journal of Agricultural and Food Chemistry (Inglett, et al., 1965). In the following 46 years (until mid-2011) 199 scientific papers have been published on the matter, one fourth of which appeared between 2009 and 2011.^*

Miracle fruit/berry - properties

• Formal name, species: Richadella dulcifica, Synsepalum dulcificum
• Common name: miracle fruit, miracle berry, Agbayun (in West Africa) (Inglett, Dowling, Albrecht and Hoglan, 1965)
• Indigenous to tropical West Africa (Inglett, et al., 1965)
• Fresh fruit is easily spoiled upon storage, but there are efforts towards development of preservation methods, e.g. covering the fruits by chitosan (chitosan is a natural substance that might e.g. be extracted from the shells of shrimp and crustaceans) (Liu, He, Xie, Yang and Liang, 2011)
• The sweetening taste is due to a protein (glycoprotein) called miraculin (Inglett, et al., 1965)
• The miraculin protein is in itself very stable and can be stored in pure form or in solution for months (e.g. 6 months) without loss of function (Gibbs, Alli and Mulligan, 1996)
• Taste modifying properties are lost upon heating, in strongly alkaline solutions (pH > 12) (Inglett, et al., 1965) and at pH below 3 (Kurihara, 1992) (note: pH 3 = is rather common in food, whereas pH > 12 is probably never encountered in food)

Traditional/historic use of miracle fruit

• “Africans have used miracle fruit to sweeten a sour gruel made from stale bread, soured palm wine, and pito, a sour beer made from fermented grain” (pitto). Also used before eating a certain type of sour corn bread (kankies/kenkey) (Bartoshuk, 1974; Inglett, et al., 1965)

Taste sensation when using miracle fruit/miraculin

Miraculin protein

• In itself miraculin is tasteless, at least not sweet tasting (several of the refs. below). I have tried the freeze dried pill version which tastes bland and very slightly herbal/”green” 
• Miraculin/miracle fruit makes sour things taste sweet. It has been tested on numerous acids, such as citric acid, ascorbic acid, hydrochloric acid, tartaric acid, acetic acid, lactic acid, phosphoric acid etc. (Inglett, et al., 1965; Kurihara, 1992)
• The effect lasts from half an hour (several refs. below and my own experience) up to two hours (Kurihara, 1992)
• When using miraculin, the total taste intensity from whatever acidic food tasted is unchanged but the sour taste is reduced (Bartoshuk, 1974)
• My own taste experience: part of the sour taste, but not all of it, is converted to sweet. The resulting sweet taste is strikingly similar to plain sugar/sucrose
• It does not reduce bitter taste (myself and (Inglett, et al., 1965; Kurihara, 1992)), even if this is claimed by some (i.e. some of those who sell it)
• Does not reduce pungent/burning taste such as chilli/tabasco (myself and (Kurihara, 1992) even if this is claimed by some (e.g. Swedish food blog Taffel)
• It does not reduce salt taste (Inglett, et al., 1965; Kurihara, 1992)
• Does not affect other sweet tasting substances present (Capitanio, Lucci and Tommasi, 2011; Kurihara, 1992)
• Slightly contradictory: Thorough sensory studies show however that other tastes are also affected, but the mechanisms are uncertain. The effect is the same as when sugar is tasted, and if that is the case we know that tastes do affect each other (e.g. salt reduces bitter taste, small amounts of salt enhances sweet taste etc.). Miraculin seems to affect sweet taste only, but the fact that our sweet taste sensation is activated should/will affect our experience of other tastes (Capitanio, et al., 2011)

Cultivation and production

dubofadubofadub @ Flickr

• It is cultivated in greenhouse in Taiwan for mass production. Made into tablets/pills based on freeze dried powdered fruit (Shimamura and Lin, 2007). Also cultivated in greenhouse in Japan, bears fruit twice a year (Kurihara, 1992)
• Typical amount of miraculin in a tablet/pill is 300 or 600 mg. Maximum amount of pure miraculin extracted from one kg fruit pulp is 250 mg according to Liu et al. (Liu, et al., 2011). Kurihara (1992) reports 36 mg miraculin from 10 g freeze dried fruit (if we assume that the fruit is 70% water this would give approx. 1 g miraculin per kg fresh fruit)
• Researchers have managed to genetically engineer organisms other than the miracle fruit to produce miraculin. This is desired because tropical species are difficult to cultivate outside their natural habitat (Hirai, Fukukawa, Kakuta, Fukuda and Ezura, 2010; Yano et al., 2010): 
• E-coli bacteria and tobacco plants produced miraculin but this miraculin did not possess taste modifying properties
• Lettuce, tomato and strawberry plants; tomato was the most efficient production-wise
• 90 µg miraculin per g tomato has been achieved which would mean close to 100 mg miraculin per kg tomato. This would give approx. 4 kg miraculin per 1000 m2 (decare, daa) land per year (some 6500 tablets/pills per decare)
• Not all miraculin produced by other genetically engineered organisms seems to have the same effect as from the miracle fruit itself (Paladino, Colonna, Facchiano and Costantini, 2010)

Use in commercial products, potential applications

justgrimes @Flickr

• Has been developed low-calorie cakes containing citric acid that can be eaten after using miracle fruit/miraculin, presumably since (part of) the taste of citric acid will be perceived as sweet (Shimamura and Lin (2007) and www.taste-m.com)
• There is a fundamental difference between miraculin and food additives because it is not necessary to add miraculin to the food itself. Hence, the amount of miraculin used may be low because you take a certain amount of miraculin and then everything tastes sweeter (in contrast with additives in which the amount you ingest is proportional to the amount of food product you eat) (Bartoshuk, 1974)
• As a general principle, miraculin might be used to make food taste “better”/sweeter, such as healthy but not very palatable foods (whatever that might be). Also potential usage for patients forced to undertake diets containing unpalatable ingredients such as yeasts and algae (Bartoshuk, 1974)
• Miraculin has been tested in controlled experiments where the test persons did not notice any difference between sucrose sweetened candy compared to miraculin + unsweetened candy (lemon juice-based popsicles). The test persons did not develop any need for energy compensation (e.g. craving for sugar) when using miraculin + unsweetened popsicle compared to the sugar sweetened variety (Wong and Kern, 2011)
• Has been tested in USA for cancer patients who had developed dulled taste sensation due to chemotherapy. A majority responded that miraculin gave enhanced taste experience. Researchers are also interested in investigating how this can be used to help people suffering from diabetes and obesity (Peregrin, 2009)
• There is a general hope that sweet and sweet-inducing proteins like miraculin may find medical applications and as one means to counter the imminent obesity epidemic because an important part of the obesity problem is due to high sugar intake. There is a hope that such substances might replace artificial low calorie sweeteners (Kant, 2005)

Health risk and governmental/public approval

• "Most of these proteins are derived from plants and have been used to add a sweet taste to food or drinks by native people and can safely be ingested. Hence these proteins may have potential as low-calorie sweeteners" (Kurihara and Nirasawa, 1994)
• Miracle fruit was originally attempted commercialized in USA in the 70’s. “However, the Generally Recognized As Safe (GRAS) petition was denied by the Food and Drug Administration due to a lack of scientific studies which would support the GRAS claim and insufficient use by consumers in the United States” (Gibbs, et al., 1996). “At this point the company's [Miralin’s] investment capital could not sustain them and Miralin folded” (Wikipedia). Web searches of miraculin will reveal stories/descriptions about sugar industry conspiring to halt/sabotage public approval of using miraculin in food
• “The intensely sweet proteins have important advantages over artificial sweeteners. Since they are natural, they could be safer than aspartame and other sweeteners” (Gibbs, et al., 1996). [my comment: the contrasting of “natural” and “artificial” seems, in my opinion, to be a strange. There is no direct link between “natural” and “healthy”, and neither “synthetic”/”artificial” and “unhealthy”]
• Miraculin “has been eaten for centuries in Africa with no reports of adverse reactions” (Wikipedia), but on the other hand this may be due to the lack of scientific studies of miraculin vs. health (my comment)
• Miraculin was considered by European food & health authorities in 1985 but was at that point not approved for use due to lack of data to support the necessary assessment (pdf). At the present, miraculin has a novel food status (two links) in Europe (the latter is a slow reacting link, be patient)
• In 2009 a case of unauthorised sale/distribution of miraculin containing product was reported to EU food authorities from Finland (appeared in an official product control). Product was detained from sale. Status today: before it may be placed on the market in the EU as a food or food ingredient a safety assessment under the Novel Food Regulation is required (pdf)

Elimination of the sweet-inducing effect

• The effect wears off with time, typically 30 minutes to one hour depending on amount taken.
• Gymnema silvestre (another taste modifier which removes sweet taste) removes sweet taste-induced by miraculin (Bartoshuk, 1974) [my comment: I’m uncertain whether the mechanism of both substances are known in sufficient detail to explain this. Also, there might be problems in calibrating the appropriate amounts; how much Gymnema Silvestre do you need to accurately counteract a certain amount of miraculin?].

Mechanism of taste-modifying activity

• Usually, large molecules do not induce taste or smell. Some exceptions do exists, and amongst these are sweet and sweet inducing proteins (note: proteins are large molecules) (Kurihara, 1992)
• Miraculin attaches to receptors for sweet taste without activating them until they are subjected to acid. Acids induce a change in the miraculin-attached receptors resulting in activation of the receptor. Hence, miraculin makes sweet receptors react when subjected to acids (Kurihara, 1992; Wong and Kern, 2011)
• Miraculin binds strongly to the receptors on your tongue which is the reason for its long lasting effect (Kurihara, 1992)
• Such interactions are often illustrated by a key-in-lock analogy. In the Swedish food blog Taffel the mechanism is described as putting the key into the lock without turning the key; the key is turned only when it is exposed to acid [my comment: In the same post, it is claimed that miraculin counteracts pungent/burning sensation in chillies. In my experience this is not the case; miraculin does not alter tastes other than sweet/sour, see previous post on fooducation]

Some other sweet and taste-modifying substances

Note: The following list is not complete, just a selection of relevant substances that I collected in the process.

• For a good overview and table with comparison, see (Kurihara, 1992; Kurihara and Nirasawa, 1994)
• Sweet proteins (several of these have strong sweet tastes comparable with strong tasting artificial sweeteners)



Stevia

• Stevia/steviosides/steviolglycosides – tastes sweet
• Used as added sweetener
• Steviol glycosides approved as food additive in Europe (scientific opinion from EFSA panel here, legislation here)
• Monellin, from West-African red berries. Sweet protein which is ca. 3000 times sweeter than sucrose on mass basis (Kurihara, 1992)
• Thaumatin, about the same level sweetener as monellin but from another West-African plant (Kurihara, 1992)
• Neoculin (Okubo et al., 2008)
• Naturally sweet protein, ca. 500 times sweeter than sucrose
• Is in addition to sweet tasting also sweet-inducing
• No structural similarities between neoculin and miraculin
• Extracted from the tropical fruits of Curculigo latifolia native to West Malaysia (Malay Peninsula) (Okubo, et al., 2008)
• Artichoke induces sweet taste in pure water (Bartoshuk, 1974)
• Reason is chlorogenic acid and cynarine present in the vegetable. Ancient folk wisdom says that artichokes are “impossible” to pair with wine. Some isomers of chlorogenic acid are also found in coffee, but I have not found definite evidence of whether these are sweet inducing
• Curculin 
• Found in an herb from Malaysia, the effect appearing when the pulp is chewed (Kurihara, 1992)
• Molecular structure of curculin does not share many similarities to miraculin
• The effect of Curculin is much more short lived than that of miraculin, declining after only 5 minutes (Gibbs, et al., 1996)
• Has a mechanism of action resembling that of miraculin, however somewhat more complex (Yamashita, Akabane and Kurihara, 1995)
• The effect is reduced by Ca²⁺ and Mg²⁺-ions (Kurihara, 1992) [my comment: the effect can perhaps be killed by eating/drinking something milk-based?]
• Is in itself sweet in addition to sweet-inducing effect. Considered 550 times sweeter than sucrose on mass basis (Gibbs, et al., 1996; Kurihara, 1992). Varying numbers, see Wikipedia
• Gives sweet effect also in pure water and tea, but sweetness is enhanced in presence of acids (Kurihara, 1992)

References

Bartoshuk, L. M. (1974). After-Dinner Talk - Taste Illusions: Some Demonstrations. Annals of the New York Academy of Sciences, 237(1), 279-285.

Capitanio, A., Lucci, G. and Tommasi, L. (2011). Mixing Taste Illusions: The Effect of Miraculin on Binary and Trinary Mixtures. Journal of Sensory Studies, 26(1), 54-61.

Gibbs, B. F., Alli, I. and Mulligan, C. (1996). Sweet and taste-modifying proteins: A review. Nutrition Research, 16(9), 1619-1630.

Hirai, T., Fukukawa, G., Kakuta, H., Fukuda, N. and Ezura, H. (2010). Production of Recombinant Miraculin Using Transgenic Tomatoes in a Closed Cultivation System. Journal of Agricultural and Food Chemistry, 58(10), 6096-6101.

Inglett, G. E., Dowling, B., Albrecht, J. J. and Hoglan, F. A. (1965). Taste Modifiers, Taste-Modifying Properties of Miracle Fruit (Synsepalum Dulcificum). Journal of Agricultural and Food Chemistry, 13(3), 284-287.

Kant, R. (2005). Sweet proteins - Potential replacement for artificial low calorie sweeteners. Nutrition Journal, 4(1), 5.

Kurihara, Y. (1992). Characteristics of Antisweet Substances, Sweet Proteins, and Sweetness-Inducing Proteins. Critical Reviews in Food Science and Nutrition, 32(3), 231-252.

Kurihara, Y. and Nirasawa, S. (1994). Sweet, Antisweet and Sweetness-Inducing Substances. Trends in Food Science & Technology, 5(2), 37-42.

Liu, C., He, C., Xie, T., Yang, Y. and Liang, T. (2011). Research on Preservation of Synsepalum Dulcificum by Coatings. Advanced Materials Research, 239-242, 2158-2162.

Okubo, S., Asakura, T., Okubo, K., Abe, K., Misaka, T. and Akita, T. (2008). Neoculin, a taste-modifying sweet protein, accumulates in ripening fruits of cultivated Curculigo latifolia. Journal of Plant Physiology, 165(18), 1964-1969.

Paladino, A., Colonna, G., Facchiano, A. M. and Costantini, S. (2010). Functional hypothesis on miraculin' sweetness by a molecular dynamics approach. Biochemical and Biophysical Research Communications, 396(3), 726-730.

Peregrin, T. (2009). "Miracle Fruit" May Be a "Sweet" Alternative for Cancer Patients. [News Item]. Journal of the American Dietetic Association, 109(6), 974-975.

Shimamura, M. and Lin, M. L. (2007). Establishment of technology on miracle fruit tablets and its applications. Chemical Senses, 32(2), 6.

Wong, J. M. and Kern, M. (2011). Miracle fruit improves sweetness of a low-calorie dessert without promoting subsequent energy compensation. Appetite, 56(1), 163-166.

Yamashita, H., Akabane, T. and Kurihara, Y. (1995). Activity and Stability of a New Sweet Protein with Taste-Modifying Action, Curculin. Chemical Senses, 20(2), 239-243.

Yano, M., Hirai, T., Kato, K., Hiwasa-Tanase, K., Fukuda, N. and Ezura, H. (2010). Tomato is a suitable material for producing recombinant miraculin genetically stable manner. Plant Science, 178(5), 469-473.

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* Numbers based on ISI Web of knowledge search containing one or more of the following key words: miraculin, miracle berry, miracle fruit, synsepalum dulcificum, richadella dulcifica.