The research, published in the Proceedings of the National Academy of Sciences (PNAS), reports a newly identified molecular mechanism for the ‘flavor tripping’ effect of the protein miraculin, which is derived from a rare West African red berry that known to many as “miracle fruit” (Richadella dulcifica).
The protein, known for its peculiar ability to make acidic and sour-tasting foods taste sweet, has been shown to transform when introduced to a low pH (sour) environment, with a team of Japanese researchers revealing that in this sour-specific confirmation, miraculin is able to stimulate a particular part of the human sweet receptor.
Dr Danielle Reed of the US-based Monell Chemical Senses Center told FoodNavigator.com that miraculin’s ability to work at lower but not higher pH “might make it a novelty sweetener for some drinks like lemonade but the pH requirement precludes its use as an everyday sweetner in (say) coffee.”
However, Louise Slade from the Food Polymer Science Consultancy, questioned the research findings, claiming that miraculin will never be “more than a party trick.”
Miracle protein?
Professor Keiko Abe and her colleagues at the University of Tokyo attempted to unravel miraculin’s secret using a laboratory assay with human cells bearing the sweet taste receptor.
They found that when cells pre-incubated with miraculin were exposed to an acidic solution, the sweet taste receptors were stimulated to a far greater extent than when they were exposed to a neutral solution.
“We succeeded in quantitatively evaluating the acid-induced sweetness of miraculin using a cell-based assay system and found that miraculin activated hT1R2-hT1R3 pH dependently as the pH decreased from 6.5 to 4.8,” explained the researchers.
They commented that although miraculin is sensory-inactive ‘per-se’ at pH 6.7 or higher, presence of the protein suppressed the response of sweet taste receptors to other sweeteners at neutral pH and enhanced the response at weakly acidic pH.
The team said that their data suggests that miraculin binds to the hT1R2- hT1R3 sweet taste receptors as an antagonist at neutral pH, but conformation changes in the protein shape at acidic pH transforms it into an agonist.
“We conclude this may cause its taste-modifying activity,” added Abe and her co-workers.
Beverage application
Reed said the potential applications for miraculin in the food and beverage industry extend only to its use as a novelty sweetener in certain beverages. She also noted that because miraculin is a protein sweetener, “like all sweeteners in this class, it is not useful in products that are heated because the protein will denature and lose its ability to stimulate the sweet receptor.”
But Slade stresses that other challenges around using miraculin in formulations include the fact that it does not have FDA GRAS status, and that the plant is relatively rare.
However Reed said that because miraculin is a protein, there is the option for it to be genetically engineered into easy-to-grow plants, “so it can be produced more cheaply than it would be by extracting it from berries.”
She added that the berries containing micacullin have been eaten in certain areas of Africa for many years. Reed said that on the back of further studies into its functioning, it may be possible for the protein to gain approval, helping to ensure its commercially viability.
But Slade said that until transgenic foods become acceptable, “there is no way to make it commercially attractive,” noting that “even if it were available, the hour long lingering effect is absolutely unacceptable for product development.”
Source: Proceedings of the National Academy of Sciences (PNAS)
Published online ahead of print, doi: 10.1073/pnas.1016644108
“Human sweet taste receptor mediates acid-induced sweetness of miraculin”
Authors: A. Koizumi, A. Tsuchiya, K. Nakajima, K. Ito, et al