They’ve been engineered to make the taste-transforming miraculin

By Janet Raloff

Web edition: August 10, 2010

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Sour busters

These tomatoes don't taste especially sugary, but eating them sweetens subsequently downed sour foods.

Credit: Kato et al/Amer. Chem. Society

Pucker no more: That seems to be one objective of research underway at a host of Japanese universities. For the past several years, they’ve been developing bio-production systems to inexpensively churn out loads of miraculin — a natural taste-altering protein that makes sour foods seem oh so sweet. Their newest biotech reactor: grape tomatoes.

Japanese dieters have begun to embrace miraculin as a weight-loss aid. They can now snack on low-calorie sour foods that won’t raise a pucker — at least as long as they first eat a few tropical berries to fool the palate. Or they can munch on dietary supplements containing the fruit’s all-natural active ingredient.

For ages, West Africans have recognized the special properties of the so-called miracle-fruit berries produced by a native shrub (Synsepalum dulcificum or Richadella dulcifica). For up to an hour after chewing on the red fruit, which itself isn’t particularly sweet, even the pulp of lemons can be savored slowly and  puckerfree.

A 1968 paper in Science identified the active ingredient as a glycoprotein that would later be named miraculin in recognition of its chemical prestidigitation: That molecular sleight of hand deceives the mouth’s taste buds into thinking sour foods are anything but. A 1989 report would eventually identify the 191 amino acids that make up the magical molecule.

In recent years, the big question has been how to ramp up production of that compound so that marketers can continue to satisfy growing international demand. Manufacturers could, of course, tap the chemical’s natural source. But that strategy might well prove a prescription for the plant’s plunder. So various research teams have been instead investigating ways to insert the gene to produce miraculin into living protein factories.

A Tokyo team initially harnessed fungi — Aspergillus oryzae — to make the chemical. Then collaborating scientists at Nara Women’s and Kyoto Universities moved the miraculin gene into E. coli (presumably a nonvirulent form of the bug). Meanwhile, researchers at the University of Tsukuba have been tinkering with field crops. Success inserting a functioning miraculin gene into lettuce was published four years ago. In a paper just posted online in the Journal of Agricultural and Food Chemistry, the Tsukuba group now describes breeding transgenic lines of tomatoes whose fruit is just about as rich in miraculin as miracle berries are.

The research team, led by Kazuhisa Kato, Hiroshi Ezura and Tsuyoshi Mizoguchi, genetically engineered a line of little miraculin-rich tomatoes and then two teensier cultivars. Their goal: diminutive plants that can be mass reared indoors. In a factory-like setting, plant managers could coddle their plants, limit any risk of infestations or blights and prevent their genetically engineered tomatoes from inadvertently sharing their genes with plants in the wild.

The top producer: a little tomato known as cross #1 offered maximum fruit yields — 73.6 kilograms (162 pounds) per square meter. In addition, the researchers observe, this tomato’s miraculin “has a similar taste-modifying activity” to the miracle berry’s protein. And just one tiny tomato has enough miraculin to switch off the taste buds’ sensitivity to sour flavors.

What Kato’s group hasn’t said it how they anticipate the little tomatoes might be used — as a pre-meal snack or as the feedstock for production of bottled capsules of miraculin.

I can, however, imagine some gustatory fallout from indiscriminate consumption of this taste-switching protein. For instance, put the new Japanese tomatoes into a salad and they’d turn my beloved sour dill pickles (eaten 10 minutes later) into the equivalent of bread-and-butter spears. Or sauerkraut could end up tasting like sweetened cabbage. Ugh!

Then again, can you imagine 'sweetening' iced tea with lemons, guzzling down pomegranate or cranberry juice to which no sugar has been added, or making lemonade without adding a sweetener? I think I'm starting to see the appeal of this dietary fraud.