Yeast make berry sweet sugar substitute.

A new study sweetens the future of a protein-based sugar substitute, providing a way for it to be manufactured commercially. Called monellin, the protein is 3,000 times sweeter than sugar and 15 times sweeter than aspartame.

First isolated by scientists at the Monell Chemical Senses Center in Philadelphia in 1972, monellin comes from a very sweet, orange-red berry native to West Africa. In the May Nature Biotechnology, Keiji Kondo and his colleagues at the Kirin Brewery Co. in Kanagawa, Japan, report that they have genetically engineered yeast to produce monellin in large quantities.

Monellin accounts for up to 50 percent of the water-soluble proteins in the yeast, concentrations rivaling those in the berries themselves. "To get that much is quite encouraging," says Monell biochemist Joseph Brand. The Kirin scientists also developed a simple method of purifying the protein, an important advantage for industrial production.

One problem with protein-based sweeteners such as monellin is that heat or acidic conditions cause them to break down, drastically limiting the kinds of foods to which they can be added. Monellin consists of two separate protein strands that wrap around each other, forming a configuration that binds well to taste receptors (SN: 5/19/90, p. 315). "If you destroy the shape, you destroy the sweetness," Brand explains.

Previous work by Sung-Hou Kim and his colleagues at the University of California, Berkeley showed that connecting the strands together at one end makes monellin much more stable without affecting its flavor. The Kirin researchers engineered the yeast to make this more stable version of monellin.

Moreover, the scientists employed a strain of yeast that's harder to work with than the others in laboratory use, Brand says, but they made that choice for practical reasons. "The yeast itself has been approved as a food additive [by the Food and Drug Administration]," says Kondo, "so it is considered to be a suitable host for production of proteins."

Thaumatin, another protein-based sweetener from a West African fruit, has been available in food products in Japan and Europe for more than a decade. So far, no one has successfully produced it in microorganisms. Although it has the same sweetness as monellin, "thaumatin is very expensive," Kondo says.

Drinkers of imported beer probably won't find monellin in bottles of Kirin anytime soon. Although the technology is promising, Kondo says, it will probably take many years for monellin to leap the high regulatory hurdles set for food additives.