Fighting a juice's bitter end.

Nobody sells 100 percent navel orange juice, because even the sweetest, tastiest fruit yields a hopelessly bitter drink. Shortly aftter it's squeezed from the fruit, the juice undergoes a chemical reaction that converts limonoate A-ring lactone into limonin, a bitter compound. Grapefruit juicers face a double whammy: the same delayed limonin development and the presence--even in the intact fruit--of a bitter chemical known as naringin. Citrus growers lose millions of dollars annually in the reduced prices these fruits bring when sold for juice. But chemistry may have an answer--several, really.

At the Agricultural Department's Fruit and Vegetable Chemistry Laboratory in Pasadena, Calif., Shin Hasewaga has identified five bacteria that will make minor changes in the molecular structure of limonin--such as the removal of a single hydrogen atom or addition of an oxygen--that render the compound non-bitter. Each "bug" has its own metabolic approach to debittering, he has found. While enzymes are their key actors, none of the 12 enzymes Hasegawa identified among the five bacteria is stable outside live cells. So he now immobilizes live microorganisms by encapsulating them in a gel and packing it into a glass column.

Fifty milliliters of navel orange juice, when passed through a column packed with 2.8 grams of bacterial cells, reduced limonin concentrations in the juice from an objectionable 20 parts per million to less than 5 parts per million--a very palatable level. Flushing the system with water between runs allows it to be used up to 20 times without a loss in debittering efficiency, Hasegawa says.

In Florida, where grapefruit presents the bitterest problem, Philip Shaw and Charles Wilson at the U.S. Citrus and Subtropical Products Lab in Winter Haven have taken an alternative approach. Their process, reported in the July-August JOURNAL OF FOOD SCIENCE, also involves running juice through a column. This column, however, is packed with beta-cyclodextrin polymer, a chain molecule with doughnut-shaped links. When grapefruit juice passes through the polymer, limonin and naringin are selectively trapped in its doughnut holes. Rinsing the column with a dilute lye solution regenerates the system for the next batch of juice.

Shaw says that before their tests ended, the system had been recycled 20 times without a loss in efficiency. And by reversing the direction of juice flow through the column, the researchers have overcome clogging by pith and other solids that cloud unfiltered juice.