Limonin consumption from synthetic citrus juices at their usual pH values (4.0-5.01) in the absence of aeration was performed with Rhodococcus fascians cells in batch and continuous immobilized systems. Limonin and its metabolites at different pH values (4.0 and 5.0) under nonaerated conditions within the synthetic citrus juice without R. fascians cells was established. Limonin and monolactone D of limonoate were the main components at pH 5.0 whereas at pH 4.0, limonin was the only possible bacterial substrate. Although R. fascians is stated as an aerobic organism, at the acidic pH of the synthetic citrus juices (pH 4.0) and without aeration of the medium, it was able to consume limonin (nearly 70% after 150 h) at a higher rate than at pH 7.0 (nearly 40% after 160 h) in batch systems. On the other hand, R. fascians cells were immobilized within polyurethane foam pads in a continuous reactor and fed on a synthetic citrus juice at a dilution rate of 2.4 10(-3) h(-1). It reached an 85% conversion of limonin after 200 h. This level was under the human detection threshold With respect to immobilization, the so-called specific rate of immobilization of R. fascians in polyurethane foam pads was calculated (K-i = 0.016 h(-1)). After 150 h, nearly no free cells were within the culture medium and bacteria were already immobilized. The optical density of the reactor bulk liquid showed that nearly no free biomass was present. Finally from the sugars and citric acid contained within the synthetic citrus juice, only fructose was slightly consumed.