Selective adsorption of limonin and naringin from orange juice to natural and synthetic adsorbents

In the majority of citrus juices, bitterness is mainly ascribed to the presence of limonoids (triterpenes) and flavanone glycosides (flavonoids), namely limonin and naringin. In this study, the selective removal of limonin and naringin from citrus juice by batch adsorption to different materials was investigated. Since the removal of reducing sugars, pigments and vitamin C may also occur, the eventual adsorption of these compounds was also investigated. The following adsorbents were tested: activated diatomaceous earths, granulated activated carbon and synthetic neutral resins (Amberlite XAD-4, XAD-7 and XAD-16). Both Freundlich and Langmuir isotherm models showed a good fit to the adsorption of limonin to the resins used. Concerning naringin adsorption, a good fit of these models was only observed when the XAD-7 resin was used. Sigmoidal profiles were obtained for the adsorption of limonin to granular activated carbon. Unfavourable isotherms were also observed for limonin and naringin adsorption to activated earths. These isotherm adsorption profiles can be explained by a multilayer adsorption phenomenon. The highest adsorption efficiency for the bitter compounds was observed when synthetic neutral resin, Amberlite XAD-7 was used. The separation factor limonin/naringin varied from 16 (with earths) to 57 (with XAD-7 resin). The adsorption of sugars and pigments to the resins was low. No adsorption of vitamin C was detected for any of the adsorbents tested. The estimated affinity and separation factors show that the neutral resins tested are adequate for the selective removal of limonin from orange juices. The estimated values of free energy of adsorption, lower than -13.3 U mol(-1) K-1, indicate that a physiosorption process occurred

Response surface modelling of the consumption of bitter compounds from orange juice by Acinetobacter calcoaceticus

In certain varieties of oranges, an increase in bitterness is currently observed in juices, after extraction, restraining their industrial use. This has been explained by the conversion of the nonbitter precursor, limonoate A-ring lactone, to a bitter compound, limonin, under acidic conditions. The aim of this study was the modelling of limonin consumption in raw and sterilized orange juices by Acinetobacter calcoaceticus isolated from soil. Response Surface Methodology (RSM) was used for modelling bioconversion and optimization reaction conditions, as a function of temperature (23-37 degreesC) and limonin content (8-16 ppm). Initial rate of limonin consumption could be described, both in raw and sterilized orange juices, by concave surfaces with a minimum at 26 and 27 degreesC, respectively. In raw orange juice, after 7 h reaction time, the amount of converted limonin, increased with temperature. Also, the highest conversions (higher than 33%) were achieved at high temperature (higher than 34 degreesC) and low initial limonin content. In sterilized juice, a maximum conversion of about 23% is expected at 31 degreesC, for an initial limonin content of I I mg l(-1). Thus, limonin bioconversion may be carried out directly in raw juice, avoiding juice sterilization. In addition, no significant decrease in reducing sugars was observed. (C) 2002 Elsevier Science B.V. All rights reserved