Determination of the oxygen-18/oxygen-16 isotope ratios of sugar, citric acid and water from single strength orange juice

The ratio of the stable isotopes of oxygen (O-18/O-16) has been measured in the sugar, citric acid and water from authentic single strength orange juices, originating from a number of different countries. The sugars and citric acid were recovered from the juices and their O-18/O-16 ratios were determined by pyrolysis/continuous flow-isotope ratio mass spectrometry (Py/CF-IRMS), The O-18/O-16 ratio of the fruit juice water was determined by the carbon dioxide/water equilibration method. The delta(18)O parts per thousand values of 45 different sugars ranged from +29.1 to +38.8 parts per thousand and 15 citric acids ranged from +18.9 to +25.4 parts per thousand. The delta(18)O parts per thousand values of the water present in the same samples ranged from -2.1 to +7.8 parts per thousand. A correlation was evident between the delta(18)O parts per thousand values of the sugar, citric acid and water from the juices. This information can be used to improve the assessment of the authenticity of commercial 'freshly squeezed' orange juices. The detection of the presence of reconstituted orange juice concentrate in 'freshly squeezed' orange juices was improved by 37% using regression analysis of the combined water and sugar delta(18)O parts per thousand ratios when compared to the use of delta(18)O parts per thousand ratios of fruit juice water alone. Copyright (C) 1999 John Whey & Sons, Ltd.