Proteins as intermolecular isotope reference for detection of adulteration of fruit juices

The fruit proteins, defined as the fraction obtained after bentonite precipitation on a centrifuged juice or diluted concentrate, are proposed as internal standard for delta(13)C measurements of fruit juices and concentrates. The range of the delta(13)C of proteins for each studied fruit is set as the following: -26.0 +/- 1.2 parts per thousand for orange, -27.7 +/- 0.8 parts per thousand for grapefruit, -25.9 +/- 1.3 parts per thousand for lemon, -12.5 +/- 1.4 parts per thousand for pineapple, and -24.7 +/- 1.3 parts per thousand for strawberry. A good correlation is found between delta(13)C of proteins and delta(13)C of sugars for a given fruit. The method presented is applicable to both fruit juices and concentrates. The delta(13)C values of proteins are much less sensitive to industrial processes than delta(13)C values of pulp, for instance. The delta(13)C values of proteins are the first link in the chain of analyses performed in the SNIP-IRMS method. The delta(15)N values of the protein fraction-of fruit juices are also presented, and their interest for geographical discrimination is discussed.