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 δ 13 C measurements of fruit juices and concentrates. The range of the δ 13 C of proteins for each studied fruit is set as the following: −26.0 ± 1.2‰ for orange, −27.7 ± 0.8‰ for grapefruit, −25.9 ± 1.3‰ for lemon, −12.5 ± 1.4‰ for pineapple, and −24.7 ± 1.3‰ for strawberry. A good correlation is found between δ 13 C of proteins and δ 13 C of sugars for a given fruit. The method presented is applicable to both fruit juices and concentrates. The δ 13 C values of proteins are much less sensitive to industrial processes than δ 13 C values of pulp, for instance. The δ 13 C values of proteins are the first link in the chain of analyses performed in the SNIP-IRMS method. The δ 15 N values of the protein fraction of fruit juices are also presented, and their interest for geographical discrimination is discussed.