Interpretation of Combined H SNIF/NMR and C SIRA/MS Analyses of Fruit Juices To Detect Added Sugar

Abstract

The site-specific natural isotopic fractionation studied by nuclear magnetic resonance (SNIF/NMR) method measures site-specific isotope contents in a variety of organic compounds by deuterium nuclear magnetic resonance spectroscopy. This technique, together with SIRA/MS (stable isotope ratio analysis/mass spectrometry) provides a powerful tool for food authentication and characterization. By using the ethanol resulting from sugar fermentation as a molecular probe, SNIF/NMR (deuterium) and SIRA/MS (13C) have been used together for authentication of fruit juices. The influence of deu terium content of the fermentation water on the iso topic parameters is shown and a means for normal izing the results is proposed. A large number of authentic juices have been analyzed to define the variation of isotopic ratios in natural juices. On the basis of these data, a set of rules was designed to enable interpretation of isotopic parameters in terms of possible adulteration of fruit juices by sugar addition. Results of analyses of Florida orange juice are presented. Orange juice samples from Brazil and Israel are included as 2 extreme cases. Assignment limits for a sample of orange juice of unknown origin also are given. These assignment limits are also provided for apple and grapefruit juices.

Stable isotope ratios of H, C, N and O in Italian citrus juices

“…The difference between the δ 13 C values for ethanol and sugars is due to isotopic fractionation related to CO 2 production during fermentation. [8] The correlation between δ 13 C in pulp and ethanol is commented on in the next section, where compliance with the internal reference method proposed by Rossmann et al [15] is discussed.…”

“…The alcoholic distillate was collected with a yield of more than 98% and an ethanol content of more than 94% w/w, guaranteeing non-significant isotope fractionation. [8]…”

“…The Concentrate values were corrected for the contribution of tap water to (D/H) I and (D/H) II during fermentation and normalised against V-SMOW reference water (D/H = 155.76 ppm) according to these relationships [8] :…”

“…Taking uncertainty of the method into account (from fermentation to analysis: 1.1 ppm for (D/H) I and 0.3‰ for δ 13 C ethanol ), all the samples fall in the reference zone of genuine juices. It seems, therefore, that the cut-off points proposed by Martin et al, [8] determined on a dataset of samples collected in Florida, Brazil and Israel, are very general and may therefore also apply to Italian citrus juices. Figure 2 shows the distribution of the difference between δ 13 C sugars and δ 13 C pulp in Italian citrus juices.…”

“…In practice, the 2 H/ 1 H ratio (expressed as (D/H) I , (D/H) II and R) measured in the ethanol obtained from the fermentation of sugars can be used to determine the addition of beet-derived sugars and geographical origin, [7,8] while 13 C/ 12 C ratios (expressed as δ 13 C ‰) indicate whether sugars and/or sweeteners from a cane or corn source (C4 plants) have been added. [8,9] Indication of the addition of tap water and geographical origin can be provided by 18 O/ 16 O (expressed as δ 18 O ‰) [10,11] and 15 N/ 14 N (expressed as δ 15 N ‰), which may indicate the illegal addition of pulp wash. [9,12] Besides these methods, it is important to define threshold values to refer to when assessing whether a juice is genuine or not. For this reason, producers from the European Fruit Juice Association (AIJN) have co-operated to set up reference guidelines, known as the European Quality Control System, [13] by which the quality and authenticity of fruit juices can be assessed.…”