Detection of Added Beet Sugar in Concentrated and Single Strength Fruit Juices by Deuterium Nuclear Magnetic Resonance (SNIF-NMR1 Method): Collaborative Study

Abstract

A collaborative study of the site-specific natural isotope fractionation-nuclear magnetic resonance (SNIF-NMR) method for detecting added beet sugar in fruit juices is reported. This method is complementary to the stable carbon isotope ratio analysis (SCIRA) (AOAC Official Methods 981.09 and 982.21), which can detect sugars derived from plants exhibiting C4 metabolism (corn and sugarcane). It is based on the fact that the deuterium content at specific positions of the sugar molecules is higher in fruit sugars than in beet sugar. The fruit juices are fermented, and the alcohol is distilled with a quantitative yield and analyzed with a high-field NMR spectrometer fitted with a deuterium probe and fluorine lock. The proportion of ethanol molecules monodeuterated on the methyl site is recorded. This parameter (D/H)I is lowered when beet sugar is added to a fruit juice or concentrate. The precision of that method for measuring (D/H)I was observed to be similar to that of other isotope ratio methods: sr values ranged from 0.19 to 0.25 ppm and SR values varied between 0.21 and 0.37 ppm. An excellent correlation was observed between the percentage of added beet sugar and the (D/H)I isotope ratio measured in this collaborative study. Consequently, all samples in which beet sugar was added were found to have a (D/H)I isotope ratio sig- nificantly below the normal value for authentic juice or concentrate of that fruit. The SNIF-NMR method for detection of added beet sugar in fruit juices has been adopted by AOAC INTERNATIONAL.