Quantitative analysis of malic and citric acids in fruit juices using proton nuclear magnetic resonance spectroscopy

¹H NMR spectroscopy was applied to the quantitative determination of malic and citric acids in apple, apricot, pear, kiwi, orange, strawberry and pineapple juices. Aspartic acid was studied as a potential interference. The effect of the sample pH on the chemical shifts of signals from malic, citric and aspartic acids was examined and a value of 1.0 was selected to carry out the determination. Integration of NMR signals at 2.89-2.95 and 3.00-3.04 ppm were used for calculating the concentration of malic and citric acids, respectively. At this pH the integrated signals were not overlapped. Sodium 3-(trimethylsilyl)tetradeuteropropionate (TSP) was used as an internal reference. The obtained results applying NMR procedures to analyze the juices from different fruits were compared to those obtained using enzymatic methods and both were in close agreement. The intra- and inter-day repeatability was tested for apple juice (7.86 g l^-1 malic acid, 0.32 g l^-1 citric acid) and apricot juice (5.06 g l^-1 malic acid, 4.79 g l ^-1 citric acid) obtaining coefficients of variation lower than 3.4% for intra-day measures (n = 10) and lower than 3.8% for inter-day measures (n = 20).