Quantitative determination of lactic and acetic acids in cider by 1H NMR spectrometry

A quantitative determination method of lactic and acetic acids in cider is proposed by means of the proton nuclear magnetic resonance (1H NMR) technique. Lactic acid gives a doublet signal at 1.42 ppm in the spectrum and acetic acid a singlet signal at 2.09 ppm, whose area is used to determine the concentration of these compounds. 3-(Trimethylsilyl)-2,2,3,3-d4-propionic acid sodium salt is added to the cider with a double purpose: as a reference for 0.00 ppm and as an internal standard for 1H NMR measurement. Enzymatic analysis methods are used to validate the method. Good correlations are found between the lactic and acetic acids concentrations obtained by 1H NMR and those obtained by enzymatic methods in different commercial ciders. For lactic acid, the 95% confidence interval for the slope is 1.05 ± 0.08 (includes 1) and for the intercept 0.27 ± 0.42 (includes 0). For acetic acid, these intervals are 1.01 ± 0.06 for the slope and 0.03 ± 0.08 for the intercept. The described method is fast and direct.