Comparison of different methods for the determination of assimilable nitrogen in grape musts

Different available methods for the determination of assimilable nitrogen in grape musts were studied. This study was carried out on 10 musts which were fermented at laboratory scale. We first validated the measurement of the actual assimilated nitrogen by yeasts according to the Kjeldahl method as a reference method. Then, the values of assimilable nitrogen obtained by Fourier Transformed Infra Red Spectroscopy (FTIR), formoltitration, [orthophtaldehyde (NOPA) + ammoniacal] nitrogen and [amino acids (automatic analyzer using ion exchange chromatography) + ammoniacal] nitrogen were compared with actual assimilated nitrogen obtained by the reference method. Results showed that the measurement of [amino acids (automatic analyzer) + ammoniacal] nitrogen is the most reliable way to measure assimilable nitrogen in grape musts. But this methodology remains time consuming and quite expensive. FTIR is also a reliable method for the measurement of assimilable nitrogen, except for atypical musts. Being fast and not sample destructive, it appears as a fitted way of assimilable nitrogen measurement on a routine basis. Formoltitration and to a lesser extent, [NOPA + ammoniacal] nitrogen underestimate assimilable nitrogen in grape musts. This study also differentiates between the nitrogen fractions and therefore allows to a better understanding of nitrogen assimilation during alcoholic fermentation.