Stuck and slow fermentations in enology: statistical study of causes and effectiveness of combined additions of oxygen and diammonium phosphate.

One hundred and seventy-eight musts from different regions in France were selected by enologists and fermented under standardized conditions. Two kinds of fermentation problems were distinguished: (a) slow and (b) sluggish (late-onset sluggish) and stuck (with residual sugar) fermentations. Slow fermentations, characterized by a low fermentation rate throughout the process, were always due to low assimilable nitrogen concentrations in the must. The advantages of using formol titration for measuring assimilable nitrogen are discussed. In contrast, sluggish and stuck fermentations, characterized by very low yeast viability at the end of fermentation, could not be predicted from the analytical data even though their probability was increased when the initial sugar concentration was high. All problems of stuck and sluggish fermentations (concerning 40% of the musts) were solved by supplying 7 mg/l oxygen and 300 mg/l diammonium phosphate at the halfway of the fermentation process, which confirmed (i) the importance of these two nutrients in enological practices and (ii) the importance of adding them at the right time.