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Applied and Environmental Microbiology, June 2004, p. 3392-3400, Vol. 70, No. 6
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.6.3392-3400.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Biomass Content Governs Fermentation Rate in Nitrogen-Deficient Wine Musts

Cristian Varela, Francisco Pizarro, and Eduardo Agosin^*

Departamento de Ingeniería Química y Bioprocesos, Escuela de Ingeniería, Pontificia Universidad Católica de Chile, Santiago, Chile

Received 17 December 2003/ Accepted 5 March 2004

Problematic fermentations are common in the wine industry. Assimilable nitrogen deficiency is the most prevalent cause of sluggish fermentations and can reduce fermentation rates significantly. A lack of nitrogen diminishes a yeast's metabolic activity, as well as the biomass yield, although it has not been clear which of these two interdependent factors is more significant in sluggish fermentations. Under winemaking conditions with different initial nitrogen concentrations, metabolic flux analysis was used to isolate the effects. We quantified yeast physiology and identified key metabolic fluxes. We also performed cell concentration experiments to establish how biomass yield affects the fermentation rate. Intracellular analysis showed that trehalose accumulation, which is highly correlated with ethanol production, could be responsible for sustaining cell viability in nitrogen-poor musts independent of the initial assimilable nitrogen content. Other than the higher initial maintenance costs in sluggish fermentations, the main difference between normal and sluggish fermentations was that the metabolic flux distributions in nitrogen-deficient cultures revealed that the specific sugar uptake rate was substantially lower. The results of cell concentration experiments, however, showed that in spite of lower sugar uptake, adding biomass from sluggish cultures not only reduced the time to finish a problematic fermentation but also was less likely to affect the quality of the resulting wine as it did not alter the chemistry of the must.

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* Corresponding author. Mailing address: Pontificia Universidad Católica de Chile, Casilla 306 Correo 22, Santiago, Chile. Phone: 562 354 49 27. Fax: 562 354 58 03. E-mail: agosin{at}ing.puc.cl.

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Applied and Environmental Microbiology, June 2004, p. 3392-3400, Vol. 70, No. 6
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.6.3392-3400.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

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