Nitrogen – Lipid Balance in alcoholic fermentations. Example of Champagne musts

Abstract

Nutrient availability – nitrogen, lipids, vitamins or oxygen – has a major impact on the kinetics of winemaking fermentations. Nitrogen is usually the growth-limiting nutrient and its availability determines the fermentation rate, and therefore the fermentation duration. In some cases, in particular in Champagne, grape musts have high nitrogen concentrations and are sometimes clarified with turbidity below 50 NTU. In these conditions, lipid deficiencies may occur and longer fermentations can be observed. To better understand this situation, a study was realized using a synthetic medium simulating the composition of a Champagne must : 180 g/L of sugar, 360 mg/L of assimilable nitrogen and a lipid content ranging from 1 to 8 mg/L of phytosterols (mainly β-sitosterol). The initial phytosterol concentration determined the amount of consumed nitrogen and therefore the population reached in stationary phase and the maximal fermentation rate. An early loss of viability was observed when lipid concentrations were very low. For example, for an initial phytosterol concentration of 1 mg/L, the viability continuously decreased during the stationary phase and its final value was only 50%. In some fermentations, 10 mg/L oxygen were added at the end of the growth phase, to combine the effects of phytosterols from the musts and the de novo synthesis of ergosterol and unsaturated fatty acids. Oxygen additions highly improved the fermentation kinetics of media with low phytosterol contents. For example, in the medium containing 2 mg/L of phytosterol, the maximum fermentation rate was increased by 45 % and the fermentation time was 70 hours shorter. In the case of media containing 3, 5 and 8 mg/L of phytosterols, the assimilable nitrogen was completely exhausted and the fermentation kinetics as well as the final populations and viabilities (higher than 90%) were identical for the 3 conditions. Impacts of lipid content and oxygen addition on acetate and glycerol synthesis were also quantified. Acetate production was lower for high phytosterol concentrations whereas, in extreme phytosterol deficient musts, oxygenation resulted in a significant increase of both acetate and glycerol synthesis. Similar results were obtained with natural musts containing different amounts of solids. Consequently, this study points out the importance of controlling the nitrogen – lipid balance, especially in nitrogen-rich musts like in Champagne, and also the interest of combining a sufficient initial turbidity with an optimized oxygenation.