Compuestos azufrados volátiles en vinos - Fundación para la ...

Nitrogen management is critical for wine Managing Director, flavour and styleEthanol is the major product of sugar fermentation.However, while DAP addition increases yeast growthand the rate of fermentation, it has little to no practicaleff ect on fi nal ethanol yield.Theoretically, DAP-grown yeast are forced to synthesiseamino acids for cell growth when compared withamino acid grown yeast. This decreases the proportionof sugar available for ethanol production (Alberset al. 1996), but in our experiments this has a minor affect on ethanol yield.Glycerol and acetic acid, which are important to winecomposition and fl avour, respond relatively stronglyto juice YAN concentration (Albers et al. 1998; Torrea etal. 2005; Vilanova et al. 2007).Figure 2 summarises general trends observed in syntheticjuice and white wine fermentations. Both glyceroland acetic acid production depends stronglyon the yeast strain used. For example, when usingyeast Vitilevure M05 DAP addition increases glycerolproduction whereas the reverse is the case for AWRI796 (Vilanova et al. 2007). Both yeast, however, producethe lowest concentrations of acetic acid at moderateYAN concentrations (range of 200-250mg/L)while higher concentrations are produced at bothlower and higher concentrations of YAN. Malic acidconsumption does, however, increase with increasingDAP concentration, irrespective of yeast strain. On thecontrary and depending on the strain, succinic acidconcentration can increase with increasing DAP addition(Coulter et al. 2004). In general, YAN can aff ect TAand the balance of organic acids which can aff ect flavour (Sowalsky and Noble 1998).Sulphur dioxide production during fermentation canalso be stimulated by initial YAN concentration, butthe response seems to be yeast strain dependent.Experimental work in synthetic media and wort suggeststhat low SO2 is produced in low YAN mediabut increases when initial YAN availability is higher(Duan et al. 2004; Osborne and Edwards 2006). SO 2production contrasts with H2S production, which isgenerally lowered by increasing YAN. Increased riskof MLF inhibition has also been associated with highYAN addition but this inhibition has not been conclusivelycorrelated with SO 2production (Osborne andEdwards 2006). Nevertheless, until better informationis available, consideration should be given to limitinghigh YAN conditions when malolactic fermentation(MLF) is required.2.2 Volatile aroma compoundsAmong the various yeast metabolic pathways that areinfl uenced by the nitrogen composition of the juice,those leading to volatile compounds are of particularimportance due to the primary role played by fermentation-derivedvolatiles in the aroma character of wine(Smyth et al. 2005). Several studies have indicated thatboth the total available nitrogen and the balance ofamino acids and ammonia can signifi cantly aff ectthe production of different groups of fermentationderivedvolatile compounds.From a practical point of view, the problem of juice nitrogencomposition is primarily linked to the frequentoccurrence of juices with suboptimal concentrationsof nitrogen, and higher risk of slow or stuck fermentation.As this problem is frequently corrected in thewinery through the addition of DAP, several studieshave investigated the implications of this commonwinery practice on the volatile composition of wine(Ayrapaa 1971, Rapp and Versini 1991; Carrau 2003; Torreaand Henschke 2004; Hernandez-Orte et al. 2005,2006; Vilanova et al. 2007). Due to the variety of yeaststrains and fermentation conditions employed, it issomewhat diffi cult to extrapolate from the literaturedefi nitive conclusions concerning the eff ect of DAPaddition on wine aroma. Nevertheless, some generaltrends relating to DAP supplementation and wine volatilecomposition are summarised in Figure 3.Higher alcohols, which are directly related to aminoacid metabolism in the cell, exhibit a characteristicbehaviour. Therefore, when total nitrogen is increasedby adding ammonium to a medium containing verylow levels of YAN, the production of higher alcoholsis initially increased, but then tends to decrease aftera peak between 200-300mg/L YAN. This activity dependson various factors, including yeast strain andfermentation conditions.Higher alcohols are characterisedby fusel-like odours, and are generally thoughtto contribute to the complexity of wine fermentationbouquet. However, when present in very highconcentrations they can have a negative impact onwine aroma, mainly because they mask fruity characters.Several authors have reported that ammoniumsupplementation can improve wine sensory qualityConcentration of yeast aroma compoundsHigher alcoholsEthyl acetateAcetate estersFatty acidsethyl estersBranched chainesters100 200 300 400 500YANFigure 3. Relationship between initial YANconcentration and fi nal concentration of volatilecompounds after fermentation.66