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

Nitrogenmanagement iscritical for wineManaging Director,flavour and styleUgliano, M.; Henschke, P.A.; Herderich, M.J.; Pretorius,I.S.The Australian Wine Research Institute, PO Box 197, GlenOsmond (Adelaide), South Australia 5064, Australia“Winemaking begins in the vineyard” is a mantrathat has widespread support amongst winemakers.It conveys the concept of vineyard or, in French jargon,terroir as an intrinsic property of grape, andconsequently the corresponding wine. There is nodoubt that many great wines are associated withgreat vineyards. So, where do yeast fi t in?The perception that fermentation yeast faithfullytransform grape must into wine has been changingin the detail over the past decades. This is a result ofscience uncovering the many roles that yeast perform,and the wider selection of strains available thatpromote these various attributes. For example, whereasmost strains produce a relatively similar, generic,fermentation bouquet only some strains possess astrong ability to hydrolyse cysteine conjugates responsiblefor Sauvignon Blanc character, meaning thatonly selected strains can enhance varietal expression(Swiegers et al. 2006).Winemakers today have many options through fermentationmanagement to enhance the varietal characteristicsof their wine, or to express further regionalattributes. Furthermore, yeast strongly respond totheir environment. It is well known that temperatureaff ects the rate of fermentation, that grape solidsenhance survival and that high osmotic stress, asimposed by a Botrytis-aff ected must, leads not onlyto increased glycerol production but also to highervolatile acidity. The latter example highlights the remarkableability of yeast to adapt to stressful (i.e. highsugar) environments. However, there is an accompanyingmetabolic adaptation which can have positive ornegative fl avour implications.At the time of inoculation, yeast are subjected to arange of stresses to which the cell must adapt in orderto exploit its new environment. Some of the knownstresses are osmotic pressure, oxidative conditions,Seminario TécnicoCompuestos azufrados volátiles en vinossulphite toxicity and temperature shock (Bauer andPretorius 2000). Nutrients, whether present in sub- orsuper-optimal concentration, can also induce stressand metabolic responses. The primary response isaimed at protecting the cell from committing to reproductionwhen key nutrients are lacking or dealingwith potential toxicity when the concentration is outsidethe normal range. The metabolic response ofteninvolves a cascade of biochemical reactions, some ofwhich can lead to altered metabolism of nutrientssuch that the yeast will secrete end-products in different amounts (Albers et al. 1998). Some of the endproducts that have sensory properties can lead tochanges in the fl avour profi le of the wine. H 2S formationis an all-too-well known example relating tonitrogen depletion stress.Clearly, the vineyard environment and interventionby the viticulturist shape the development of the vineand especially the composition of the grape. Becausethe viticulturist attempts to balance a long list of prioritiesin order to produce fruit to specifi cation, mostattention will focus on those factors that cannot bemodifi ed once the fruit has been harvested.Therefore, yeast nutrients, especially nitrogen, mightnot be optimised for fermentation, largely in the beliefthat nutrients can be easily corrected in the winery. Giventhat we estimate that up to 500t of diammoniumphosphate (DAP) could be used each year to produceAustralian wine, is this winemaking input being usedeff ectively? Our current state of knowledge on theimplications of controlling vineyard nitrogen as opposedto fermentation nitrogen on fermentation performanceand wine composition has been recently reviewedby Bell and Henschke (2005). In this article, wewill focus on the role that fermentation nitrogen hasin modulating metabolism and some of the changesthat this can have on wine fl avour. We will fi rst summarisecurrent best practice for managing fermentationnitrogen and then describe the main fl avourchanges that are aff ected by nitrogen.Finally, we will consider the fl avour implications of nitrogenfor white and red wine fermentations.CURRENT BEST PRACTICE FORMANAGING FERMENTATION NITROGENA common practice amongst winemakers is to makea standard addition of DAP to the juice or must (100-300mg/L) at inoculation without measuring the nitrogenconcentration. This article will show that DAPaddition has signifi cant fl avour consequences andthat measuring the initial nitrogen concentration providesthe opportunity to adjust DAP addition not onlyto achieve an adequate fermentation rate, but also tomore reliably guide the fl avour profi le and style ofwine required. This work is still in a conceptual stageTextos asociadosWine Industry Journal > Vol 22 No 6 > November / December 200763