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

When the heat is on, yeast fermentation runs out of puff4(a)2007 20084(b)250120200100YAN concentration (mg/L)15010050YAN concentration SD (mg/L)806040200Region ARegion B0Region ARegion BFigure 4(a). The average YAN concentration in juices from two regions in South Australia for the 2007vintage and the 2008 vintage. For region A, n = 855 in 2007 and 1272 in 2008; for region B, n = 414 in 2007and 788 in 2008. Figure 4(b). Standard deviation in average YAN concentration from wine regions A and Bwas greater in 2008 than in 2007.concentrations of inhibitors in the stuck wine, especiallyethanol. It is, however, critically important thatthe yeast not be starved of sugar or nitrogen duringthe stepwise acclimatisation procedure. The cultureshould also be briefly aerated at each step.Fresh yeast lees from successfully completed fermentscan also be useful for reinvigorating or restarting fermentswhen a relatively small amount of residual sugarremains. A small addition of DAP and limited aeration,once the yeast is actively fermenting, promotesgood activity. This procedure provides a more convenientrescue option when yeast lees are available.Looking ahead, the AWRI is also alerting winemakersto the longer term effects of the 2008 heatwave. If redwines contain higher than usual amounts of residualsugar, and other nutrients added as fermentationstimulants, for example, the Brettanomyces/Dekkerayeast, can strike later in the maturation cycle or evenafter bottling of unfiltered wines. The higher alcoholconcentrations of 2008 wines might slow the growthof this spoiler meaning that greater vigilance shouldbe maintained further on down the process chain.Malolactic fermentation is also adversely affected byhigh alcohol levels such that Brettanomyces/Dekkerayeast can have a greater opportunity while the wineis unprotected by sulfites. The more alcohol tolerantlactobacilli, which are capable of forming biogenicamines, will also thrive if the pH is not maintained below3.5, which is more favourable to Oenococcus. Theimpact of March 2008 might continue if not managedwith greater care due to the perturbed nature of thisyear’s wine chemistry.Stuck fermentations – what you can do• Increase the concentration of sulfur dioxideadded to grape bins. Th is will keep microbial‘bugs’ at bay and slow oxidation whichcan speed up at higher temperatures.• Sanitise bins between loads. Cover binsduring transport to keep the sun off fruit.Clean all equipment to stop bacteria.• Choose yeast known to have high toleranceto alcohol and prepare it properly. Rehydrationof dried yeast with complex inactivatedyeast nutrients can be benefi cial forstressful ferments.• Dilute very high sugar musts with lowstrength juice (LSJ) before fermentation toimprove wine balance, in accordance withwine statutory regulations.• Add tartaric acid as soon as must tanks aremixed and acidity levels are known. Checkagain after thorough mixing to ensure levelsare correct.• Take steps to reduce pH to improve the efficacy of sulfur dioxide. Lower pH increasesits antimicrobial and antioxidant properties.It controls the growth of unwantedmicroorganisms including lactobacilli,some species of which are known to inhibityeast development.• Monitor pH during fermentation and maintainit between 3.4-3.5.• Measure YAN, preferably on the last vineyardmaturity sample before harvest, sothat DAP can be added accordingly. If thisis not possible, add DAP as a precautionagainst low YAN, especially in dry years andwhen sugar concentrations are high, sincemore YAN is needed by yeast to achieve lowresidual sugars.• Aeration of ferments around the mid pointof fermentation can be highly benefi cial byrestoring lagging rates and has no knownadverse impact on wine fl avour, rather itcan prevent quality losses which sometimesresult from stuck fermentations.• Increasing juice turbidity consistent withtarget wine style can considerably improvegrowth and fermentation activity.• Consider adding lysozyme to must of fermentsto control bacterial development.80