Issue link: http://winesandvines.uberflip.com/i/1007939
42 WINES&VINES August 2018 WINEMAKING PRACTICAL WINERY & VINEYARD This is why the nitrogen that is first absorbed by yeast is in an inorganic, ammonium form. The correction of organic nitrogen at the start of fermentation is highly recommended because specific carriers are not inhibited by ammonia. Amino acids can spread within the cell, signifi- cantly stimulating the formation of enzymes and proteins. The yeast is able to use the amino acids directly, without modifications, entering them into the protein synthesis process. Entry of YAN into the yeast cell occurs by means of active transport and therefore involves energy consumption at the expense of adenos- ine triphosphate (ATP). Alcoholic fermentation, with its corresponding increase in ethanol, al- ters the plasma membranes. The active trans- port process is greatly restricted, resulting in a reduction of nitrogen flow into the yeast cell. In other words, it is only at the initial stages of fermentation, when the concentration of etha- nol is low that the yeast is able to rapidly as- similate the nitrogen that then will be used throughout the entire fermentation. Yeast either can use the amino acids in this form or perform deamination, releasing the nitrogen and the respective higher alcohol. The released nitrogen will then be used for synthe- sis of the amino acids needed for protein synthesis. Addition of amino acids at the initial stage, at the expense of ammonia, can thus increase the deamination process of the yeast, which needs nitrogen for its protein synthesis. In this case, there will be higher alcohol pro- duction that can adversely affect the aromatic profile of the wine. YAN correction prior to the start of alcoholic fermentation, as well as avoiding dreaded stuck fermentations accompanied by sudden in- creases in volatile acidity, minimizes production of reduced sulfur compounds. Nitrogen defi- ciency does not, in fact, allow the yeast to produce sulfur-containing amino acids such as cysteine and methionine through the use of sulfates, sulfites and sulfur anions. Thus, with a low YAN value these anions enter into a reduc- tion chain that results in acquiring the degree of minimum oxidation in the form of sulfur ion, causing the characteristic smell of rotten eggs. What are the stages at which YAN analysis is important? The answer depends on the kinet- ics of alcoholic fermentation. Generally speak- ing, there are two crucial points at which YAN analysis must be performed: prior to the onset of fermentation, to evaluate the appropriate supplementation rates, and at the exponential growth stage of the yeast, when the almost complete depletion of nitrogen can become a limiting factor in the fermentation process. Valuable tools against climate change Wine and grape-juice analyses are crucial ele- ments to achieve a high-quality wine. The winemaking process has changed drastically over the years thanks to enological research and new analytical techniques. Enzymatic and colorimetric analyses play an important role in improving winemaking management, and the ability to conduct them in "real time" can give a winemaker a more accurate vinification control, besides the "standard" enological analyses, with climate change. YAN and gluconic acid constitute two im- portant analyses for the management of alco- holic fermentation. In years when weather conditions are extreme, these analyses are even more important to obtain a quality wine. Their execution, in real time, allows an enolo- gist to better design the fermentation protocol, providing appropriate SO 2 addition and plan- ning adequate nutrition for the yeasts. Simone Bellassai is a food and beverage analysis expert with a double degree in chemistry and enology from University of Florence. CDR srl offers CDR WineLab, an innovative analysis system for grape juice and wine. To see the bibliography for this article, go to winesandvines.com and search under Magazine › Features › August 2018 LIQUID NITROGEN DOSING regardless of your closure preference Minimize dissolved oxygen Extend shelf life Purge O 2 from empty bottles Purge O 2 from headspace Since 1958 4 Barten Lane, Woburn, MA 01801 T 781-933-3570 F 781-932-9428 sales@vacuumbarrier.com vacuumbarrier.com