Issue link: https://read.dmtmag.com/i/705606
sometimes it doesn't. If there's a deficiency, It can alter the yeasts' metabolic pathways and cause off- flavors in the wine, or it can result in stuck fermentations, both of which are a winemaker's night- mare. Basically, yeasts don't want to commit themselves to reproduc- tion if there aren't enough nutri- ents, so they just stop dividing and fermentation stops. For this reason, supplementing musts with added yeast nutrients is widely practiced. Yeast nutrition is highly significant in winemaking, because while yeasts are adaptable and can adjust to differing must conditions, the accompanying metabolic adap- tations they make can result in dif- ferent sensory impacts. NITROGEN The chief nutrient requirement (and the one that's most often defi- cient in grape juice) is nitrogen, easts make wine, with, in many cases, a bit of help from bacteria. They're adapted to feed on sugar, converting it to energy by a rather wasteful process (fermentation) that doesn't derive the maximum energy possible from the sugar, but that, by producing heat and alcohol as a by- product, kills off the competition for this precious resource. It's a form of habitat engineering that's been hijacked by humans for the produc- tion of wine. But the environment yeasts work in during wine produc- tion is a stressful one: They begin in a solution containing a high concen- tration of sugar, which they gradually turn to alcohol, and this eventually causes them to die. If they're to com- plete fermentation — particularly in an age where alcohol levels in fin- ished wines have crept up because of the tendency to pick grapes later — they need adequate nutrition. Grape must often contains all the nutrients needed by yeasts, but BY DR. JAMIE GOODE + Yeast nutrition is highly significant in winemaking. + The chief nutrient requirement is for nitrogen. + Low-levels of yeast- available nitrogen (YAN) are problematic. + Nutrients can aid yeast to create reliable and complete fermentations. AT A GLANCE Winemaking nutrients and a close watch on fermentation can make or break your end results. T i m i ng I s Ev e r yt h i n g which is used for the production of amino acids (and thus proteins) by the yeasts. The nitrogen that's needed by yeast is called YAN (for yeast-available nitrogen) and con- sists of free amino nitrogen (FAN, which is made up of alpha amino acids and small peptides) and ammonium ions. A lack of sufficient YAN is a key risk factor for the development of reductive off-odors in wines. The wine fault "reduction" is caused by the presence of volatile sulphur compounds (VSCs), which are usu- ally produced by yeast during fer- mentation. During fermentation, the VSC produced first is hydrogen sulphide (H 2 S). This has a distinc- tive eggy/blocked drain smell, and it's relatively common during early stages of fermentation at low lev- els, but later disappears. When this H 2 S persists, is made later when it's less likely to blow off or devel- ops into more complex VSCs such as mercaptans or disulfides, then there's a real problem. VSCs aren't all bad. Some are actually positive in wine, with the best examples being the roast cof- fee character from furfurylthiol in some red wines and the famous polyfunctional thiols 3-mercapto- hexan-1-ol (3MH) and 3-mercap- tohexyl acetate (3MHA), which contribute passionfruit and grape- fruit aromas to Sauvignon Blanc. There's also a current trend for toying with matchstick-smelling 4 8 V I N E YA R D & W I N E RY M A N A G E M E N T | J u l y - A u g 2 016 w w w. v w m m e d i a . c o m