Care and feeding of Wine Yeast (i.e. nutrition)
I recently asked Bruce Zoecklein what he thought was the primary driver of volatile acidity in Virginia Wine. He mentioned two things: lack of proper hygiene (including monitoring of hygeine practices) and poor yeast nutrition. To be honest, I was prepared for the first one, but surprised by the second. This does make sense when you recall that improper feeding of yeast during fermentation leads to stress, and when yeast are stressed, they produce VA. So this year, I will be more careful about YAN.
There are many articles and reviews already written about YAN. There is probably something about this topic in your favorite enological products catalogue. Here is a recent review from Penn State’s Enology Extension. YAN can be measured in-house if you have a spectrophotometer, but is also a relatively inexpensive analysis for the service labs to complete. As you move into harvest, it is a good idea to review your YAN targets, make sure you have a good yeast nutrient in-house and have a plan for proper addition, because timing really does matter with nutrition. There are also specialized yeast nutrients on the market to increase precursors for aroma and flavor. Some of these (especially the thiol-increasing kind) require different timing than your standard protocol, so be sure to consult the product information or the representative of the manufacturer for instructions.
When you are taking your juice samples for YAN, I recommend the following:
- For white and Rose wine, take your juice sample after juice clarification and racking, just prior to inoculation. Juice solids may alter the available nutrients.
- For red varieties, take your juice sample after any cold soaking or delay in inoculation. The activities of non-Saccharomyces yeast can alter available nutrition, so this allows for as accurate a reading as possible of the available nutrients.
In a his presentation to the Virginia Wineries Association in 2018, Ken Hurley of the Virginia Tech Analytical Services Lab Presented the following data on YAN values in Virginia.
These three graphs reveal a few important aspects of nutrient management.
- YAN in different every year. YAN is driven by a number of factors, including vintage variation as well as vineyard management. In discussion with Ken about YAN, he remarked that the data were more consistent per vineyard than per region or vintage. Meaning, to truly understand the YAN in your grapes, you need to test them for your own vineyard.
- If 140 mg/L is taken as a minimum, many of our white varieties often have sufficient YAN to complete fermentation. However, remember that fermentation security is not the only goal of proper nutrition, and most of the YAN targets will be higher based on the starting Brix of the juice as well as the conditions of fermentation. It is one thing not to starve the yeast and another to ensure they are expressing the full suite of enzymes to convert precursors into aromas. The red varieties are less likely to contain sufficient YAN to complete fermentation.
The bottom line is that it is very difficult to know based on comparisons with other years or neighboring vineyards what your YAN values are. If you want to get the most out of your fermentation, it is worth the effort and expense to test YAN and make your decisions accordingly.