It may be an obvious statement, but for a vine to grow and for grapes to ripen, adequate water and nutrients are required. The nutrients required are predominantly found in the soil. The good news is that vines themselves require pretty low levels of nutrients. However, those nutrients do get depleted by viticulture, so that’s why grapegrowers are constantly testing their soils and monitoring nutrient levels.
Nitrogen – This guy has a dominant impact on vine vigor and, thus, grape quality. The nutrient itself is actually a component of proteins and chlorophyll—extremely important for photosynthesis. So, too much Nitrogen and you risk excessive vegetative growth (which will eventually hinder the grape-ripening process and even, later down the line, create too much shading in the fruit zone, further impeding grape development and poor ventilation, which could lead to diseases). Too little Nitrogen means a reduced vine vigor and yellowing of the vine leaves. Again, this will affect the vines ability to photosynthesize and ultimately affect the quality of the grapes grown and wine produced. But, here’s the kicker, grapes that have low levels of Nitrogen can also be problematic for fermentation. Bingo. I’ll explain why further on.
Potassium – This dude helps regulate the flow of water in the vine, so pretty essential to vine growth. Too much Potassium in soils can actually impede magnesium uptake—another nutrient found in chlorophyll and, therefore, super important to photosynthesis. Minus that magnesium means minus those grape yields and overall ripening—which will, of course, affect wine quality. Furthermore, too much Potassium in soil means too much Potassium in the grape (duh), which is linked to high pH—which can also cause trouble down the line, as high pH is linked to less stability. Bingo. Low levels of Potassium will lead to poor sugar accumulation in the grape as well as reduced yields and just overall unhealthy and unhappy vines. Bummer.
Phosphorus – Vines only need this guy in small amounts, so usually the natural Phosphorus levels found in the soil are good enough. However, a deficiency in phosphorus could lead to stunted root systems which would mean that the vine isn’t very good at taking up water and nutrients (reducing vine growth and grape yields).
Calcium – Kind of like human bones, calcium helps build the structure of plant cells, as well as aids in photosynthesis. Calcium deficiency is rare, but can have a negative influence on fruit set. (Like, the grape wouldn’t be able to get pregnant.)
Magnesium – As stated above, Magnesium is another nutrient found in chlorophyll and, therefore, super important to photosynthesis. Too little Magnesium means reduced grape yields and overall ripening—which will, of course, affect wine quality.
Oh, and as a footnote: “Several other nutrients play a role in vine growth and reproduction. These include sulfur, manganese, boron, copper, iron and zinc.”
How do those nutrient levels translate into the winery and winemaking aspect? Walk with me through the vineyard and into my humble little winery…
Let’s talk about what fermentation is in the first place. Alcoholic fermentation is the conversion of sugar into ethanol and carbon dioxide. It’s carried out by yeast in the absence of oxygen. An important note is that this process also produces heat, which will also have to be monitored, as too high a heat (above 95°F) can also lead to a stuck fermentation.
When fermentation kicks off, yeast feed off of oxygen, but once oxygen is used up, the process becomes anaerobic. The yeast feed off the sugars in the grape/must and produce alcohol and CO2. But for all of this to take shape, yeast need access to nutrients—specifically Nitrogen.
If Nitrogen levels in the must are too low, this will lead to a stuck fermentation which can result in a rotten egg-like smell from the stressed out yeast producing sulfur compounds. I kind of think of it as the yeast stress-farting…
So, if your yeast is stress farting, what can you do? Give it some Gas-Ex? Sort of. There’s something called Diammonium phosphate (DAP) that can be added to the yeast nutrients. It’s basically supplemental nutrients to help keep those yeasts happy and non-farty.
NOTE: The risk of poor Nitrogen in the must is not JUST a result of soil and/or vine health. Must that is heavily clarified is also at risk at reducing the amount of Nitrogen in the must and thus at higher risk for a stuck fermentation situation.
Too Sweet for Me!
Grapes/must higher in sugar content also struggle to start, or complete, fermentation. And, as a side note, grapes/must with higher sugar content have lower acidity levels and—you guessed it—higher pH levels, which again is linked to microbiological instability (i.e. you’re at greater risk for spoilage). This is a higher risk situation for those grapes that are very mature and/or harvested from extremely warm regions where sugar accumulation in the grape is quite high. Of course, if you’re making a sweet wine, that may not be a problem—you’d want the ferment to stop while there are still some sugars in the must. But for those looking to produce a dry wine, this is certainly not ok—yeast may struggle to transition all sugars into alcohol and will die off before a complete, dry ferment is accomplished. Or, barring that, the higher pH levels in the resulting wine will most likely need to be adjusted (via acidification). But this is slightly outside the scope of this question…
Monitoring that Temperature
As I mentioned earlier, the temperature at which must ferments into wine is extremely important. So, we’re in my imaginary winery and we want to make sure that the temperature isn’t too hot. Typically, you’ll find:
- Fresh, fruity white and rosé wine ferments between 54 and 61°F — promotes production and retention of fruity aromas and flavors
- Lighter style reds or medium bodied whites ferment between 63 and 77°F — this reduces the formation of fruity esters (thus typically utilized for non-aromatic white wines)
- Powerful red wines ferment between 79 and 90°F — higher temps mean greater extraction of color and tannins (but also a loss of some of the fruity flavors)
But, temperatures above 90°F interfere with the yeast enzyme; too cold, and the yeast won’t be able to reproduce fast enough to create a “fermentative population.”
Always love to hear when a winery/winemaker utilizes native fermentation. Because that is *not* an easy task. And one of the reasons why that is: stuck fermentation.
Let’s pause a moment to talk about different yeast types. Saccharomyces cerevisiae is the most common species of yeast used in winemaking for a reason: it can withstand high acidity levels, high alcohol levels, is fairly resistant to the presence of SO2, and it most reliably ferments must to dryness. So, basically, everything you can imagine that goes on in a winery. There are several strains and winemakers can choose strains based on specific properties that can enhance, or restrain, specific varietal aromatic compounds. Interestingly, other species can be used for specific wine styles, like Saccharomyces bayanus for must with high potential alcohol or for re-fermenting sparkling wine.
In many of these cases, the winemaker is using a “commercial” yeast, i.e., one that has been manufactured and can be added to the must.
But some folks really want their own “funk” to the wine, if you will.
Ambient yeasts, or wild or native yeasts, are those that are pre-existing in the the vineyard and winery. There’s a broad range of species that are included in this category, but FUN FACT: most natural yeasts die out as soon as alcohol levels rise past 5%. At that point, the Saccharomyces cerevisiae take over anyway.
The bummer, is that these fermentations may start slowly, putting the must at risk for accumulating unwanted volatile acidity as well as spoilage yeast (like Brett) and bacterias—all of which can lead to an even funkier funk than originally intended. These fermentations, once kicked off, can take longer, which sucks if time and space is an issue. And, as noted, there is an increase risk that the fermentation will completely stop all together, which means you won’t have any wine.
ENSURING A HEALTHY FERMENT MEANS…
- keeping your soil healthy (good levels of Nitrogen)
- harvesting at the proper time (and check those pH and sugar levels)
- not over clarifying your must
- using yeast nutrients (DAP) when needed
- allowing enough oxygen at the beginning of the ferment to ensure rapid yeast population growth.
- monitoring the temperature of the fermentation every step of the way
- rack and return a red wine fermentation early on may be preferable to pumping over (introduces more oxygen to the yeast)
- warming up a stuck white wine ferment to “enliven” the chilly yeast; you can also add additional yeasts of either the same or a more robust species to help re-start (or kick off in the first place) the yeast reproduction
Oh my goodness, we made it. That was a long exposé. How did I do? Did I miss anything? Any comments or questions? Let me know…
Thanks again for studying with me!
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