Successful Wine Fermentation - Fermentation Monitoring
Posted May 2012; updated November 2015
Winemakers all have varying opinions on the best, or most efficient, way to monitor fermentation. These opinions will be largely dependent on several variables such as facility size, harvest pace, staffing, and the type(s) of ferments used.
My main goal is to track as much as possible about individual batches so that I can not only ensure successful fermentation, but also gather data that will be useful in future vintages. So, I tend to be rather meticulous in my fermentation monitoring.
Pre-fermentation Analysis
My first priority is getting accurate analysis prior to inoculation. The completeness of this analysis really depends on the laboratory, but ideally I like to have brix, pH, TA, YAN, free and total SO2, malic acid, temperature, and VA. These analyses should take into account any amendments made to the juice/must since arrival at the winery, providing a good baseline to track that the ferment is proceeding healthily and a good starting point for determining any additional ferment amendments desired.
Nutrients
I do not agree with the practice of standardized nutrient additions before or during fermentation. Plenty of juice/must does not require additional nutrients, why waste the money and suffer in down-the-line processing?
Nevertheless, nutrient additions are often beneficial. When they are to be made, such amendments should always be made before brix levels drop too low. I usually aim for 1/3 or 1/2 sugar depletion for secondary additions of fermentation nutrients and nitrogen supplements.
If undesirable odors and/or flavors appear later in fermentation, additional amendments can be made. It is uncertain at what level yeast become unable to synthesize nutrients due to low sugar levels and high alcohol levels, so try to ensure any amendments are completed prior to brix falling below 2/3 depletion.
Fermentation Charts
My colleagues will tell tales of my semi-obsession with spreadsheets and charts. We all have our quirks.
A basic fermentation chart comparing dates versus brix and temperature gives a good visual of how the fermentation is proceeding, aid with down line processing: amendments, temperature adjustments, cap management (for red must), stopping fermentation (if residual sugar is desired), and planning fermentation tank turnover time for future vintages.
Barrel vs. Tank
Timing of analysis can be important. Typically, I work on a 24-hour ferment check for all tank ferments. This applies not only to lab analysis, but sensory. If anything appears strange, then further analysis may be desired.
Barrel ferments are often more difficult to track accurately compared to those in tank. This is due to variability between barrels and less temperature control. You can't expect laboratory staff to complete a representative sample on 100-barrel wine lots. Depending on the winery size, 24-hour ferment checks will likely prove too time consuming.
Random representative sampling and increased periods between analysis are the standard practice granted the ferments are proceeding healthily.
Speed of Ferment
I find that ferment speed is either immaterial or of the utmost importance depending on the winemaker. The major concern is the initial lag phase before the first brix drop and the final stages as the ferment moves towards dryness if that is the goal. A general rule of thumb for healthy fermentation is a 1° brix drop per day, but this is highly dependent on the winemaker's choices involving these four variables:
- Type of inoculation, or lack thereof. Typically Indigenous yeast will have the longest lag phase and biggest issues with completing fermentation, while wet inoculated ferments will have little or no lag time and whiz straight through dryness without issue. Dry inoculated ferments fall somewhere in the middle.
- Inoculation rate. Within reasonable range (< 7mcell/mL), inoculation rate and fermentation speed should have a direct relationship. The higher the rate, the quicker the ferment.
- Temperature. Again, direct relationship with ferment speed within reasonable values.
- Nutrient availability. Low levels of nutrients may slow fermentation down, but is more likely to cause stress, but poorly managed additions of diammonium phosphate (DAP) can lead to big fluctuations in ferment speed. Too early or too much will cause a massive spike in yeast populations, rapidly increasing ferment speed that will drop off again due too overpopulation and lack of food supply.
Stop Ferments & Residual Sugar Checks
If stopping fermentation with residual sugar is desired, I'll create a brix target to mark the beginning of residual sugar (RS) checks. This will hopefully be a couple days in advance of the ferment reaching the target RS to ensure it is under control and stopped on target. For ferments aimed at dryness (less than 2 g/L RS), RS checks can begin once the ferment reaches -2° brix (or maintains a negative brix reading for several days).
