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PRE-FERMENTATION

Pre-fermentation is a strategic time to selectively extract color, amend grape must, and establish the conditions for a desired wine style. Beginning with cold soaks, winemakers can take advantage of anthocyanin’s polar nature to selectively extract color before tannins. This is the only selective phenolic extraction tool, and it is very useful for advancing and maximizing color extraction in red wine. The cold soak temperature influences extraction efficiency and also acts as a selective mechanism for microbial activity. 

COLD SOAK

Cold soaking is the only selective extraction mechanism winemakers have to distinguish between anthocyanins and tannins. Its key advantages are advancing color extraction, increasing peak total anthocyanin extraction, and changing wine flavor. The utility of cold soaking is subject to grape quality and their phenolic balance. Free anthocyanins and iron-reactive phenolics will extract readily during cold soak because free anthocyanins are water soluble and because non-flavonoid iron-reactive phenolics extract readily from the pulp. Protein-precipitable tannins from the skins and seeds will not extract until the start of fermentation when alcohol and temperature increase. If the intention is advance color extraction, then the best candidate is fruit with abundant color and tannin (mountain fruit, high tannin cultivars, etc.). By extracting color early, you can manipulate the fermentation variables to moderate subsequent tannin extraction (low fermentation temperature, less movement, shorter maceration). A cold soak extraction lower than 30% is considered inefficient while greater than 50% is considered highly efficient. The remaining color will extract within the first 10 days of an active fermentation, if not much sooner. The following are general criteria for a typical cold soak.

  1. Duration: 1-10 days
  2. Temperature Range: Low 5-9°C (41-60°F), Medium 10-12°C (50-54°F), High 13-16°C (55-61°F)
  3. Movement: 1-2 volume anaerobic pumpovers, 1-2 times a day. 

Note: It is fundamental to maintain an inert headspace during cold soak. Oxygen solubility increases with cooler temperatures, and higher dissolved oxygen can be deleterious to quality. Add a healthy scoop of dry ice pellets on top of the cap at the end of each day or as necessary.  

If the intent behind cold soaking is to increase peak color extraction, then we also need to stabilize that color within 100 days post-crush. Cold soaks have reliably proven to extract higher peak colors during primary fermentation, but research has also demonstrated the finished wines have the same concentration of color. This is because peak total anthocyanin extraction is finite and temporary. Winemakers can take advantage of the color potential by using warm primary and secondary fermentation temperatures to stabilize color within the first 100 days post-crush. Color stabilization, aka bound anthocyanin formation, will polymerize increasingly fast above 21°C (70°F). The best demonstration of this is the opposite of a cold soak, flash détente. Flash détente heats unfermented grape must up to about 85°C (185°F) before rapidly cooling the wine in a vacuum chamber. Even after a very short time, these extreme temperatures form high concentrations of bound anthocyanins. A classic example of this is megapurple.

Amendments

Any additions added during this time should be carefully considered. For instance, nutrients should be added after the start of fermentation to avoid feeding non-fermentative yeasts and bacteria. Sterile filtered water should be used when watering back a high Brix must to prevent contamination. There are also unadvertised side-effects of many winemaking additives described below.  

Sulfur: While its use is ubiquitous, there are lesser-known consequences to adding sulfur before fermentation. The key concept worth considering is that sulfur isn’t providing much antimicrobial activity in red musts. Adding 50 ppm at the crusher will result in 0 ppm free sulfur by the end of fermentation. When free anthocyanins are present, they are readily bleached by sulfur dioxide thus restricting its antimicrobial behavior. In addition, sulfur-bound pigments are restricted from polymerizing with tannins and other IRPs in the fermenting/finished wine. This can have a significant impact on the resulting mouthfeel of the wine. With proper grape quality and sanitation practices, winemakers should be confident fermenting clean wines without early sulfur addition. For reassurance, Cathy Corison has abstained from adding SO2 until after malolactic completion for 40 years. Her reason is that sulfur extracts harsh phenolics, so she takes a proactive approach to monitoring for off-flavors and uses ETS laboratory’s scorpion test for validation. She also harvests earlier than most of her neighbors which aids in maintaining grape quality (link here).

Pectolytic Enzymes: While these enzymes are often marketed as being selective towards color or tannin, their true nature is much less selective. Pectolytic enzymes are powerful for increasing the speed of extraction when needing to turn a tank, but not for selecting color or tannin. All phenolics tend to be released indiscriminately.

Lysozyme: While lysozyme is marketed towards removing gram-positive bacteria, there is no mention of its powerful tannin fining capabilities. If added before fermentation, a winemaker is likely to never notice the impact of lysozyme on tannin structure. Post-fermentation, however, lysozyme is perhaps the most powerful tannin fining agent available. It has been observed dropping tannin concentrations upwards of 50% depending on the dosage. This might be complementary to a hard press fraction, but not to a quality free run wine. Furthermore, our experience suggests that long polymeric pigments are also subject to removal from lysozyme.

Flavor

If the intention of cold soaking is to introduce flavor and mouthfeel complexity, then leaning towards a warmer cold soak may be of interest to you. Higher cold soaking temperatures of 13-16°C (55-60°F) increase the diversity of microbial activity which can aid complexity and mouthfeel. As per usual, flavor changes are highly subjective to each wine and the winemaker’s style. The best approach is to experiment with different temperatures and make decisions accordingly. In any case, winemakers should track the phenolic evolution during cold soak as well as the following parameters to understand the quantitative changes in their must pre-fermentation.

Measurement

Volatile Acidity (VA): Depending on the quality and integrity of the fruit, winemakers may want to measure volatile acidity several times during a prolonged cold soak. A generally good practice is to measure VA within the first couple days of cold soak to gauge what VA came from the vineyard. Around day 3-4, assuming more berries have broken, a second measurement can help inform winemakers of any VA being generated by the microorganisms in the cold soak. Generally, by this time, VA decreases due to dilution from the grape juice, but in as a result of extreme heat events levels can get as high as 0.4-0.8 g/L. In hot vintages, a VA of 0-0.05 g/L is typical pre-fermentation. Depending on your starting values, fermentation can decrease VA throughout fermentation as yeast metabolize it and/or it blows off with CO2.

Yeast Assimilable Nitrogen (YAN): Measure YAN before cold soak and immediately after the start of fermentation. This informs winemakers of the nutrients that are available to their fermenting yeasts and thus if they need to make any additions. From experience, it has been determined that 90-120 ppm is an effective minimum, but this is ultimately subject to each fermentation. We encourage winemakers to test for YAN in their finished wines as a measure of nutrient demand. Targeting a concentration below 20-30 ppm is typical, but unchecked it is common to see concentrations over 100 ppm in finished wines. This is likely due to the thought that reduction in wine fermentations is due to low YAN. We suspect that this has more to do with H2S-nutrient interactions because what coincides with nutrient addition is a sharp drop in oxidation-reduction potential (ORP). One would assume a drop in ORP would produce more H2S, but it almost immediately dissipates. We, therefore, suspect that the nutrients are either reacting with the H2S or that a drop in ORP signals yeasts to stop producing H2S. Either way, nutrients should not be added late in fermentation. Winemakers should instead focus on the complexity of H2S formation as a combination of yeast stress due to a variety of factors (temperature, alcohol, nutrients, etc.) and/or due to negative ORP due to IRP reductive strength.

Conclusion

In summary, pre-fermentation is a very valuable time for a winemaker to impart stylistic decisions according to the qualities of their fruit. Not only are cold soaks the only selective extraction tool to distinguish between anthocyanin and tannins, but temperature is also a biologically selective tool to facilitate the activity of non-fermentative yeast and bacteria. In general, cold soaks do not make every wine better and they consume a lot of energy with the high refrigeration demand. Choosing how to treat/amend your must pre-fermentation can result in both good and bad quality changes that persist into the finished wine.