High Brix Red Fermentations
What a difference a year makes! Whereas last year I received inquiries about much juice can be chaptalized, this year I am getting questions about managing fermentations with high potential alcohol. Some growers are seeing high Brix accumulation while waiting for phenolic and flavor ripening. If you find yourself calculating potential alcohols that seem high, there are a few things to keep in mind as you formulate your winemaking plans. Following are notes from a conversation I had with Nichola Hall of Scottlabs on this subject. Recommendations from other product reps and sources are also included.
Potential alcohol estimation
Brix is a good indicator of potential alcohol within normal ranges, however as you get to higher brix (above 25), it is more accurate to calculate potential alcohol from glucose/fructose numbers. Nichola Hall from Scottlabs (personal communication) explains that that at high brix, the proportion of soluble solids due to glucose and fructose is less consistent, and often higher, meaning that potential alcohol estimates from brix alone to be too low. The OIV calculation for potential alcohol is
Glucose/fructose (g/L)/16.83 = % Ethanol
Choose your yeast carefully. Yeast have differential ability to tolerate high alcohol levels and complete fermentation due to differences in their ability to produce transport proteins and compounds to stabilize cell membranes under high alcohol conditions. Yeast manufacturers will list a tolerance range for each strain. Also, choose yeast with lower nutrient requirements, if possible (see #5 below). If you want a recommendation for your specific needs, contact the company representative and he/she will be happy to help. Table 1 lists yeast strains generally recommended by representatives from several companies.
Raise your yeast to go the distance
Proper rehydration conditions (nutrients and temperature) and aeration early in fermentation help yeast produce healthy cell membranes that will enhance ethanol tolerance later in fermentation.
Eglantine Chauffour from Lamothe Albeit cautions that at higher brix, and especially with shriveled grapes, the ratio of glucose:fructose may be different. In his article on glucose:fructose ratio in wine grapes, Snyman (2006) explains that glucose is preferentially produced in the grape prior to veraison, but after veraison, fructose is produced at a rate that leads to equilibrium between these two sugars at maturity. When grapes become overripe, some cultivars will experience a glucose:fructose ratio less than one. Most yeast preferentially take up glucose over fructose, though both sugars are transported when they are in a mixture. If there are higher levels of fructose than glucose, as in overripe grapes, co-transport stops once the glucose is used up, leaving residual unfermented fructose. To counteract this effect, Chauffour recommends inoculating with a Bayanusstrain of yeast at 3-5° Brix to finish fermentation. Bayanus is a fructophilic strain that will preferentially utilize fructose.
High Brix wines often have lower levels of nutrients, while the amount of nutrients needed by yeast to complete fermentation is higher. Most yeast manufacturers provide guidelines for nutrient additions according to brix level. In general, Zoecklein (2001) provides the following:
21 brix = 200 mg/L N
23 brix = 250 mg/L N
25 brix = 300 mg/L N
27 brix = 350 mg/L N
High temperature further destabilizes cell membranes. So, if the yeast you are using is near the limit of its alcohol tolerance, high temperature may prove to be too much stress on the cell membrane. Nichola Hall recommends keeping fermentations below 82°F if you are at high potential alcohol.
When yeast are stressed, they produce more acetic acid. Monitor high Brix fermentations carefully for VA accumulation.
Lowering the conversion rate
If you are looking for strategies to lower the conversion rate of sugar to alcohol, Nichola Hall reminded me that non-Saccharomyces yeast such as Torulaspora delbtrueckii and Metschnikowia pulcherrimia have low fermentative capacity, but still metabolize sugar. Generally used as bio-protection during cold soak or to increase complexity in the wine, these can also be used to lower the potential alcohol of the wine. Keep in mind, though, when using non-Saccharomyces yeast you will also need to do a second inoculation of Saccharomyces. Make sure to read the manufacturer’s instructions for rates and timing of addition. Use a longer time frame for more Brix depletion.
If your wine ends up with noticeable imbalance due to high alchol, polysaccharides can help integrate the heat. If you anticipate this ahead of time, choose a high polysaccharide producing yeast. Aging on lees is another way to add polysaccharides. Enological companies also sell yeast lees products and mannoproteins.
If you plan to water back the juice, be careful with your calculations (you can’t take the water back out), only add 60-70% of the water at first, and acidulate the water with 4-6 g/L tartaric acid (or a mix of tartaric and malic acids).