Most wineries utilize active dry wine yeast to directly inoculate their juice/must. Many winemakers are beginning to in-house yeast propagation, a process that was far more prevalent several decades ago.
Active dry wine yeast strains are propagated from pure cultures of yeast that are known to produce quality wine. Propagation is conducted using specialized equipment, and the yeast is supplied with adequate nutrients and oxygen through growth phase until they are dehydrated from approximately 70% water content to just 8%. The dried yeast is is then tested for several qualities, including % solids/proteins, stability, viable population, bacteria, and killer factor. Killer factor refers to yeast that excretes a toxin that kills other yeasts sensitive to it. Then, the yeast is vacuum-packed and ready for sale.
Yes, there is quite a process behind creating active dry wine yeast and such products are user-friendly for wineries. By implementing a yeast propagation program, a winery can benefit greatly.
A well-organized yeast propagation program can produce significant cost savings, particularly for medium- to large-scale operations. Such wineries are using thousands of dollars worth of active dry wine yeast for dry inoculations during a given vintage.
When I have worked with yeast propagation in commercial winery settings, we regularly propagated 2 kilograms of active dry yeast to inoculate 250,000 liters (approx. 66,000 gallons) of juice. Using direct inoculation at 250 ppm, this would require 62.5 kilograms of active dry yeast. Depending on the yeast strain, one may be paying $50-100 or more per kilogram; if you're fermenting a few million liters of juice, you're talking about a lot of money. Add in the cost of using yeast rehydration nutrients and the savings nearly double.
Fortunately, winemaking is not always about the financial bottom line. For most winemakers, they are far more interested in wine quality benefits than financial gains. So can propagated yeast help?
During the inoculation process, yeast are subject to numerous stress factors, including temperature difference, population dispersion, and sugar toxicity. During direct inoculation procedures, active dry wine yeast is rehydrated, quickly acclimated, and added to the juice/must.
When propagating yeast, they are given optimum fermentation conditions. This includes making sure the sugar levels are not too high and the pH is well within the safe range. The culture can then be slowly acclimatized over 1-2 days prior to inoculation of the juice/must, giving the yeast time to build up tolerance to stressors.
This means propagated yeast cultures, also referred to as yeast starters, tend to have:
Juice/must is highly susceptible to undesirable microorganisms prior to fermentation, from native yeast such as Pichia and Kloeckerae to other microbes such as Lactobacillus and Brettanomyces.
Direct inoculation usually results in 1-2 days of lag time before fermentation truly kicks off, while yeast starts will begin fermentation immediately upon inoculation. The faster the chosen yeast strain begins actively fermenting, the more protected the juice/must will be from undesired activity.
Many desirable flavor and aroma compounds in juice/must dissipate quickly prior to fermentation. The faster the fermentation starts, the more likely these positive compounds will be retained. The difference in fermentation onset when inoculating with a yeast starter and with direction inoculation discussed above can mean a far duller wine.
Manufacturers of active dry wine yeast typically suggest an inoculation rate of 250 ppm. Following recommended rehydration procedures, the inoculum volume is typically 1-3% of the juice/must volume, with a final viable yeast population between 2,000,000 and 5,000,000 cells/mL. Fermentation kinetics will be significantly different at the higher and lower end of this spectrum.
This variability is easily eradicated within a winery's yeast propagation program. The viable yeast population is accurately analyzed and tracked throughout the entire process, allowing the winemaker to be assured that inoculum is added at the desired inoculation rate. Since the yeast starter is acclimatized for significantly longer (sometimes 100 times longer), the inoculum volume tends to be be significantly higher with the added benefit of relieving issues with population dispersion.