But before we add wine yeast to crushed grapes (also known as the must), we have to first kill off any existing wild yeasts and microorganisms by adding sulfite. If not, these can contribute to off flavors and spoilage during the fermentation process. Two days after adding sulfite, wine yeast is then introduced to the must.
Yeast Gets Hungry
Very much like baking sourdough, you first make a starter by activating the dry yeast in warm water that is supplemented with some nutrients. (I used Go-Ferm.) Then you feed it with sugar; in this case, a 1:1 ratio of must and water. Temperature control is important as the yeast is quite thermosensitive.
Several hours later, the yeast would feast on the sugar and start to multiply. A healthy starter expands in size and bubbles happily. However, if the yeast is starving, it tells you in the most dramatic pitiful way, which was what happened to me.
Thankfully the fix was simple enough. I just had to add more juice to the starter, and within a half-hour, the starter came back to life. But just in case, it is good to have spare yeast and nutrients around.
Slowly acclimatize the starter by putting the bowl on the must in the primary fermentor for a few hours before pouring it in. Soon, the primary fermentor should feel really warm, and the must should be bubbling away. If you are really quiet, you could almost hear your yeast having a blast with all the sugars in the juice.
Sugar Alone Ain't Enough
While sugar may be delicious, it alone can't keep the yeast healthy. Other nutrients are also needed, and an important one is nitrogen. Unfortunately, Washington grapes are notorious for having a very low nitrogen level, which could inhibit yeast activity and cause a stuck fermentation.
We did an additional measurement on the must this year, known as the Yeast Assimilable Nitrogen (YAN). True enough, our YAN reading was at 33.7 mg N/l, a far cry from the targeted 250 mg N/l. Under the guidance of more experienced winemakers, I measured the amount of nutrients that I needed to add to the must.
|Hydrometer records 0 Brix|
I went with Fermaid K, which consists of a complex formulation of nitrogen, vitamins, and minerals. In addition, I threw in a small dose of diammonium phosphate (DAP) for extra nitrogen. The nutrients and supplements gave the yeast such a nice boost in the fermentation process that I reduced the second dose and didn't even bother with a third dose.
By the tenth day after harvest, the juice was fermented to dryness. Alcoholic fermentation was completed. The must was no more, and I officially had wine.