Problems with Fermentation Rate
I may be having problems with a primary fermentation.
Here is the background: I have set out to make a Chardonnay from a 15 L "premium" wine kit that is supposed to contain a mixture of concentrated juice and fresh juice. The juice was reconstituted to 23L according to the instructions and placed in a sanitized carboy. I substituted the yeast that came with the kit with 2 packets of another yeast. The yeast was rehydrated exactly as instructed on the packet and added to the juice. I did not include nutrients. The must was stirred, fitted with an air lock and then left in a dark place at 75°F for 24 hrs then in a cooler place at 60°F. The specific gravity was measured daily and plotted (e.g. on the vertical axis, time in days on the horizontal axis). Staring at 1.090, it dropped about 10 units/day for the first three days. The rate is now slowing down to less than 5 units/day and the curve is becoming more and more horizontal. At day 6, it is reading 1.040. As the curve is becoming increasingly horizontal, I find I am becoming increasingly anxious that it will actually completely flatten out and I will be left with a sweet, partially fermented brew that is stuck. One winemaking book I consulted simply stated that the primary fermentation rate slows after a few days but it did not explain how or when or at what rate it is supposed to slow down. So perhaps I am needlessly concerned. However, in another book, I found a graph showing yeast cell population growth and specific gravity plotted against time. In this graph, the two curves are shown as mirror images of each other. If this is really the case, then my specific gravity curve should not flatten out until nearly the end of the primary fermentation when the s.g. approaches about 1.010. If this is true, then my fermentation is becoming stuck, in which case I should take measures to prevent this misfortune. The information in these sources does not seem compatible. Which is correct? What would you advise me to do? You are to be complimented on the effort that you are putting in to monitor your fermentation by building a good data bank. Your data seems to make sense. During the growth phase of the yeast there will be a rapid drop in sp. Gr. And increase in alcohol production. Yeast produce several times as much alcohol per yeast cell per unite time during this growth phase than it does when it reaches the stationary phase. So it is to your advantage to keep the yeast growing as long as possible The growth phase levels off at 100 - 150 million yeast cells per ml and becomes the stationary phase when there is about 5 % alcohol. The yeast remain alive and eat away at the sugar, producing alcohol, to obtain energy to stay alive but they have more difficulty as the alcohol level builds above 5%. The production of alcohol will be at a slower rate and it will continue to slow down during the last 1 - 3% sugar, 1.015 & below sp. Gr. You are right to be concerned about stuck fermentation. Some juice in wine kits is very low in nutrients for the yeast and low in solid matter that usually assists in keeping the yeast in suspension even during active fermentation, especially under cooler fermentation temperatures. With your future fermentations it would be a good idea to add some type of balanced yeast nutrient such as Fermaid K at 0.1 - 0.2 grams per liter plus 0.2 - 0.4 grams of diammonium phosphate (DAP) per liter. This will supplement a balance of the yeast nutrients to grow and build healthy bodies. Recent studies show that the best way to add the nutrients is to divide the total amount of nutrients into several portions and add it over the first several days of fermentation. Another factor to consider regarding juice from concentrate and kits is the fact that there is usually very little to no insoluble, particulate matter present. The absence of particulate matter allows the yeast to settle rapidly during the period that there is very little fermentation activity. This is especially true with cool temperature fermentations. It is advisable to stir the must several times during the first 24 - 48 hours and again during the last days of fermentation to keep the yeast up in suspension where all of the action is. Another very important yeast health consideration is oxygen. Yeast needs oxygen during the growth phase and by leaving the airlock off for the first 2 - 3 days. The yeast needs oxygen to produce lipids in their cell walls to protect themselves against the alcohol toxicity near the end of fermentation. To help your current fermentation and prevent it from sticking it may not be too late to leave the airlock off for a couple of days provided the yeast are still showing signs of fermenting. It would help the fermentation, if you agitate the must in the carboy several times over the next few days to get the settled yeast on the bottom up in suspension where they can do the most good. With an unhealthy fermentation you will definitely have a long slow horizontal tail end of fermentation. With a healthy fermentation (adequate oxygen, agitation and nutrients added over the first half of the fermentation), the yeast will remain alive and healthy to the very end of the fermentation so there will be very little plateau at the end of the sugar curve.