What Is One-Third Sugar Depletion?
updated 12/12/2025
A common recommendation for the addition of yeast nutrient is to add two-thirds at yeast inoculation time, and to add the remaining one-third at “one-third sugar depletion”. This provides a boost to the yeast in the mid-point of the reproduction cycle (not the fermentation cycle). It also helps prevent hydrogen sulfide (H2S) formation, which can happen if the yeast is stressed by factors such as insufficient nutrient. Overall, the practice of adding additional nutrient at one-thirds sugar depletion makes the yeast happier, which results in a better fermentation.
However (and you knew there’d be a “however”), recently a new winemaker asked, “What does ‘one-third depletion’ mean?”
Something common to experienced members in every field is that things basic to us don’t necessarily make sense to beginners. In other words, what experienced practitioners consider ‘basic’ is not as basic as we often believe. Hence this post.
‘One-third depletion’ means that one-third of the sugar in the must has been consumed by the yeast. This more-or-less means that the fermentation is one-third complete. We use the Specific Gravity (SG) of the must to determine this.
Note: Winemakers treat SG as a measure of the sugar in a must.
This is not exactly correct.
SG is the relative density of the must with respect to distilled water, whose SG is 1.000. The density of a must reflects any sugar present, any alcohol present, plus all other constituents. However, treating SG as a measure of sugar is sufficiently accurate for the purpose of determining one-third sugar depletion.
So how do we determine one-third sugar depletion? Once a must is created, measure and record the SG, referred to as Original Gravity (OG).
For this example, let’s assume the OG is 1.090.
Take the 3 digits to the right of the decimal (“090”) and treat it as a whole number, e.g., “90”.
Divide by 3 and multiply by 2 to get 2/3 of that number:
90 / 3 * 2 = 60.
Divide by 1000 and add 1, which results in 1.060
Once the SG reaches 1.060, we consider this to be one-third sugar depletion. At this point in the fermentation, the remaining one-third of the yeast nutrient should be added.
If the OG was 1.080, then one-third sugar depletion is:
80 / 3 = 26.7
26.7 * 2 = 53.4
1 + (53.4 / 1000) = 1.0534
Note: Don’t focus too much on the exact SG. In the first example (OG = 1.090), any reading between 1.050 and 1.070 is sufficiently close to one-third depletion. In the second example, anything between 1.045 and 1.065 is sufficient. Basically, plus or minus 10 “points” is close enough.
An alternate mentioned in the Finer Wine Kits instructions is to add the second dose of nutrient 48 hours after inoculation. When making an overnight starter, in the last four years I have had 100% success in having fermentation evident (sight and smell) within six hours of inoculation, so this method works fine.
If inoculating by any other method (sprinkling yeast on must, simple rehydration, etc.) a fast start to the ferment is less likely, so adding nutrient 48 hours after inoculation may not work.