Homogenizing a Must

This post is in response to yet another common question among new winemakers: Why does my SG change from day 1 to day 2 with no fermentation?

This has been asked by grape, fruit, and kit winemakers. The answers (yes, this is plural) to this question differ depending on what type of wine is being made.

In November 2022 I checked my notes from the previous one and and a half years, making a table of the Specific Gravity (SG) of must on Day 1 (when must created) and Day 2 (just before inoculating). Nine out of the 10 musts had a different SG the following day.

Two of the ten (marked in red) had the SG go down on the second day, one (marked in blue) had the SG remain the same, and seven (marked in green) had the SG rise on the second day.

I thought I was stirring enough. There’s nothing like practical evidence to convince me that I am NOT stirring as well as I believed.

Note that these wines are all kits, so they start with a concentrate that’s around 68 brix (percent sugar) and are diluted to roughly 21 to 24 brix. The concentrates are very thick and hard to stir. I use a drill-mounted stirring rod which works well in general, definitely better than hand stirring.

I surmise the concentrate was not fully dissolved and/or not evenly distributed in the must. Nothing was added or removed overnight, yet the SG changed in 90% of the instances.

After reflecting on this, the one answer that comes to mind is that the must wasn’t stirred enough. Overnight the must homogenized itself more, and a final stirring prior to checking SG helped ensure the concentrate was better dissolved and more evenly distributed.

Assuming I’m correct, how do we fix this?

Note: My overall process typically calls for creating a must on Day 1, and inoculating with an overnight yeast starter on Day 2.

Kit Wines

Kits normally contain a high brix concentrate that is diluted with water to a useful level.

I expect it’s common to put the concentrate into a primary fermenter, fill with water to the correct dilution level (typically 6 US gallons / 23 liters for most kits), then stir to dissolve. If this isn’t working, then it makes sense to start stirring sooner.

This is my current process for kits and any wine involving a concentrate. While honey is not a concentrate, I use the same technique when making mead to ensure it dissolves.

1. Add the concentrate to the primary fermenter and rinse the bag, emptying the bag into the primary. This is typically 4 liters of water (more or less).
2. Stir with a drill mounted stirring rod for 1 minute, changing direction half way through.
3. After each additional gallon, stir again for 1 minute.
4. When at full dilution (typically 6 US gallons / 23 liters), stir for 2 minutes, changing direction every 30 seconds.
5. Check the SG in 3 places in the primary, simply out of curiosity. After this much stirring the readings may differ a bit, but should be within 1 or 2 points of each other.
6. In the morning, stir again for 1 minute, changing direction half way through.
7. Check the SG in 3 places. If the readings are not within 1 point of each other, stir again. If 2 readings differ by more than 5 points, stir for 3 minutes.
8. Repeat until the readings are within 1 point.

At this point, continue with the winemaking process.

Grape and Fruit Wines

Grapes and fruits don’t have concentrate, so the answer must be different.

Discussions with fellow winemakers illuminated two points:

1. The must is thick so it takes more effort to ensure it’s homogeneous.
2. The sugar in the fruit takes time to come out. Cold soaking (fruit is crushed but kept cold so it doesn’t ferment) for at least an overnight gives the sugar time to come out of the fruit in to the liquid.

This indicates the answer is both stirring and time.

Grapes and fruits have a lot of solids, and especially with grapes and fruits made with no added water, the must is thick and hard to stir.

The answer to that is perseverance, as my method indicates:

1. Put the fruit into the primary fermenter along with any necessary water.
2. Stir, punch down, and/or mix for at least 3 minutes. The thicker it is, the longer it should be stirred. [We’ll call this “stirring” to make the wording easier]
3. Check SG in 3 places. If any 2 readings differ by more than 5 points, stir again for 3 minutes, and check SG again.
4. Repeat until the readings are consistent
5. In the morning, stir again for 3 minutes. Check SG is 3 places. In most cases the SG will be higher than the previous evening.
6. Repeat until the readings are within 1 point.

If the readings are within 1 point continue with the overall procedure.

Chaptalization

The previous sections provide basic processes that work. But they don’t include chaptalization, e.g., increasing the SG in a must that is low is sugar.

This part is not as clear cut — a judgment call must be made.

I do not blindly follow a recipe with regards to added sugar. I start with the SG of the must, consider the desired target SG (which varies by wine type), and use that information to calculate how much sugar I need to add. [I have a separate post in which I explain how I chaptalize.]

The difficulty here is deciding when to use the SG of the must to hit the target SG. I use my experience to make that judgment call, and do my best to explain how I do this:

Stir well on Day 1, and if the SG readings are consistent, that’s the starting point. Use that to make the sugar calculation.

An important point — the target SG is not an absolute requirement. If I’m within 2 points of that (high or low) I’ll probably call it good. For higher SG musts, even 3 or 4 points off is probably sufficient.

Then add about 3/4 of the calculated sugar amount in a thin stream while stirring. Keep in mind another important fact: it’s far easier to add more sugar than to take some out. This is another thing that may sound humorous at first, but it quite literally true.

Then stir well. And then stir again. After that, check the SG in 3 places and look for consistency.

Here’s the judgment call: if the SG is lower than the target SG, decide to wait until the morning to add more sugar, or add it now. If there is any concern, wait for the following day.

If more sugar is needed in the morning, stir well, doing the process twice. If the SG readings are consistent, the must is ready for inoculation.

Conclusion

The above works well and has been in use for a couple of years in 10+ batches of wine.

I use a similar process when adding glycerin at bottling time. When doing the final racking I decide how much glycerin to add, and direct the siphon hose into the sanitized glass containing the glycerin. This dilutes it as it’s added to the wine. Then depending on how large the batch is, the must is stirred during the racking, and again when the racking is complete.

We do the same process if backsweetening the wine.

The results indicates this works well.