If you use a 1-5 gallon (3.8–19 L) yeast packet for 1 gallon as opposed to 5 gallons, it is likely that your fermentation will proceed faster, have a more yeasty aroma and go to completion (go “dry”) more quickly. It’s also likely that the fermentation will occur at a higher temperature because yeast generate a lot of heat when they’re working fast and furiously. When concentrated in a small area — say, in 1 gallon as opposed to 5 — the density will cause the temperature of the wine to rise. As long as you ferment to dryness (which means all of the sugar is consumed by the yeast), more yeast in solution won’t cause a higher final alcohol content, because that’s determined by the initial sugar concentration of your juice, regardless of volume. However, if you’re starting with a very high initial sugar (over 26 °Brix), adding more yeast may give you a higher final alcohol level. This works because a stronger fermentation will result, and the yeast may be able to ferment it all to dryness. If you had a high-sugar must or juice to begin with, the yeast may be stressed by the sugar level (and by the resultant increased alcohol concentration) and might die before the fermentation is complete. This shortcoming would leave you with residual sugar as well as a lower alcohol level.
If you have a challenging fermentation and dryness is your aim, I would suggest using the higher dose rate. Some conditions that can contribute to a less-than-ideal fermentation include: high initial sugar (over 26 °Brix), cold temperatures (under 60 °F or 15.6 °C at inoculation), damaged fruit, and moldy or infected fruit or juice.
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