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I recently tried fermenting pure apple juice. I didn't add sugar to reach my goal for ABV. I took enough juice out to use to make a starter, but my yeast was weak. It was close to expiration, so I added fermax and other boosters and rehydrated the yeast. After pitching this into the main must, I let it sit for 60 days to be sure all the sugar was used up. When I racked the cider, I checked the alcohol level and it read around 5% alcohol content. It has a bold, rich flavor because I used zero water, but it's weaker than I expected. Can I pitch fresh yeast to bring the alcohol content up to 10+%?

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It is possible that by adding a yeast that is more alcohol tolerant and better at consuming simple sugars, such as a champagne yeast, you could get somewhat higher ABV than this result from whichever yeast you used (was not specified). However, really the only way to achieve 10% ABV or more in a cider is to add quite a bit of sugar. Apple juice on its own can easily achieve 6 to 7% ABV in most cases. Your case was a little less but not unheard of, for a juice with relatively low sugar and perhaps a worn-out yeast. For next batch, I would consider another sugar source, including regular beet or cane sugar, honey, maple syrup, grape juice, etc. to increase original gravity towards 1.075 (about 18 Brix) or higher. The original gravity of regular apple juice is usually 1.040 to 1.050 (about 10 to 13 Brix) which will only create 6 to 7% ABV at most.

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Two things that stood out to me: Zero Water, and lack of pH buffering.

Brulosophy has tested the impact of water mineral balance in its "exbeeriments", which often dramatically invalidate some brewing steps. They repeatedly found that water mineral content is one of the most important factors in a brew. Removing all of the minerals via reverse osmosis ("RO") as in Zero Water, can actually make it harder for the yeast to survive. Now in one test, it had no identifiable impact, but that was a beer, which has a wealth of natural nutrients that fruit juices often don't.

Separately, yeast fermentation causes the pH of a brew to drop (become more acidic). Beer malts naturally stabilize this effect, so the pH never goes too low, but in many other fermented products, especially fruits and sugar washes, the pH can drop below the level where yeast survives easily. 12+ month aging times often overcome this, but it can be prevented entirely by using an additive. K2C03 (potassium carbonate) is favorite, adding 1/8 of a teaspoon at the beginning per pound of sugar (for apple juice, it would be the sugar content of the juice).

Lastly, there's not really any such thing as "not enough sugar" for fermentation. You can have too much, or so little that is has no noticeable effect, but you never need at add extra sugar to make it work in the first place. You can ferment plain apple juice if you want, provided it doesn't have yeast-killing preservatives, and has a pH and nutrient profile that yeast can handle- the final product just might not be very strong.

EDIT: To add, you mention that you "checked alcohol level". It's a common mistake to use a hydrometer that has "ABV" labels and believe it indicates the alcohol content. A hydrometer just measures the density of the liquid, and sugar and alcohol each affect this, so without knowing the difference in levels, you don't know how much of the reading is from alcohol and how much is from sugar. The only way to know the alcohol content is to know the difference between the starting and finishing gravity.

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