I re-researched this every fall when it came time to backsweeten the previous year’s cider, so I wrote this guide to future me.
If you add sugar to hard cider and don’t want that addition to restart fermentation (which would increase alcohol and leave the cider even drier), you’ll need to stabilize it. The most common method in home cidermaking is to add both potassium sorbate and potassium metabisulfite (“K-Meta”). This guidance from BYO magazine on backsweetening provides background on the approach.
For each gallon of cider, use 1/2 tsp of potassium sorbate and 1/2 tsp of 10% sulfite solution (an extra step, but worth making – the solution is easier to work with than dry potassium metabisulfite).
How much potassium sorbate?
Winemakers talk more about sorbic acid, the relevant chemical; potassium sorbate is 74% sorbic acid. There are legal limits of 0.2 g/L (Europe) and 0.3 g/L (America); the sensory level for perceiving this chemical’s flavor is reported at 0.135 g/L.
Adjusting these from sorbic acid -> potassium sorbate (what a homebrewer weighs) gives legal limits of 0.26 g/L and 0.4 g/L, respectively, with a taste threshold of 0.18 g/L.
The container from the homebrew store says “1/2 tsp per gallon.” My high-resolution scale says a typical 1/2 teaspoon of sorbate weighs 1.1g (this container has been opened many times, and perhaps has absorbed moisture). Thus the container recommendation translates to 0.29 g/L.
BYO recommends a sorbate dosage rate of “0.5 to 1.0 g/L”. This is much higher than all other recommendations, with no justification provided.
This winemaking guide, quoting Peynaud (1984), notes that sorbate is more effective – and thus less is needed – at lower pH and higher alcohol %. Unfortunately for us, cider is generally much less alcoholic than wine (pH is similar … I think?).
My approach: just go with the recommendation on the label: 1/2 tsp per gallon, aiming for 0.3 g/L – the limit allowed in American winemaking.
How much potassium metabisulfite?
More complicated process, but more coherent estimates. Sulfites are discussed in terms of ppm of sulfur dioxide (SO2), as this is what matters and there are several ways to add sulfur dioxide to wine or cider. Professionals have procedures for estimating free SO2, as they add sulfites at various stages to maintain a desired SO2 level. If you haven’t yet added sulfites, let’s assume you have no free SO2 in your cider.
BYO recommends a sulfite addition of 30 mg/L.
Try Winemaker Mag’s sulfite calculator, treating your cider as a white wine – you can enter the actual pH and ABV, so this doesn’t seem like a misapplication. When I punch in BYO’s recommended goal of 30 mg/L of free SO2, the calculator suggests I target 36-42 mg/L instead, based on pH.
To achieve this, the calculator suggests adding about 2.6 mL of 10% sulfite solution per gallon of cider – which is just slightly more than 1/2 tsp per gallon. A 10% solution is the easiest way to add sulfites, as the math is simpler and you don’t have to worry about dissolving your sulfites each time. Here’s how to make a 10% solution. I keep a small bottle of it.
Note that a solution of 10% potassium metabisulfite is functionally a 5.76% solution of SO2. When the potassium metabisulfite molecule, K2S2O5, dissolves in water, it gives off two molecules of SO2, comprising 57.6% of the original molecule’s weight. Using a calculator like the one linked above will include this adjustment when recommending a volume of 10% solution, but if you’re working out your own numbers you’ll want to keep this adjustment in mind. (The above-linked article “Solving the Sulfite Puzzle”, with instructions for mixing and using a 10% solution, leaves this out of their example calculation, which confused me for a while.)
The US legal limit for sulfites in cider is 300 mg/L, counting all additions. You shouldn’t be hitting this as a home cidermaker, but if you use sulfites up front (e.g., Campden tablets) to control wild yeasts and microbes, do the math to be sure. Commercial winemakers add sulfites at various stages (including when racking) to reduce oxygen pickup; I don’t know any home cider makers who do this.
My approach: I go with about 1/2 tsp of 10% solution per gallon of cider.