Using ascorbic acid to scrub dissolved oxygen

Q: Can I put ascorbic acid in my cans before I fill them with beer to scrub any dissolved oxygen? Also, how would I dose it?
— Darren O’Day • Philadelphia, Pennsylvania

Mr. Wizard says…

A: Ascorbic acid, also known as Vitamin C, is indeed an antioxidant often discussed in the context of beer stability because of its ability to scavenge oxygen in the headspace of packaged beer. Its mode of action is somewhat different from many other antioxidants. Ascorbic acid does not directly bind oxygen. Instead, it donates hydrogen atoms to reactive oxygen species such as peroxide radicals, thereby neutralizing them. This transfer of electrons is the fundamental chemical process we call oxidation.

However, the behavior of ascorbic acid can be complicated by the presence of transition metal ions, particularly iron and copper (Fe²⁺ and Cu²⁺). In the presence of these metals, ascorbic acid can actually promote the generation of reactive oxygen species rather than prevent it, effectively reversing its antioxidant role and causing oxidative damage. For this reason, successful use of ascorbic acid requires very low levels of these metal ions. Unfortunately, in brewing, keeping metals out can be difficult. Common brewing inputs such as hops, malt, brewing water, and filtration aids can all be sources of copper and iron. But for the sake of discussion, let’s assume a beer with minimal problematic metal ion content.

When used under the right conditions, ascorbic acid is safe for beer, though flavor limits must be respected. At higher concentrations, it imparts a noticeable tartness. The threshold for this effect varies depending on beer style: Light, delicate beers can show tartness at concentrations above about 10 mg/L, while more flavorful or heavily hopped beers can tolerate slightly higher levels. As is often the case in brewing, bench trials are invaluable for determining the appropriate dosage for a specific recipe. A common working range is 5–10 mg/L.

The most practical way to add ascorbic acid is as an aqueous solution. Because the compound itself is reactive toward oxygen, the solution should be prepared in deoxygenated water — ideally water that has been boiled, cooled, and stored in a sealed container to minimize oxygen pickup.

The dosage calculation is straightforward when using metric measurements. For example, suppose you are working with 20 liters of beer and wish to dose at 10 mg/L. That requires 200 mg of ascorbic acid in total. Since the solubility of ascorbic acid in water at 68 °F (20 °C) is about 330 g/L, there is no risk of creating an overly concentrated stock solution. A convenient approach is to prepare a 40 g/L stock solution. 20 mL of this solution will contain the required 200 mg of ascorbic acid for the 20-L batch. Unless you have a highly accurate scale, mixing up 4 g of ascorbic acid in 100 mL of water is easy to measure and gives you plenty for bench trialing and dosing.