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<span style="text-decoration: line-through;">A storage solution of 1 g citric acid and 2 g potassium metabisulfite per L (or 1 lb potassium metabisulfite and 0.5 lb citric acid per 225 L barrel<ref>[http://www.thebrewingnetwork.com/the-sour-hour-episode-5/ The Sour Hour with Tim Clifford of Sante Adairius]</ref> (~40 min in)) is often used.</span>
''Update:'' The lined out instructions above have sometimes been advised by wine makerswinemakers, and presumably taken from their instructions <ref>[http://www.juicegrape.com/documents/documentview.ashx?id=412fa046-2de0-4051-9d41-a1ee012cea11 M&M Wine Grape Co. Citric Acid & Sulfite Barrel Preparation. Retrieved 03/06/2016.]</ref>. Jay Goodwin from The Rare Barrel, however, reported sulfur off-flavors in beers from using too much potassium metabisulfite in their barrels. The beers needed extended aging for the sulfur to be volatilized off. Eric Salazar from New Belgium Brewing advised that they cut the potassium metabisulfite powder down to 1 ounce per 59 gallon barrel (~0.017 ounces per gallon/~0.127 grams per liter) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1648119741882900/?comment_id=1648160011878873&reply_comment_id=1648559265172281&comment_tracking=%7B%22tn%22%3A%22R0%22%7D Mike Makris.
Milk The Funk Facebook group. 04/12/2017.]</ref>, and to check the barrel once a month and change the storage solution once a month if needed <ref>[http://www.thebrewingnetwork.com/the-sour-hour-episode-28/ The Sour Hour Episode 28 (~49 minutes in). Jay Goodwin from The Rare Barrel. 02/17/2016]</ref><ref name="makris" />. The amount of citric acid is the same: 0.5 lb per 59 gallon barrel (1 gram per liter/0.13 ounces per gallon) <ref name="makris">Private correspondence with Mike Makris from The Rare Barrel by Dan Pixley when asked to clarify the amounts that Eric from NBB recommended. 03/06/2016.</ref>. Before using a barrel holding this sort of storage solution, the barrel should be intensely sprayed and rinsed (not just filled up with hot water and dumped).
Andrew Zinn from Wicked Weed reported that the recommendation above was not enough potassium metabisulfite to prevent microbial growth. They found that 0.3 grams of potassium metabisulfite per liter was the best balance between inhibiting microbial growth while limiting residual sulfur aromas in the barrel. Barrels with this holding solution have been reportedly free of microbial growth for 6+ months. Barrels are only filled with holding solution if they are going to sit for longer than a month; anything earlier than that is sulfured by burning an SO<sup>2</sup> tab <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1691672454194295/?comment_id=1691712440856963&comment_tracking=%7B%22tn%22%3A%22R%22%7D Andrew Zinn. Milk The Funk Facebook group. 05/19/2017.]</ref>.
This method is not recommended by some brewers due to the residual sulfur that can be difficult to rinse from the barrel, and the over-extraction of the barrel's flavor compounds into the storage soltuion <ref></ref>.
===Maintenance===