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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>. They also found that first rinsing the sediment out of barrels with cold water briefly before rinsing with hot water or steaming helps remove residual sulfur <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1886645328030339/?comment_id=1887660384595500&comment_tracking=%7B%22tn%22%3A%22R1%22%7D Andrew Zinn on rinsing barrels with cold water first. Milk The Funk Facebook post. 11/18/2017.]</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.