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[[Blending ]] is a fundamental part of traditional spontaneous beer production (and typically of wood aged sour beer production in general). In barrel aged mixed fermentation beer, and especially spontaneously fermented beer, there is a high potential for variability in different barrels/fermentation vessels, even those resulting from the same hot side process. To help create a more balanced and complex product, producers of sour beers often blend barrels (of both the same and of different vintages) together into one final product. The homebrewer can employ the same techniques and blend to reach the desired final product from beers of different vintages and different carboys/vessels of the same brew.

Frequently a non-trivial amount of beer is dumped at spontaneous beer breweries. The exact amount depends on the conditions of the brewery and the willingness of the brewer to try to blend in batches that might not taste as good and/or have mild off flavors at the expense of the overall quality of the blend, but commercial brewers have reported duping levels of 5% (and possibly up to 15%) of total production <ref name="Beer Temple interview with De Garde">[https://vimeo.com/127084279 The Beer Temple Interviews #264 with Trevor Rogers of De Garde]</ref> (~13 minutes in). This may be due to an imbalance in the microbes <ref name="Beer Temple interview with De Garde"></ref> (~14 minutes in) or a bad barrel resulting in off woody flavor <ref name="Spontaneous Sour Hour" /> (~1:31 in) or excessive O2 exposure. In addition to the beer inside such barrels being dumped, the barrel itself is also often discarded <ref name="Beer Temple interview with De Garde"></ref> (~14 minutes in). Homebrewers who are fermenting spontaneously may expect that from time to time they will need to dump a batch.