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In the past, some yeast manufacturers (namely Wyeast) did not recommend making a starter for mixed cultures. Reasons varied from "throwing off the ratio of microbes" to "creating acetic acid". In regards to the first concern, throwing off the ratio of microbes will likely not be a real concern. It is possible that this will change the profile of the fermentation (for example the beer might turn out more sour because the lactic acid bacteria were allowed to grow more cells in the starter), but if treated properly all of the microbes will have a higher cell count after a starter is made, and the yeast (''Brettanomyces'', ''Saccharomyces'', and any bacteria) will be more viable after a starter. Pitching a second culture of fresh ale yeast isn't necessary with mixed cultures that have their own ''S. cerevisiae'', and making a starter ensures the cell count is high enough. However, some mixed cultures might not come with ''Saccharomyces'' out of the package, or in the case of a [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|re-using a sour yeast cake]] the ''Saccharomyces'' might be dead. In such cases the brewer may choose to pitch fresh ''Saccharomyces'' separately (see [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|re-using a sour yeast cake]]). Oxygen does not inhibit lactic acid bacteria in general, so this is generally not a concern.
If the package is fresh, then a starter is generally not required. If the package is nearing expiration or expired and buying a new package is not an option, or if the batch of beer is much larger than what the package is intended for, then the viability of the yeast and bacteria is probably compromised and we recommend making a starter for all mixed cultures to (including Wyeast Roeselare). In general, the brewers yeast and lactic acid bacteria benefit from a starter the most(''Brettanomyces'' doesn't need a high cell count in order to affect the beer, but some strains of lactic acid bacteria benefit from a higher pitching rate), but a starter is insurance for all of the strains in the blend. We recommend a 500-1000 mL starter (~1.040 OG with DME) for a 5 gallon batch. Run a stir plate on the lowest setting, or use occasional manual stirring with no automatic stirring. If the package contains ''Brettanomyces'', then run the starter for 4-6 days; if the package does not contain ''Brettanomyces'' then run the starter for 24-48 hours. Do not use hops in the starter unless inhibiting the lactic acid bacteria is desired. One may want to have an anaerobic starter in the case that the brewer wants to inhibit the yeast in the starter, especially ''Brettanomyces''. Remember though that yeast (particularly ''Brettanomyces'') needs oxygen in order to grow viable cells, so yeast growth under anaerobic conditions won't be optimal, and limiting oxygen during the starter will favor the growth of ''Saccharomyces'' over ''Brettanomyces'' (see [[Mixed_Cultures#Effects_of_Mixed_Cultures_on_Growth|Effects of Mixed Cultures on Growth]] below) <ref name="martyniak" /><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1660420660652808/?comment_id=1660436453984562&comment_tracking=%7B%22tn%22%3A%22R8%22%7D Pat Tkacz and Justin Amaral. Milk The Funk Facebook group. 04/21/2017.]</ref>.
In regards to the concern about oxygen and acetic acid production, oxygen does encourage acetic acid production in ''Brettanomyces'' (see [[Brettanomyces#Starter_Information|''Brettanomyces'' starters]]). However, a starter can be decanted to remove the starter beer if it smells highly acetic. Some brewers have reported not decanting the starter at all, and the small amount of acetic acid present doesn't make a large impact on the final beer due to dilution. Therefore, in general, it is good to use aluminum foil to cover the starter instead of using an airlock. If the starter is going to be stored for longer than 2 or 3 weeks, then use an airlock after 3-6 days to limit the acetic acid production and store it at refrigerator temperatures. Long-term exposure to acetic acid and low pH will lower the vitality of ''Brettanomyces'', ''Saccharomyces'', and lactic acid bacteria. The viability of the microbes will get worse the longer the starter is stored. Some strains of ''Brettanomyces'', saison yeast, and lactic acid bacteria might remain more viable than regular brewer's yeast over time in these harsh conditions, but a starter that is 6+ months old should be treated like a [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|sour beer yeast cake]]) and a fresh starter should be made to make sure the cell count is adequate at pitching time. Low pitching rates in general are sometimes acceptable for sour beer fermentation, as long as adequate growth of each species can occur during fermentation.