[brewsumore]

I'll be sour worting this weekend.  I know that the common practice after collecting the mash runoff is to use lactic acid to drop pH to 4.0 to 4.4 (or 4.6) before pitching a starter into the wort to sour it.

I will be pitching 2000 ml of lacto starter made with WY 5335 Lactobacillus culture, into 13.5 gal wort, at ~92F.

I don't have any lactic acid on hand and was wondering if it would be ok to use phosphoric acid instead to drop the pH. 

I plan to treat my mash and sparge water to create a 5.2 pH wort as the starting wort.

Martin, are you there??!?! 

[mabrungard]

Sure, you can use the phosphoric. But you will be loosing the opportunity to bolster the lactate ion content which is an important flavor component in this style.

As you imply, souring the raw wort below pH 4.5 is important for avoiding off-flavor producing organisms. But in your case with a good-sized lacto starter, that may be less of a concern. As long as you've brought the temp of the raw wort well above 150F for 15 minutes, it should be reasonably free of spoiling organisms and the need to acidify the wort pH to 4.5 with phosphoric is reduced. You could reduce the use of phosphoric and allow the lacto to do its thing and bring the pH into the sub-3.5 pH range needed for a good Berliner or Gose.

[brewsumore]

Thanks a bunch!  I'll be making a sour worted plum saison (plums added at the end of saison yeast ferment), so souring the full volume wort, followed by boiling the wort as normal.  Yes, I'd already planned to run off the mash/ sparge into my kettle and boil for 5 minutes before chilling to 92F and pitching the starter.

So I'll just target mash pH of 5.2 and call it good.  That makes more sense now too.  I plan to sour the full volume wort for ~1.5 days, or until I hit 3.6 pH, and then do the boil.

Awesome!

[Stevie]

The pre-souring is also to help in foam stability. I haven't tried it both ways myself.