Guinness knock off?

I am interested in brewing a Guiness knock off.
I read in "The Joy of Homebrewing" that Guiness puts 3% pastureized sour beer in the product.

If, God forbid, I have a sour batch, can I save it for a stout? Has anyone tried that?

How would I pastureize it w/o ruining it?

usually what you would do is save a portion of the wort and sour it. Do this by brewing as usual. Save a small amount of wort from the fermenter. Put it in a bucket with a handfull of 2 row. In a day or so, it will smell absolutely awful. once you have reached desired sourness, boil it to kill off the lacto. Now you have a small soured portion of unfermented wort to add to your now fermenting beer once it is cool.

Thanks, That makes sense.

You can also add a little bit of acidulated malt or lactic acid to the mash. The flavor won't be quite the same, but it's easier than souring beer, IMO.

Bob