A friend has a still so as an experiment we made 15 gallons of an Indian Red Rye Ale and split into two kettles (10 gallons in one and 5 in the other). We did not add any hops to the 5gal batch with the idea of running it through the still after primary fermentation. Well the beer ended up being left in the fermenter longer than planned and went sour. Before throwing it away we tasted a small sample and were rather impressed with the flavor. So we figured we’d bottle it and maybe end up with a "Happy Accident Sour’. So we racked to a secondary and added some SO2 to kill the bugs. A week later there is another pellicle starting to form which I’m guessing means it is still fermenting / souring.
My question (finally!!) is how to I stop the souring process? Was SO2 the wrong approach and if so what should I do. Or did we just not add enough SO2 - we put in approx 1/4 teaspoon into 4 gallons of beer. A local friendly winery tested the SO2 levels in the beer for us and it came back a essentially 0ppm.
Greatly appreciate any suggestions / advice.