Unrefrigerated starter lost rising power
Pardon the long story, but 15 years ago or so my sourdough starter (1848 + San Francisco mixed) suddenly developed enough rising strength that I stopped adding commercial yeast, and since then I have been making great bread. I refresh the started over night with 3 C water and 3 C flour, adding another 3 C flour in the AM, and adding remaining flour and kneading about noon. One more rise (2-3) hours and I made loaves and let them rise for about an hour, then baked.
Last month my starter was unrefrigerated for about a week (I live in Santiago, Chile… we lost electricity with the earthquake). When I reactivated the starter it was still alive, looked and smelled the same, but had lost its vigor… it no longer produces a strong enough ferment to raise the dough without added yeast.
What do you think happened (1) 15 years ago to boost its fermenting power and (2) last month to diminish its power while unrefrigerated? How can I make it strong again?
Could it be that I infected the starter with commercial yeast 15 years ago and had a sourdough-yeast starter ever since? What would happen if I re-introduced commercial yeast into the starter?
I appreciate your thoughts and suggestions.