Problem with natural yeast

Mazzidante,

Let's start with your starter or biga.  You are maintaining it at 50% hydration, as expressed in bakers percentage (one part water, by weight, to two parts flour, by weight).  Good for you for weighing your ingredients!  It makes a lot of things easier to work with.  A 50% hydration biga (or starter, or levain, or mother, or chef, or natural yeast--this stuff has a lot of names!) will be quite stiff, as you have noticed.  That's perfectly alright.  After you feed it, how many hours does it take to double in volume at room temperature?  And what is your room temperature?

You mention that your starter (I'll try to use that word consistently) is about 1 month old.  That is still quite young for a starter; the populations of yeasts and bacteria probably haven't stabilized yet, which could lead to the different rates of activity that you are seeing.

You also mention that you feed two parts of starter with one part flour and 1/2 part water, all measured by weight.  I think your starter might be malnourished.  Try feeding one part starter with two parts flour and one part water, by weight, for the next few days.  You'll probably want to leave it in a covered container in your kitchen, without refrigeration, and feed it twice a day.  That should help get all of the yeasts and other micro-organisms well-fed and ready for action.  I would also suggest that you discard all but a few grams of starter before you feed it.  For example, let's say you have 50 grams of starter.  Throw away 30 grams (or keep it for use in some other baking) and feed the remaining 20 grams with 40 grams of flour and 20 grams of water.  At the next feeding, discard all but 20 grams.  Feed that remaining 20 grams with 40 grams of flour and 20 grams of water.  And so on.  When you need to bake with it, take a portion and feed it two or three times with increasing quantities of flour and water until you have enough biga/levain for your bread.  Feed the remaining portion as per the usual routine so that you have a supply to work with for a future baking session.

The feeding regimen described above offers several benefits.  First, you maintain a relatively small amount of starter, so the discards aren't a significant economic drain.  Second, you don't keep doubling or trebling your starter quantity with each feeding, which means you won't need to buy a swimming pool to store it in.  Third, you get rid of most of the starter, which has exhausted it's food supply, and replace it with lots of fresh food for the organisms in the remaining starter. 

There may be some other issues that need to be addressed, but begin by focusing on the care and feeding of your starter.  Once the starter is routinely able to double its volume in a predictable amount of time, maybe 4 or 5 hours, you'll be able to bake with the confidence that your starter is dependable.  Do remember that starters are very sensitive to temperature changes.  A decrease of just a few degrees in the ambient temperature can slow a starter that doubled in 4 hours at warmer temperatures to the point that it needs 7 or 8 hours to double. 

Paul