Bubbles at surface when proving in banneton
Wondering if anyone can help but I'm getting large air bubbles at the surface (seam side) of my sourdough when proving at room temp before baking - these appear sometimes within minutes of sticking it in the banneton.
I'm working off a tartine recipe but today had to be a bit more flexible due to other commitments. It goes as follows:
450g strong white flour, 50g wholemeal and 350ml water to autolyse for approx 20mins
Add in 100g starter (approx 1:3:3, has doubled within 4 hours of feeding) and mix in, then add further 17g water with 10g salt.
I mixed it all together and today knew I couldn't attend to it so for the first few hours so I kneaded in the bowl for like 8 minutes or so. The dough temp was 25.4°c at the start of bulk.
I did a stretch and fold and some coil folds at 2, 4 and 6 hours bulk, with the dough temp remaining at the 25°c mark. It had risen by at least 30%, was bubbly on the surface and had clear bubbles on the sides and bottoms of the container.
I turned it onto the counter to preshape and it was airy albeit a little sticky. After 20 mins rest I did another preshape as felt it wasnt as strong as a previous loaf I did yesterday.
After another 20 min rest I flipped it over and shaped into a boule, there was a good amount of tension and small air bubbles were coming to the surface (smooth side) of the boule. I flipped it over and put into the banneton and within 20 minutes perhaps sooner, larger air bubbles seemed to accumulate on the top (seam side) of the boule, which became progressively worse. After 40 mins or so I decided to turn it out onto baking paper and put it in a dutch oven, i think had I not been quick with the transfer it would've become a total pancake.
I dont quite understand where I've gone wrong here. I did a sourdough yesterday with almost exactly the same recipe, though the temp of the dough was consistently at 27°c for the bulk and not 25°c like today, but the bulk today was about an hour longer and the end point felt compatible to yesterdays. I did retard yesterdays loaf (though my fridge was at 9°c so not great, and to be fair it did also have some bubbles but I guess being cold helped it keep its shape when turning out.
It's annoying as I see on YouTube people turn out their doughs from the banneton and they have wonderful structure and hardly seem to deflate at all, and I wonder just what I'm doing so incorrectly. For further context yesterday I was able to attend to the dough a bit more, and did regular stretch and folds at half and hour intervals (but didnt do any kneading in the bowl prior to bulk).
Any help would be appreciated. It's very frustrating. I feel for the first time I'm actually not under fermenting during bulk but who knows, perhaps I should just keep pushing with the bulk. It just seems that when I compare methods to ones i see on instagram, and in the tartine book, a decent bulk should take around 4 hours or so with the temperatures I'm at currently. My starter is fed usually twice a day at 6 and 6 and never goes in the fridge. I've made a fair amount of similar hydration non sourdough loaves so I dont think my shaping is so awful as to warrant what I think is a total lack of structure. Feels like the dough was overproved the minute I put it into the banneton.