So, to cut a long story short, I made a big batch of 77% hydration country sourdough, and was aiming to bake 4 X 250g loaves and 1 X 950g loaf. Oven broke down before the fourth loaf but I did end up with three comparison bakes.
From left to right: Baked in a Dutch Oven, baked in a glass casserole dish, and baked on an open shelf with a tray of steam. The fourth bake was going to be with my oven's superheated steam feature so maybe next time!
The short conclusions:
The glass casserole disappointed me most, because in this test I didn’t get the great results I have had before. I was hoping to prove that it doesn’t matter what you bake in, but clearly there is a difference here. In spite of evidence to the contrary, I’ve done beautiful loaves in the glass casserole and will do it again to prove I’m right!
The tray of steam method surprised me, mostly because the rise was as good as the other methods, although a little more triangular and less rounded on top. The crust was thinner too, although the crumb was practically identical.
The Dutch Oven method is perhaps the most expected, as this looks like the loaves that line the glossy pages of all those expensive artisanal bread books. It’s got the colour, the crumb, the crust. Perhaps because so many people use this method it is de facto how we expect a sourdough to look. I’ll admit the bursting crust is impressive, and they taste as good as they look.
And the gratuitous crumb pic:
Not sure if I'm allowed to link, but there is a long write-up on the process on my blog twistybakes.kitchen
I love it when people take the time to organise experiments like this, and share the results - so many thanks!
One of the interesting things about glass versus cast iron, from a material science perspective, is that it has double the specific heat capacity (750 J/kg-K versus 462 J/kg-K) but a VASTLY lower thermal conductivity (1 W/m-K versus 55 W/m-K).
This means that a glass cooking vessel is very poor at delivering heat energy into a lump of cold dough by conduction i.e. to give the loaf that big hit of energy at the start of the cooking process, which is what gives you the 'oven spring'. So putting the loaf in a glass casserole will actually have had the effect of insulating it from the heat of the oven throughout the bake.
The effect of steam is also a very interesting process. The significant factor here is water's very high latent heat of vaporisation (2,260 J/gramme). This is the 'extra' thermal energy that has to be put into 1cc of liquid water to convert it into steam at the same temperature - and conversely the energy that is released when that steam condenses back into liquid water. So the relevance of this to your bake is that when the cold dough goes into the oven, all the time while its surface temperature is below 100 deg C, hot steam condenses on its surface - thereby releasing all that latent heat of vaporisation into the bread raising it's temperature much more rapidly than the normal heating processes in the oven.
Note however that there's a catch with the 'tray of water' in the oven... which is that most domestic ovens only produce about 2.5 kW - so the effect of putting a whole tray-full of boiling water in at once really kills the temperature of the oven, until it's all boiled away. And this happens precisely during the period where you want to hit the loaf with as big a whack of heat as possible (!). People think it's OK because the water is "already boiling" when it goes into the oven - but that's because they don't understand about the extra 2,260 Joules per CC required to convert liquid water at boiling point into steam. This is what kills the oven heat. Ideally therefore you need (like you have) a steam injection oven that is able to deliver pre-heated steam into the oven without the oven itself having to vaporise it. If you haven't got one of these, you really want to put in as little water in the tray as you can get away with... remembering that the steam has NO further beneficial effect once the surface temperature of the loaf goes above 100 deg C (because the steam can no longer condense on it). So you want to use just enough water to get you to that point.
Incidentally another very good trick to try, is to put something with a big thermal mass (ideally a good thick slab of something like aluminium - or a layer of fire bricks) to heat up fully in the oven before you bake, and then load the dough directly on top of them (I do this by proving on a sheet of baking parchment, which I can then slide straight onto the bricks). This can give you a very good oven spring - just like traditional bakers loading loaves directly onto the sole of the oven.
Enjoy!
Wow, it's fascinating to hear the science behind it all! Thank so much for that.
For the tray method, I usually throw only a cup/250ml of water in to the tray, and it's gone within the first minute. Have to be fast to close the door!
I had no idea about any of that regarding the glass. I've had some quite successful bakes in the glass (not reflected in this experiment!) so it's nice to hear the reasons behind it.
Great post, and I loved all the extra details on the linked page. I believe every data set is helpful in our understanding. One problem I have when I run similar tests is that I worry that some of the differences are caused by differences in shaping and scoring. As a weekend baker, I find there is a great variance in how I shape and score each loaf.
If you run another test, try one with a cold dutch oven - many here had reported no difference, and that should further the risk of scars from a hot DO .
before I got my cast iron D.O., I used to bake in a borosilicate (generic Pyrex) glass casserole. I preheated it.
After I got the cast iron D.O., I found out you're not supposed to preheat glass, since the cold dough could shock and break the glass.
Did you preheat the glass casserole for this test?
IMHO the pre-heated temperature of the glass isn't really the issue: it's the extremely low thermal conductivity of glass (Pyrex included). Consequently when cold dough touches hot glass the surface temperature of the glass falls almost immediately to that of the dough - even though a few mm below the surface, the glass is still extremely hot. Thats why glass oven doors work: they don't allow the heat to escape from the oven (not by conduction, anyhow).
The other point - and the reason that a cold DO also works - is that the thermal mass of the cooking vessel is not that large compared to a loaf: so the energy stored in it by pre-heating is rapidly "used up" by the large lump of cold, wet dough, without raising its temperature very much. Of course with a heavy cast iron DO and a smallish loaf that can still be a help tho.