The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

steam vs. water vapor

ericb's picture

steam vs. water vapor

Baking instructions published in books, online, and in forums, almost exclusively used the term "steam" to describe what we should do to the oven when baking bread. We all know that there are a variety of methods of "steaming" the oven: spritz the side walls, pour boiling water into a hot pan, ice cubes, rocks, injection... the possibilities are endless.

My understanding of the term "steam" is that it describes water molecules in a gaseous state. If I'm remembering my early physics lessons correctly, this can only occur when water molecules exceed a temperature of 100 C, which can typically only occur under pressure, or very briefly at atmospheric pressure (for example, steam exists just beyond the opening of a tea kettle before it turns into visible water vapor).

So, my question to those who know about such things is this: when baking instructions call for "steam," do they actually mean "steam," or do they simply mean "water vapor?" If they indeed mean "steam," as in super-heated gaseous water which is likely attainable only in commercial ovens, are we home bakers wasting our time with by trying to recreate these conditions on our leaky home ovens? And if they mean "humidity" or "water vapor," why don't they use this language instead?

I know this is all academic, and ultimately, it doesn't really matter if we are satisfied with the bread we bake. I am happy pouring water into a hot pan. Others get perfectly good results from spritzing. Still, I'm curious what the more scientifically inclined minds among us think about this.


G-man's picture

Yes, the "steam" referred to is mist, humidity or water vapor, not steam in the scientific sense. It's likely, though don't quote me on this, that folks say steam because it's just easier that way. Also because not all bakers are scientists and folks don't usually make the distinction.

Edit: Actually, normal 'wet' steam is just water that's been heated up past water's boiling point. 100 degrees celsius (212 F) is very easy to achieve and maintain in an oven. Perhaps you're thinking of superheated or 'dry' steam of the sort that can start fires.

asicign's picture

Steam really isn't a scientific term.  Water (like most all substances) exists in 3 states: solid, liquid and gas.  At 100 degrees Celsius at sea level, liquid water will evaporate, and continue to do so until saturation is reached.  In an oven, the temperature is higher than 100 degrees Celsius, so any liquid water will evaporate, never reaching saturation because the water vapor can leak out.  The water vapor could be considered 'dry steam' because there wouldn't be any liquid water dispersed in it.  Water vapor is colorless.  When you open the oven, you'll see a burst of steam because the water vapor is condensing as the temperature of the gas drops, and the saturation point is reached.

I've never used a commercial steaming oven, but I wonder if they allow pressure to build up.  If so, then a higher concentration of water vapor would be possible than in the oven I use for baking, and this might make a difference in how the bread behaved.  I'm guessing that the steaming techniques I've tried never saturate the baking area with water vapor.

yy's picture

Steam and water vapor are the same thing in a physical, chemical sense. As for the temperature at which water vaporizes to become steam, that depends on the surrounding environmental pressure. At 1 atmosphere of pressure (say, at sea level), water will boil pretty much at 100 C. Under higher pressure, water will boil at a higher temperature (look up the formal definitions of vapor pressure and boiling point on wikipedia).

I'm not sure how important the temperature of the steam is when it comes to bread. My impression is that it's all about the quantity of steam, and saturating the oven with steam, rather than achieving steam that is above 100C. Practically speaking, though, I wonder whether the two go hand in hand. In other words, in practice, maybe it's only possible to achieve the proper amount of steam under a high pressure environment. I'm definitely curious to hear a more knowledgeable person's explanation.  

pmccool's picture

of water, it's called steam.  It may or may not be contained in a pressure vessel, it may be saturated or superheated, but it is still steam.

In a commercial oven with steam capability, the water is boiled in a boiler and the steam is kept bottled up at pressures exceeding atmospheric pressure until it is released into the oven cavity.

In a home oven, we boil water by a number of mechanisms and the steam is at atmospheric pressure.

In either case, the resulting fluid (water in a gaseous phase) is maintained at temperatures in excess of water's boiling point so long as it stays within the oven and so we call it steam.  As it is vented, it cools below water's boiling point and (usually) condenses back to a liquid.  What we see are actually tiny droplets of water drifting in the air.  And yes, I am aware that water can exist in a gaseous phase below its boiling point.  We're talking semantics here, though, not physics.

Does that clarify things, or make them foggier?


DerekL's picture

And it's really only kept above atmospheric pressure because then all you need is a valve and some piping - and the steam will then transport itself when the valve opens and it vents to the oven which is at atmospheric pressure.  Much easier than pumps or a seperate pressurization system.  If the overpressure at the end of the injection cycle is low enough, they won't need a pump to refill it either - simple line pressure will do.

Engineers love to make things simple, and corporate accountants love simple things because they're less expensive.  (I don't know if commercial ovens do or don't have pumps, just illustrating a principle.)

SteveB's picture

In a commercial oven with steam capability, the water is boiled in a boiler and the steam is kept bottled up at pressures exceeding atmospheric pressure until it is released into the oven cavity.

Hi Paul,

Not all commercial ovens produce steam in this way.  Some commercial deck ovens produce steam by spraying water on a large heated metal bar located in a chamber adjacent to the baking deck.  It is conceptually similar to the home baking technique of throwing water on a heated cast iron pan.




pmccool's picture

Every time I visit here, I learn something new.



Chuck's picture

Water heated well above 100C is typically invisible. That whitish mist (hot fog?) is steam on the verge of becoming not-steam again. We typically see a cloud of steam surrounded by cooler air, which means the edges of the steam cloud condense into that whitish stuff. So we've gotten used to locating steam -even though we can't see most of it- by the whitish stuff around its edges. But as all too many people can attest, the worst burns come from the invisible stuff!

It seems to me that when bakers talk about "steam", they mean all water that's above 100C, both invisible and visible. Trying to narrowly define "steam" as only the whitish stuff is too precious. Visible or invisible matters to our eyes, but not to bread.