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Causes of Oven Spring

colinm's picture
colinm

Causes of Oven Spring

For a long time I have been puzzled by the variety of explanations offered for oven spring. So finally I decided to search for scientific papers which might shed some light on the subject. Even in the scientific literature there is more than one opinion but I think that it is not hard to figure out the truth. In case anyone else is interested, I thought that it might be useful to record what I found. The full discussion is a bit technical but there is a summary at the end. To make the results more concrete I have calculated the size of the effects for a typical loaf of homemade bread.

In reading books, and articles accessible on the internet I found two main schools of thought. The first was due to Professor R. C. Hoseney of Kansas State University and the second, more recent, one was propounded by A. H. Bloksma of the TNO Cereals, Flour, and Bread Institute of the Netherlands. I have not been able to locate copies of the original papers by these authors but I was able obtain a copy of Chemistry and Physics of Baking, edited by J.M.V. Blanshard, P. J. Frazier, and T. Galliard, 1986, which is the proceedings of a symposium held in 1985 and which includes papers by both Hoseney and Bloksma. A free copy is linked here. Both authors provide some rough calculations of gas production and Bloksma shows a graph, but without any details of the underlying calculations. However, I also found a useful related paper by Fan, Mitchell, & Blanshard (Journal of Food Engineering 41 (1999) p69) which presents the equations for a detailed model of oven spring and also includes the dynamics of bubble growth. The link is here.

To provide estimates of the relative importance of the different contributions I will assume the following parameters. The nominal homemade loaf has a hydration of 70%, and is proofed to double its initial volume before baking. If the dough density before proofing is 1.2 g/cc, the initial loaf volume divided by the water volume is (1.7/1.2) / 0.7 = 2.0. After bulk fermentation the dough is saturated with CO2 and therefore contains approximately 1.7 g of CO2 per liter of water. I will assume an initial temperature of 20° C and that the oven spring continues up to a temperature of 70° C.

There are several possible contributors to oven spring:
1. The final burst of yeast fermentation
2. Thermal increase of gas volume as the dough temperature rises from room temperature to about 70° C.
3. CO2 dissolved in the liquid phase of the dough being forced out of solution as the temperature rises.
4. Ethanol evaporating.
5. Water evaporating.

Yeast Fermentation. This is a popular explanation but Hoseney computes that it contributes only 1% of the total rise in the case he was considering. Bloksma does not even provide a value, presumably because it is so small. In my own experiment I saw no indication of a significant burst of yeast activity. Hoseney and Bloksma agree on this one.

Thermal increase of gas volume. To a first approximation the gas volume expands in proportion to the absolute temperature as it rises from room temperature , 20° C or 293 K, up to the end of oven spring at about 70° C or 343 K. The expanded gas volume is thus (343 - 293)/293 = 1.17 times the original gas volume. The expansion is therefore 8% of the original volume of the proofed loaf, which is 50% gas by volume. Hoseney and Bloksma agree on this one, too.

Dissolved CO2.A saturated solution of CO2 in water contains about 1.7 g of CO2 per liter of water at 20° C but only 0.5 g/L at 70° C. Each liter of water therefore expels at least 1.2 g of CO2, which takes up a volume of 0.8 L at 70° C.  The expelled gas is thus 80% of the water volume. Since the volume of water is 1/4 of the volume of the proofed loaf, the expelled gas is 20% of the volume of the proofed loaf. Again, Hoseney and Bloksma agree.

By this point, we have accounted for a substantial amount of rise, of about 30% of the volume of the proofed loaf, but this is still short of the observed rise in the examples considered by Hoseney and Bloksma. So there must be another substantial contributor.

Ethanol evaporation. This is the solution suggested by Hoseney, since ethanol is produced in a quantity equal to the CO2. One might guess that it could potentially expand to the same volume as the CO2 above its boiling point of 78° C if it could somehow be separated from the water. However, Hoseney offers no mechanism for this, and Bloksma dismisses ethanol. Doc Dough points out that an ethanol-water mixture boils at a single temperature near 100° C. In other words, the mixture behaves essentially like pure water. In the absence of any plausible mechanism for ethanol, I conclude that ethanol has no significant effect on oven spring, in agreement with Bloksma, but not Hoseney.

Water evaporation. This is the final contribution proposed by Bloksma but completely ignored by Hoseney. At first glance it might seem implausible because unleavened dough does not rise significantly during baking, although there is plenty of water. But the trick is that the evaporation of water into the bubbles of CO2 produces an increase in volume which is proportional to the initial gas volume. So unleavened bread with no CO2 will not show any effect of water vapor while risen dough will expand significantly. In addition, the expansion of the bubbles will reduce the partial pressure of CO2 and thus bring more CO2 out of solution.

To calculate the expansion, we consider a bubble of CO2 in equilibrium with liquid water. At room temperature the saturated water vapor pressure is low. However, at 70° C the saturated water vapor pressure rises to roughly 0.3 atmospheres. As the water vapor enters the bubble, the bubble must expand to maintain the pressure of approximately one atmosphere. For a water vapor pressure of 0.3 atm, the required expansion factor is 1/(1 - 0.3) = 1.4. Taking the previous effects into account, the expanded CO2 volume is 80% of the volume of the proofed loaf volume. The effect of water vapor is to increase this to 1.4 x 80 = 112% so the contribution from water is 32%, about the same as the previous two contributions added together. The total potential oven spring is then 62% of the volume of the proofed loaf.

The relative magnitudes of these three calculated contributions agree fairly well with the graphs shown in Bloksma’s paper, giving some assurance that the calculations are correct. Of course they must be considered only as approximations because they are based on a simple equilibrium model while the temperature inside a real loaf varies dramatically from place to place and over time.

In summary, a typical home-made loaf has the capacity to expand its volume by about 60% in the oven, although the dough may not permit the full amount before it starts losing gas. The three main contributors are 1) thermal expansion of gas (~10%), 2) expulsion of dissolved CO2 (~20%), and 3) evaporation of water (~30%). The contributions from the final burst of fermentation and from ethanol are negligible.

GaryBishop's picture
GaryBishop

Thanks for digging this out. I find it fascinating. 

Gluten-free Gourmand's picture
Gluten-free Gourmand

Hi colinm,

Thank you for this in-depth recap of oven spring.  It confirms a lot of the ideas I've had about it, and also tackles the sticky subject of yeast production, which I've been thinking about a lot lately.  In Bread Science: The Chemistry and Craft of Making Bread, Emily Buehler writes:

"Most oven spring occurs in the first 10 minutes that the bread is in the oven. Many factors contribute to oven spring:

  • Chemical reactions speed up.  Enzymes work faster. Fermentation reactions produce a final burst of CO2.  More gas means more expansion of the dough.
  • ....Alcohol and water both vaporize at hotter temperatures.  Again, more gas means more dough expansion.
  • Gases expand as they heat. The bubbles of CO2 expand, pushing the dough out.  Other gases, such as water vapor and ethanol, also expand."

I bring this up because I was having a discussion with someone about oven spring and they cited this page of Buehler's book to state that the final burst of fermentation is critical to oven spring.  I think the author is mixing all the elements up in a way that reads like that, whereas your analysis separates the different gas components to clarify each role more.  But from my observation, there is an increase in oven spring if the fermentation is still going at an exponential rate when the bread is put in the oven - that is to say, if it's a little underproofed.  That observation seems to suggest that oven spring does rely somewhat on the final burst of CO2, or there is another issue going on,  with the physics of the dough.  Maybe if the dough has already stopped expanding as quickly due to slowing yeast activity, it's difficult to speed it up again due to inertia.  What are your thoughts?

I also wanted to note that there is a kind of "oven spring" that happens with unleavened dough.  Griddle spring maybe?  Mexican tortillas, when cooked by someone very skilled (not me), can puff up like a pita bread.  Both corn and flour tortillas can do this.  There are dishes where this is done to the tortilla purposely so that it can be stuffed with beans for example for panuchos. That does demonstrate that the water evaporation may play a large part in oven spring.  

Gina

colinm's picture
colinm

Hi Gina,

I don’t think that Emily Beuhler has any independent information. At the end of the section you quote she references a paper by Moore and Hoseney (1985). I have not been able to find a copy of that paper but it is also given as a reference for the chapter by Hoseney in the book I linked. So we appear to be be describing the same experiment but I agree with you that her choice of words is a little misleading in that she leads with the least important cause and mentions the most important as an afterthought at the end.

Like you, I have also noticed that slightly under proofed loaves have more pronounced oven spring but I don’t think that’s an argument for the importance of yeast. After all, the main drivers of oven spring are still present, although in slightly different proportions. I always just assumed that an under proofed loaf could expand more before the dough reached its limit.

And there is nothing better than a fresh tortilla. Other flat breads, such as roti, puff the same way. I’m just guessing here but I think that the puffing happens at higher temperatures, near 100° C, when the crumb is set and there is plenty of steam. So the puffing inflates the bread as a whole but does not affect the crumb. The puffing is also encouraged by pressing down on the bread, maybe to break the connections between the top and bottom. I like your naming it griddle spring.

 

Colin

MikeV's picture
MikeV

Hi Colin,

Thanks for the very clear analysis! I have always had a "gut feeling" skepticism about the yeast-activity theory but had never taken the time to properly research or analyzee the problem.

If you want to look at the original Bloksma papers, the TNO has an online publication repository:
https://repository.tno.nl/islandora/search/bloksma?sort=mods_originInfo_dateSort_dt%20asc

The papers are not directly accessible online, but there is a link to request them ... especially for the older articles where journals may be long out of print and/or not available online, it can't hurt to ask! It looks like the majority of his publications were in English rather than Dutch.

I assume in the industrial bread-baking world this should all be quite well-understood, there are still academic groups quite active in studying dough rheology and bread baking, and the consequence for final bread volume... for sure in the Netherlands it is (sadly) clear that maximization of bread volume is a very important steering parameter for industrial bakers.

Cheers, Mike

colinm's picture
colinm

Mike,

Thanks for that link! I was able to get a copy of the 1990 Bloksma paper "Rheology of the Breadmaking Process", which contains the specification of his model, as well as a detailed comparison with that of Hoseney. Oddly, despite the obvious difference, he seems too polite to say plainly that Hoseney was wrong. In any science that I have been involved in, such a difference would have been examined and argued until it was clear where the truth lay.

I agree that it is sad that the academic cereal world is so focused on making something fluffy that resembles bread, completely ignoring flavor. And the Netherlands is far from being alone in that. On the other hand, if I could easily buy the breads I like, I might never have started baking.

 

Cheers,

Colin

alcophile's picture
alcophile

That’s a thorough analysis you’ve made of the data presented in those studies. What surprises me is the different conclusions reached by Bloksma and Hoseney from similar systems. I’m also surprised that Bloksma invokes the law of Gay-Lussac when discussing the expansion of a gas with temperature. Shouldn't that be Charles’s Law? I’m surprised that wasn’t caught by an editor.

I haven’t done the math, but could ethanol expansion (not evaporation) also contribute some to the oven spring? I know there is little ethanol compared with water and CO2 but, in combination with the water present, will it expand with the water as you described? Or is there too little ethanol to matter?

I have also been disappointed in Emily Buehler’s Bread Science book. I'm a retired chemist so I was hoping for a more rigorous treatment of the chemistry of bread. Even though the book is meant for a general audience, I think the science could have been more thoroughly researched and then explained to the audience in understandable terms.

colinm's picture
colinm

You are right, I believe, that it should be Charles's Law. As a retired physicist, I am also surprised and dismayed that such a substantial difference in conclusions should exist without any attempt to settle the matter.

 

The ethanol is produced in equal molar quantities to the CO2, so it should produce equal gas volume above the boiling point of 78° C if it could be separated from the water. The amount of ethanol will depend on the details of the fermentation, but it is small, <0.5%. I am not an expert in distillation but I think you would have to work very hard to extract the ethanol. So the mixture boils at a single temperature near 100° C, producing water vapor with an enhanced but still small percentage of ethanol vapor coming along for the ride, not enough to make any practical difference. Does that make sense?

Cheers,

Colin