Reason for the stall

[Smokesman]

Wow, you guys have been busy! Started reading this last night and had to return to it over coffee this morning. Thanks for a great and interesting read!

 

[TactTm1]

Great reading. I am waiting for the "Cliff Notes" recap to print and stick in my BBQ binder.


Have a great day, everyone.

TT1

 

[ZILLA]

If we assume that the test and texture of good BBQ depends on breaking down the collagen then it seems logical that if you lower the temperature after you hit stall then you are going to break down more collagen than if you had a higher temperature and thus better Q. The other conclusion from the assumption is that after you get out of stall you can raise the temperature as much as you want to get the internal temperature to the proper level in the shortest amount of time.

I missed this last time around.... I think there is a limit to how much collagen we should convert to gellatin. If we convert too much then we get an overcooked mushy brisket, I think...

 

[infernooo]

Well, I posted this question on the eGullet forums and we were fortunate enough to have a number of extremely intelligent people discuss it. One of whom boasts the following credentials: prize winning pitmaster, food scientist, billionaire, PhD by the age of 23 and worked under Stephen Hawking. Another contributor is a PhD and author of a ground breaking new Sous Vide book.

http://forums.egullet.org/index.php...es-equipment/page__view__findpost__p__1756170

 

[Brewmaster]

I missed this last time around.... I think there is a limit to how much collagen we should convert to gellatin. If we convert too much then we get an overcooked mushy brisket, I think...

A great thread!! I disagree with this. I think you get mushy brisket from over cooking the meat not the collagen. I think you will get brisket that falls apart if you over cook the collagen.

Now think about this. What if you approached your cook differently in the beginning rather than the end. Say you cooked/ grilled you butt in the beginning to bring it up to 160 degrees quickly. Then change your pit temp to say 225 while the collagen breaks down. I wonder if this would save you from the meat breaking down and getting mushy. Kind of a hot and fast combined with a low and slow.

Thoughts?

Cheers,
Nate

 

[cmmcp2]

Good discussion. I know I was floored by the stall the first time I smoked butts (doing 25#'s of them my first go didn't make things any easier to comprehend) and almost lost faith that they were ever coming off 165 after 4 hours passed. Ultimately it took about 8 hours.

I'm duplicating my first effort as I type with 29#'s of butts. I got smart this time and started them at 9PM on Friday so they will be ready for dinner tonight instead of breakfast on Sunday. :becky:

 

[cmmcp2]

ignore this please

 

[ZILLA]

The reason that I suggested that when too much collagen breaks down you get an over cooked piece of meat is because out of the 28 different types of collagen that there are the five most common are what holds the entire cut together. When all 5 break down you get mush.

 

[D0ughB0y]

Well, I posted this question on the eGullet forums and we were fortunate enough to have a number of extremely intelligent people discuss it. One of whom boasts the following credentials: prize winning pitmaster, food scientist, billionaire, PhD by the age of 23 and worked under Stephen Hawking. Another contributor is a PhD and author of a ground breaking new Sous Vide book.

http://forums.egullet.org/index.php...es-equipment/page__view__findpost__p__1756170

That's really Nathan Myhrvold posting in those threads?

 

[infernooo]

Yep!

 

[infernooo]

For those of you who have not read the link I posted, it turns out the stall is NOT due to collagen breakdown, but wet bulb vs dry bulb temperatures.

Essentially, the interior of the meat cannot be hotter than the exterior. Until the outside of the meat dries out enough, evaporation prevents the exterior from reaching certain temperatures. Once it does dry out, it can continue increasing in temperature, as can the inside.... quoted from my link:

I think the stall is more likely due to the fats melting than collagen to dissolving to gelatine.
Nathanm would know the definitive answer, since he is also a prize winning BBQ pit master


Thanks for the vote of confidence! We have done studies of meat in a differential scanning calorimeter (DSC). This is a device that looks for "stalls" as you heat things. It puts constant heat in, then watches how the temperture increases.

The most dramatic sort of stall is a phase change (ice melting, water boiling) but a DSC is made to have great precision in how it looks for ANY difference in heat versus temperature.

When you have a chemical reaction - like collagen converting into gelatin - it will show up with a DSC.

We have measured this transition and it is real, as is fat melting. Both contribute. However, neither one of them is big enough to really account for the "stall" observed by BBQers.

I think that a better explanation is wet bulb temperature. Basically, if you have a wet thermometer, it reads a lower temp than a dry one, due to evaporation (unless relative humidity is 100%).

Food mostly cooks at wet bulb temperature - until the outside is totally dry. I suspect that is that is the origin of the stall.

As far as I know, the conversion of collagen to gelatin is endothermic. So, if the amount of thermal energy you are putting into the meat is small enough and there is sufficient collagen present, there would be a plateau in the temperature of the meat while the reaction happens. Pork fats melt at around 40C +/-5, and of course this would take fusion energy, but if that were to cause a "stall" we would then expect to see it at around 40C. But of course, melting pork fat and rendering pork fat out of adipose tissue are not the same thing. Adipose tissue, meanwhile, is made up of fat and collagen and so rendering fat out of the adipose tissue will be dependent in part upon breaking down the collagen. The observed temperature of the "stall" at 74C, which is much higher than the melting temperature of pork fat,makes me believe that the "stall" does not meaningfully result from melting fat.


I basically agree, but the issue is complicated.

Pure fat (i.e. rendered lard or suet) does have a relatively low melting point. It is not a sharp number because it is a mixture of different fats as pointed out in other posts.

When we talk about melting fat during cooking, we really mean rendering, which is largely about converting the collagen in the fatty tissue. Raw fat as it occurs in piece of pork or beef is NOT pure fat - it has lots of connective tissue in it, which is mostly collagen. So rendering fat out of a piece of meats is about both melting the fat, and breaking down the collagen. This is all pretty obvious if you try to render a piece of beef or pork fat - it takes a long time and a lot of heat. Compare that to melting some (already rendered) lard or suet.

The conversion of collagen into gelatin occurs across a wide range of temperatures, at a rate that increases exponentialyl with temperature. That is why we must cook 48 hours at 55C/130F to get tenderness that we might get in an hour at much higher temperature.

Anyway, it does take some heat to melt fat, and it does take some heat to convert gelatin. That will cause SOME stall, but I don't think that is large enough to be a real explanation.

My guess is that in a typical smoker cooking a big pork shoulder or brisket, the wet bulb temperature stays below 74C until such time as the crust of the meat is very dry, at which point dry bulb is more important. This is, of course, discussed at length in my book. This seems like a much better explanation to me.

If your wet bulb / dry bulb hypothesis is correct, this temperature stall would happen in a BBQ oven, but not if you cooked an identical cut of meat sous vide (or braising btw) at a water temperature higher than 74°C. Did you do the experiment?
If memory serves me, collagen is enzymatically converted to gelatin by collagenase, which gets inacivated at about 60°C; a slow process which takes one to several days depending on age-related cross-linking of the collegan. At or above 60°C collagen fibers which have not yet been enzymatically converted to gelatin will swell and shorten (a process reversible by cooling or stretching of the fibers) and squeeze liquid out of the muscle fibers. At about 74°C collagen undergoes a physical phase transition (like melting ice) which is much faster than the enzymatic conversion to collagen, and which would explain the temperature plateau.

I think that a better explanation [of the BBQ "temperature stall" is wet bulb temperature. Basically, if you have a wet thermometer, it reads a lower temp than a dry one, due to evaporation (unless relative humidity is 100%).

Food mostly cooks at wet bulb temperature - until the outside is totally dry. I suspect that is that is the origin of the stall.

Heat gets into food from the surface. If the surface is wet, it will be at the wet bulb temperature. This depends on two things - the food staying wet near the surface, and also the relative humidity in the air around the food. If the humidity is 100%, as it is inside a SV bag then wet bulb = dry bulb. If the food is underwater, then there is no air to worry about and the temperature is again the same.

The surface is wet because there is water in the meat, but over time the surface layers dry out. This tends to raise the temperature until with a very dry crust on the meat very little evaporation will occur and the temperature will be at the dry bulb temperature.

The reason this affects the interior temperature is that the interior can't be any hotter than the surface. So if the surface is stuck at say 74C, then the interior can't get any hotter. As the surface dries out, the surface temperature rises, and sometime later the core temperature will too.

As Pedro points out this would not happen inside a SV bag (or in a combi oven or CVAP oven in low temperature steam mode.) Yes we have done the experiment (sort of) - we have recorded temperature in meat cooked SV or in combi for long intervals and we do not see a stall.

I say sort of because if you really wanted to check this out you would need to take two identical pieces of meat (say brisket), treat them indentically (dry rub and so forth) and then cook on in a smoker. We haven't cooked a whole brisket side by side. We did smaller pieces of meat, and we didn't do perfect comparisons. So a skeptic could still.

Most smokers / BBQs have no humidity control, and people don't know what wet bulb temperature is. We show in my book to rig up a wet bulb thermometer. We think that is how you should control your BBQ, and once you do that we think that reports of the "stall" will disappear.

I have several smokers and we have wet bulb temperature in them and you don't see a stall.

We have also done experiments using a convection oven, or a very fancy smoker, where you can show the wet bulb effect easily. Take a convection oven (or combi oven in convection mode), set the temperature to say 60C/140F and put meat in, it plateaus at the wet bulb temperature.

Finally, if you put meat in a DSC you can see a bit of a stall due to the various chemical changes that are happening but it is NOT a single fixed plateau, nor is the total amount of heat big enough to cause the reported hours-long "stall" that BBQers see.

Conversion of collagen into gelatin is complicated it does NOT occur at a single temperature - it is a reaction that occurs at an increasing rate over a range of temperatures, all the down to just above the animal's original body temperature. It does not occur "at" a certain temperature.

Very interesting. So let me see if I get this right...

For a given piece of meat in a barbecue-style smoker the temperature of the meat will slowly rise until it reaches a kind of "temporary equilibrium temperature" where evaporating moisture maintains the surface of the meat at a more or less constant temperature via the "wet bulb effect" (the inside of the meat also plateaus since peak internal temperature is dependent on surface temperature). The temperature of this temporary plateau should be somewhat different depending upon the ambient temperature and humidity in the smoker, but I imagine these variables are similar enough in barbecue-style cooking to account for the broad similarity observed in this temperature plateau. So the meat sits at the plateau temperature until the surface of the meat dries out sufficiently so that evaporation is no longer able to cool the surface of the meat effectively in the ambient environment (this is now "dry bulb"), at which point the surface temperature and consequently the internal temperature can rise. The time that the meat spends at the "temporary equilibrium temperature" until the surface dries out is the observed "temperature stall."

That is not only incredibly interesting, but also a fun debunking of what everyone thought was a revelatory explanation of the science behind this observed effect! I wonder the extent to which the "temporary equilibrium temperature" could be altered by introducing moisture into the smoking chamber.

We did the stall experiment. We took a brisket, and cut it in half - one piece was sous vide vacuum packed, the other not. Both pieces were put in a combi-oven in convection mode at 90C/194F. We had temperature probes in the air, in the core, and at the surface of each one.

The stall occurred for the un-bagged brisket. The surface temperature of the bagged and unbagged meat started to diverge at 72C/161F. Below that point the bagged one was slightly lower (due to insulation effect and slick surface of plastic), but they crossed over at 72C and from that point forward the bagged one was hotter (both surface and core).

The flat part of the stall was at 78C/172F. The flatest part of the stall curve lasted about 2.5 hours, then gradually creeped upwards. The temperature of the un-bagged brisket was still lower than the bagged brisket after 5.5 hours.

I think this conclusively shows that wet bulb temperature effects (i.e. oven humidity and drying) are the source of the "stall" in doing BBQ. Uncovered meat will stall due to evaporative cooling, which gradually goes away as the surface dries.

This also points something out that I am not sure everybody knows - you can cook sous vide in a convection oven. The reason that we all use water baths is that most convection ovens have poor temperature control compared to a water bath. The heat transfer rate to the food is also much worse, because water is about 1000 times denser than air. However you can do it. In this case we did it because I wanted the two brisket pieces to be under the same temperature and heat transfer rate - the only difference was surface evaporation.

So, in a 100% humid envrionment, we will not observe a stall but we will observe a much gradual temperature increase over a longer period of time depending on the size of the meat?


Yes - a 100% humidity environment is what you have inside a sous vide bag, or in a covered pot, or in a combi-oven in steam mode or CVAP oven. In that case there is no evaporative cooling, and there is no stall.

Thermal Properties of Meat

We model the temperature inside the meat using the heat equation. The only parameter in the heat equation is the thermal diffusivity of the meat.1 Between refrigerator temperatures and boiling, we usually assume that thermal diffusivity is constant even though it change slightly with temperature.

The change in thermal diffusivity with temperature depends on the specific heat capacity of the meat. As Nathan mentioned, one way of measuring the change in specific heat capacity is with a differential scanning calorimeter (DSC).2 The specific heat capacity tells how the temperature changes with the addition of energy. Kemp et al. (2009) showed that the specific heat capacity of muscle fibers and sarcoplasmic proteins is essentially flat from 30C to 85C, connective tissue has a hump peaking around 60C, and fat is essentially flat over 50C. Therefore, the `stall' in smoking at around 74C isn't caused by chemical changes in collagen or the melting of the fat.

There are huge change in thermal diffusivity of meat outside 0C to 100C. Melting/freezing takes a huge amount of energy: it takes about as much energy to heat beef from -20C to 0C as it does from 0C to 70C. It takes even more energy boil away the water in the meat, but we don't have to worry this when cooking sous vide so I'll discuss this later.

DRY OVENS AND SMOKERS

While the meat at the surface still retains most its water, we can use the same heat equation and take the thermal diffusivity to be constant. The main differences are that h is much lower and how quickly the surface temperature increases is proportional to the difference between the wet-bulb temperature in the oven and the current surface temperature of the meat. (In the water bath, the surface temperatures rate of change was proportional to the water bath's temperature, which is always equal to the wet-bulb temperature, and the surface's current temperature.) As Nathan's explained, the wet-bulb temperature is cooler than the dry-bulb temperature in an oven when the relative humidity is less than 100% because of evaporative cooling. Evaporative cooling is so important because it takes a huge amount of energy to change the phase of water to water vapor --- it takes five times as much energy to evaporate a mass of water as it does to heat it from 0C to 100C!

When the meat at the surface has desiccated (dried out), we can no longer assume that the thermal diffusivity is constant --- indeed, it changes dramatically. This makes it very difficult to calculate the core temperature of the meat and so is beyond the scope of this post. This is also when, as Nathan discussed, that the dry-bulb temperature in the oven or smoker becomes more important. Since this doesn't really apply to sous vide cooking, I'll leave it for a later discussion; if you're really interested in how we deal with this `moving boundary value problem' then either Nathan or I can direct you to the relevant literature or discuss it on a more relevant eGullet thread.

Here is the graph showing BBQ stall is due to wet-bulb temperature effects (humidity). Chris Young, one of my co-authors on the Modernist Cuisine book ran these experiments last night and this morning.

We took one brisket, cut it in two. One was sous vide cooked, the other was not covered. Both were put on a wire rack in a Rational combi-oven in convection mode at 90C/194F, with a temperature sensor in the center (core).

The wet-bulb temperature comes from an home-made wet bulb sensor (details of the sensor, and this whole topic are given in my upcoming book).

Dry bulb temperature is from a sensor in the oven. You will notice that the sous vide sample actually is higher than the oven temperature. How can that be? The reason is that the oven temperature varies at various places in the oven by a few degrees.

The sous vide brisket makes a smooth transition from its initial temperature up to 90C/194F, reaching that point in about 10,000 seconds (2.78 hours). There is NO STALL in the sous vide sample.

The glitch in the dry bulb and wet bulb temperatures near the start is caused by opening the oven to put the meat in.

The wet bulb temperature starts out at about 60C/140F. The WB temp rises as evaporation from the meat makes the oven higher humidity. It reaches a peak of about 80C, then declines over time becaues the meat is drying out, and the oven is venting air and moisture.

The non-bagged brisket (labeled traditional) has a stall between about 7500 seconds and 15000 seconds (between 2 hours and 4 hours after the start). Eventually the surface of the meat dries out, so that the the surface reaches dry bulb temperature - at that point the stall ends and the meat temperature rises again.

I think this pretty conclusively shows why there is a stall.

The actual temperature of the stall will depend on the humidity in the oven, and a couple other factors.

 

[Dave Russell]

Ok, so it's a simple matter of surface evaporation stalling the transfer of heat into the core of the meat, and nothing to do with fat and colagen and connective tissues?

Does this mean that in a dry environment the evaporation will be greater, but for a shorter period of time, and vice versa for a wsm w/ water pan, for example? IOW, there's no effect on the length of the stall if using water in a pan or not, right? Only way to reduce the stall is to wrap in foil?

 

[ZILLA]

From what I can tell wet bulb and dry bulb temperature phenomenon has to do with air temperature and the relative humidity and rise of temperature of the air. This process seems to be used in the food industry in the dehydration of foods like making dried potatoes and such air dried food items that have to really dry. Even after a brisket is injected with more liquid the wet bulb effect is not sustained for a longer time. The science of collagen breakdown has been studied extensively by the meat industry and if anyone cared to pay for the research articles available we might get a more clear picture. WB DB may however play a role in some situations but the collagen stall is real. .

 

[Dave Russell]

From what I can tell wet bulb and dry bulb temperature phenomenon has to do with air temperature and the relative humidity and rise of temperature of the air. This process seems to be used in the food industry in the dehydration of foods like making dried potatoes and such air dried food items that have to really dry. Even after a brisket is injected with more liquid the wet bulb effect is not sustained for a longer time. The science of collagen breakdown has been studied extensively by the meat industry and if anyone cared to pay for the research articles available we might get a more clear picture. WB DB may however play a role in some situations but the collagen stall is real. .

I wonder if "the collagen stall" and the "evaporative cooling stall" are two different things that just happen to occur at roughly the same time, together working to make the IMT stall as significant as it is. It seems unlikely, but both concepts seem to have science behind them.

 

[infernooo]

Zilla: From what Nathan has said, collagen conversion is definitely real and will cause SOME stall, but not enough to account for the magnitude of the typical stall (hours). If it were purely due primarily to the collagen, we would see a plateau when cooking at 100% humidity, but as you can see from the above experiment, we don't.

 

[mikeTRON]

so how do you set up a thermometer for wet bulb?

 

[ZILLA]

http://www.meathaccp.wisc.edu/assets/SlideRuleMethod.pdf

 

[Northwet Burner]

Wet bulb/dry bulb is only germaine if the discussion is about gas. It has no meaning in a discussion of liquid/solid temperature measurements. It is good information about stack conditions as moist air can carry more energy (specific heat) which means that food will cook quicker.
This has been a learning experience and I appreciate your insight. Now- one more question. How does the location of the fire affect this (convection vs Radiant) heat?
Seems like the radiant energy would penetrate the meat better but what effect would that have on the colagens?

 

[Dave Russell]

Now- one more question. How does the location of the fire affect this (convection vs Radiant) heat?
Seems like the radiant energy would penetrate the meat better but what effect would that have on the colagens?

I must say that the magic of a uds (if you don't mind all that "fat in the fire flavor") is to make JUICEY 'Q, REAL QUICK. The colagens do just fine.:becky:

 

[ZILLA]

According to this article there seems to be an error in the way these guys measured the temperature of the surface of the meat which they state is the wet bulb temp. Wouldn't this cause a discrepancy in their outcomes?

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2621.1976.tb14386.x/abstract

It also seems to me that when cooking Sous Vide or in a pressure cooker, there are other effects at work that would change the dynamics of cooking that they fail to discuss or take into consideration at all. Comparing a wood fired, free draft smoker to an electric/gas Convection oven and on top of that one product is in a Vacuum sous Vide bath? It seems wrong to compare the two cooking methods and then draw conclusions that way. Apples and oranges it seems to me.... I'm not trying to defend the Phase Change theory at all, as I really don't care what ends up causing the stall as long as I can get to the bottom of it. I just can't find any reference that leads to their conclusions at all. Wet Bulb Dry Bulb temps seem to be relevant to gas atmospheres and those that rely on them for drying and dehydrating to maintain a level of safety from bacteria growth.