Flame On - Brew In A Bag best brewing practice - or - is adjusting temp mid-mash a good idea?

I used to be the guy that chased exact temp mashing.

As a home brewer does the final product benefit to any noticeable degree from being mashed at any constant temperature for the entirety of the mash? Does kettle mashing have any drawbacks compared to cooler mashing? Based on conversion facts what do we need to do to make good beer?

It is a proven fact that when mashing grain, 90% of the conversion from starch to sugar occurs within the first forty-five minutes. For the non-scientific heat transfer explanation it is also proven that the mash insulates itself and is further insulated by the metal kettle. In any mash tun, because of thermal mass and heat transfer laws the mash resists sudden temperature fluctuations. I use the word fluctuate because not only does the mash resist temperature decrease, it also resists increase.

Basic Processes

With the exception of the bag and insulation reference this process is the same for all brewers. The strike water is heated to 159º or so, the bag is inserted, grain is added, (12 to 18 pounds) stirred, covered, and the kettle insulated. The temperature should be about 152º which is in the “zone” for most recipes.

Conversion starts the very second the grain is added to the water. Most brewers don't start their timers until after insulating the kettle, and for me that's about 10 minutes. I set the timer for 60 minutes, which is really 70, and walk away and I don’t open the lid until the timer goes off.

Newbie Test

I didn't always walk away. In the beginning I checked the temp every fifteen minutes. That meant I took the insulation off, removed the lid, stirred for minute or so, recorded the temp using the sidewall kettle thermometer, and then covered it all up again - for another fifteen minutes. Funny that every time I went through that process the temperature dropped a couple of degrees.

I have a bimetal sidewall mounted thermometer in my aluminum kettle and it's about 2º low compared to my new digital thermometer. Every time I calibrate it, it’s still 2º off one side or the other, so I’m thinking it’s my eyesight. I didn't always have two thermometers to calibrate one to the other. So, in my early days of brewing I'd look at the bimetal thermometer in the kettle and freak out that 2-3º was lost. That meant my beer would suck, so I needed to get that temperature back to 152º ASAP before conversion stopped!

So the guessing game would begin. How long to apply heat to correct that drop? And at what temperature will the 3” of wort in the kettle need to be heated to raise the temp of all the grain in the kettle back to my mash temp? 10º, 15º, 20º???

To prevent the bag from melting, I'd hoist the bag a few inches off the bottom, fire up my 160,000 btu KAB4, and watch the thermometer. It would rise very quickly, like 10-15º in just two minutes. I'd guess that was good enough, shut off the burner, lower the bag and stir like crazy.

That process resulted in confusion. According to the sidewall thermometer, the temp of the wort would go up 10-15º, but after stirring, the mash temp didn’t change or went over my target of 152º - not good at all.  Hmmm, I don’t get this, so to the internet I went and found thermodynamics - From Wikipedia - “A thermodynamic operation is an artificial physical manipulation that changes the definition of a system or its surroundings. Usually it is a change of the permeability or some other feature of a wall of the system,that allows energy (as heat or work) or matter (mass) to be exchanged with the environment. For example, the partition between two thermodynamic systems can be removed so as to produce a single system.”

The Keys to Understanding

I'm not a scientist - but I don't have to be in order to understand the inherent considerations of insulation - and in this case we're talking about a blanket and bag of wet grain preventing changes in temperature. Also that fire + kettle + grain + water is not a single system.

The problem with trying to recover even 1º with a kettle full of wet grain is that the mass of grain is nearly impermeable to the heat applied to it without risking burning the sugars in the wort. Short of starting over with a new set of calcs by draining the bag, heating the wort in the kettle and putting the bag back in with full consideration of the grain temp etc, trying to correct tempo mid-mash is a crap shoot.

To further emphasise crap shoot aimed at volume and energy applied. Put ½” of water in any size pan and place it on your stove at full flame and then do the same with a quart, and a gallon. You’ll see that the energy / time needed to boil that water changes with the volume of water. So when we consider that grain bills are inconsistent the complexity of determining the final temperature is nearly impossible. For the sake of consideration the typical propane  flame burns at 3,630º - that my friends will melt your cat, let alone your bag!

So when we apply heat to our boil kettle knowing there are only three to six inches of wort - or if you don’t lift the bag - zero inches of wort the “excess” energy superheats that small volume of wort. We all know that sparging at 170º stops conversion and I’ll bet two shiny nickels that the temperature at the bottom of the kettle exceeds 170º, otherwise, how can we raise the temperature of the mass if the temperature is less than the mass? So every time we apply heat we kill off a portion of the enzymes, likely burn or alter the sugars in that space, and ultimately detract from the very perfection we’re trying to achieve.

There are charts to figure this kind of stuff but the variables for all of us are too many. Why? Because with every brewer and every brew, the grain volume changes, along with water depth, strike temp, ambient temp, sugar concentration, and on and on.

We can't calculate the temperature of the flame relative to the time it takes to burn the wort in every kettle and circumstance, but we know that sugar burns at 350º and so does the polyester material - unless there is sugar free liquid surrounding it, and then it does not melt or burn. Have you ever seen the video of the styrofoam cup filled with water sitting in the coals of a campfire? - not melting. Do a search, you’ll be amazed.  

What Can We Do?

Strike temp relative to the grain bill, ambient temp, grain temp, and proper insulation can be known and used to control the initial and thus somewhat predictable final mash temp. And that is the key to mashing at proper temperature. Almost every brewing calculator considers these factors when creating a recipe and process sheet.

Another point worthy of consideration is the temperature conversion scale range of mashing. Enzymes do their converting of starches to sugars from as low as 95º on up to 165º with different types of sugars created in specific temperature ranges. Because of the sophisticated malting techniques we are now beneficiaries of, most grains available to homebrewers are modified to react well between 143º and 163º without rests at lower temperatures that would produce different additional sugars.

So depending on the mouthfeel they’re after, style of beer, etc. most home brewers focus their mash temperatures in that range with the majority of advice pointed at the 152º mark. 152º actually overlaps the conversion range of short chain more fermentable sugars (130º - 152º) and long chain less fermentable (152º-158º). Again, popular advice tells us that if we mash at 152º our beer will be “good” and will not be too dry or too heavy (sweet), and that advice is correct in my brew house.

Facts is Facts

So let’s say that you hit your strike temp, mashed in and hit the mash temp as well and then thirty minutes into the mash you check the thermometer and notice a 2º loss. Since you’re on target today and are not about to lose 2º no matter what, you decide to burn your wort and fire up to get that 2º back. Was it worth it and what did you gain? No, and nothing.

The reality is as long as the mash temperature is in the range for the style of beer you’re after, everything is just fine. The range at the lower end is 152º to 130º with optimum temperature for Beta sugars produced at 143.6º, and at the higher end 156º to 163º with optimum temperature for Alpha sugars produced at 152.6º.  As stated above, the majority of conversion occurs in the first 45 minutes and we’ll add that at 30 minutes it is ~75%  complete.

If you insulate your kettle or use a cooler, by the time you notice a temperature drop you’re fighting a war that’s 75% over and the damage caused by superheating the already converted wort is far greater than what’s to be gained by firing up that burner.

Strike water temperature is just the beginning of the mash temperature. If your grain temperature is lower than assumed the mash temperature will drop, sometimes by 3º to 5º more than calculated, and that can change fermentability to some degree. In order to hit your mash temp consistently you have to consider temperature fluctuations where you store your grain, or if you buy grain from the LHBS or otherwise, stick it with a thermometer before input to the software, or leave it for an hour or so where you know what the ambient temperature is.

Also, bear in mind that “hitting the numbers” starts at the beginning with mash temperature / conversion / fermentability and completes all the way to the end of the cycle. If you expected 1.007 and only got 1.012 that may be a result of too high a mash temperature which resulted in a portion of the wort being unfermentable.

Don’t Burn the Bag!

Using a fabric filter can shorten your brew day, make cleanup easier, and prevent a stuck or slow sparge, but it can’t compensate for lack of knowledge or understanding.  Remember, the filter does not make the beer, but choosing the right one sure does make the brew day easier.

Below is an photo of a bag that was burned in the kettle by turning the flame on with a bag full of grain. You can see that the burns are equal distance and look similar indicating the construction of the burner or false bottom that touched the bag. Sugar burns at 350º and so does polyester - dead enzymes - dead bag. The other photo is a result of exposure to open flame.  

You can also watch this video that shows the material being held to the bottom of a boiling pan - without melting or burning. 



Older Post Newer Post

  • Terry Wray on

    Lots of great info in here! From now on, I’m going to grind my grain all the way and stop worrying about my mash temp after the grain goes in. Just gonna set that timer and keep the lid on! Oh, and make sure my grain is room temp before mashing, too. Keep these posts coming! Loving my brew bag!

  • The Brew Bag on

    Thanks for your input and we appreciate your logic. Our process logic does not involve using a false bottom. The point we make is, unless your intent is driven and can be confirmed by the consistency of the final product, correcting temp losses of 1º to 3º over 60 minutes is of little value to homebrewer’s. Of course using a false bottom upsets the logic of the article – in our opinion using a false bottom is redundant equipment cost.

  • Roger Burns on

    I agree with your general consensus in the article, however, there is a flaw in the logic that I would like to point out. Each of the points you mentioned for BIAB brewing, and heating in place, are resolvable through modifications in your equipment. I have been brewing BIAB for years, and have never had any burnt bags, while heating my mash to maintain temps, or mash out. Firstly, a false bottom is needed, always, to provide that liquid barrier. Second, I have had great success in using aluminum bottomed (or all aluminum) kettles. The aluminum transfers the heat very well, compared to SS where hot spots can develop. I regularly heat my wort up to 168-170 F, with grains in bag, while stirring, and reducing the viscosity so that when I raise the bag, it drops out most of the sugar without sparging.

Leave a comment

Please note, comments must be approved before they are published