Mash efficiency

[YeastFeast]

I use Brewfather online for my recipes and it gives me a guess at OG and FG. For whatever reason I haven't determined yet, I'm always a 2-4 points low. ....But this last beer I did I was 16 points low! Just wanted to get some opinions on why my mash efficiency was so off.

To me the most obvious would be a bad grain crush. I have it done at a local brew shop and is one of the few things I'm not doing myself yet.
But I also noted during the brew session that I had a low pH (as measured 10 minutes into the mash). I usually shoot for 5.4 - 5.6 measured at room temperature. It was actually a pH of 5.3 at room temperature. Could that low pH affect mash efficiency?

One other thing, I sparged at a much slower rate than I had before on my 3 kettle system. There was some discussion in another thread that suggested if you sparge TOO SLOW, it will actually have a negative consequence as far as extracting all the sugars.

Thanks for any input/thoughts/experience!

 

[DBhomebrew]

For whatever reason I haven't determined yet, I'm always a 2-4 points low. ....But this last beer I did I was 16 points low!

How many batches 2-4 pts low? Are they all roughly the same OG?

The 16pts low, much bigger OG?

Are all of these on the new system? Or is the -16pts batch on the new with the others on the old?

 

[YeastFeast]

How many batches 2-4 pts low? Are they all roughly the same OG?

The 16pts low, much bigger OG?

Are all of these on the new system? Or is the -16pts batch on the new with the others on the old?

I've done 8 batches on the new system, all but the last one were 2-4 points low, pretty consistent. Yes, all but one were Hazy IPA recipes and all had a very similar OG. The one that wasn't an IPA was a stout a few batches back which was also 3-4 points low.

I know there's a lot of variables here... just trying to get some idea if low pH and/or sparge speed had anything to do with it.
Thanks.

 

[DBhomebrew]

And the -16pt batch? How did it compare to the other in regards to OG?


All same batch volumes?

 

[YeastFeast]

And the -16pt batch? How did it compare to the other in regards to OG?


All same batch volumes?

The -16pt batch was almost the same exact recipe as the other 7 before it. Just rotation of different hops and slight changes in grains. All 5 gallon batches.

 

[hotbeer]

You also have to take into consideration if you got the same volume of wort at the end of the boil that your recipe showed you to have. If it's more then that can easily give you a lower OG. If it's less you'll probably have a higher OG.

And a little bit of water more or less can make the difference of a few points of SG to a lot of points.

But grain crush and things you do while mashing can make a difference too.

 

[YeastFeast]

You also have to take into consideration if you got the same volume of wort at the end of the boil that your recipe showed you to have. If it's more then that can easily give you a lower OG. If it's less you'll probably have a higher OG.

OK, been keeping track of that the last 5 batches. Just checked notes, all within .25 gallon of each other.

 

[doug293cz]

Mash efficiency is equal to conversion efficiency times lauter efficiency. To diagnose why your mash efficiency is low, you need to determine whether your conversion efficiency is low, your lauter efficiency is low, or both.

Conversion efficiency is the fraction/percentage of the maximum potential sugar (rigorously extract) that you actually created in the mash. It is affected by crush fineness, mash time, mash temperature, and mash pH. Crush affects how long conversion takes - coarser crushes convert slower. Coarse crush can be compensated by mashing longer, as long as the amylase enzymes don't denature before the end of the mash. The rate of conversion is primarily limited by the rate of starch gelatinization (starch cannot be converted to sugar until it is gelatinized) and gelatinization proceeds faster at higher temperatures. Higher temperatures also cause enzymes to denature faster, so you have to balance gelatinization rate vs. the denaturing rate. pH plays a relatively minor role, as long as your pH (measured at room temp) is between about 5.2 and 5.8. Below pH 5.0 alpha amylase denatures much more quickly. So, it is unlikely that a pH of 5.3 had much effect on your big OG miss.

Lauter efficiency is the fraction/percentage of the sugar/extract that was actually created in the mash that got transferred to the BK. Lauter efficiency is affected by sparge process, grain absorption rate (gal/lb of grain), and undrainable wort volume (what's left in the bottom of the MT and in plumbing.) Sparge process is by far the biggest variable. No-sparge is the lowest lauter efficiency, then single batch sparge, double batch sparge, and the highest (if done correctly) is fly sparging. A poorly done fly sparge can have lower lauter efficiency than a batch sparge (in which case you are wasting time with your fly sparge.)

Lauter efficiency decreases as the ratio of grain bill weight to pre-boil volume increases (higher OG beers) similarly for all sparge processes. Unfortunately, most brewing software does not do predictive modeling of this effect, and expects the user to specify their mash or brewhouse efficiency (and derives mash efficiency from the input brewhouse efficiency.) This is a sad situation because predicting lauter efficiency is relatively straight forward. The chart below shows how lauter efficiency varies with the grain weight to pre-boil volume ratio for various numbers of batch sparges and two different grain absorption rates. Fly sparging is a few percentage points better than a triple batch sparge @ 0.12 gal/lb absorption rate (a bit higher than the top solid line in the chart.) The chart is for zero undrainable mash volume, and any undrainable volume will shift the lines lower.

Conversion efficiency can be measured using the method here, and sampling SG during the mash with a refractometer makes monitoring conversion during the mash practical. To determine your lauter efficiency, you divide the actual mash efficiency (calculated by your brewing software) by your conversion efficiency.

If conversion efficiency is below about 95% then you should be looking at your crush and/or mash time to make improvements (and pH if outside the 5.2 - 5.8 range.) If lauter efficiency isn't meeting the expectations of the chart above, then you should be looking at your sparging process to make improvements. If you want to do your own mash efficiency modeling, you can use my spreadsheet (download a copy into Excel or LibreOffice.)

Brew on

 

[YeastFeast]

@doug293cz
Quite informative and educational! Thanks for that! Guess I have some work to do!

It appears you've ruled out relatively low pH, which is good to know, but will need to be more careful with my lactic acid additions in the future.

My Sparge process / lauter efficiency is where it seems I need to concentrate. I use a Blichmann system (3 kettle) with their AutoSparge system so I guess that would be considered a single batch sparge. Of course I'm in full control of pushing the sparge water through the mash so I'm probably causing the problem in some way. Yet to be determined.

 

[balrog]

Everyone's system and process, when done the same way every time on the same recipe with the same crush, will have a brewhouse efficiency, which BrewFather knows nothing about. If you calculate what the efficiency is, and simply use that going fwd, your should expect (keeping all things equal) the same results. If something suddenly changes, like different crush at the LHBS or using higher percentage wheat in the grist, then you can expect some differences. If it's always a couple points low, personally I would simply change BrewFather's expectation brewhouse efficiency on that recipe to be a skosh less. Repeatability is more important to me that a couple percentage points in efficiency. If I know a recipe is 76% or 68%, I know how to adjust the grist to get the OG I want.

 

[VikeMan]

I use a Blichmann system (3 kettle) with their AutoSparge system so I guess that would be considered a single batch sparge.

That's a Fly Sparging device (not batch). Done properly, a fly sparge will give better lauter efficiency than any number of batch sparges.

Of course I'm in full control of pushing the sparge water through the mash so I'm probably causing the problem in some way.

With fly sparging, the slower the better, from an efficiency standpoint. Also, the more evenly and gently the water lands on the surface of the wort, the better, because it can help avoid channeling.

 

[eric19312]

The -16pt batch was almost the same exact recipe as the other 7 before it. Just rotation of different hops and slight changes in grains. All 5 gallon batches.

My understanding is you tend to run 3-4 points low on SG but one batch with similar grain bill to other batches came in quite a bit lower than expected. The 3-4 percent low every batch suggests you have really good repeatability on that system. Just build that adjustment into your recipe design and you will be coming out right on target every time.

Assuming these batches were all similar size (target OG) I'd give you three possible reasons for the single really low batch...
Crush - finer crush usually increases efficiency but increases risk stuck mash. Always inspect your crush and make sure you are not seeing a lot of uncracked grain.
Measurement - happens, somebody left out a couple pounds of grain, probably between the bins and the mill, by accident.
Lautering speed - slower is better. I build my recipes around what I get with 45-60 minute lauter but know 90 min will be more efficient while 20-30 minutes I will see a significantly lower efficiency.

 

[Deadalus]

@doug293cz
Quite informative and educational! Thanks for that! Guess I have some work to do!

It appears you've ruled out relatively low pH, which is good to know, but will need to be more careful with my lactic acid additions in the future.

My Sparge process / lauter efficiency is where it seems I need to concentrate. I use a Blichmann system (3 kettle) with their AutoSparge system so I guess that would be considered a single batch sparge. Of course I'm in full control of pushing the sparge water through the mash so I'm probably causing the problem in some way. Yet to be determined.

About a quart/minute into the BK is the rate I have seen bandied about for fly sparging.

I have a rotating sparge arm that is fed with a DIY float valve almost the same as the autosparge. I actually took better notes and measurements when I brewed yesterday and I had noted the time I started sparging. I was going really slow at the 30 minute mark, still had about 3.5 gallons to go...so I sped it up at the end some but it still took about an hour. I think that's my usual on that as I know the flow rate from observation. I happened to be five points over but the number I have been using for BH efficiency is somewhat conservative. However, I used a slightly different mash schedule this time which included a protein rest and semi-double crushed the grain. I ran my grain through and at the end, noticed that one of the set screws had loosened, and I had a V-shaped gap. I remilled the grain as the grain only crushed on the one side of the rollers. My end of running gravity was 1.011 and I overshot my preboil volume by a quart but I did boil it off anyway.

I can't say I have ever seen that argument about going too slow with fly sparging being less efficient. Maybe it's based on if there is cooling in the mash column if really slow? I have a 3V EHERMS (30 amp). I can maintain the sparge water at 170 without a problem but I can't direct fire the MT. I ran a glacially slow fly sparge on a recent dopplebock as part of a plan to address the high gravity, lower efficiency issue and that seemed to help.

How was the tempertature of your sparge water maintained or not? Maybe if you were slower sparging, that cooled off some. I don't know if that can cause a noticeable change however.

I'd be checking and calibrating my thermometers though if I was off that much.

 

[Teufelhunde]

I use Brewfather online for my recipes and it gives me a guess at OG and FG. For whatever reason I haven't determined yet, I'm always a 2-4 points low. ....But this last beer I did I was 16 points low! Just wanted to get some opinions on why my mash efficiency was so off.

To me the most obvious would be a bad grain crush. I have it done at a local brew shop and is one of the few things I'm not doing myself yet.
But I also noted during the brew session that I had a low pH (as measured 10 minutes into the mash). I usually shoot for 5.4 - 5.6 measured at room temperature. It was actually a pH of 5.3 at room temperature. Could that low pH affect mash efficiency?

One other thing, I sparged at a much slower rate than I had before on my 3 kettle system. There was some discussion in another thread that suggested if you sparge TOO SLOW, it will actually have a negative consequence as far as extracting all the sugars.

Thanks for any input/thoughts/experience!

Having moved from extracts to all grain about a year back, I can tell you that all of the people that tell you that you need to crush your own grains are 100% correct. You will never get consistent mash efficiencies with either a LHBS or online HBS crushing the grains for you. Just get it over with and buy a mill......

 

[YeastFeast]

That's a Fly Sparging device (not batch). Done properly, a fly sparge will give better lauter efficiency than any number of batch sparges.



With fly sparging, the slower the better, from an efficiency standpoint. Also, the more evenly and gently the water lands on the surface of the wort, the better, because it can help avoid channeling.

Interesting. I guess I always thought fly sparging had to involve some device that created something like a rain shower above the grain. My Autosparge maintains 1-2 inches of water over the top of the grain (at least until the sparge water runs out...by the end, the grain bed is completely exposed) with the water simply coming out of a tube.

 

[YeastFeast]

My understanding is you tend to run 3-4 points low on SG but one batch with similar grain bill to other batches came in quite a bit lower than expected. The 3-4 percent low every batch suggests you have really good repeatability on that system. Just build that adjustment into your recipe design and you will be coming out right on target every time.

Assuming these batches were all similar size (target OG) I'd give you three possible reasons for the single really low batch...
Crush - finer crush usually increases efficiency but increases risk stuck mash. Always inspect your crush and make sure you are not seeing a lot of uncracked grain.
Measurement - happens, somebody left out a couple pounds of grain, probably between the bins and the mill, by accident.
Lautering speed - slower is better. I build my recipes around what I get with 45-60 minute lauter but know 90 min will be more efficient while 20-30 minutes I will see a significantly lower efficiency.

Never considered they screwed up the grain amount. Definitely can't rule that out. Guess that's where weighing & milling my own grain comes in!
Thanks for the other thoughts.

 

[YeastFeast]

Everyone's system and process, when done the same way every time on the same recipe with the same crush, will have a brewhouse efficiency, which BrewFather knows nothing about. If you calculate what the efficiency is, and simply use that going fwd, your should expect (keeping all things equal) the same results. If something suddenly changes, like different crush at the LHBS or using higher percentage wheat in the grist, then you can expect some differences. If it's always a couple points low, personally I would simply change BrewFather's expectation brewhouse efficiency on that recipe to be a skosh less. Repeatability is more important to me that a couple percentage points in efficiency. If I know a recipe is 76% or 68%, I know how to adjust the grist to get the OG I want.

Thanks. Adjusting Brewfather is a good plan.

 

[VikeMan]

Never considered they screwed up the grain amount. Definitely can't rule that out. Guess that's where weighing & milling my own grain comes in!

And...make sure your scale is calibrated.

 

[YeastFeast]

How was the tempertature of your sparge water maintained or not? Maybe if you were slower sparging, that cooled off some. I don't know if that can cause a noticeable change however.

I'd be checking and calibrating my thermometers though if I was off that much.

I am circulating the water through the HLT to maintain temp, although once it runs out, it may be cooling in the mash tun?
Not a bad idea to check therms for accuracy.

 

[YeastFeast]

Having moved from extracts to all grain about a year back, I can tell you that all of the people that tell you that you need to crush your own grains are 100% correct. You will never get consistent mash efficiencies with either a LHBS or online HBS crushing the grains for you. Just get it over with and buy a mill......

I think I'm convinced. Will probably get a mill before my next brew. More research, not I gotta figure out which one!

 

[eric19312]

I don't believe sparge water temperature has big impact on efficiency. Know it is best practice and part of the idea is better mobilization of sugar from the grain but plenty of homebrewer anecdotal experiments out there of people sparging with room temp water without taking big hit on efficiency. I heat my sparge water to the wort going into kettle is as hot as it can be so I can get to a boil in short order when lautering is done.

On lautering rate it seems that the 60-90 min lauter duration is applicable regardless of vessel size. So the rate in L/min will be quite different for a 5 gallon mash than it will be for a 15 gallon mash.

 

[Deadalus]

I am circulating the water through the HLT to maintain temp, although once it runs out, it may be cooling in the mash tun?
Not a bad idea to check therms for accuracy.

Yes, I have temp probe on the MT out valve and while I don't watch the the reading much there is definitely difference going out. I'm usually more focused on the flow rate but on my last brew I think the temp was in the 130's or 120's going out. Not saying that makes a difference, it's probably fairly consistent for similarly sized grain bills.

 

[doug293cz]

About a quart/minute into the BK is the rate I have seen bandied about for fly sparging.

I have a rotating sparge arm that is fed with a DIY float valve almost the same as the autosparge. I actually took better notes and measurements when I brewed yesterday and I had noted the time I started sparging. I was going really slow at the 30 minute mark, still had about 3.5 gallons to go...so I sped it up at the end some but it still took about an hour. I think that's my usual on that as I know the flow rate from observation. I happened to be five points over but the number I have been using for BH efficiency is somewhat conservative. However, I used a slightly different mash schedule this time which included a protein rest and semi-double crushed the grain. I ran my grain through and at the end, noticed that one of the set screws had loosened, and I had a V-shaped gap. I remilled the grain as the grain only crushed on the one side of the rollers. My end of running gravity was 1.011 and I overshot my preboil volume by a quart but I did boil it off anyway.

I can't say I have ever seen that argument about going too slow with fly sparging being less efficient. Maybe it's based on if there is cooling in the mash column if really slow? I have a 3V EHERMS (30 amp). I can maintain the sparge water at 170 without a problem but I can't direct fire the MT. I ran a glacially slow fly sparge on a recent dopplebock as part of a plan to address the high gravity, lower efficiency issue and that seemed to help.

How was the tempertature of your sparge water maintained or not? Maybe if you were slower sparging, that cooled off some. I don't know if that can cause a noticeable change however.

I'd be checking and calibrating my thermometers though if I was off that much.

Sparge water temp makes no difference when batch sparging, and shouldn't make a difference with fly sparging. Even in room temp water the solubility of maltose is 44% by weight (or 44°Plato) which corresponds to an SG of 1.197. If you mash at 1.0 qt/lb, the max SG of the mash wort will be about 1.115, so there are no solubility issues, since the sugar is all dissolved as it is created (there is no sugar that needs to be dissolved during the sparge.)

One caveat for cold water sparging being equivalent to hot water sparging is that either the conversion efficiency has to be ~100%, or a mash-out needs to have been done to cut off any enzymatic activity during the sparge. If there is any conversion remaining to be done at the start of the sparge, it will continue during the sparge until all the enzymes are denatured. In this case a hot water sparge would be somewhat more efficient than a cold water sparge (because conversion happens faster at higher vs. lower temperatures.)

One difference between hot and cold water is the rate of diffusion of dissolved solids (sugar/extract in our case.) Molecules move faster at higher temperatures, so diffusion (mass transport across distances without solution movement) also occurs faster. In the case of a fly sparge, you want to minimize the vertical mixing of the concentrated wort and the sparge water. This is so that the sparge water will push the most concentrated wort out the bottom of the MLT, leaving behind the least concentrated wort/water. Mixing of the wort and sparge water reduces the effectiveness of the "sweeping" action of the sparge flow.

An argument could be made that cold water might be more efficient than hot water for a fly sparge, as there will be less diffusion of extract upwards in the MLT during the sparge. Any upwards diffusion of extract will reduce the sparge efficiency.

Upwards diffusion of extract during a fly sparge might also explain the observation, by some, that very slow fly sparges have lower efficiency. If this effect is real, then it would indicate that fly sparging should be done as fast as possible consistent with avoiding any channeling thru the grain bed (which is a much more detrimental effect than any upwards diffusion of extract during the sparge.)

Of course the biggest advantage to hot sparging is that the time to heat to a boil is minimized. But, this is a bigger advantage when batch sparging, as you should have lots of time to be heating the BK during a fly sparge (if you start heating as soon as you have a couple inches of wort in the BK.)

Brew on

 

[hotbeer]

@doug293cz I was all set to pose a question about conversion of starch to sugar and the temp dependent ratios of alpha and beta amylase that usually have us heating water to about 154°F (68°C).

Then I realized you said sparge water, not mash water temperature. For some reason I still tend to lump sparging and mashing together as the same process when they are in fact two different processes.

 

[Deadalus]

Sparge water temp makes no difference when batch sparging, and shouldn't make a difference with fly sparging. Even in room temp water the solubility of maltose is 44% by weight (or 44°Plato) which corresponds to an SG of 1.197. If you mash at 1.0 qt/lb, the max SG of the mash wort will be about 1.115, so there are no solubility issues, since the sugar is all dissolved as it is created (there is no sugar that needs to be dissolved during the sparge.)

One caveat for cold water sparging being equivalent to hot water sparging is that either the conversion efficiency has to be ~100%, or a mash-out needs to have been done to cut off any enzymatic activity during the sparge. If there is any conversion remaining to be done at the start of the sparge, it will continue during the sparge until all the enzymes are denatured. In this case a hot water sparge would be somewhat more efficient than a cold water sparge (because conversion happens faster at higher vs. lower temperatures.)

One difference between hot and cold water is the rate of diffusion of dissolved solids (sugar/extract in our case.) Molecules move faster at higher temperatures, so diffusion (mass transport across distances without solution movement) also occurs faster. In the case of a fly sparge, you want to minimize the vertical mixing of the concentrated wort and the sparge water. This is so that the sparge water will push the most concentrated wort out the bottom of the MLT, leaving behind the least concentrated wort/water. Mixing of the wort and sparge water reduces the effectiveness of the "sweeping" action of the sparge flow.

An argument could be made that cold water might be more efficient than hot water for a fly sparge, as there will be less diffusion of extract upwards in the MLT during the sparge. Any upwards diffusion of extract will reduce the sparge efficiency.

Upwards diffusion of extract during a fly sparge might also explain the observation, by some, that very slow fly sparges have lower efficiency. If this effect is real, then it would indicate that fly sparging should be done as fast as possible consistent with avoiding any channeling thru the grain bed (which is a much more detrimental effect than any upwards diffusion of extract during the sparge.)

Of course the biggest advantage to hot sparging is that the time to heat to a boil is minimized. But, this is a bigger advantage when batch sparging, as you should have lots of time to be heating the BK during a fly sparge (if you start heating as soon as you have a couple inches of wort in the BK.)

Brew on

Good information, I was wondering how the argument referenced, that a slower fly sparge was potentially inefficient, was constructed. It doesn't follow anyting I've noticed or read about previously, I just threw temperature out there as more of an opening discussion. I follow the points you are making here and it's helpful that you have removed solubility from the discussion. So you are saying there is a diffusion rate, which is modified by temperature, and a flow rate. And I am not at all disagreeing with anything you wrote, but the way I was thinking of the problem setup is different in that I was thinking that the sparge water is a fixed temperature and that the mash temp drops over time. Nearly all of my recipes I mash out at 168 as well (without getting on a tangent regarding whether that is necessary). Call it 170 for simplicity for that and the sparge water. I think it is fair to say that even though a continuous stream of 170 water is being added to the mash tun, that going slower will result in an overall average temperature in the mash tun lower than if going faster (Less energy entering the system when slow, more time to lose heat.) Setting up the problem as I have then, a slower sparge results in a lower average MT temperature, and there would be less upward diffusion. The diffusion rate (upward component) however is counteracted by the flow rate, while also noting that the diffusion rate changes due to the flow rate in the manner I postulated. I don't have any idea about what the diffusion rate is nor the rate of heat loss from the MT itself to ballpark any of this. I do know from a modeling viewpoint that when variables interact it's necessary to fix one or more of the variables at a specific value in order to determine the change that occurs by varying the values of the variable of interest. Interaction here meaning the variables are multiplied in the model. Seems like a partial differential equation problem maybe, not my usual modeling situation!

 

[doug293cz]

Good information, I was wondering how the argument referenced, that a slower fly sparge was potentially inefficient, was constructed. It doesn't follow anyting I've noticed or read about previously, I just threw temperature out there as more of an opening discussion. I follow the points you are making here and it's helpful that you have removed solubility from the discussion. So you are saying there is a diffusion rate, which is modified by temperature, and a flow rate. And I am not at all disagreeing with anything you wrote, but the way I was thinking of the problem setup is different in that I was thinking that the sparge water is a fixed temperature and that the mash temp drops over time. Nearly all of my recipes I mash out at 168 as well (without getting on a tangent regarding whether that is necessary). Call it 170 for simplicity for that and the sparge water. I think it is fair to say that even though a continuous stream of 170 water is being added to the mash tun, that going slower will result in an overall average temperature in the mash tun lower than if going faster (Less energy entering the system when slow, more time to lose heat.) Setting up the problem as I have then, a slower sparge results in a lower average MT temperature, and there would be less upward diffusion. The diffusion rate (upward component) however is counteracted by the flow rate, while also noting that the diffusion rate changes due to the flow rate in the manner I postulated. I don't have any idea about what the diffusion rate is nor the rate of heat loss from the MT itself to ballpark any of this. I do know from a modeling viewpoint that when variables interact it's necessary to fix one or more of the variables at a specific value in order to determine the change that occurs by varying the values of the variable of interest. Interaction here meaning the variables are multiplied in the model. Seems like a partial differential equation problem maybe, not my usual modeling situation!

I don't know if fly sparging too slow can actually reduce lauter efficiency, but apparently the claim has been made. (If anyone has a link to the thread discussing this that was mentioned by the OP, please post it.) There are several possibilities that could lead to someone believing that sparging too slow is detrimental lauter efficiency:

• Measurement errors were made in the case of the slower sparging, and the efficiency wasn't really lower, but the brewer thinks it was.
• The efficiency was actually lower, but something other than the sparge speed also changed, and that was the reason for the lower lauter efficiency, not the slower sparge rate. For example a larger grain bill for the same batch size, which reduces lauter efficiency all else being equal.
• The slower sparging actually did result in lower lauter efficiency.

If we assume for the sake of argument that the last of the three possibilities can actually happen, then there has to be some physical effect that would reduce the lautering efficiency at lower sparge rates. What could that effect be? It's highly unlikely to be channeling, as the slower you sparge, the less the forces that lead to channeling. After thinking about it, the only thing I could think of was more mixing of the mash wort and sparge water during the sparge. If that is in fact what reduces efficiency, then what physical process causes the mixing? Again the only thing I could think of was diffusion, which occurs whenever a concentration gradient exists. So, I speculated around that possibility.

The net flow rate of liquid down thru the grain bed during lautering, does not affect the diffusion rate of extract in the opposite direction, as long as the liquid flow is laminar (not turbulent.) Think of the diffusion rate as analogous to the airspeed of an airplane. If you have a headwind, the ground speed of the airplane slows down, and with a tailwind the ground speed increases, even tho the speed thru the local air mass is the same. The ground speed varies because the air mass is moving relative to the ground.

In the vessel during lautering, the liquid mass is moving faster than the diffusion rate (otherwise the wort SG at the top would be increasing over time). This is like an airplane flying at an airspeed of 100 mph into a 150 mph headwind. The airplane moves backwards over the ground, but it is still flying (all the airplane cares about is how fast the local air is flowing over the wings.)

I discounted temperature effects, since good mash tuns only lose a couple of degrees an hour. If you sparged for two hours instead of one hour, you'd only lose a couple of degrees more. I don't think that is enough to affect the sparge efficiency. If you take 170° wort out the bottom at the same rate you add 170° water to the top, you don't remove any net heat. The heat lost thru the sides of the vessel will remain the same as if there was no flow (i.e. the same as during a mash rest without heat added to make up for conduction/convection losses.) In reality, the wort coming out the bottom will be slightly cooler due to the vessel wall losses, and the sparge water will then represent a net heat input to the vessel.

You are correct that a rigorous model of fly sparging would require you to get into differential equations, which is way too much trouble for home brewing.

Brew on

 

[day_trippr]

Skeptical Cat is Skeptical wrt slow fly sparge being less efficient...

Cheers!

 

[doug293cz]

Skeptical Cat is Skeptical wrt slow fly sparge being less efficient...

Cheers!

I am also skeptical, as I haven't seen any real data, but I am willing to accept something counter intuitive if it is supported by good data.

Brew on

 

[day_trippr]

As would I.