Guest Post: Dispelling Brew in a Bag Misconceptions

biab brewing

by Rex Slagel – from – read more about Rex below

Because the brew in a bag process is still fairly new in the USA, there is still some confusion about how to use a fabric filter in the brewing process. Some folks still call it a “method”, as if the resulting product is different than when employing a sparge to wash sugar from grain, and they also suggest its use for only single kettle BIAB, but it is currently being used by sparge brewer’s as well.

Essentially, the fabric replaces the grain bed as the filter and that alone allows many steps of the process to be modified – with the same (or better) results as traditional three vessel sparge set ups. The filter also eliminates the need for a manifold, false bottom, or braided cord. When we acknowledge the capability and apply it to filtering wort, it’s easy to understand the operational advantages of using a fabric filter.

Most writers explain that the advantages us using a bag include lower equipment expense and time savings, but that the drawbacks are lower efficiency, cloudier wort, a messy bag of grain, and the need for a larger kettle or mash tun. They also say “if you’re on a tight budget, you can still make good beer with minimal equipment”, which implies that in order to “really brew” you need to spend additional money for more equipment.

But in most of these articles, what is not generally conveyed, relative to conversion, extraction, and transfer of wort, is that there are some significant advantages compared to three vessel sparge home brewing, but that topic is another article.

This article is aimed at the sources of authority in the industry that, through lack of understanding or perhaps time to edit existing information, perpetuate misconceptions associated with using a fabric filter.

If you do some light research on the subject of “brew in a bag” and “efficiency” you can find plenty of websites that contain false statements. I’ll say in advance that I believe these articles were written using information understood to be accurate and that no one purposely set out to mislead the reader.

However, now that we have a greater understanding of how to use the bag, and proven results that contradict these writings, simple modifications to these articles should be administered. I’ll also say that I’m in favor of brewer’s doing things their way – that’s one of the inherent philosophies of home brewing! But, for those just getting into brewing, having bad information at the start is frustrating and wasteful, and bad information does not allow accurate comparison of pertinent factors.

There is inaccurate information in a number of articles on the internet, but for the sake of concise responses, and because the AHA article has more information than others, it also contains the most contradictions in one piece, I’ll use it as a reference when stating the facts we now know about using a fabric filter (or brew in a bag). The numbers beginning each point are indicative of the placement in the AHA article.

Just to be clear, this article is not meant to be derogatory and is is not aimed at the AHA. I’m a big fan of the organization. They have helped advance homebrewing in a professional consistent manner and lead the way as an information resource.

The following bold text is directly from the AHA’s pdf article on BIAB. My comments follow.

1. “Because of the simplicity of the process and equipment, BIAB has become a popular means of all-grain brewing for homebrewers new to mashing, living in small confines, pinching pennies, or brewing small batches.” ( true – but there are brewers using a fabric filter in fifty-five gallon barrels as well. Better stated “…for homebrewers of nearly any batch volume.”

2.“The ideal BIAB bag will be able to fit around the circumference of the boil kettle ***while not resting on the bottom to prevent scorching and will retain most of the grain sediment so as not to have too many solids left for the boil.” ( Sizing statement regarding circumference is correct, but “not resting on the bottom to prevent scorching” is not correct using confirmed procedures. There is a myth that maintaining exact mash temp for the entirety of the mash is critical for the homebrewer and the language “scorching” implies the need to fire up with the bag in the kettle to maintain temperature.

When properly insulated the kettle mash temperature will only drop about 2º over 60 minutes – which equates to about half a degree every fifteen minutes – and we all know that the majority of conversion occurs inside of thirty minutes, and if the mash is within conversion temp range this loss is inconsequential. Secondly, the bag will not melt [HBF editorial note – Rex is speaking specifically of The Brew Bag] or scorch at boiling temps – however, when firing up with the bag full of grain in place, the space between the bag and the bottom superheats and causes the sugars and the material to burn. You can watch a short video here of the material being held to the bottom of a pan while it’s on the flame boiling. Thirdly, the openings in Voile (the most commonly used material for bags) are ~ .0083” – while grain mills are set between .020 and .045 – so grain sediment in the boil is null.

3. “Squeezing is not recommended.” ( actually totally not true. Another myth is that squeezing the mash bag will extract tannins and also make the wort cloudy. Squeezing the bag of grain and tannins are not synonymous. Excerpted and paraphrased from the books “Water – A Comprehensive Guide for Brewers” and “Malt – A Practical Guide From Field to Brewhouse”: Tannins, a subset of polyphenols, are present in grain husks and cell walls. They are released at mash temps and bind with proteins to form haze. In conjunction with a pH above 6, excess tannins are extracted and impart an astringent flavor – they cannot be produced by pressure.

4. “One of the biggest downsides to brew in a bag is the efficiency compared to fly or batch sparging in a mash tun. It is not uncommon to have efficiencies in the 50-60 percentile.” ( again, totally not true, although, not mashing at proper WTGR (Water to Grain Ratio) or over-sparging will produce low numbers, but that’s the case with any method. They do offer offer some hope for other brewer’s following the above sentence. “That being said, many BIABers are achieving efficiencies comparable to the traditional forms of mashing in the 70-80 percentile.”

5. “Run your grains through the mill twice.” ( of little value unless the mill is adjusted down, and if the mill is adjustable, why not just lower the setting and run once. You can confirm this information by listening to or reading from a presentation at the AHA in 2014 by Jennifer Helber here.

6. “Increase mash rest duration. Some homebrewers have found longer mash durations allow for more conversion and ultimately higher efficiency.” ( conversion is a finite process, and as stated the same logic applies to any mash not just BIAB. Duration of the mash does impact conversion – but only to the degree that there is available starch to convert. Regarding higher efficiency, what generally happens is the brewer does not test conversion at regular intervals and does not control the results. So they may have a low reading on a brew and then the next brew using different grains, WTGR, and temperature – they rest longer, see an improvement and then state that a longer mash time produced higher conversion. For more infomation on this subject you can go to

7. “Sparge. Sparging is one of the best ways to ensure all the sugars have been rinsed out of the mashed grains.” ( not true, normal sparging techniques are not going to “ensure all the sugars have been rinsed out of the grains”. Even if that were the case due to possible excess tannin extractions, we would not want to achieve 100% efficiency. The fact of the matter is sparging is not necessary to achieve good efficiency. BIAB brewers can achieve as good or better efficiency vs brewers that sparge.

Remember that when using the grain bed as the filter and any other type of pick up or manifold the opportunity for a stuck sparge exists. When using a fabric filter – you’re using a true filter, so there is no need to “set the bed” with a coarse grind and there is no thought of a stuck sparge. Before the fabric filter, brewing processes were built around the avoidance of the stuck sparge and could not be optimized for efficiency!

8. “Calculate recipes with a lower expected efficiency. This will allow an increased grain bill to make up for any shortcomings due to low efficiency.” ( not true, in fact the opposite is true, but your processes will determine the outcome for your system. But you can count on a finer grist aiding conversion and efficiency. I have personally experienced lower grain bills and higher efficiencies, many in the upper 80’s and one in the low 90’s.

Also from RexBrew In a Bag – Bag Material – Does it Matter?

About the author: Rex Slagel is the founding member of Brew In A Bag Supplies LLC. in Plainfield, IL which manufactures The Brew Bag®. He has brewed over one hundred batches, including over sixty using a fabric filter. He can be reached at

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23 thoughts on “Guest Post: Dispelling Brew in a Bag Misconceptions

  1. Matty M

    I’m brewing 6-11 gallon batch sizes all with BIAB. My wife made my voile bag with sewn in webbing material for bag support and lifting loops. The system flat out works! There is definitely more cloudiness in the wort at flame out BUT it drops clear at cold break. I keg and gelatin fine so I have absolutely no haze or debris issues in my commercially clear beer. It’s fantastic. I see nothing to gain on the homebrew level by spending hundreds or even thousands on a 3 tier contraption. Too much of the homebrewing world is wrapped up in make believe commercial brewery techniques and equipment. I say use what you want but for me that’s the cheap, reliable method of brew in a bag. Don’t let ANY homebrewer say you can’t make exceptional beer with BIAB.

  2. Kyle

    A good article. I’ve done a few BIAB brews myself, and it will probably be my method of choice going forward. I did have one nit to pick though. I’ve noticed that my pre-boil wort is not nearly as clear as when I use a traditional false bottom. It doesn’t bother me much since a little wirfloc and time clear it up enough by the time I’m ready to serve. Regarding point #2, although the mill settings are higher than the mesh holes, you still get a lot of grain particles that are significantly smaller than the roller gap due to fracture mechanics. The roller gap just ensures that no pieces larger than the gap make it through. Recirculating wort through a grain bed will clear out most of those particles. I haven’t tried vorlauffing with a bag filter, but I’m guessing it might help…unless the squeezing undoes most of the filter.

    1. The Brew Bag

      I’d agree that some particulates make it through the bag openings, but I’ve never found that to be an issue at any stage of my brews. You notice any difference in your final product?

  3. Brogala

    Regarding #4, this statement has me a bit confused:

    “again, totally not true, although, not mashing at proper WTGR (Water to Grain Ratio) or over-sparging will produce low numbers, but that’s the case with any method.”

    I was under the impression that the WTGR for BIAB was dependent on batch size, as you’re adding all of the water needed for the batch at once, and then adding the grain to that. Am I missing something?

    1. The Brew Bag

      BIAB WTGR ratio is variable based on batch size, but if your batches are consistent in volume, say 5 gallons, the only variable will be the calc for grain absorption. Generally the low efficiency statements are derived from sparging methodology combined with bag use – meaning, the brewer does not understand full volume mashing and thus does not account for actual volume. Because of the difference in grain bed flow, sparging a BIAB is less effective than using full volume and squeezing. Also, it nearly impossible to record 50% to 60% efficiency for an average batch unless the water volume is too high no matter the wort transfer method.

    2. Jim

      WTGR does affect the mash efficiency. I know a few BIAB brewers who mash for an hour, because that’s what they are told to do. They get good efficiency. I know brewers, like myself, who mash for an hour using a manifold lautering system because that’s what we’re used to doing.

      The fact is, that a proper WTGR and the proper pH can usually contribute to a mash conversion in a MUCH faster amount of time.

      Doing a full volume WTGR BIAB might still result in complete conversion over an hour, but it might not be able to get done in the same amount of time as when you do a recommended 1.25-1.5:1 ratio. It simply takes longer for the enzymes to find the starches and do their thing.

      If you do BIAB at the more efficient WTGR, then you are leaving a higher concentration of sugars behind because the wort is more concentrated. In that case a sparge might be a good idea to release those sugars from the grain and into the Boil Kettle.

      However, BIAB brewers generally can crush much finer without risk of a stuck sparge, and they don’t generally lose anything to the equipment (deadspace) so they can leverage that HIGHER efficiency against the residual wort concentration.

      The bottom line is that any combination of methods *might* produce different efficiency number in the end. The best thing to do is to take careful measurements of conversion, volume, and gravity, and come up with a process that YOU like best.

      1. The Brew Bag

        Regarding your point of conversion “might” get done in an hour – please see the info below from a recent batch.

        Also, the following is from

        “Brewers that don’t mill their own grain will not be able to effect the tightness of the crush and will have to accept lower conversion efficiencies or ensure that the mash has enough time and “strength” to achieve an acceptable conversion of the starches. If the mill gap spacing can be controlled the conversion efficiency can be improved through a tighter crush. But at some point the crush might be to tight for a reasonable run-off speed.

        The thickness of the mash doesn’t seem to effect the fermentability of the wort that is produced but thinner mashes can significantly improve the conversion efficiency. As a result brewers who see low efficiency from their mashing may try to use a thinner mash (3-4 l/kg or 1.5 – 2 qt/lb) as they were shown to convert more starches.”

        My notes – Just finished a brew and tracked temp loss and gravity. BIAB 15 gallon kettle 14 lbs 13 oz grain for a 8.5 gal batch. Kettle insulated with duct wrap and draped with a moving blanket. Stirred every measurement. I’m not sure that I’m calculating the % completion correctly . I simply divided 15 / 38 – rather than 1.015 / 1.038 in which case the % = 97.8. So this is a good post for folks with greater understanding of the calcs to respond to. Strike temp is a bit high in that it dropped 3.6º as compared to my anticipated 6 – 8º. But I was shooting for a full bodied IPA, so I let it go.
        Directly after dough in gravity was
        0 min – 153.9º – G 1.015 – 39.4%
        15 min – 153.1º – G 1.028 – 73.7%
        30 min – 153.0º – G 1.033 – 86.8%
        45 min – 151.7º – G 1.036 – 94.7%
        60 min – 150.4º – G 1.037 – 97.3%
        75 min – 149.0º – G 1.038 – 100%
        I stopped the mash at 75 min assuming any additional increase would be negligible and the volume was .37 gal over target, so these numbers would be slightly higher. The target gravity was 1.038 providing a mash eff of 85.4% – Barley Crusher set to .020 and brewhouse efficiency of 77.6%

        Regarding leaving sugars behind and a higher concentration residusla sugars using a bag filter – actually, since I’ve never seen measurements of tun gravity from sparge brewers, I have no idea what get’s left behind, but i can say that based on software, the collective BIAB community averages in the high 70’s to low 80’s brewhouse efficiency using less grain. in addition, after squeezing the bag, lost wort is roughly 7 oz per pound while lost wort due to grain absorption in sparging can not be recovered and averages about 16 -18 oz per lb, and while deadspace is variable for sparge brewers it accounts for additional loss.

        But you’re right – do what suits you.

  4. John

    I use both methods. When I biab it is a smaller batch usually 3.5 gallons designed for a short brew day. I love the method and have never been below 80%.

    I have noticed I get significantly more trub using biab. It looks like standard hot / cold break materials. The beers also seem to take longer to clear on a whole. Any thoughts on the trub?

    1. The Brew Bag

      I really don’t know the answer to that. I’ve never sparged a batch so I have nothing to compare and I always use a brew bag for hops, so the only trub in my kettle is proteins and fats – which have little to no impact on volume.

  5. Meow

    Just to chime in, I highly recommend BIAB. I’ve been doing it for 40+ batches, and I tweak something every time still, it seems. I notice lower efficiency on 1.070+ beers, but sparging seems to help that. I cut my mash times to 15-30 min depending on the brew size, and don’t bother sparging anything 1.05 or lower. I planned on getting a gravity or pump system, but stovetop BIAB has been working great for me.


    ps- My beer quality skyrocketed going from extract partial boils to full boil BIABs. Switch now.

    1. The Brew Bag

      All good stuff !! I’m sure you’ll find that ALL set ups suffer efficiency loss when mashing big beers because the WTGR drops which prevents complete transfer of the wort. Sparging will push more sugar, but also more water, so boiling off is about the only way to hit the target.

  6. Sittig

    Done the BIAB for about 5 brews now. It’s a very workable system and I’ll stick with it. Still working, though, on quantities, as Beersmith does not seem to get what we are doing.
    As for cleanup, I dump most of the grain then let the bag dry out, then just shake and brush it. A rinse finishes the job.
    As for scorching, it CAN happen, but a small false bottom solves the problem.

    1. The Brew Bag

      That software needs to be set to “your equipment” BIAB and the calcs for evap need to be adjusted as well. I’ve found that grain absorption if squeezing well, is about 7 oz per pound. For clean up I dump, rinse and let air dry then give a few quick snaps to release any grain husks. Every few brews I toss it in the sink with Oxy and let it sit overnight.

      1. Sittig

        thanks for the info about Beersmith setup. I’ve done the equipment customizing but still need to tweak it. Your data helps!

  7. Kirby Thiessen

    I still use a pump to recirculate during the mash. I typically had a lot of problems with small, and sometimes not so small particles getting into my pump, causing it to stop. I then had to take apart the pump clean it and try again, and again, sometimes many times again. I don’t know why it took so long to get the idea but I bought the brew bag to use as a filter, I still use the false bottom because I feel it keeps the wort flowing evenly through the grains, but I have not had a pump failure since. My cleanup has been easier, I have been using it now for over a year in all of my brews. Love it, love it!

  8. da kine

    I get 70-75% efficiency using my Brew-in-a-Bag (TM) and 80-85% efficiency with my three-vessel system. I don’t change the size of my grain mill, though, and I expect I could bump my BIAB efficiency up if I did a finer crush without the risk of a stuck sparge.

    1. The Brew Bag

      Yes – you could grind finer and modify your brewhouse efficiency numbers. I have mine set at 78% and grind at .020 That’s one of the game changers that 3V brewers often don’t understand about filtering this way, so they don’t modify their processes and the results are skewed. But still, nothing wrong with 70-75%.

  9. Kurt

    Regarding #7. I agree with what the AHA article was trying to say and Rex’s comments as well. I think the article is trying to say that sparging is the most optimal way to extract additoinal sugars from the grains. The higher the gravity of the wort, it becomes less effective extracting additional sugars. Regardless, BIAB can achieve great effeciency by modifiying your process to take advantage of it’s strengths as Rex mentions.

  10. Jeff

    Great information guys, thanks for posting it.

    I just thought I’d mention that I have been doing BIAB for somewhere between 10-20 batches now. I started doing small batch all grain due to size/ space constraints. When I switched over to BIAB, for the first couple brews I did increase my grain bill in preparation for this “reduced efficiency” I had been warned about, only to wind up with considerably more beer than planned (not really a complaint, haha). Now I’d say on average my efficiency in 75- 80%, maybe more some days.

    Overall, couldn’t be happier. Efficiency is great, and the process between mash and boil is much, much, easier.

    I would be interested to hear how most clean their bags between brews though.

    Thanks again!

    1. Craig

      All I do is dump the grain, rinse off the clinging grain bits, and let it air dry. After dry, I will shake the bag to free any remaining grain bits stuck to the bag. But that is it.

  11. Will Tuttle

    I just bought a Brew Bag. I was interested in using it to reduce costs by going all-grain without adding equipment. Once I had “stepped into” all-grain with the bag, I was planning to add equipment over time to get to that “authentic” all-grain brewing experience.

    SCREW THAT! Now I am EXCITED to go whole-hog into all-grain home brewing by simply adding a bag and a pulley. My friend’s chickens get more spent grain, I make the same quality (or better) beer for less per batch, and I haven’t added any equipment, costs, space or even much time to my process. WIN-WIN-WIN-WIN.

    Thank you HBF and BIAB!!



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