Misconceptions - Brew Bag Myths Explained



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 might also suggest its use is 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 fabric filter also eliminates the need for a manifold, false bottom, or braided cord, or bazooka tube. When we understand and utilize the capabilities of the fabric and apply it to the transfer of wort, it’s easy to comprehend the operational advantages of using this type of filter.

Most writers explain that the advantages of using a fabric 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 fabric filter 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 when deciding what to buy or even space considerations.

There is inaccurate information in a number of articles on the internet, but for the sake of concise responses, and because the published AHA article found here https://www.homebrewersassociation.org/wp-content/uploads/How-To-Brew-in-a-Bag.pdf has more information than others I’ve read, it also contains the most contradictions in one piece, so I’ll use it as a reference when countering with 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 incorrect if using confirmed mashing and temperature maintenance procedures. There is a myth that maintaining exact mash temp for the entirety of the mash is critical for the homebrewer. Is recirculating to maintain constant temperature a good idea, absolutely, but not many brewers have this capability.

The language “scorching” implies the need to fire up  with the bag in the kettle to maintain temperature. When properly insulated or when using a cooler as a mash tun the mash temperature will only drop about 2º over 60 minutes. That equates to about half a degree every fifteen minutes, and we all know that the majority of conversion occurs inside of forty-five  minutes. And if the mash is within conversion temp range, no matter the type of beer you’re brewing, this heat loss is inconsequential to the final product.

To clarify, a polyester or nylon bag will not melt or scorch at boiling temperature in water. However, when firing up with the bag full of grain in place, the space between the bag and the bottom of the kettle superheats and causes the sugars and the material to burn. Sugar burns at 350º as does polyester. But here’s the most critical information against trying to recover a degree or two. When you flame on and superheat the wort in the bottom of the kettle to above 170º the enzymes in the wort in that space are denatured and are then incapable of converting additional starches.

The openings in Voile (the most commonly used material for brew bags) are ~ .0083” - while grain mills are set between .030 and .045 - so grain sediment in the boil is null because it doesn’t go through the fabric.

  1. 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.

I’ll add that John Palmer’s recent trials aimed at isolating the flavor/aroma results of steeping, concluded that squeezing mashed chocolate / roasted barley produced a repugnant burnt coffee and gravy flavor. The degree of bad flavor was not so evident if the barley was not squeezed, and this flavor was also evident after boiling.  So if using a high percentage of chocolate / roasted barley in your recipe, consider setting it aside and steeping alone without squeezing, then adding the “tea” back to the wort after boiling the rest of the wort.

  1.  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 or over-sparging will produce low numbers, but that’s the case with any method. A recent study by a homebrewer using the single kettle and fabric filter method over 33 batches revealed an average pre boil efficiency of 77.5%. But the AHA article does offer offer some hope by following the above sentence with - “That being said, many BIABers are achieving efficiencies comparable to the traditional forms of mashing in the 70-80 percentile.” I’ll add as a side note that a study by BYO in 2013 indicated that over 75% of brewers - usually, occasionally, or would like to learn - how to (BIAB) use a fabric filter.
  2.  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.
  3.  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 over a few batches. 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 Braukaiser.com
  4.  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.  As stated above BIABer’s 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. Therefore, since there is no thought of a stuck sparge the grain can be milled to flour, something not possible without a fabric filter. Before the fabric filter, brewing processes were built around the avoidance of the stuck sparge and could not be optimized for efficiency!

From Troester: braukaiser.com  “In addition to that, the gelatinization of starch is also slower and happens at higher temperatures in thick mashes and as a result thinner mashes are known to give more fermentable worts at normal mashing temperatures. Figure 7 (his site) shows data from mash experiments done by Windisch, Kolbach and Schild. It shows how the thinner mashes were able to convert more of the malts starch and also produce more fermentable sugars. But the ratio between fermentable extract and total extract (i.e. fermentability) remained largely constant over the range of mash thicknesses that were tested. Also note the data for the permanently soluble nitrogen. This data uses the right hand scale of the chart and shows that thicker mashes show better proteolytic activity.”

A water to grain ratio of 2.6 or higher aids in washing the grain and coupled with squeezing the bag adds additional gravity wort to the kettle without dilution. This process, which produces undiluted pH controlled wort can't be duplicated to the same degree when sparging. You can sparge by holding back some of the total volume, but the resulting gravity will be the same. In all methods and conversion being equal, how you get the wort into the boil kettle is the variable. Depending on the sparge setup, the maximum water volume in the kettle is the deciding factor on when to stop adding sparge water. And, both methods leave sugar behind in the bag or tun.  

  1. 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. Using a fabric filter you can grind to flour 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.

The bottom line for using a fabric filter is that you can use it in any mash tun setup and continue with your normal processes. The bag never clogs, is easy to clean, allows a finer grind, and the mash converts more thoroughly in less time.   If you’re going to do a full volume mash do a bit of research on proper technique to achieve the best results.  


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  • A.Johnson on

    You seem to have some contradictory statements here. The rather small holes in voile fabric versus the that of a large grain crush is provided as evidence for less sediment, but then you advocate for a tighter crush to increase efficiency. I use one of your bags (cooler size) for every brew, and certainly have grain sediment present. I’m not particularly concerned about it though as the yeast seem to enjoy it. Cheers!

  • Greg Turley on

    I love my BIAB. After several years of extract I went this route. I had read an article by a local brewery about a no sparge batch. Since I am not in this for profit, I don’t have to find a balance between efficiency and taste. That being said, my last BIAB Saison came in at 82%

  • Lisa Watts on

    Thanks for the clarifications!

  • Pier on

    Excellent and useful article.

  • The Brew Bag on

    We’d suggest that you can not increase efficiency by dilution. Holding back some of the total volume and sparging will provide the same gravity as mashing in the total volume and squeezing the bag. There are a finite amount of sugars to be passed into the brew kettle. Sparging does not move more than exist.

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