Brew In A Bag Mash Efficiency

New and experienced all-grain brewers moving to Brew-In-A-Bag (BIAB) techniques often have concerns about how it will affect their mash. BIAB traditionally uses a full volume mash, which means the ratio of water to grain is much higher than a traditional mash.

Will BIAB brewing require more grains due to a lower efficiency? Will it affect the quality or flavor of the beer? How will it affect my mash pH and water balance? I’m going to take a look at these key questions to explore how BIAB affects all-grain beers.

BIAB Mash Differences

A traditional mash is done at relatively low water-to-grain ratios of 1.25 to 2 quarts per pound of grain (2.6 to 4.1 L/kg). This results in a fairly thick mash. However Brew-In-A-Bag is mashed in with the full boil volume. So for a typical 5-gallon batch (19 L) we might have as much as 7.5 gallons (28 L) in the mash for a ratio of over 3 qts./lb. (6.2 L/kg). So we have between two and three times as much water per unit of grain resulting in a much thinner mash.

There are several concerns expressed by brewers with using a thinner mash. One you will find on many brewing forums is that you could have a lower level of enzymes active as they could be diluted in the thinner mash. The fear is that this could result in lower efficiency in the mash.

Another concern often found online is the theory that a thinner mash results in higher yeast attenuation and a thinner tasting beer. Ironically this concern is almost the opposite of the previous one — since higher attenuation means you have a more complete starch conversion during the mash.

A final concern is the impact of a thin mash on your mash pH. Using a high water-to-grain ratio is associated with a higher mash pH than a traditional mash, so extra care is needed to manage the pH of the mash.

BIAB Efficiency In Practice

I’ll start the discussion by noting that brewing beer with a thin mash is nothing new. In fact European brewers have been doing decoctions at high water-to-grain ratios for hundreds of years. Many traditional German decoctions were done at water-to-grain ratios nearly identical to those used in modern BIAB, and no one complains about the efficiency of a three stage decoction or argues that German beers brewed a traditional way are too thin or lack a malty body.

Beyond the historical record, a number of brewers have studied the efficiency and attenuation of BIAB beers. One of the best of these studies was done by Kai Troester in 2008 (on his website entitled “Evaluation of the Effect of Mash Parameters on the Limit of Attenuation and Conversion Efficiency in Single Infusion Mashing.” In the study he tested the impact of a wide variety of mash parameters on efficiency, to include mash thickness. For mash thickness he compared a traditional mash and sparge against a no-sparge method, which uses a full boil volume mash at the same water/grain ratio as a BIAB.

His findings were that while traditional mashes had a slight variation in efficiency with mash temperature, they consistently delivered a modest brewhouse efficiency between 58-60% for a mash thickness of 2.57 L/kg (1.25 qt./lb.) on the system tested. At double the water-to-grain ratio of 5 liters/kg (2.4 qt./lb.) the efficiency varied significantly with mash temperature but was always higher than the traditional mash. Efficiency in his experiments peaked at 68%, which is nearly 8% higher than the traditional mash.

Earlier work in Brigg’s book Brewing Science in Practice in 2004 (section 4.3.7) also addresses mash thickness and extract efficiency. He also finds an increase in efficiency as the water/grain ratio is raised. Going from 2 to 4 L/kg (roughly 1–2 qt./lb.) raises the extract from 71.7% to 80% in the mash, and at 5 liters/kg (2.4 qt./lb.) extract remains at 80%. He concludes that a concentrated mash actually inhibits starch conversion.

Troester and Brigg’s findings mirror my own experience. I’ve found that on average I get either the same or higher brewhouse efficiency when brewing with BIAB. I think several factors work in BIAB’s favor for efficiency. First, highly modified modern malts have a very high enzyme content which is well in excess of the minimum needed for conversion of sugars. So raising the water/grain ratio has a minimal impact on the enzyme concentration required for starch conversion.

Secondly, the full-volume mash results in a very long contact and soak time for the grains which aids in extraction. The entire water volume is in contact with the entire grain volume for the entire mash time. This is in contrast to a traditional mash and sparge where the sparge water is run through the grain bed relatively quickly and has less contact time to extract sugars from the grain. I think these processes work in BIAB’s favor in terms of extracting starch from the grain.

BIAB Attenuation

Some brewers have anecdotally reported both higher and lower attenuation using BIAB. Others say simply that BIAB beers taste “thinner.” However a thinner beer would only happen if we had lower efficiency (i.e. a lower OG) or higher attenuation of the beer (i.e. more of the sugars ferment to alcohol). I showed in the previous section that BIAB often results in higher and not lower efficiency when mashing at high water-to-grain ratios, so the OG should be equal to or higher for BIAB. Now let’s look at attenuation.

The theory for thinner BIAB beer is that larger water-to-grain ratios favor beta amylase. Beta amylase is the enzyme tied to a more complete starch conversion and higher attenuation. For example you can enhance beta amylase by mashing at a lower mash temperature, and this is a common way to increase the attenuation in lagers and other light beers to get a cleaner, lighter-bodied beer.

If a thinner BIAB mash resulted in more beta amylase activity we would expect to see a higher yeast attenuation in the finished beer. Troester measured the attenuation of the beer using identical batches and varying mash thickness. In this case he could find no measureable difference in attenuation. The high water/grain ratio mash and traditional mash both attenuated the same as long as both were mashed at the same temperature. Neither resulted in a thin beer.

In Brigg’s book we find evidence that fermentability actually decreases as mash thickness decreases. At 158 °F (70 °C) he shows a relatively flat fermentability up to 3 L/kg (1.4 qt./lb.) but attenuation drops by several percent when we get to 4 L/kg (1.9 qt./lb.). However the effect is less pronounced for lower mash temperatures, so we would see little difference with mash thickness in attenuation for a mash temperature of 148 °F (64 °C). In either case BIAB would not result in a thinner beer, but might actually result in a beer with slightly more body if mashed at the higher temperature of 158 °F (70 °C).

Mash pH and BIAB

The final concern when brewing BIAB is the effects of a high water-to-grain ratio on mash pH. Specifically the use of high water-to-grain ratios will give you a higher mash pH. In this case, the concern is very real and supported by science.

The chemistry here is pretty simple. Brewing water is typically slightly alkaline (pH a bit above 7) and drives mash pH up, while malts are acidic. When we mix the water and grain the acidic grains will lower the mash pH resulting in a mixture with lower pH than the starting water.

The ideal mash pH range is roughly 5.2–5.6 for best beer flavor and stability. Darker grains are more acidic, so mash pH is generally lower for darker beers. However for many lighter beers the pH often has to be adjusted as the malts don’t provide enough acidity to reach the target 5.2–5.6 range. Acid malt or acids such as lactic acid and phosphoric acid are often used for adjustment.

Now let’s consider an ideal situation where we are using brewing a dark beer using a traditional mash and slightly acidic water. The mixture of water and grains results in a perfect 5.3 pH level and needs no adjustment.

However using the identical water and grain for a BIAB brew we now have twice the amount of water and the same amount of grains. Since the water is alkaline, and we’re using twice as much water we are definitely going to see a higher pH than the traditional mash. Twice as much water and the same amount of grains will raise the mash pH.

So when we’re brewing using BIAB techniques we need to take extra care in monitoring and adjusting the mash pH. Since mash pH is difficult to accurately predict in advance, I recommend using a pH meter to measure pH and then use lactic acid or acid malt to adjust the pH back to the desired 5.2–5.6 range.


Unfortunately there are a lot of myths regarding BIAB mash techniques floating around the Internet, so research the Web with caution and skepticism. Despite what you may have read, BIAB does not have to result in lower mash efficiency, and in fact can result in slightly higher efficiency at homebrew scales than traditional mash techniques.

Another unsupported myth is that BIAB will give you “thin” beer or beer with poor attenuation. Experimental data instead supports identical or slightly higher attenuation at higher water/grain ratios, especially when mashing at higher mash temperatures in the 158 °F (70 °C) range.

Finally, mash pH concerns are not a myth. Mashing at twice the water-to-grain ratio will give you a higher mash pH due to fact you are working with twice as much alkaline water with the same amount of acidic grain. Therefore take extra care to measure and control mash pH when brewing using BIAB, and acid malt or lactic acid may be needed to reach an ideal mash pH range.


Issue: May-June 2016