Author: Matt Del Fiacco

Presumably due to the availability of cheap 6 gallon fermentation buckets, 5 gallons/19 liters has become the standard batch size in modern homebrewing, which also happens to fit conveniently in corny kegs. Of the various wort production methods that exist, there seems to be ubiquitous agreement that boiling the full volume produces a higher quality beer, hence the gear upgrade aspirations of many a new homebrewer. While relatively common in extract brewing, it seems partial boils are rarely practiced by those relying on all grain methods, perhaps due in part to the bad rap it’s received over the years.

Often criticized as a method that produces a darker beer with a higher chance of contamination due to the addition of dilution water, there are a number of conveniences to partial boils as well. For example, a full 5 gallon batch of beer can be boiled in a smaller and less expensive kettle, cold dilution water can eliminate the need for a wort chiller, and hitting a precise OG is as easy as making small adjustments to the dilution volume.

For the purposes of this article, the term partial boil refers to the method of producing a high OG wort that’s approximately half the intended batch volume and diluting it with water to the target batch volume after the boil to reach the proper volume and OG. There are quite a few factors to consider when using this method such as the water to grist ratio, hop utilization in the boil, and pH of the wort, all of which are believed to effect beer character. Curious of the impact of a partial boil, and admittedly interested in methods for simplifying my brew day, I was excited to put this one to the test!

| PURPOSE |

To evaluate the differences between a beer made using the partial boil method then diluted with water to the proper volume and one where the full volume of wort was boiled.

| METHODS |

After several high-gravity ale brews, I wanted to put a flavorful lager on tap and decided on my standard Munich Dunkel recipe for this xbmt.

It’s A Slam Dunkel

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 4.5 gal 60 min 21.7 IBUs 10.7 SRM 1.051 1.018 4.3 % Actuals 1.051 1.014 4.9 % Fermentables Name Amount % Munich I (Weyermann) 4.312 lbs 50.36 Munich II (Weyermann) 3.875 lbs 45.26 Caramunich I (Weyermann) 6 oz 4.38 Hops Name Amount Time Use Form Alpha % Magnum 9 g 60 min Boil Pellet 11.6 Tettnang (Tettnang Tettnager) 12 g 30 min Boil Pellet 4 Yeast Name Lab Attenuation Temperature Urkel (L28) Imperial Yeast 73% 52°F - 58°F Notes Water Profile: Ca 21 | Mg 4 | Na 8 | SO4 31 | Cl 30 Download Download this recipe's BeerXML file

I made a starter of Imperial Yeast L28 Urkel a few days ahead of time, my first time using this strain.

To start my brew day off, I added the full volume of water (6.5 gallons/25 liters) to both kettles, adjusted each to the same profile, then removed precisely half of the water from the partial boil kettle.

I then turned the heating elements on.

While the water was warming up, I weighed out and milled my grains.

As expected, the smaller volume of water reached strike temperature before the full volume batch, so I mashed in on it first. Once both were going, the difference in mash thickness was very apparent.

I set the controllers on each batch to maintain the intended first mash step temperature of 146°F/63°C.

At 15 minutes into each mash, I pulled samples of wort to measure the pH and noticed a predictable difference.

At 30 minutes into each mash, I set the temperature controllers to the second mash step and checked 10 minutes later they hit my target of 160°F/71°C.

Once each mash was finished, I removed the grain baskets and let them drain while the wort began heating. Again, due to the smaller volume, the partial boil wort reached a boil before the full boil batch.

The worts were boiled for 60 minutes with hops at the appropriate times. Once complete, I quickly chilled each batch by running it through my counterflow chiller directly into fermentation kegs.

It was at this point I noticed what seemed to be noticeably more break material in the full boil kettle.

Once chilled, I set the fermentors in my chamber then proceeded with boiling the dilution water I’d previously saved from the partial boil batch for about 15 minutes to sterilize. After letting it cool for a bit, I added it to the dilution batch then took hydrometer measurements of both worts showing a 0.001 SG difference. Not too bad for a first attempt.

The worts were left in my chamber until they stabilized at my desired pitching temp, at which point I split the yeast starter equally between them. I noticed activity in each 12 hours later and let them ferment for 5 days before slowly raising the temperature to 70°F/21°C over the course of 3 days.

After a 3 day diacetyl rest at this warmer temperature, I transferred the beers to serving vessels, attached spunding valves, and eased the temperature down to 36°F/2°C where they lagered for 2 weeks. Hydrometer measurements at this point showed the FG of the partial boil batch was 0.002 SG higher than the full boil beer.

The beers were placed in my keezer and briefly burst carbonated before I reduced the gas to serving pressure. After another week of cold conditioning, the beers were ready to serve.

| RESULTS |

A total of 23 people of varying levels of experience participated in this xBmt. Each participant was served 2 samples of the partial boil beer and 1 sample of the full boil beer in different colored opaque cups then asked to identify the unique sample. While 12 tasters (p<0.05) would have had to identify the unique sample in order to reach statistical significance, only 9 (p=0.35) made the correct selection, indicating participants in this xBmt were unable to reliably distinguish a partial boil Munich Dunkel from one made using the full boil method.

My Impressions: Out of the 6 triangle tests I attempted, I correctly picked the odd-beer-out only twice, which is exactly inline with random chance. Despite my best efforts, I just could not tell these beers apart. I expected the full boil beer to be a touch more bitter due to decreased hop utilization from the higher gravity of the partial boil wort, but at the end of it all, everything about these beers was identical to my senses.

| DISCUSSION |

Of all of the recommendations given to new brewers looking to make their beer better, moving from partial to full boils is pretty high on the list. Like I imagine is true for most, my initial brewing setup included a small 4 gallon kettle, limiting me to partial boils if I wanted to make 5 gallons of beer. Like I also imagine is true for most, I was convinced by what I’d heard from more experienced brewers that getting a larger kettle and performing full boils would drastically improve the quality of my beer. When I eventually made the transition, I did think my beer was better, and I believed this was due in part to my departure from partial boils. Admittedly, the results from this xBmt showing neither participants nor I could reliably distinguish beers made using either method left me dumbfounded. Where did these supposed truths originate? How did it become commonly accepted that a partial boil produces worse beer than a full boil?

I don’t doubt there are certain objectively measurable differences between beers produced using either partial or full boils, but these results demonstrate both methods can create beers of equal quality on a perceptual level. Of course, an obvious caveat to these findings is that both beers were produced using an all grain method, leaving open the question of whether the results hold true when brewing with extract, which I look forward to exploring in the future.

If you have any thoughts about this xBmt, please share them in the comments section below!

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