2015 Coolship Experiment - Cooling Rate


As previously mentioned in a post about My Spontaneous Beer Evolution, 2015 was my first coolship season where I felt comfortable enough with the process to begin playing with its variables. At that point in time, I had two 12-gallon batches under my belt (and actually ended up dumping the entire first batch) so maybe my confidence was a bit misplaced, but I was motivated to learn more. Before getting too deep, lets talk about what a coolship is and how it is used...


Cantillon's copper coolship has a total
volume of ~64BBL (1,971 gallons, 75HL)
A coolship (or koelschip in Dutch) is a vessel used in a brewery that serves the purpose of chilling and inoculating the wort with ambient microbes. They are traditionally very wide and shallow with an open top so that the wort remains exposed to the local microflora that is floating around in the outside air or in the brewery. Typically, at the end of the boil, a brewer using a coolship will transfer the near-boiling wort into the coolship. It is common to see a hop-filtering basket (or similar apparatus) attached to the coolship, especially when the brewer uses whole hops. The hot wort is left in the coolship for an extended period of time (often in the range of 8-16 hours) and allowed to cool naturally, slowly trending towards reaching thermal equilibrium with its ambient environment. In this time, organisms floating around in the air (or in the brewery) are allowed to fall onto the surface of the wort and provide inoculation for the upcoming fermentation. From the coolship, the wort is often transferred to a blending tank (sometimes called a horny tank) where the wort is mixed to ensure homogenous inoculation throughout the entire volume of wort. From the blending tank, the wort is moved to a fermentation vessel, which in the case of a Lambic brewer would most often be a well-used wine barrel. In the production of Lambic or any other spontaneously fermented beers, there would be no additional yeast added. This is the most common modern application of a coolship, but historically they would have been more widely used due to chilling technology available at the time. It was only about 95 years ago that the plate heat exchanger became commercially available so it is safe to assume that brewers prior to that were chilling by other means, possibly using a coolship.

De Dolle Brouwers in Esen, Belgium uses
their koelschip to make non-Lambic beers.
I am a big fan of split-batch experiments where one process variable is manipulated while all others remain the same or as close as possible (I think the Brulosophy team does an excellent job of executing these types of experiments). A split batch where only one variable is manipulated serves to highlight the impact this process variable has on the resulting beer and provides valuable insight to the brewer(s). During 2015 there had been quite a bit of discussion on the Milk The Funk Facebook page about the importance of a coolship's inoculation time versus its surface area to volume ratio (SA:V). In the homebrewing community, it was somewhat of a previously held belief that the SA:V ratio of a coolship was the critical variable of the coolship process. As a result, homebrewers frequently used wide and shallow vessels, such as a hotel pan, to attempt to maintain the ideal SA:V ratio. However, there were more and more people reporting successful spontaneous fermentation (at the homebrew scale) by using coolships of equal depth and width, like the typical 10-gallon kettle. It was argued that the rate of cooling and contact time with the ambient environment was the most critical variables of the coolship process. Wanting to gain some firsthand experience with how cooling rate affected the resulting spontaneous beer, I developed an experiment in order to attempt to highlight the differences between a coolship that cools more quickly and one that cools more slowly.


My approach for this coolship experiment takes advantage of both sides of the argument above (inoculation time versus its SA:V ratio). I believed it was a reasonable assumption that the top SA:V ratio (area of wort in contact with ambient environment) was important to some degree. If this variable wasn't important then you should be able to completely cover the top of a coolship and expect to get the same results. However, I think that would be a very unreasonable assumption. That being said, I was able to match Cantillon's top SA:V ratio of 33.33cm^2/L reasonably closely with the two vessels I selected. I chose a 10-gallon beverage cooler (Coolership) with a wort volume of 6 gallons (22.71L) for a top SA:V ratio of 37.7cm^2/L as well a 10-gallon aluminum kettle (Kettleship) with a wort volume of 7 gallons (26.5L) for a top SA:V ratio of 40.18cm^2/L. The math for calculating SA:V works out as follows:

Cantillon's SA:V Calculation (cm^2:L)

Coolship dimensions: 500cm x 500cm x 30cm
Surface area of wort exposed to air: 500cm x 500cm = 250,000cm^2
Surface area of wort to coolship bottom: 500cm x 500cm = 250,000cm^2
Surface area of wort to coolship sides: 500cm x 30cm x 4 = 60,000cm^2
Total Surface Area: 250,000cm^2 + 250,000cm^2 + 60,000cm^2 = 560,000cm^2
Volume: 75HL = 7,500L
Top Surface Area:Volume = 250,000cm^2:7,500L = 33.33cm^2/L
Total Surface Area:Volume = 560,000cm^2:7,500L = 74.67cm^2/L

The Coolership is being filled with
wort following a two hour boil
The Coolership and Kettleship are
left outside with no cover over them

Coolership SA:V Calculation (cm^2:L)

Cooler inner radius: 16.51cm
Wort Depth: 26.7cm
Surface area of wort exposed to air: 3.1416 * 16.51cm^2 = 856.3cm^2
Surface area of wort to coolship bottom: 3.1416 * 16.51cm^2 = 856.3cm^2
Surface area of wort to coolship sides: 26.7cm x 103.7cm = 2,768.8cm^2
Total Surface Area: 856.3cm^2 + 856.3cm^2 + 2,768.8m^2 = 4,481.4cm^2
Volume: 6 gallons = 22.71L
Top SA:V = 856.3cm^2:22.71L = 37.7cm^2/L
Total SA:V = 4,481.4cm^2:22.71L = 197.33cm^2/L

Kettleship SA:V Calculation (cm^2:L)

Kettle inner radius: 18.41cm
Wort Depth: 24.89cm
Surface area of wort exposed to air: 3.1416 * 18.41cm^2 = 1,064.8cm^2
Surface area of wort to coolship bottom: 3.1416 * 18.41cm^2 = 1,064.8cm^2
Surface area of wort to coolship sides: 24.89cm x 115.7cm = 2,879.77cm^2
Total Surface Area: 1,064.8cm^2 + 1,064.8cm^2 + 2,879.77m^2 = 5009.37cm^2
Volume: 7 gallons = 26.5L
Top SA:V = 1,064.8cm^2:26.5L = 40.18cm^2/L
Total SA:V = 5,009.37cm^2:26.5L = 189.03cm^2/L

A colander is used in the direct-fired mash tun to help
separate the turbid runnings from the main mash
The First turbid pull is being 
heated in the secondary kettle
Keeping the SA:V ratio for both vessels as close as possible served to reduce the impact that this variable had on the resulting beer. Additionally, both coolships would receive the same amount of ambient inoculation time to eliminate further variability. It was assumed that the insulated beverage cooler would cool at a much slower rate than the un-insulated aluminum kettle and would potentially produce two distinctly different beers, but exactly how these beers would vary was unclear to me at this point. Would a slower cooling rate and a longer stand at above 86°F (30°C) result in more dominant populations of lactic acid producing bacteria and a lower final pH? Would a faster cooling rate and a longer stand below 86°F (30°C) result in more dominant populations of yeast and more brettanomyces characteristics? I was eager to find out and learn more. I did modify the recipe I used for this batch and those details can be found in the Batch 3 section of my last post.

Recipe Specifications

Beer Name: MOambic 2015 (Spontaneous Batch #3)
Brew Date: 11-21-2015
Batch Size: 12.0 gal (45.4L)
Estimated OG: 1.050
Estimated: Color: 3.4 SRM
Estimated IBU: 11.8 IBUs
Brewhouse Efficiency: 70%
Boil Time: 120 minutes

65.2% - 15lbs (6.8kg) - Belgian Pilsen
34.8% - 8lbs (3.62kg) - Raw Triticale (locally grown)

Boil: 120min - 7.2oz (0.6oz/gal) Aged Hop Pellets [0.9%] - 11.8 IBUs
WARNING: 0.6oz/gal is at the high end of what I would recommend for homebrewers. I would encourage first time spontaneous brewers to start with 0.15-0.3oz/gal of well aged hops and adjust up or down from there. A recent post titled 2017 Coolship Experiment - Hopping Rate explains why I am issuing this caution.

Inoculated Via Coolship (no yeast pitched)

Turbid Mash 
Dough In - 0 min @ 118°F (47.7°C)
          Pull 1/2 gallon of turbid runnings
          Add 2 gallons water
Rest - 15 min @ 133° (56.1°C)
          Pull 2 gallon of turbid runnings
          Add 3 gallons water
Rest - 45 min @ 152° (66.6°C)
Rest - 45 min @ 162° (72.2°C)
Mashout - 10min @ 174° (78.8°C)

Add all contents of secondary kettle to the mash tun
Spare with water of at least 180°F (82.2°C)

Add 2g gypsum and 2g calcium chloride to 4gal strike water prior to Dough In step.
7.2oz. of aged hops added at
the beginning of a long boil

No acid was used to adjust the mash pH.
Collected 16 gallons @ 1.022 in my 20-gallon kettle.
Due to unexpectedly low mash efficiency, I added 5lbs 4oz light dry extract to kettle to raise pre-boil gravity to 1.036 @ 17 gallons.
Once boil is achieved, add aged hop pellets.
Boil until gravity of 1.049 is achieved (two hours).
6 gallons boiling wort placed in empty 10-gallon beverage cooler.
7 gallons boiling wort placed in empty 10-gallon aluminum kettle.
Set both outside at 10PM.
By 8AM Kettleship wort was 50°F and Coolership was 100°F.
Place in fermentation chamber at 75°F.
No acid was used to pre-acidify either of the worts.
pH strips indicated that both worts were somewhere between 4.6pH and 5.2pH going into the carboys.


Left to Right: Kettleship, blended, and Coolership
carboys 60 hours post coolship
Following 10 hours of inoculation time, the Coolership reached a terminal temperature of 100°F (37.7°C) and a gravity of 1.052 while the Kettleship reached a terminal temperature of 50°F (10°C) and a gravity of 1.051. The graph below includes hourly weather (temperature and wind speed, if you think any additional weather data would be meaningful please let me know) as well as an approximated trend line for the cooling rate of each vessel based on known initial and final temperatures of the wort. After resting in the coolships, the wort was transferred into three carboys. The first 3-gallon glass carboy was filled with wort from the Coolership, a second 3-gallon carboy with wort from the Kettleship, and a third 6-gallon carboy with a 50/50 blend of both worts. The carboys were then placed in a fermentation chamber set to 75°F (23.8°C) for two weeks. The Coolership wort was the first to begin showing signs of fermentation; developing a one-inch deep krausen 48 hours post coolship. The blended wort reached the same vigor 60 hours post coolship and the Kettleship reached the same vigor 72 hours post coolship. After the fermentation chamber, the carboys were moved into my basement where temperature swings from 58°F (14.4°C) in the winter up to 73°F (22.7°C) in the summer. The beers were aged for 25 months before final evaluation. Tasting notes and additional data points throughout this period can be found below.

Approximate cooling rate of coolships and hourly weather report

January 16, 2016 Tasting Notes

Kettleship Wort
Aroma: Moderate cheesy hop funk, low pineapple
Flavor: Moderate cheesy hop funk, low wheat malt
Overall: Still tastes fairly sweet, I'm guessing it is approximately 1.030. Most funky of the three. No sourness or tartness detected.

Blended Wort
Aroma: Low overripe melons, Medium-low cheesy hop funk
Flavor: No comments
Overall: Seems more attenuated than the other worts. No sourness or tartness detected.

Coolership Wort
Aroma: Medium unfermented wort
Flavor: Medium unfermented wort
Overall: Seems more attenuated than Kettleship, but less than the blend. Most malty and least funky of the three. No sourness or tartness detected.

January 16, 2016 - From left to right: Kettleship, blend, and Coolership

July 13, 2016 Notes

Kettleship Wort
3.68pH, seems the least attenuated, least fermentation activity over time, lightest color

Blended Wort
4.09pH, fairly well attenuated,

Coolership Wort
4.10pH, fairly well attenuated, most fermentation activity over time

July 13, 2016 - From left to right: Coolership, 50/50 blend, Kettleship

December 10, 2017 Tasting Notes

Kettleship Wort
1.014, 3.32pH
Aroma: low rotten vegetables, very low peach skins, medium-low aged hops
Flavor: medium lemon, medium pine, low peach skin
Overall: medium body, high acidity

Blended Wort
1.006, 3.58pH
Aroma: medium-low peach, very low aged hops, low acidity
Flavor: medium-low oak, medium lemon
Overall: medium-light body, medium acidity

Coolership Wort
1.002, 3.53pH
Aroma: medium-low peach, light and clean malt, low aged hops, low acidity
Flavor: medium-low peach, low aged hops, 
Overall: medium body, medium acidity

Approximated pH trend for each wort

At the beginning of this experiment I didn't own a pH meter, but acquired a MW-102 meter in the summer of 2016. All pH data after that is measured with a calibrated pH meter. The pH plot above includes the three data points I have for this experiment as well as an approximated trend line for the sake of illustration. I think three data points is insufficient in accurately representing what happened over time, but it helps paint a vague picture in this case. For future experiments like this I will plan on taking pH readings at 0, 3, 6, 9, 12, 18, 24, 30, and 36 months. It is observed from the graph generated that the Kettleship exhibited the fastest drop in pH as well as maintaining the lowest pH at the end of the experiment.

As seen in the tasting photo above from July 13, 2016, the Kettleship and blended carboys produced beers with a deeper orange color than the Coolership beer. Oxidation is the obvious first guess, but all three were maintained in glass carboys. I have never noticed this sort of color change in my other golden sour beers aged in these same types of carboys, but those beers weren't hopped as heavily as this batch so maybe that is the difference. Another possibility is that the unique microbes in each carboy are responsible for the change in color. I believe the color change seen is a result of both oxidation and microbial influence.


High Krausen
Final pH
Apparent Attenuation
50/50 Blend

I probably could have let this experiment go for another year, but I believe there was enough consistency in my observations over the duration of this experiment that 25 months was sufficient. The Coolership, slower cooling rate and more time spent above 86°F (30°C), yielded a beer with a much lower FG and a higher pH, 1.002 and 3.53 respectively. The Kettleship, faster cooling rate and less time spent above 86°F (30°C), yielded a beer with a much higher FG and a lower pH, 1.014 and 3.32 respectively. This is the exact opposite of what I had expected. I assumed more time spent above 86°F (30°C) would allow lactic-acid producing bacteria to become more dominant in the spontaneous fermentation, but that doesn't appear to be the case. However, the exact organisms at work are unknown, as I never performed any microbial population analysis during this experiment. This is something I would like to incorporate into future experiments. Additionally, I expected the carboy that began fermenting first (48 hours post coolship) and exhibited the most vigorous fermentation over time to be the most attenuated beer. However, this was not the case with the Coolership beer.

The Coolership did achieve a much more comparable cooling rate to that of a larger commercial coolship, but I believe its cooling rate was a little too slow. The overnight low during this coolship experiment was 23°F (-5°C), colder than is typically recommended by Jean Van Roy of Cantillon, but even after 10 hours of cooling the Coolership temperature was still only at 100°F (37.7°C). Another two hours of inoculation time would have gotten the wort closer to fermentation temperature, but I don't think it would have been below 86°F (30°C). Additionally, the Coolership I used was made of plastic, which is more prone to harboring unwanted yeast and/or bacteria than stainless. For these reasons I would recommend that homebrewers not use a plastic beverage cooler as a coolship if they have other options.

The Kettleship grossly undershot the target fermentation temperature, as expected, and ended up at a terminal temperature of 50°F (10°C) post coolship. What I found most interesting about the resulting beer was the reduced attenuation. I would assume that this was a result of the yeast being inhibited by the lower pH of this beer. The Kettleship beer also had more of a lemony/rotten vegetable character where the Coolership beer was more peachy and hoppy. Based on the laws of thermal physics, I would assume that the cooling rate of a hotel pan used as a coolship would be significantly faster than the cooling rate I observed in my Kettleship.

The carboy that received a 50/50 blend of both worts post coolship really surprised me. I assumed its behave would be halfway between the two worts it was composed of, but overall it behaved nearly identically to the Coolership beer and the two beers were nearly indistinguishable in the final tasting. This blended carboy wasn't part of the original intent of this experiment, but produced some interesting data nonetheless.

When a brewing vessel of unique geometry has been used for over a hundred years, it is not difficult to assume that the geometry of that vessel is a critical part in creating the resulting beer. After all, the coolships used by Lambic breweries are typically wide and shallow vessels with open tops, not something you see at your average local brewery. However, I don't believe a coolship of this shape is an optimal choice at the homebrew scale unless the brewer is able to control the cooling rate of the coolship so that it maintains an average rate of change of 12°F/hour (6.7°C/hour) and doesn't grossly undershoot the target fermentation temperature. My conclusion on the inoculation time versus SA:V debate is currently inconclusive, but I currently believe that inoculation time (cooling rate) is more important than SA:V ratio. Further experimentation is required to verify this, but based on the results of this experiment, I believe a hotel pan coolship will behave very similarly to an un-insulated Kettleship. I guess a more finite conclusion for this experiment would be that each beer produced in this experiment was quite interesting and worthy of blending. My preferred coolship approach for all batches following this experiment has been a modified version of both the Kettleship and the Coolership. I use a stainless kettle (Mine are 20-34gal or 80-136L) and insulate as needed in order to achieve fermentation temperature following 12 hours of time in the coolship. Unless I receive a spark of inspiration before the next coolship season, my next experiment will be to use two identical coolships with electric heating elements in them and manipulate the cooling rates and terminal temperature of each wort.


  1. Interesting read, Caleb. Do you think this can be replicated in a deep plastic bus tub? I think it would cool slower than a metal hotel pan, but still provide a large surface area. You have inspired me to try an experiment of my own. Dan.

    1. I think plastic tubs are a viable option, but the greatest thermal losses in a coolship are from the top surface of the wort. Therefore, I would expect the plastic tub would cool similarly to a hotel pan simply due to the large top SA:V ratio. Maybe someone has tested the two side-by-side, but if not it sounds like a worthy experiment. I would like to purchase a handheld thermo-couple data logger to do some similar testing for a future post.

  2. Caleb, I stumbled upon your site and wanted to thank you for putting all your experiments and findings on here. Have you had any luck this fall with the electric setup? I've often wondered if I can do the same with my rig. I'm curious how traditional cooling vs whirlpooled and temp controlled wort would compare. i.e. whirlpool+chill down to 100F, then turn the PID to 68F and let it whirlpool for a few hours. This should give plenty of time for any bacteria to inoculate between 68F and 100F as well as a nice long rest for yeast (plus plenty of O2 from whirlpool) once the wort is at a consistent 68F. This kind of setup should increase the "spon season" for brewers too. I might be able to do this and would be willing to share my findings (just have to get the ok from the SO/brewing partner).


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