Experimentally measure the effect of coffee bloom on dissolved CO2 in coffee brewed with Aeropress inverted method.
What is coffee bloom?
During the coffee roasting process CO2 is formed and released through the formed macropores of the roasted coffee beans. There is an existing correlation between the degree of the roast, released and residual CO2 (1). Part of the residual CO2 is believed to be trapped within the formed porous matrices left from the evacuated cells as occluded gas, which is in equilibrium with the absorbed/adsorbed CO2 in oil, polysaccharides, and moisture (2). Once the roasting process is completed, the beans then begin to slowly release these gases in a process called "degassing" that continues for several weeks. When hot water touches the coffee grounds, it purges out and replaces the carbon dioxide thus creating the "bloom."
Why is bloom needed?
It is widely spread as a hypothesis that blooming is mandatory because if CO2 is not properly released, it will block the contact of water with grounded coffee. If water doesn't have adequate exposure to the grounds, the extraction will likely be weak. It is also believed that if CO2 is not fully released it dissolves in the brewing water forming carbonic acid. Carbonic acid is tasted by the sour-sensing taste cells on the tongue leaving a popping and acidic sensation. Dissolved in water CO2 is in constant equilibrium with the carbonic acid (3):
Hypothetically, if the coffee is not allowed to bloom, during the brewing process, more CO2 will be dissolved into the water, leading to the formation of carbonic acid, carbonates and hydrogen carbonates ions.
A higher concentration of carbonic acid will worsen the coffee taste.
My coffee project and my entire research and experiment are based on these two hypotheses and my goal is to prove or disprove them. However, the measurement of CO2 is not an easy task, and this could be the reason why I didn't find any scientific literature on the topic. Accurate measurement of the dissolved CO2 in the form of carbonic acid, carbonates, and hydrogen carbonates ions will provide an answer to the question - How important is the blooming step during coffee brew? Such an experiment will clearly show whether the CO2 is efficiently removed from the coffee during bloom using different techniques like Aeropress, V60 or Chemex. This is where my idea comes that this experiment could be performed with the so-called Total organic carbon (TOC) analyzer.
What is TOC?
Total organic carbon (TOC) is a measure of the carbon content of dissolved or undissolved organic matter which is present in different matrices. In this case, the matrix is water. Here we must explain three different definitions:
TC (total carbon) - The sum of organically and inorganically bound carbon present in water, including elemental carbon.
TIC (total inorganic carbon) - The sum of carbon present in water consisting of elemental carbon (C), total carbon dioxide (CO2), carbon monoxide (CO), cyanide (CN−), cyanate (NCO−) and thiocyanate (NCS−). The TOC analyzer mostly registers as TIC only the CO2 originating from hydrogen carbonates (HCO3−) and carbonates (CO32−).
TOC (total organic carbon) - The sum of organically bound carbon present in water, bounded to dissolved or suspended matter. Inorganic carbon compounds as Cyanide, Cyanate, Thiocyanate, and particles of elemental carbon (soot) will be determined together with TOC.
A typical analysis of total carbon (TC) measures both the total carbon present and the so-called "inorganic carbon" (IC), the latter representing the content of dissolved carbon dioxide and carbonic acid salts. Subtracting the inorganic carbon from the total carbon yields TOC (4).
Just a little remark for the non-chemists reading this article. Almost everything that the water is extracting from the coffee bean is organic compounds containing carbon (TOC) and inorganic compounds containing carbon (TIC). Quite a few from all hundreds of extracted compounds could not contain carbon. This means that TOC is capable of analyzing almost 100% of all compounds contained in the coffee.
Much more precise, accurate and exhaustive compared to a simple TDS (total dissolved solids) meter, that could never measure undissolved matter and oils, nor could it ever distinguish inorganic from organic compounds.
In this experiment, TOC-L model from Shimadzu Corporation was used. It works on the principle of combustion where half of the sample is injected into a chamber where it is acidified with phosphoric acid, to turn all the inorganic carbon into carbon dioxide as per the following reaction, pulling the equilibrium to the left side:
This is then sent to a detector for measurement. The other half of the sample is injected into a catalytic combustion chamber which is heated to between 600-700 °C. Here, all the carbon reacts with oxygen on a catalyzer, forming carbon dioxide. It's then flushed into a cooling chamber, and finally into the detector. The detector used is a non-dispersive infrared spectrophotometer. By finding the total inorganic carbon and subtracting it from the total carbon content, the amount of organic carbon is determined.
Sample preparation
A series of samples were prepared by the following procedure:
1. 15g coffee of very fresh coffee roasted 3 days before the experiment, ground medium-coarse for Aeropress;
2. 85 °C Water temperature;
3. A total of 200ml of water was used for all samples;
4. The total brewing time of 2:30 min for all samples;
5. Aeropress inverted method;
6. 30 sec for plunging;
7. 10 gentle stirs on 1:00 min mark;
8 samples were prepared - 4 with a blooming time of 40 sec and 50ml of water followed by the addition of the rest 150ml. The other 4 without blooming where the entire amount of 200ml of water was added as quickly as possible. The brewed coffee was transferred to 50 ml centrifuge tubes filled to the top and firmly closed, so no CO2 could be dissolved from the air and CO2 degassing was minimized. Tubes were placed immediately at 4 °C in the fridge. Lowering the temperature will draw the CO2 equilibrium to the dissolved CO2 and carbonic acid formation. Samples are kept in the fridge until analysis. For the TOC analysis 1ml from each sample was transferred to 50ml centrifuge tubes and it was diluted with 49ml of ultrapure water. Samples were gently stirred 2-3 times and were loaded on the TOC instrument. An injection volume of 300 microliters was injected for TOC analysis based on the measurement of TC and TIC. The instrument measured 3 times each sample for better statistical results.
On the next day, the same measurement was repeated but instead of cooling the samples, they were measured immediately after preparation. This experiment doesn't require the creation of a calibration curve because it is a comparison, not quantitative analysis. Together with all samples tap water, filtered water, and ultra-pure water was analyzed, so the results could be evaluated more correctly. As I said, no quantitative analysis is needed so for the results, peak area was evaluated as an absolute value of Area Units (AU).
The surprising results
Immediately after obtaining the first results I was a bit surprised. To be honest, I believed that there will be a clear difference between two methods for coffee preparation and the entire experiment was created to prove this hypothesis.
However, analysis after analysis, the results were confirmed.
There was absolutely no difference in the concentration of inorganic carbon contained in all samples. The average value for all 4 repetitions for coffee brewed with bloom was 666 AU, and the average for coffee without - 716 AU (Table 1).
And before anybody says that there is a clear difference, I must mention the results for all kinds of measured water samples in (Table 2). Tap water 1 and 2 are samples from the same water source taken at different times. So, the same water measured twice has a bigger difference in the content of inorganic carbon of 68.8 AU than coffee with or without blooming. 50 AU is the difference between tap water and the same water filtered through a water filter jug. In the city where I live and measure the samples, we have a classification of very soft water with a value below 2.5 dH (German degrees). Differences between two Ultra-pure water and tap water were in the range of 160 AU (Graph 1).
Measuring a new batch of samples on the next day immediately after coffee preparation showed absolutely the same results. Coffee with bloom time and coffee without bloom differ from each other with around 70 AU (Table 3). Considering the value for Total Carbon measured of 150,000 AU this difference on IC is a negligibly small value. This is 0.047% of the total yield of the extraction. Yes, there is a difference but this difference could never be seen or felt without such a high precision instrument as TOC. This particular instrument has a detection limit of 4 ppb (parts per billion) or microgram/L and the value of 70 AU is in the ppb region.
Conclusions
All experimental results are showing that the difference of CO2 content represented as dissolved carbonic acid and measured as Total Inorganic Carbon between coffee brewed with and without bloom are negligible. This means that with or without blooming, the CO2 removal is the same if all other variables are kept constant - temperature, total brew time, brew ratio, stirring and plunging. Both hypotheses that the blooming procedure is helping the extraction and improving the coffee taste (because of reducing the dissolved carbonic acid) are wrong.
The difference is so small that nothing except very sensible and expensive laboratory equipment could detect it.
Many will immediately disagree with the results, but I must clearly say that the facts speak for themselves. I know that some wouldn't believe in this experiment because blooming myths are told for years in the field of specialty coffee and are so well accepted. Many things were said for coffee bloom by experts and famous people that everybody is taking it for granted.
Who is going to believe me? I don't know, however, I do know that as a scientist I believe in the experimental results.
One thing I know is that this research will continue as I plan to extend it further for the Normal Aeropress method, V60, Chemex and other drip brewer methods. As I'm posting this article I'm currently working on these samples. But I have the feeling that the results are already clear. During this experiment, something more interesting was observed! This was the Total organic carbon content, which represents the extraction precision and repeatability, extraction yields and the difference in coffee extraction with and without bloom. However, this is a story for another time...
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Literature:
(1) Effect of Roasting Conditions on Carbon Dioxide Degassing Behavior in Coffee XIUJU WANG, LOONG-TAK LIM* Department of Food Science, University of Guelph, Guelph, On, N1G 2W1, Canada
(2) Illy and Viani 1995; Schenker 2000; Clarke and Vitztbum 2001.
(3) Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 310. ISBN 978-0-08-037941-8.
(4) Lenore S. Clescerl; Arnold E. Greenberg; Andrew D. Eaton (1999). Standard Methods for Examination of Water & Wastewater (20th ed.). Washington, DC: American Public Health Association. ISBN 0-87553-235-7. Method 5310A
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