Contrary Hypothesis How Brewing Temperature is Affecting Flow Rate of Espresso
...thanks NOT to dissolved gases.
There are several reasons why I'm writing this response to a recently published article from Barista Hustle, regarding brewing temperature effects on the flow rate. If you didn't read the original post, go find it HERE. First of all, it is because I vote for science when it comes to a scientific article. And in coffee science, I'm constantly reading and hearing about not well-described effects and hypothesis. Secondly, for a long time, I wanted to start a coffee science blog. As a scientist who is working in the field of analytical chemistry, physics, and instrumental analysis, I have the knowledge and the experience to write about coffee science. Additionally, I have the opportunity to work with a wide variety of high-end analytical instruments.
Dissolved vs not dissolved gases
Why is nobody thinking about degassing, dissolving gases and everything that is happening? And why is nobody thinking about correct scientific terms?
In the beginning, I want to start with the so often referred in the past weeks dissolved gases that are responsible for so many things in coffee extraction. BH is mentioning that dissolved gases are the obvious reason why the flow rate is decreasing with the higher brew temperature. Right after that, they mentioned that bubbles formed in the puck should be the reason. But if you think for a second, formed bubbles are coming not from dissolved gases, but from the not dissolved ones. And the gas will stay dissolved until the conditions are changed so the water couldn't hold it anymore. This is an equilibrium that is dependent on the conditions. Two major conditions could change in the brewing process - a decrease in temperature and pressure. The first will lead to more gas to be dissolved in water. This means that during extraction more and more gas will be introduced into the water. The effect of degassing will be decreased. So the gas bubbles are not formed by the dissolved gases. But from the gas pushed out by the water from the coffee beans that could not be dissolved.
The second major condition is pressure. The pressure drop will lead to more gas to be degassed from the water. But we expect pressure drop after the water exits the coffee basket and this couldn't explain the formation of the bubbles in the coffee puck.
Of course, you can argue that in the beginning, the temperature will increase as the first contact of the water with coffee will cool it down. But also at this moment, the pressure will start to drastically increase and this will negate the temperature increasing effect on gas dissolving. After the initial conditions, the temperature could only decrease. Or in the best espresso machines will stay constant. In this case, the conclusion will stay the same. So I think that gas bubbles are not coming from the dissolved in water gases.
So I think that gas bubbles are not coming from the dissolved gases in water.
Density vs Viscosity vs Compressibility
"Bubbles that increase resistance to the flow of water." Is this really what is happening?
Here comes the biggest contradiction that leads me to write this post in the first place. The authors are making the hypothesis or assumption that the creation of bubbles increases resistance to the flow of water or resistance in the bed. I couldn't disagree more on this assumption that wasn't covered or supported by any literature or physical explanation. Gases can't generate higher resistance compared with liquid. There are three major factors that are supporting this.
Gases have much higher compressibility than liquids. Compressibility is a measure of the relative volume change of a fluid (gases are also fluids) or solid as a response to a pressure change. The particles in a gas are much farther away from each other allowing them for easier movement and more compressibility. Another effect is the atoms/molecules attraction. The higher the atoms/molecules attraction to each other, the harder it is to compress. From the commonly used solvents, water has really low compressibility and one of the reasons for this is the weak H-bonding. Often it is being assumed that water is incompressible. Incompressible fluids possess only one coefficient of viscosity because, by definition, no changes in volume can occur. If such a fluid contains air bubbles it becomes compressible, and any changes in volume involve a contraction or expansion of the bubbles which is resisted by the ordinary viscosity of the surrounding fluid (1). This means that liquid with a high amount of dissolved gases or a mixture of liquid with gases will have higher compressibility.
Gases have a much lower viscosity than liquids. Viscosity is a material property that describes the resistance of a fluid to shearing flows. It corresponds roughly to the intuitive notion of a fluid's 'thickness' (2). Just compare Carbon dioxide or Air viscosity of 14.90 and 18.90μPa.s respectively with 890μPa.s for water. Even at a high temperature of 100 °C viscosity of water is more than 10 times the one for CO2 or Air. The kinematic viscosity (aka momentum diffusivity) will likely fall if air bubbles are present because the bubbles will interfere with the diffusion of momentum by the collision between the molecules of the liquid. However, depending on the corresponding change in density, the dynamic viscosity, which includes the kinematic viscosity and density, may not change by the same ratio. As a rule, the dissolved gases decrease the density of water (3).
Let's investigate the dissolved carbon dioxide. At a low temperature of 25 °C dissolved CO2 causes an increased velocity by <1.5%. However, this GWR (Gas to Water Ratio) effect is reduced with increasing temperature and diminished at around 60°C. Then the CO2 effect on the velocity of water reversed: reducing water velocity with increasing GWR, and increasing temperature (4).
Gases have a much lower density than liquids. The density (more precisely, the volumetric mass density; also known as specific mass), of a substance, is its mass per unit volume (5). In comparison, the density of Air is 1.2kg/m3 compared with 1000kg/m3 for water. On the other hand, density is related to the Volume flow rate with the formula Mass flow rate=Density*Volume flow rate. So the density and volume flow rate are inversely proportional. The lower the density, the higher the volume flow rate.
There is an effect of density increase of the Aqueous Solutions of CO2, but only when water is saturated with carbon dioxide and the solution doesn't contain bubbles. This density increase is quite negligible - around 2-3% (6).
Just to summarise, gas bubbles in water decrease viscosity, and density. All two lead to a higher flow rate of a mixture of a liquid with gas. With that in mind can anyone explain how a gas bubble can create higher resistance? I'll give also a practical example from the liquid chromatography. In LC we have a pump that is pumping liquid trough a small capillary (0.01-0.3mm ID) and trough a chromatography column, which consists of small densely packed balls. Schematically it is similar to water pumped through a coffee puck and coffee basket. The column is generating back-pressure as the liquid passing through it. In practice introducing even the smallest amount of gas (dissolved or as a gas bubble) leads to pressure drop. It could never lead to a pressure increase.
So, is there a proof that gas could increase resistance? If not, I think this hypothesis is wrong, as it couldn't be supported by any proof, observation, literature or scientific data.
The new hypothesis
What is my hypothesis?
I would like to give a different explanation of the observed results from Barista Hustle. During the roasting process, coffee releases oils, and darker roasts contain more of them. With time coffee beans start to lose these oils from evaporation, oxidation, and other processes. Also, around 30% of the coffee bean is soluble in water. The higher the content of oils and solubles in the coffee, the more viscous and dense will be the extracted water from the brewing process. If the viscosity and density of the fluid increase, the flow rate will decrease. The effect must be stronger to a coffee with higher oil and solubles content like fresh or dark roasted coffees. Increasing the brewing temperature will lead to faster and more complete oil extraction and solids dissolution. The viscosity and density of the extracted water will rapidly increase and this will lead to an even lower flow rate. In addition, higher brew temperature will speed up the process of extraction and dissolution and both processes will end earlier. This will speed up, even more, the flow rate during the second part of coffee extraction. And that is exactly what the team from Barista Hustle observed.
This is just a simple explanation of the observation, but more of the claims could be measured or checked in the available scientific literature. This is only a hypothesis and experiments have to prove it right or wrong. My idea is only to provide a different explanation for the observed effect and open a discussion on the topic. I'm not claiming that my explanation is 100% correct.
What about incompleteness of data?
Strangely only 3 types of coffee were used in this experiment - Fresh light roast, Old light roast, and Fresh dark roast. To perform a better experiment, create a better hypothesis and take into account all the variables it is mandatory to have a representative sample from all types of coffee - Fresh light roast and Old light roast, Fresh medium roast, and Old medium roast, Fresh dark roast and Old dark roast. Only then the measurement and collected data will reliable enough.
The conclusion
It is great that more and more questions are asked linked with the coffee brewing process. More people are interested to better understand the coffee extraction and everything that is happening, which is great. This inevitably will lead to progress in this field. But we have to be aware of the false conclusions as many could believe and rely on these experiments and results. Many will take these conclusions as a reliable source without checking the hypothesis or test the explanation. And their explanation shouldn't be against fundamental physic or chemistry laws.
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Literature:
(1) The Two Coefficients of Viscosity for an Incompressible Fluid Containing Air Bubbles
Geoffrey Taylor, Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences Vol. 226, No. 1164 (Oct. 21, 1954), pp. 34-37
(2) Symon, Keith R. (1971). Mechanics (3rd ed.) Addison-Wesley. ISBN 978-0-201-07392-8.
(3) The Influence of Dissolved Gases on the Density of Water, H. Watanabe, K. Iizuka, Article in Metrologia 21(1):19 · January 2005
(4) Velocity and density of water with dissolved CH4 and CO2, De-hua Han and Min Sun*, RPL, University of Houston, DOI http://dx.doi.org/10.1190/segam2013-111
(5) The National Aeronautic and Atmospheric Administration's Glenn Research Center. "Gas Density Glenn research Center". grc.nasa.gov. Archived from the original on April 14, 2013. Retrieved April 9, 2013.
(6) Density of Aqueous Solutions of CO2, Julio E. Garc´ıa, Graduate Student Research Assistant, Earth Sciences Division, Lawrence Berkeley National Laboratory, October 11, 2001