by Nasko Panov / NP’s Coffee Science

When chasing the perfect brew, we sometimes fail for a reason that is unclear or strange. Even with a good preparation technique, from time to time it happens that the final beverage is cloudy or turbid. Often this leads to a specific taste profile related to some unpleasant or undesirable notes such as high harshness and astringency. But what exactly went wrong with the extraction that caused it to remain unclear? There are a few speculations on this topic, however, they do not have any experimental evidence. The presence of a bad brew shows us that we have to learn more and research more.

The decrease in transparency of a liquid caused by undissolved particles or suspended solids is called turbidity. It is also known as a haze. This is not a well-defined physical entity but still can be standardized and measured.

There are three other beverages that are strongly influenced by turbidity – beer, wine, and tea. However, there is no simple way to evaluate the turbidity in them, and many research groups are working on this topic in different ways. Among them, it appears that wine is the most researched from and this is normally based on the price and quality demands from the customers. There are a few very interesting articles on this topic and one, in particular, was made by a research group in Navarre, Spain (1). It was found that the turbidity of red wine during its aging in oak casks has an influence on the accumulation of volatile compounds and, thereby, on the wine’s aroma. It was suggested that some particles in suspension and that precipitate during the wine’s time in the cask, are able to retain certain volatile compounds that are responsible for the wine’s aroma.

It is not hard to imagine that the same may happen in coffee as well. Suspended particles could retain some aromatic compounds and influence the aroma and taste. Fine coffee particles are often related to astringency and a harsh taste that is undesirable. Yet, coffee turbidity is one of the effects, which is not well researched. About a year ago Jonathan Gagne made some experiments on this topic and withdrew some conclusions as well. According to his study “measuring coffee, turbidity would potentially be extremely useful; it may allow us to put an objective measurement on astringency, and help us find out how to better manage channeling.”(2)

With this experiment, I wanted to create a quick and reliable method for measuring the coffee turbidity and analyze the amount of turbidity in different AeroPress methods. In particular, I wanted to understand in depth how the following things influence the coffee turbidity:

• Does layering 2 papers reduce turbidity compared to using just one paper?
• Is turbidity lower if you allow a coffee bed to settle onto the paper before you plunge in comparison with starting plunging right after inverting the AeroPress.
• Does squeezing the last bit of air out make any difference to the turbidity.
• What’s the difference in turbidity if you plunge very slowly and gently compared with pressing fast and hard?

First, I had to find a method that could precisely measure the coffee turbidity, but also this method must be fast and reliable. In his article, Jonathan used a turbidity meter, but it seems that without some kind of internal or external correction the measurement is strongly influenced by some factors to a point, where the results are controversial and the uncertainty is high. I found some technical notes by Ph.D. Kevin L. Goodner (3 and 4), where he used a UV spectrophotometer for measuring turbidity in tea. Thus, I decided to take his approach and use a UV spectrophotometer for this experiment. Again, I was quite lucky because I had one to play around in my office.

Secondly, I had to validate the method used by Ph.D. Kevin L. Goodner for coffee in order to prove that my measurements are correct. I used the same correction factor at 750nm and cornstarch to introduce suspended particles into the coffee sample for preparation of standards. This was used for method validation that will show the linearity of turbidity measurement.

Suspended solids in coffee block photons and appear as absorbance in UV spectrophotometric measurement.  Analyzing the coffee spectrum showed that absorbance at 750nm could be used for reference in order to correct for turbidity. It was shown that absorption at this wavelength comes mainly from the particulates and not from the coffee. The final calculation is based on a correlation between the absorption at different wavelengths that are differently influenced by coffee absorption and particulates absorption. This was observed by comparing the spectrums of coffee and filtered coffee through 45-micron or 20-micron filters.

All the experiments were performed with an AeroPress inverted method with differences in the method steps.  And after a few sleepless nights, here are the results.

I started with just a casual Inverted AeroPress recipe – 15g coffee, 250 water at 90°C, 10 times stirred at 30 sec mark, close the cap at 1 min, leave the coffee to settle before plunging, slow plunge for 45-50 sec., no full plunge. All samples were measured after reaching room temperature and 3 repetitions per sample were conducted for a better statistical result. (Yes, I did experiments at different temperatures and the results were similar). In Table 1, there are the results for the standard recipe used for comparison with all other samples. In the same table is the comparison with the same recipe but using 2 paper filters.

As expected, when two paper filters are used in AeroPress the turbidity is less. This is due to the additional filtration through the second filter and the higher probability for fines to be retained. The difference is not big, but still detectable. This is also a confirmation that the method is working – the experiment is confirming the observation.

Illustration: AeroPress using two filter papers. 

The next experiment is to change the plunging start time. Is there a difference in turbidity if I start to plunge the AeroPress right after I invert it, in comparison to letting the coffee bed settle? The idea is to see if the coffee bed itself could retain the fine particles and reduce the turbidity. Here are the results:

Results showed that there is little to no difference in turbidity in both methods. This means that the coffee bed cannot filter fine particles and the time to settle the coffee bed is irrelevant to turbidity in the final coffee brew.

Illustration: Plunging the AeroPress immediately after inverting, without settlement of particles.

Illustration: Plunging the AeroPress after particles have settled.

Next, I addressed the question whether squeezing the last bit of coffee and air from the AeroPress contributes to higher turbidity. This was something that I observed in practice during my work with an AeroPress. In most of the cases, I see a change in turbidity when the last drops of coffee and air are squeezed. There is also a clear change in taste and the coffee feels harsh, astringent, and sometimes earthy. Prior to conducting the experiment, I expected that the turbidity would be higher with this method. This is what the measurement showed:

The turbidity for this method was comparable to the standard recipe, showing that squeezing the last air out from the AeroPress does not influence the turbidity. And then this happened – the calculated turbidity for the third repetition was 0.8356 – a significant increase in the result. Afterwards, I made the fourth and fifth measurements. The results were 0.8034 and 0.8280 respectively. This was the moment where I started rethinking the entire procedure. If there are fine particles small enough to pass through the filter, they will pass no matter what. This was the reason for similar results achieved for all methods. Why did I get this strangely high result on repetitions 3 and 5? There should be a second reason for the higher turbidity, but I’ll come back to this later.

The final experiment includes a hard plunge. This is a comparison of a gentle and slow plunge with a fast and hard plunge. Does applying high pressure on the AeroPress influence the turbidity of the final brew? The results were as follows:

As seen from Table 4, fast and hard plunge leads to a major turbidity increase and this was visible with a naked eye. After observing this, I started thinking that the reason for the increase of turbidity is because there is another place for the particles to pass the AeroPress filter. The only possibility was that the hard plunging opens a small hole between the filter and the plastic, leading to huge channeling and bypassing the filter.

Illustration: Channeling is created by pressure when plunging hard and fast, allowing more fines to get through the filter paper and into the brew.

I had to prove this hypothesis and made a few more experiments. This time, I intentionally hard tightened the AeroPress filter cap. Afterwards, I made some measurements with a loose cap, but still reasonably tight. If I was correct with my assumption, there should be a significant difference between the two methods when the plunging pressure is high.

Results from Table 5 confirmed what I was thinking. The tighter the cap is placed, the less turbid is the coffee. This means that the main reason for turbidity increase is not the way we prepare AeroPress, but the possibility for the water to pass between the filter, the filter cap, and the AeroPress body. It seems that the only method directly influencing the coffee turbidity is applying hard pressure on the plunger. This opens a gap, allowing for the fine particles to bypass the filter, ending in your cup of coffee.

In conclusion, this was an interesting experiment that will help better understand coffee preparation with an AeroPress. Controlling turbidity is important as it is related directly to the coffee beverage taste and aroma. With growing customer demands for quality, the expectations towards barista skills are also increasing. To address this, baristas should understand the weak points in coffee preparation. In this research, it was shown that in immersion brew like Inverted AeroPress there is almost nothing related to the AeroPress method itself influencing the final beverage turbidity. Of course, there are other things leading to higher turbidity like coffee roasting level, grinder performance, and maybe the coffee beans themselves. In AeroPress, there is one weak point that could potentially be crucial for the end result and could increase the coffee turbidity to a point where coffee feels harsh, unpleasant, and astringent. It is related to the tightness of the filter cap and the potential for fine particles to bypass the filter through a small gap between the filter, filter cap, and AeroPress body. This weak spot is only apparent if the plunging pressure is high enough to open these gaps between the plastic parts. Increasing the tightness of the cap decreases the turbidity, but still a fast and hard plunge always leads to higher turbidity. This means that applying lower pressure when plunging is the best way to keep the extraction consistent without the risk of introducing harshness and bitterness.

• Nerea Jiménez Moreno, Public University of Navarre.
• Jonathan Gagne, Coffee Ad Astra blog, The Turbidity of Coffee
• Kevin L, Goodner, Ph.D., Sensus Technical Note (SEN-TN-0008)
• Kevin L, Goodner, Ph.D., Sensus Technical Note (SEN-TN-0010)