Acidity Extraction and Espresso

Acidity Extraction and Espresso

Acidity is one component of coffee that can be a very desirable trait. When kept in good balance with other tasting sensations such as sweetness, body, and tannins, acidity can become the defining character of that coffee. Careful acidity extraction can help to transform a coffee from good to excellent. While using the Strada EP, we have found some patterns that help us to design pressure profiles.

Many of these discoveries we discuss take place while experimenting with different pressures, and tasting lots of coffee. We tend to spend several hours each week brewing coffee with different parameters, just to see how the variables change flavor. Some patterns have emerged, and we find those traits to stay consistent across many different coffee offerings. This article will cover acidity extraction, and our attempt to control it.

Acidity Extraction Happens First

When brewing espresso, the first half of the process is where the majority of extraction occurs. This makes sense when discussing solubility and the extraction process. Certain chemical reactions happen quickly, and others occur later in the brew. This is covered well by Royal Coffee in this article about understanding extraction.

One of the first chemicals to be extracted in the brewing process are acids. These acids are easily soluble, and attribute many flavor compounds to the cup. While acids add flavor, for the most part they lack body.

During the second part of extraction, lipids are released. These add texture and body to the coffee, often described as mouthfeel. The combination of acids and lipids contribute to the first part of tasting espresso. Our brain first picks up on the flavor and type of acidity, followed closely by the texture as the coffee coats our mouth.

The final part of extraction will cover sugars and other chemicals like tannins. Non-soluble materials extract at this point which round out the espresso experience. When in correct proportions, all of these components contribute to a well balanced drink.

Manipulation of Acidity

Although all of these extraction properties happen at different times, they all contribute to the coffee experience as a whole. An espresso with lots of acidity and lipids may be full of flavor and body, but it isn't always well balanced by the sweetness we look for. An espresso heavily weighted towards the early extraction components we sense as sour with an unpleasant finish.

Likewise, an espresso weighted towards the body and sugar extraction side will taste bitter and thin, with little unique acidity and a long unpleasant finish. If we are able to carefully design our brewing parameters to take balance into consideration, a well rounded and optimal espresso should result.

With careful manipulation of pressure during the early brewing process, we can alter the perception of acidity. We have mentioned many times that an increase in pressure results in an increase in extraction. If we increase pressure early in the brew, we will in turn increase extraction of acids.

While this general rule of acidity extraction being up front holds true across most of our experience, it is not always a desirable trait. Each coffee is unique, we seek to strike a balance between acidity, the the other components of espresso flavor. A coffee with a high acidity tends to benefit from extraction at a lower pressure up front than a coffee with lower acidity. Alternatively, a coffee with very low acidity tastes best when kept at a lower pressure. This helps to avoid entering the "Tannin Zone" we discuss in our last article.

A Case Study

Now we will apply these concepts to a real-world profile design. For this, we will use the example of an Ethipia Sidama Natural, and a Mexico Decaf. Both of these coffees contain different concentrations of perceived acidity. We must adjust our brew pressures to accommodate these traits to achieve proper balance in the cup.

The Ethopia Sidama undergoes natural processing. This process tends to increase the perceived acidity, so careful attention to brewing is necessarily to balance out the extraction.  Although this article concentrates on acidity extraction, the final brew stage can not be ignored in a discussion of pressure profiles. If you have not already, we encourage you to read the prior article which covers the Sweetness Curve, and the Tannin Zone.

It becomes very obvious that the Ethiopia Sidama benefits from a long pre-infusion to completely saturate the coffee grounds. After 8 seconds at a low pre-infusion pressure, the pressure is ramped up to 10 bar. We hold the pressure for only a few seconds, and gradually back off down to a final brew pressure of 5 bar. This recipe creates a cup with beautiful berry acidity,  syrupy body and a sweet sugar finish that lingers briefly.

Adjusting Acidity Extraction Up or Down

Through experimentation, it becomes clear that brewing at a lower pressure results in a significant lack of acidity. The cup becomes dull. Although it may have good body and a sweet finish, there is no acidity to round out this espresso and define its character. With an initial brew pressure of 8 bar, acidity becomes significantly muted. When brewed to a pressure of 6 bar, we find almost no perceived acidity in the cup.

Alternatively, an initial pressure of 12 bar is overwhelmingly acidic. The Ethiopian Sidama at this pressure suddenly becomes tart and sharp. Just a slight change in this initial pressure seems to have a drastic change in flavor perception. Even though the peak pressure lasts for only a few seconds, careful attention to this extraction parameter is important to ensure optimal flavor.

A Note About Acidity Quality

Up to this point, we have focused only on the quantity of acidity during extraction. There is another factor at play here. We must also consider the quality of the acidity. It is our experience that slight adjustments to the initial pressure will actually change the flavors extracted. These flavors are all part of the acidity spectrum, and slight changes in extraction pressure will alter them.

A quick example may help to make this point clear. Recent experimentation with developing a pressure profile for a washed Kenya helped to create some insight for us. This particular offering had distinct notes of black berries when brewed as a filtered coffee. We set out with the objective of finding the same notes in a well balanced espresso.

In order to control for the variable of pressure, we kept all brewing factors constant. The dose going in was 19.5 grams, with a yield of 52 grams in 30 seconds. The only variable that changed was the pressure during the first part of extraction.

The Experiment

With the recipe above the espresso had a great balance between acidity, sweetness and body. The goal was to change the flavor in the coffee without any major changes to the balance found in the original cup.

The first iteration of this experiment resulted in a very pleasant flavor, but it was not the delicate berries that we found in the filter coffee. The flavor was not very clear. It was bright, but it did not pack the distinct berry notes that we had come to love. To try and coax the berries out, we decided that we would increase the initial extraction pressure from 9 bar, to 10 bar.

The second attempt was successful in finding the berries we were chasing, but it presented a new problem. This second coffee now had a beautiful berry flavor, but the acidity was no longer in balance. The acidity had come to define the cup.

The Solution

Thankfully, the third attempt solved this problem. When we increased the initial pressure to extract the berry acidity, we also needed to find a way to reduce the overall acidity. It became clear that the higher pressure was beneficial to flavor, but the general rule of increasing acidity with the increase in pressure held true.

To solve this issue, we designed our next extraction to take place at 10 bar, but we reduced the pressure immediately after hitting this goal. Originally, we stayed at peak pressure for about 5 seconds before reducing down to avoid the Tannin Zone. With the increase in peak pressure, we decided to compensate for the increased acidity by dropping pressure just after hitting our 10 bar goal.

This time around, everyone at the tasting  was in agreement. This resulted in a perfectly balanced coffee, and it had that berry acidity we were looking for. In the end, we only changed one variable and still stayed within our defined parameters.

Final Observations

Acidity extraction is a tricky subject. When introducing pressure as variable, the concept becomes significantly more complicated. However, with a firm understanding of the factors involved, a wide range of flavor potential is possible. Designing pressure profiles for coffee can be tricky. When we think about designing pressure profiles, acidity extraction is an important consideration. It is also important to ensure that we balance extraction against the other factors involved, such as sweetness and body.

In the next article, we plan to attack the concept of brewing ratio and espresso recipes. Armed with this new knowledge of acidity quality and how to balance the cup, it may be possible to push the limits of extraction further without the usual negative attributes. What if we could brew an espresso with a high extraction yield while still maintaining balance? It is time to break out that refractometer and begin logging some objective data.