Introduction to Espresso Coffee Pressure Profiling

By Ross Spencer             17th October 2011

Profiling is an emerging area in specialty coffee that could potentially revolutionise the industry. For many years, it has been considered that optimum espresso extraction is obtained by pumping water heated to about 92°C through a cake of ground coffee at a fixed 9 bar of pressure to produce around 30 ml of espresso in about 25 seconds. The exact values seem to vary somewhat, but the one constant in conventional thinking is fixed pressure, and this is evident in the design of most electrically driven pump espresso machines. Profiling advocates challenge this fixed assumption, and argue that 9 bar is not necessarily the optimum pressure for all coffee blends, and that 25 seconds extraction time does not always produce the best results. The essence of profiling is to not accept conventional parameters, but search for alternative pressure (both fixed and variable) and timing combinations to produce the best possible espresso extraction from a particular coffee.

Even though profiling is a relatively new topic, the fundamentals have been used, possibly by accident, for at least as long as lever machines have been available. The variable pressure used in lever machines is a consequence of their design, and operating these machines in such a way is simply how they are used. It was not until modern pressure profiling machines emerged that people began to focus on and analyse the profiles used in lever machines. As such, it seems that the lever machine’s profile has become a form of benchmark that people aim to replicate.

The availability of information and research on profiling is limited, but this will inevitably change over time as the availability of equipment capable of profiling increases. The aims of this article is to contribute to the pressure profiling knowledge-base by sharing experimentation data and results, as well as attempt to provide an objective standard that will enable others to replicate and compare results. It is recognised that “taste” preferences vary somewhat from one person to the next, but it is anticipated that patterns will emerge over time to reveal consistent and repeatable profiling effects.

It should also be noted that all experimentation is conducted with the Rossa Hand Espresso fitted with the Profiling Adapter. However, the principles and profiling data presented in this article apply equally to all machines and devices capable of controlling and manipulating the necessary variables.

Definition of Profiling

Profiling is typically defined in terms of variable pressure, which is why it is often referred to as “pressure profiling”. However, any comprehensive discussion or analysis of profiling should include all possible variables, including temperature, flow rate, grind size and consistency, coffee roast, and any other variable that influences espresso. Rather than challenging convention, profiling is an extension or improvement on what is already known.

The aim of profiling is to seek the specific combination of variables, whether fixed or otherwise, that reveal or enhance the desirable characteristics of a particular coffee. Profiling will not necessarily make bad coffee taste good, but it does provide more scope or options to discover otherwise hidden or subtle flavours, or to enhance or reduce specific flavours. Therefore, profiling can be defined as the process of seeking the optimum combination of fixed or variable temperature, pressure and timing which produces an infused espresso stream that flows at a rate that produces the preferred characteristics from a particular coffee.

Grind size and quality

Grind size in profiling requires a different perspective than that described in conventional espresso theory. The grind size is selected primarily for the taste it produces, rather than a specific flow rate.

Grind quality is important for all espresso extraction, and is vitally important in conventional machines where grind size is used to control the rate of flow. However, profiling enables a broader range of grind sizes to be used. The grind size remains important, but more so for taste rather than pressure or flow rate. Similar pressure and flow can be obtained from a broader range of grind sizes than that used in a conventional machine by manipulating the pre-infusion and pressure ramp-up profile. This will be discussed in more detail below.

Grind size consistency is crucial for effective profiling. The grind size distribution should have minimal deviation so that water penetrates each particle to the same depth to ensure consistent extraction. Consistency on this variable helps isolate and enhance or soften individual characteristics. A large grind size distribution causes more variation which results in a more blended extraction. This makes it more difficult to isolate certain characteristics, but results can still be of a high quality.

Extraction temperature

The importance of water temperature during extraction is viewed as the most critical factor to some, and quite a tolerable variable to others. Regardless of these views, what is certain is that water temperature decreases to varying levels during extraction. This can easily be tested by measuring the temperature of a shot immediately after extraction.

The temperature drop is caused by lower temperatures of the group-head and the ground coffee puck. Even though it is standard practice to preheat the group-head, it will rarely reach or maintain full temperature. Additionally, coffee is ground at ambient temperature or cooler, which will cool heated water as it works through the puck. The temperature equalisation effect of the cooler group-head and coffee puck will cause the water temperature to drop by varying levels during extraction. The rate of temperature drop is dependent on the equipment used and its method of operation.

Temperature drop does not cause any real problems; otherwise its effects would be more widely discussed. It is recognised that the difference is not of any real significance, and it is raised here for completeness only. However, it is possible to influence espresso taste by controlling the rate of temperature drop in the Rossa Hand Espresso.

All things being equal, the rate of temperature drop can be influenced by varying the Rossa’s preheating procedure. For example, preheating the unit just enough to obtain the desired temperature will have a higher rate of heat loss than when the unit is preheated for an extended period and the water allowed to cool to the desired temperature before extraction. The first preheating method is quicker, therefore less heat penetrates the brass mass. The second method is longer, therefore more heat penetrates the brass mass. Even though the water used for extraction is set at the same temperature, the rate of drop will be less for the second technique. The difference is minimal, and for the record, the author prefers the first technique.

Stages of a profile

The following terms define the various stages of a typical profile. They relate specifically to pressure and timing. Temperature is varied independently of the profile, but all variables and their interrelating effects should be considered.

Initial water contact – The moment that water first touches the coffee puck.

Pre-infusion – The period from initial water contact to commencement of pressure ramp-up. Used to allow water to soak into the coffee puck prior to extraction.

Pre-infusion time – The duration of time from initial water contact to commencement of pressure ramp-up.

Pre-infusion pressure – The pressure used during pre-infusion to enable water to penetrate the coffee puck.

Pressure ramp-up – The rate of pressure increase from pre-infusion to extraction pressure.

Upper pressure – The highest pressure used during extraction.

Pressure ramp-down – The rate of pressure decrease from the upper extraction pressure to the lower extraction pressure.

Lower pressure – The lower pressure used during extraction.

Profiling Equipment

The availability of machines or devices capable of profiling is currently limited, but this is likely to change as more people are introduced to the possibilities that profiling offers. The Rossa Hand Espresso, with or without the Profiling Adapter, is currently the only low cost device capable of controlling the variables to the level required for effective profiling, and is possibly the only currently available device that is capable of manipulating the variables to the level described in this article.

The Profiling Adapter provides more accuracy, and enables objective sharing of data. However, hand profiling with the Rossa is certainly possible for those with good feel. Crank arm turning effort is surprising accurate and repeatable, but it does take some practice to develop proficiency.

How to Profile

It is advisable to begin with a standard extraction using conventional variables. While this is not absolutely necessary, it will provide a reference to which you can compare experimental shots. From here on, there are no set rules. Experimentation is the key, but taking notes is advisable due to the number of interrelated variables.

Setting the Grind

A good starting point for grind size is to begin with the same setting used for the standard shot as described above, and then vary as necessary to find the size that results in the best taste for the particular coffee used. Choose a quality grinder that enables very fine adjustment. The Rosco Hand Grinder’s locknut grind setting adjustment design is ideal for profiling because it enables the size to be set at any point, as opposed to a predetermined index position.

It is possible to use a much finer grind with profiling, but this can sometimes cause a taste similar to a poor quality grind that produces numerous fines. Testing so far has shown that no more fines pass through into the cup between the finer grind and the optimum grind, but the taste could lead one to think otherwise. The key is to find the exact size from which taste begins to deteriorate, and then use a slightly coarser grind setting. If using the Rosco, use a setting about 0.01 – 0.02 mm coarser than the point from which taste begins to deteriorate.

Pre-infusion

The purpose of pre-infusion is to prepare and set the puck by permitting enough water to penetrate the individual coffee grounds in the correct time to achieve the desired extraction characteristics. Pre-infusion occurs at low pressures ranging from ambient pressure (zero gauge reading) to approximately 2 bar. The duration also varies from a few seconds to 30 seconds or more. These variables are subjective and certainly open to experimentation. As with all profiling variables, there are no fixed rules.

Pre-infusion pressure and duration should be viewed as being unrelated to extraction pressure and flow rate for the purpose of seeking optimum taste, but they do influence flow to varying degrees. In general, the finer the coffee, the more pre-infusion required to produce a specific flow rate, and the coarser the grind, the less pre-infusion required to produce the same flow rate. However, the pressure ramp-up speed can influence the upper extraction pressure.

Pressure ramp-up

Pressure ramp-up is the process of increasing pressure from pre-infusion to extraction pressure. The ramp-up variables are primarily the speed or rate of increase, and whether or not the ramp-up rate is even or variable. For example, the pressure increase rate can start slowly, and then increase as pressure rises, or inversely, a rapid start followed by a slowing rate as the upper pressure is approached. Regardless of the speed of ramp-up or whether or not the rate is constant or varied, the key is to maintain smoothness. Abrupt fluctuations can potentially disturb the puck causing the release of unwanted flavours.

The maximum obtainable pressure and flow rate can be influenced by a combination of pre-infusion variables and ramp-up speed. High pressures are easily obtained with fine grinds, but coarser grinds require careful manipulation to obtain both the required pre-infusion duration (to allow water sufficient time to penetrate the individual coffee grounds) and the desired flow rate. As a general rule, a coarser grind requires a faster pressure ramp-up in order to obtain full pressure.

It appears that a rapid increase forces the pre-infused coffee grounds more tightly together, which produces more resistance to flow. This increased resistance enables higher pressures to be obtained during extraction. The inverse also applies. Finely ground coffee can be extracted within the desired pressure range by using a slow pressure ramp-up. This seems to enable water to flow more easily through the puck (without channelling). Fine tuning this method requires experimentation with various combinations of pre-infusion variables. It should also be noted that limitations to the maximum and minimum grind sizes still apply, but this method enables a relatively broader range of sizes to be used.

Upper pressure

Conventional espresso extraction uses 9 bar of pressure. However, when pre-infusion is applied, it appears that pressures above 6 or 6.5 bar release unfavourable flavours. These flavours can be eliminated when higher pressures are used by shortening or skipping pre-infusion (similar parameters as conventional espresso extraction), but it is difficult or impossible to extract the subtle flavours or produce a rich and balanced shot to the same level without pre-infusion. It should be noted that different coffees will generally respond differently. As always, experimentation is the key.

The current theory to explain why it is necessary to use a lower than conventional extraction pressure when pre-infusion is applied is as follows: Pre-infusion enables water to penetrate into the individual coffee particles before commencement of extraction, therefore requiring less pressure to extract the soluble elements of the coffee. When no pre-infusion is used, the coffee particles remain dry until extraction. Therefore more pressure is required to penetrate water into the individual coffee particles far enough to extract the same amount of soluble elements.

Regardless of the accuracy of this theory, experimentation is the key. The effects of this variable can be tested easily without needing to alter other variables. Pressure is perhaps the most objective variable to measure, therefore enabling accurate and repeatable testing and experimentation.

Pressure ramp-down

Pressure ramp-down is the process of decreasing pressure from the upper pressure to the lower pressure. This step is not always required. For example, a fixed pressure profile requires no pressure change, and a profile that begins with a lower extraction pressure before increasing pressure requires no pressure ramp-down step.

The primary ramp-down variable is speed or rate of pressure decrease. Pressure can be decreased quite rapidly, or it can be a gradual reduction, and both methods produce quite different results. The ramp-down profile is a significant variable that should be viewed in conjunction with both the upper and lower extraction pressures and their durations. Similar to the ramp-up process, the key is smoothness without abrupt fluctuations.

Lower pressure

The lower pressure is a lower extraction pressure preceded by the pressure ramp-down and upper pressure. There are no specific rules, but results to date indicate that the lower pressure should not drop below 4 bar. Fixed pressure testing has shown that 4 bar is about the lowest pressure capable of a reasonable extraction, but different coffees will inevitably produce different results. As with the previous steps, experimentation is the key. 

Putting it all together

Each of the above steps is equally important, and each can be varied relatively independent of each other. However, accumulation of all steps ultimately affects the resulting shot, and the number of potential variations is seemingly infinite. Therefore it is advisable to work on one step at a time in the overall profile, and take note of changes. As you become familiar with and confident in the changes of one step, move on to the next, and so on until you become a profiling master. Replicating the profiles shown below should save time and possibly many wasted shots until such time that your confidence and proficiency increases.

Where to from now

The current limitation of machines capable of manipulating variables to the degree as described in this article is a significant factor holding back people’s awareness of the potential that profiling offers. However, in light of the above discussion, it should be obvious that effective espresso extraction comprises significantly more potential variations than what conventional extraction theory suggests. The conventional extraction method is arguably the fastest way to produce specialty coffee, and considering the economics of running a busy cafe, making great coffee fast does make sense. However, if you want better than “great”, and seek the best possible coffee, it would be naive to ignore what profiling can and does offer.

It has also been suggested that current coffee roasting practices limit profiling potential. Coffee is currently roasted to suit machines using conventional extraction variables, but finding new roasting variables to compliment profiling could potentially reveal otherwise unknown characteristics of coffee that ultimately produce better espresso.

The final point or suggestion is to document and share your experiences. Copy the chart format shown below. Upload your profile onto coffee forums or anywhere that others can access, and describe your results. As noted in the introduction, the purpose of this article is to contribute to the pressure profiling knowledge-base by sharing experimentation data and results, as well as attempt to provide an objective standard that will enable others to replicate and compare results. It is hoped that this article achieves these objectives so we can learn from each other, and hopefully uncover the potential that profiling offers.