The Science of Perfect Espresso: The Quest for the Ultimate Expression of Coffee Art

The perfect espresso is not just a drink, it is a combination of precise science and art. An ideal cup of espresso should have rich flavor, smooth taste and beautiful crema (the foam on top of the coffee). When you place a small piece of sugar on top of a perfect shot of espresso, it will briefly float to the surface for a few seconds before slowly sinking — this is considered by many professional baristas to be the classic test for judging the quality of espresso. Let’s take a deeper look at the science behind making the perfect espresso.

Parameters for a perfect espresso

Professional baristas generally agree that the ideal espresso is not a single shot but a double shot, about 45 ml (1.5 oz). Here are the key parameters for making the perfect cup of espresso:

• Coffee powder amount : Use 14-16 grams of ground coffee beans
• Extraction time : 28-30 seconds
• Extraction process : There should be a 3-5 second delay after pressing the start button, and then it will develop from a slow drip to an even stream
• When to stop extraction : Stop immediately when the coffee flow changes from brown to golden yellow
• Machine pressure : A standard espresso machine should maintain a pressure of 9-10 bars

The key role of grind size

Coffee grind size is considered the most critical factor in making the perfect espresso. Grind size directly affects three important factors: contact time, extraction rate, and flow rate.

The soluble components in coffee beans have different solubilities, and their extraction order is as follows:

1. First extraction : Fruit acids (bright, fruity flavors)
2. Mid-term extraction : light aromas produced by the Menard reaction and sugar browning during roasting (nuts, caramel, vanilla, chocolate, etc.)
3. Final extraction : Heavier organic matter (wood, ash, malt, tobacco, etc.)

When you grind your coffee, you’re actually drastically increasing the surface area of ​​the coffee that’s in contact with the water. The grind size does not change the components that are extracted, but it changes the order in which they are extracted. Therefore, you need to adjust the grinding size at any time according to the extraction time:

• If the coffee flows too fast (less than 25 seconds), make the grind finer.
• If the coffee flows too slowly (extraction time exceeds 30 seconds), make the grind coarser

It is important to adjust only the grind size and not the force with which you tamp the powder to control the flow rate. Due to the hygroscopicity of coffee, changes in humidity throughout the day can significantly affect the extraction time, so baristas need to adjust the grinding settings in time according to changes in extraction time.

The Science of Ideal Extraction Yield

"Soluble yield" or "extraction" is defined as the percentage by weight of coffee grounds that can be dissolved in water. In simple terms, it’s how much coffee ends up in your cup.

The ideal extraction rate is widely accepted to be 20% ± 2% (18%-22%). This value was originally calculated by the Coffee Brewing Corporation (CBC) led by Professor EE Lockhart of MIT in the 1950s and confirmed by subsequent testing by the Specialty Coffee Association of America (SCAA).

• Extraction rates below 18% are considered "under-extracted", specifically "under-developed", as the desirable components are not fully extracted. This type of coffee often feels sour because the acidic components are extracted early, while the balanced sugar (sweetness) and bitter components are extracted later.

• Extraction rates above 22% are considered "over-extracted", especially when bitterness is evident, as the bitter components continue to be extracted after the acidity and sugars have basically been extracted.

It’s important to note that caffeine is extracted earlier in the brewing process, so a higher extraction rate does not produce a more caffeinated coffee, just an over-extracted coffee. Once all the ideal components are extracted, the coffee stream will turn from brown to golden yellow, which is why professional baristas stop extracting when they see the color change.

Importance of Coffee Grinders

Any professional barista will tell you that the grinder is the most important piece of equipment in the chain of espresso making. When choosing a grinder, give priority to models with larger grinding discs, heavier overall weight, and slow grinding speeds.

There are two main types of burr grinders (not blade grinders): flat burr grinders and conical burr grinders.

Advantages of conical blade grinder :

• Increases the surface area of ​​each coffee particle, increasing the amount of flavor that can be extracted
• Because the blade is longer, it can rotate at a lower speed, generating less heat
• Longer life: Conical burrs process approximately 2,000 pounds of coffee before needing replacement, while flat burrs process 600 pounds before needing replacement

Espresso requires very fine and consistent coffee particles because it is extracted at high pressure (9 bars, about 135 psi) in a short time. Inconsistent grinding will not only result in uneven extraction, but it can also cause severe channeling. Channeling can cause some parts of the coffee cake to be over-extracted and other parts to be under-extracted, resulting in a coffee that tastes both bitter and sour at the same time.

The Science of Crema

Coffee oil is one of the unique characteristics of espresso and an important indicator for identifying the quality of espresso. It is formed by high pressure that forces the oil in the coffee beans to dissolve and form a colloid in water, in which small oil droplets are suspended in the water.

An ideal coffee crema should have the following characteristics:

• Mainly dark in color but with some white streaks, giving it a tiger-striped appearance and with small bubbles
• A few millimeters thick, completely covering the surface of the espresso
• Double espresso should be golden brown to dark brown in color
• The color may be slightly lighter where the coffee is poured
• The coffee oil should be kept for at least 3-5 minutes or even longer

Coffee crema is formed from the carbon dioxide released when espresso is extracted under high pressure. A rich, persistent crema is usually a sign that the coffee is fresh and properly roasted and extracted.

Practical tips

1. Consistency is key : When tamping powder, maintaining a consistency of about 30 pounds of pressure is more important than how much pressure you actually use.

2. Watch for changes : Monitor your grind size 24/7 and adjust as your brewing time changes. Changes in humidity can affect the hygroscopicity of the coffee, causing changes in extraction time.

3. Watch the color change : Learn to recognize the exact moment when the coffee stream changes from brown to golden yellow, which is the best time to stop extraction.

4. Invest in a grinder first : If you have a limited budget, prioritize investing in a high-quality grinder instead of an expensive coffee machine.

5. Sugar float test : A simple test is to place a small piece of sugar on top of freshly brewed espresso. If the sugar floats to the surface for a few seconds, this is usually a sign of good crema quality.

Making the perfect espresso is a perfect marriage of science and art. By understanding and mastering these scientific principles, you can continually improve your coffee-making skills and create amazing espresso experiences.