Why Does Espresso Spray Out? How to Make Standard Espresso at Home?

I remember when FrontStreet Coffee first used a naked portafilter, imitating the instructor's operations before pressing the extraction button. Seeing the first drop of espresso slowly fall into the cup, I happily thought this shot was successful. But unexpectedly, the vertical liquid stream suddenly shot out a pale, thin stream of water, and the coffee flow rate became erratic... At that moment, I heard the instructor standing beside say: It's spraying again, isn't it~

(Exaggerated for dramatic effect~)

Why does espresso suddenly spray?

First, we need to know that the extraction of a standard espresso离不开三要素：细研磨、高压强、短时间，which means using high pressure to force hot water through finely ground and firmly tamped coffee grounds within tens of seconds.

Just as people tend to avoid difficulties when faced with challenges, under high pressure, water flow will also prioritize the path of least resistance. Once there's an uneven density distribution in the coffee puck, the pressurized hot water will rush directly to the weakest spot, then quickly spray out from one corner of the bottom of the portafilter basket.

(The escaping coffee liquid~)

If you're using a naked portafilter at this time, you'll notice that this part of the coffee quickly becomes pale and white, and as extraction continues, the liquid release becomes more intense, even splashing everywhere. This is what baristas commonly refer to as "channeling."

In addition to spraying, channeling also brings uneven extraction of coffee. When we remove the portafilter, we sometimes see water accumulation in the basket or small holes in the coffee grounds. When tasting the espresso, it typically exhibits weak aroma, thin body, off-flavors, and uncomfortable tastes such as sharp acidity, watery texture, bitter notes, and astringency.

In fact, channeling can be ubiquitous. Even experienced baristas who skillfully master various brewing equipment and techniques still encounter "channeling issues" when making espresso, though the intensity of "spraying" might not be as exaggerated, and they can quickly adjust based on their bar experience. However, for home baristas without much operational experience, it's necessary to first identify the problem before they can "prescribe the right remedy."

What situations cause channeling?

1. Not paying attention to details during distribution

Whether in video tutorials or educational articles, we're always told about the importance of distribution for espresso extraction. The purpose of distribution is simple: to evenly distribute coffee grounds in the portafilter basket, allowing hot water to penetrate each particle fairly, thus keeping their extraction state as consistent as possible. But often, the simpler the details, the more easily they're overlooked.

For example, some friends are always too impatient during operation, starting to tamp before the coffee layer is evenly distributed. At this point, gaps or inconspicuous small pits will form at the edges of the coffee puck. When attached to the group head for extraction, these seemingly insignificant depressions will give hot water an opportunity, causing perforation and spraying.

2. Bumping the portafilter after distribution

It's important to know that not only does perforation in the coffee puck count as channeling, but gaps between the coffee puck and the portafilter basket also count as channeling. Bumping the portafilter after distribution is often the main culprit causing "holes."

According to FrontStreet Coffee's observations, these friends usually have larger movements, not finding the right angle when locking in the portafilter and just pushing it up against the group head. At this point, the originally firmly tamped puck has likely been shattered, greatly increasing the possibility of "channeling." If your distribution technique is fine but you frequently experience spraying when making espresso, you might want to check if you have this bad habit and avoid it promptly.

3. Insufficient coffee amount or grind too fine

Theoretically, with even distribution, the greater the resistance water encounters, the slower the flow rate, and typically the richer the extracted flavors. However, when coffee grounds are ground too fine, the density of the tamped coffee layer becomes very high, creating enormous resistance to water. When it comes to the actual extraction phase, the high-pressure hot water urgently needs an outlet, so it finds a relatively weak spot in the coffee puck to drill through, or escapes along the edge of the portafilter basket.

Additionally, insufficient coffee amount can also lead to coffee liquid spraying issues. When there's too little coffee, a certain distance forms between the coffee puck and the shower screen. During extraction, the surface coffee grounds will experience剧烈翻滚，at which point channeling can occur at any time. After removing the portafilter, we'll find that the surface coffee grounds appear muddy, accompanied by perforation.

If we're unsure whether we've found the right grind setting, we can also refer to the time in the espresso extraction formula: 1:2 coffee-to-liquid ratio, time 25-30 seconds, with coffee amount depending on the basket capacity. Like the portafilter basket FrontStreet Coffee uses has a reasonable capacity of 20g, so we'd extract 40g of coffee liquid from 20g of coffee grounds, controlling the time between 25-30 seconds. If the coffee liquid takes too long to drip out (over 7 seconds), and the final extraction time exceeds 35 seconds, with the latter part of the espresso obviously turning pale, it indicates that the flow rate is too slow due to too fine a grind, and we need to grind the coffee coarser.

4. Coffee ground clumping

During the process of grinding espresso coffee, intense friction generates static electricity (the grinding quality of the grinder also affects this), resulting in lighter coffee particles clumping together and forming clumps. This phenomenon is most likely to occur, especially on humid rainy days, when grinding dark roast coffee beans rich in oils.

Although we can't see any issues after firm tamping, these coffee clumps usually have a more compact structure. The water that originally needed to pass through here must find another path, choosing places with less resistance to flow away, causing "perforation" again. The solution is simple: use a needle distributor or similar tools (like toothpicks) to stir the coffee grounds in the portafilter basket, helping to break up clumped coffee particles and make the density distribution of the coffee layer more uniform.

Can a spouted portafilter solve the "channeling" problem?

Reading this, some friends might think, if using a naked portafilter easily causes spraying when making espresso, then switching to a spouted portafilter would solve it, right?

Obviously not. Although the mechanism of a spouted portafilter can indeed make coffee liquid release along two fixed channels as expected, if the coffee puck is uneven (or any of the above situations occur), hot water will still find gaps to drill through. On the surface, the espresso flow rate might not seem to have much problem, and it won't spray everywhere like a naked portafilter, but internally, some parts are already over-extracted while others are under-extracted. When removing the portafilter, you can also see conclusive evidence of "channeling."

Therefore, the practice of "switching to a spouted portafilter to avoid channeling" is actually more like self-deception~

- END -

FrontStreet Coffee

No. 10, Bao'an Qianjie, Yandun Road, Dongshankou, Yuexiu District, Guangzhou City, Guangdong Province

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