Is Pre-infusion Necessary for Coffee? What is Espresso Pre-infusion Extraction? How to Perform Pre-infusion?

Typically, when extracting espresso, we use the coffee machine's default pressure of 9bar for a continuous, uninterrupted extraction process. However, as people's pursuit and research of coffee have evolved, a completely new espresso extraction method has emerged in recent years—that is, "pre-infusion."

Understanding Pre-infusion

Pre-infusion involves using a small amount of hot water to moisten the coffee grounds in advance or controlling the pressure at its minimum to allow the coffee grounds in the portafilter to have a soaking time. After the soaking is complete, the coffee machine then begins to apply more pressure and water volume to dissolve the flavor compounds from the coffee. The purpose of pre-infusion is quite simple: to moisten the coffee beans and allow them to degas. Therefore, pre-infusion is often compared to the bloom phase in pour-over coffee. But many friends wonder: if their regular espresso extraction already tastes great, what's the significance of using pre-infusion?

Why Pre-infusion Matters

Under the high extraction pressure of 9bar, espresso extraction can indeed overlook the impact of large amounts of carbon dioxide and extract flavor compounds. However, everything exists for a reason, and the existence of pre-infusion precisely indicates a demand. In certain specific situations, using pre-infusion can indeed help improve coffee quality. But to know when to use pre-infusion, we must first understand it thoroughly.

The Purpose of Pre-infusion

The purpose of pre-infusion is twofold: one is to increase extraction rate, and the other is to reduce the occurrence of uneven extraction. As we all know, the most common negative phenomenon during espresso extraction is channeling. Because dry coffee grounds are hydrophobic, this property allows inert water to flow more easily to places with less resistance.

If the coffee puck can be evenly tamped, then there's not much impact. But if the puck's tamping is not uniform enough, it will lead to some lower-density channels appearing in what should be an airtight puck. Then, under the influence of pressure and water's inertial properties, hot water will concentrate its flow to these areas, thereby forming channeling. The coffee grounds within the channel range are over-extracted, while the coffee grounds outside the channel are unevenly extracted. (The holes appearing in the coffee puck in the image below are all channels)

Through pre-infusion, the coffee puck can be fully moistened and expand before large amounts of water arrive, reducing hydrophobicity and thereby minimizing channeling. Simultaneously, because the coffee grounds are soaked, carbon dioxide is released in advance, allowing the subsequent hot water to extract more efficiently with a faster flow rate (the presence of carbon dioxide slows down flow rate). This means we can also adjust the grind to be finer, further improving extraction efficiency.

When to Use Pre-infusion

Actually, from FrontStreet Coffee's explanation above, many friends have already guessed when to use pre-infusion. That's right! Light roast and freshly roasted coffee beans can both be candidates for pre-infusion. Light roast coffee beans, due to their higher density, are difficult to extract. Therefore, we need to use finer grinds to increase the surface area of coffee grounds in contact with water, thereby improving extraction rate. However, this often leads to slow flow rates due to overly fine grinding, making the process more difficult to control.

At this point, we can use pre-infusion to increase flow rate while making extraction more efficient. The same principle applies to freshly roasted coffee beans, although FrontStreet Coffee would recommend letting them rest for a period before use. Since each coffee bean and each machine has different characteristics, there is no standard for pre-infusion time and pressure adjustment—everyone needs to adjust according to their specific machine and beans.

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