Prewetting: When to do it, when not to

A comment in the recent hand-pour post prompted the idea of writing a short post about prewetting. I think prewetting is a bit misunderstood (aren’t we all?), and seems to have gone from something almost never done, years ago, to something always done for every type of brew, even immersion. Let’s talk about why we prewet, its effects, and when perhaps it’s pointless, or even detrimental, to prewet.  

Why prewet?

Prewetting offers a few benefits in percolation brewing: it helps decrease channeling in the coffee bed, increases extraction (usually desirable), and improves the uniformity of extraction. In espresso preparation, prewetting (usually referred to as preinfusion) offers the additional benefit of decreasing fines migration.

Prewetting and channeling

If a barista were to skip prewetting and simply pour water over a bed of grounds in a V60, the water would always wet some grounds before others. Unfortunately, once some areas are wet and others are dry, the water will favor the wet areas, and the dry areas will resist wetting. Eventually all areas of the coffee bed will be wet, but passive pouring without prewetting or stirring tends to create channels, and the grounds along the channels will not only have access to more than their fair share of the extracting liquid, but also cleaner (ie, lower-TDS) extracting liquid, enhancing the unevenness of extraction. Once such uneven wetting has occurred, it is difficult, if not impossible, to “fix” the unevenness to a satisfactory degree.

Prewetting and extraction uniformity

From one perspective, there are two types of coffee grounds: particles with no intact cells and particles with one or more intact cells. By cells, I mean the tiny, closed chambers formed by the matrix of cellulose strands that make up a coffee bean. The extractable coffee solids and oils of a coffee bean coat these strands. Particles with no intact cells (the definition I prefer for “fines”) extract almost instantaneously, as all of their strands’ surface area is exposed to the brewing liquid. The brewing liquid easily washes the exposed coffee solids off of the fines’ surfaces. Coffee solids within the intact cells extract much more slowly, as the brewing liquid must pass through the pores of the cellulose to access and dissolve the coffee solids within the chambers, and the solids must then diffuse back out through the pores in order to mix in the slurry and ultimately leave the coffee bed. The diffusion process to extract solids from the cells is slow and passive. Probably the most important benefit of prewetting is that it provides extra time for the brewing liquid to begin this diffusion process before the pouring phase removes (much) extract from the coffee bed.

Prewetting and extraction level

It’s a not quite accurate to say that prewetting increases extraction, as prewetting also increases contact time during brewing, and extra contact time increases extraction. Does prewetting, as opposed to simply the increase in contact time it causes, increase extraction? I’m not sure. I simply think of prewetting as a method that tends to increase both the quantity and quality of extraction.

Espresso preinfusion

Espresso preinfusion is similar to prewetting during a hand pour or batch brew. Ideally, espresso preinfusion occurs at a zero bars of pressure and a low flow rate (say, 2--3 ml/second), and pressure does not increase until the entire coffee bed is wet*. As in prewetting, a low-pressure preinfusion soak improves extraction uniformity by allowing water to find its way to all pockets of the coffee bed before the high-pressure phase removes solids from the bed. Espresso preinfusion also decreases fines migration, decreases channeling, and allows use of a finer grind, which increases total extraction slightly.

*One of the innovative, and frankly, amazing, features of the Decent Espresso Machine is that the machine will automatically detect the end of preinfusion and wait until that moment to ramp up pressure. Every other machine in the world has either no real preinfusion phase, a too-short preinfusion (which may be worse than none), or a time-based preinfusion, with no flow control or way of indicating when preinfusion is complete. “Smart” preinfusion is one of many features that will enable the Decent Espresso Machine to pull better espresso than any other machine in history.

Prewetting and immersion brewing

I believe prewetting an immersion brew may be pointless, or perhaps even counterproductive. Let me explain. Let’s say we are to make two french presses with a 17:1 brew ratio and 95℃ water, one with a 45-second, 2:1 prewet, one without. Both brews will have a total contact time of 4:30. During prewetting some grounds will be wetted by fresh, clean, very turbulent, low-TDS water falling directly from the kettle. Other grounds will be wetted more passively by liquid already packed with coffee solids. The grounds wetted by the clean, turbulent water will get an insurmountable head start in extraction, ensuring some unevenness. Many years ago I had the realization that if one pours all of the water over the grounds at once during an immersion brew, all of the grounds will be wetted by clean, turbulent water at a known, precise temperature. Pouring 17 parts of water per part grounds allows all of the grounds access to low-TDS water; but when prewetting at a 2:1 ratio, there’s just not enough clean water to go around. Prewetting also ensures the initial extraction will happen at a much lower temperature than would be the case with no prewet, as the large relative mass of room-temperature grounds acts as a significant heat sink. For example, if the brewing system has two prewet inputs--grounds (20g at 25℃) and water (40g at 95℃)--ts average will be approximately 72℃.

“But…”

I fully expect the usual protests, such as “But my best immersion brews involve prewetting” or “some barista won the Brewer’s Cup by prewetting an immersion brew.” I have no doubt you can make a wonderful immersion brew by incorporating a prewet phase. As with most coffee-making techniques, though, the question isn’t whether a method can produce a good cup; it’s what method produces a good cup most reliably. Two of the fundamental principles of good coffee-making technique are to achieve the most uniform extraction, and to ensure the bulk of extraction occurs within the desired temperature range. Based on those criteria, prewetting is, on balance, beneficial for percolation brewing but not for immersion brewing. If you choose to try some side-by-side immersion brews with and without prewetting, please note that the best results for each method may require different grind settings and/or different kettle-water temperatures. A fair amount of experimentation and extraction measurement may be in order. As always, feel free to protest. I don’t have all the answers, and I would love to hear others’ experiences and perspectives.