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Æ 6.06 Pressure, Flow, and Solubility

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Now that we’ve established how to measure and adjust the pressure, let’s look at how changing the pressure affects flow and extraction.

Since Flow = Pressure Drop / Resistance, as we learned in Lesson 6.02, increasing the pressure in the pump can be expected to increase flow through the puck. However, as pump pressure increases past a certain point, typically somewhere between 5 and 7 bars in the puck, the flow plateaus and then begins to decrease again. This happens because the pressure causes the bed to become more tightly packed, which increases the resistance in the puck (B R Corrochano et al, 2015). The exact pressure reading at which this reduction in flow happens will depend on the grinder, machine, and technique used.

As the flow increases, extraction also increases, so the highest extraction can be obtained at the pressure that gives the highest flow rate (S Andueza et al, 2002). The higher flow rate also allows baristas to grind finer to achieve a given shot time, allowing them to achieve higher extractions still. We’ll explain how to make the most of this phenomenon in Lesson 6.06.

Changing the pump pressure also has an impact on flavour in the espresso. For example, S. Andueza et al. (2002) found that a brewing pressure of 11 bars led to bitter and astringent espressos. However, it’s unlikely that this effect  is due to pressure affecting solubility of coffee compounds in water. The solubility of solids and liquids is generally not affected by pressure. Gases do become more soluble in liquids at higher pressures, but these would contribute to crema or aroma, rather than bitterness and astringency.

The pressure in espresso brewing does, however, help to emulsify the oils in coffee (M  Petracco, 2005).

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