Advanced Espresso

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Æ 5.06 Solubility

It’s well known that raising the brewing temperature increases extraction. There are two reasons for this: the first is that the molecules in hotter water are moving faster. This means they are more likely to collide with soluble compounds in the coffee, so the rate of extraction increases. However, the temperature also affects how much in total of each compound can be dissolved in water. This amount is called the solubility of the substance.

The effect of temperature on solubility is different for different compounds. Gases tend to be less soluble at higher temperatures, which is why boiling a kettle drives off dissolved gases. Many solids are more soluble at higher temperatures, but some are less affected, and some even become less soluble at higher temperatures.

For example, the solubility of table salt doesn’t change much with temperature. In other words, hot water will dissolve salt more quickly because the water and salt molecules collide more often, but given enough time, cold water will dissolve nearly the same amount of salt in total.

This means that changing the brewing temperature won’t just change the overall extraction, it will also change the relative proportion of different compounds that are extracted. Some flavour compounds will become more soluble at high temperatures, while others won’t change as much. The result of this is that changing the brewing temperature will change the flavour, even if the total extraction is identical.

In espresso brewing, increasing the temperature has been shown to have a bigger effect on the extraction rate of less polar aroma molecules than of the more polar or more volatile compounds (JA Sánchez López et al., 2016). This may be because higher temperatures increase the extraction of coffee oils and lipids, which can help dissolve non-polar compounds. This effect will change the balance of aroma compounds in espresso made at higher temperatures, with a higher proportion of the less volatile and less polar compounds, even if the overall extraction is kept the same.