I really don’t see the point of this. I presume they wet the grounds before locking the group in?
Gases are generally more soluble in cold water than hot water. However, there are several reasons I don’t think pre-wetting with cold water will release much of the CO2 from the ground coffee (relative to hot water):
- pressure influences the solubility of carbon dioxide much more than temperature (higher pressure increases solubility).
- increasing the temperature of a gas causes it to expand. So, if there is CO2 in the pores of the ground coffee, heating it up will force much of it out of the pores (and into the water). That won’t happen unless the water is at a significantly higher temperature than the ground coffee.
- it’s possible there will be carbon dioxide adsorbed (that is, stuck to the surface of) the ground coffee. The amount of CO2 which can adsorb to a surface is affected by temperature. Higher temperatures = less capacity to adsorb the gas = CO2 released in hot water (but not cold).
That said, there could be other reasons for the influence on taste.
For starters, the solubility (and the rate of dissolution) of coffee solids is lower in cold water. So the cold water will not dissolve as much coffee as the equivalent amount of hot water. This basically means you’ve diluted the resulting shot.
Additionally, the average extraction temperature will be lower, because there will be a mixing of the cold “pre-infusion” water, with the hotter extraction water. Which means the average extraction rate will also be lower = lower yield and lower TDS.