Lid vs. No Lid
Do you need to use a lid on the French press? Does it make a difference? To compare the effects of using a cover, we prepared two sets of brews: one with and another without a lid. This table shows the Total Dissolved Solids % of each test we did, with and without a lid.
From these tests we found that extraction yields were on average, 12% higher when we prepared a French press without a lid. Some of this difference must be attributable to evaporation, i.e. the brews with the lids removed would have allowed more water vapor to escape and so become more concentrated for that reason. Nonetheless, this difference in extraction is considerably larger than the expected water loss due to evaporation. We can only speculate that some interaction between the water vapour and the crust at the top of the coffee may cause a change in how extractable the coffee becomes when a lid is used. For this reason, we recommend you only use a lid after breaking the crust of your coffee when you brew a French press.
Plunging-Only vs. Breaking the Crust
Some cafés prefer to set up French presses and deliver them to customers, sometimes accompanied by a timer, immediately after adding the water. The customers do the plunging after a specified amount of time. This process can be particularly effective in large catering environments where many people must be served coffee simultaneously. One problem with this approach is that carrying a French press to a customer has the effect of ‘breaking the crust’, which affects the brewing process. It’s best to avoid any movement of your French press in the period between adding the water and breaking the crust.