They have a special compression chamber that keeps the coffee at extreme pressures so it doesn’t boil. At 425℃ (about 800℉), the McDonald’s employees are to set the machine at a pressure of 100MPa ±5MPa (approx. 990 times the pressure of the atmosphere at sea level, plus or minus 50 atmospheres). This is to guarantee nothing but the most warm coffee in the industry, and has test-marketed very positively amongst consumers with long commutes.Unfortunately, this machine is extremely expensive to run and when it fails, is catastrophic. Safeguards are in place to prevent this from happening, like in compressed gas cylinders. Interestingly, there’s an additional release in case of high pressure where some of the heat is redirected into the fryers! It’s pretty clever.Once one did fail because an employee disconnected some of the safety catches for maintenance, but what he didn’t know was that another employee had accidentally disconnected the safety to the fryer. This led to the tragic 2008 Harrisburg incident, when the compression valves failed and the liquid, suddenly allowed to be at atmospheric pressure, immediately vapourised and was inhaled by all those in the restaurant. They suffered what Dr Niel Chesterson, a burn specialist in Regional Medical Center at Memphis’s Firefighters Regional Burn Center, called “absolute liquefaction of the alveolar, ocular, and sinus tissues” and of the victims’ deaths, that “It appears that some more distant from the dysfunctional machine tried to scream, but this allowed more of the gas to penetrate their trachea and lungs, and probably within a second they were choking on their own melted throats”.McDonald’s settled with the families, paying compensation for their losses out of court. The site, however, also experienced fires as a further complication from the incident and was destroyed in February 2009.
tl;dr: They used to squish it down in a canister but they don’t do that anymore Also I’m a dirty liar.
NW Espresso Machines 