Most neighborhood espresso bars in Italy have no seating and only a stand up bar that will accommodate three or four patrons at a time. It is not uncommon for an Italian worker to slip out of his workplace into the neighborhood espresso bar several times a day for a five minute break and a quick pick-me-up.
He walks into the bar and pays for the espresso and hands the receipt to an elderly barista, who acknowledges his order by placing a saucer and a tea spoon on the counter in front of the customer. The barista will then turn away, draw the espresso into a ceramic cup, and place the cup on the saucer. The customer will spoon two heaped spoonful of sugar into the cup, give it a stir, and finish the espresso in three quick gulps. He then heads back to work, and a new customer takes his place at the counter.
Espresso in North America
The espresso market in the US and Canada looks nothing like that in Italy.
First and foremost, the vast majority (90-95%) of espresso drinks are milk-based drinks. The espresso machine in a North American cafe uses much of its energy and water to steam milk. Brewing espresso often looks like an after-thought.
The second difference is the lumpy order flow. Most North American cafes get a big chunk of business during the morning rush. The rest of the day is much less busy.
During the morning rush, machines are often running flat out- possibly well beyond their design capabilities. Machines are then under-utilized for the rest of the day, including some stretches where they may remain completely idle.
The Single Boiler and the Italian Market
A single boiler machine will do fine for the Italian set up.
In single boiler machines, steam produced in the boiler is collected, pressurized and stored above the water level. Water normally boils at 212 deg F, but will no longer boil at that temperature when it is under pressure. It is "super heated" to temperatures above 250 deg F for it to boil under pressure and continue to produce steam.
That super heated water, though, is too hot to brew espresso with; it will simply scald the coffee. Since a single boiler machine can't use boiler water to make espresso, machine designers have used several techniques to bring cold water from the outside and heat it to brew temperature with varying degrees of success. The most common method uses what is referred to as a "heat exchange" mechanism to produce brew water. The machine brings in cooler water from the outside, heats it by passing it through a copper tube buried in the boiler water, and sends the newly-heated water to the brew head.
In heat exchange machines, brew water temperature thus obtained will depend on several factors, including the temperature of the incoming water, any preheating used, and the actual heat exchange technique. To a very large extent, temperature of the brew water is determined by how long the incoming water stayed in the copper tubing in the steam boiler and absorbed heat through heat exchange. When espresso is produced at a steady pace, cool water spends the same time in the heat exchanger between drinks and gets heated up to the same temperature. That explains why those single boiler machines function well in Italy.
Why North American Coffee Houses Should Use Double Boiler Machines
Single boiler machines, though, are found seriously wanting in the North American model of the business.
First, let us examine what happens when the machine has been idle for a time. Cool water in the copper tube has been stagnant for a while and it has absorbed enough heat to reach the temperature of the boiler water, which we already pointed out to be too hot to brew coffee with. That problem is easy to handle. The barista need only bleed an ounce or two of water from the brew head before locking the portafilter in place. That will cool the machine down.
But, look at the other end of the spectrum, when the machine is running flat out during the busy period. Cool water brought from the outside simply does not have enough time in the copper tube to absorb enough heat to get to brew temperature. In those cases the espresso is being made with water that is not hot enough. Unfortunately, there is no quick fix for this problem. One just cannot make high quality espresso with water that is just not up to temperature.
This is where "double boiler" machines become critically important. This machine design has two (or more) boilers- one for producing steam and separate ones for brewing espresso. Temperatures of all boilers are controlled, with the brew boiler being held to +/- 0.1 deg F.
Regardless of whether the machine has been idle for a while or it is being run flat out, the brew boiler is always ready to dispense water at the correct temperature.
Because consistent brew temperature is crucial for the production of high quality espresso, we always recommend that shops focused on serving high quality espresso deploy double boiler machines on their bars.