The most basic lever machines operate via a single boiler, which feeds the group head a dose of water through either steam pressure or by gravity. Steam-pressure lever machines need a higher boiler temperature than espresso would prefer, to generate the force needed to route water up to the group head. As such, temperature regulation is nigh on impossible, and on most machines of this kind the brewing temperature increases with the more shots you pull.
Gravity-fed lever systems see the boiler located above where the coffee extracts from, allowing water to simply flow down into the group head. No steam pressure is required, so the boiler can be set to a lower temperature. These machines produce a far more consistent temperature for brewing with, but they’re no good for heating milk: the lower boiler pressure is insufficiently equipped to deal with the temperature and pressure requirements of a steam wand.
THE SINGLE BOILER ELECTRIC PUMP
The most simple electrically pumped machines can make espresso and steam milk, but not as well as you might like. It does it by using two thermostats (temperature controllers): one for the steam and one for the brew water. A push button is used to control which thermostat takes priority, engaging the heating element when steam is required and switching it off (to cool down the water) when you want to make espresso. It’ll work ok if you’re making only one or two drinks, but beyond that it becomes very frustrating waiting for the boiler to adjust itself.
Modern espresso machines usually have two pressure gauges; one for the steam boiler and one for the group-head pump.
If only milk and espresso didn’t taste so delicious together, then the espresso machine would be a much simpler piece of equipment. The problem with both elements is that they have unique requirements. Espresso needs 90–95°C/194°–203°F water delivered at a pressure of 9 bar/130 lbs per square inch (psi). Milk on the other hand, needs a jet of steam delivered at a minimum of 100°C/212°F and at a pressure of between 0.8–1.2 bar/12–17 psi. There’s little room for negotiation from either side. Furthermore, these conditions might need to be met with consistent precision, perhaps across multiple group heads, over the course of a few hours, preferably without breaking down.
HEAT ExCHANGERS vS DOUBLE BOILERS
Heat exchangers provide one solution, consisting of a steam boiler operating at high temperature with a layer of steam floating on top that can be directed to the steam wand for milk foaming. Water for brewing is fed through a thin copper pipe, connected to a cold-water feed that passes through the interior of the steam boiler, heating it as it travels through. The exact length and diameter of the pipe are carefully calculated so that the water reaches the correct temperature when it arrives the group head. In addition to this, the temperature of the group head is stabilized by way of a thermosiphon that continuously runs water from the boiler to the group head then back to the boiler. In this way the large brass and steel mass of the group head maintains temperature consistent with that of the boiler.
One thing a good barista must be mindful of on a heat exchange machine is the tendency of the group head to overheat slightly if left to sit for too long. This can be overcome by a quick flush of (slightly cooler) brew water through the group head, which will cool it down the components to the correct temperature for brewing. Exactly how much water needs flushing through hinges on multiple factors, not least of all the model of machine itself and how long it has been idling for, but 100–150 g/3½–5¼ oz is about right.
The heat exchanger design is epitomized by the E61 group head (see page 89). Now in its sixth decade, it is time-tested and, with a little practice, highly effective in its operation. The E61 has a small lever that is used to commence and end extraction, as well as control pre-infusion.
A double boiler overcomes the conflicting dual-purpose nature of the espresso machine in perhaps the most obvious way – one boiler for brewing coffee and another for steaming milk. The first commercially machine of this kind was the La Marzocco GS. The GS was released in the 1970s, but it’s fair to say that double boiler machines didn’t really catch on for another 20 years. Most modern commercial espresso machines are now double-boiler models, though, boasting superior levels of temperature control and improved consistency over heat exchanger models.
FINE-TUNING
The obvious next step on from this is a machine that has an individual boiler tank for each group head, allowing for the further refinement and tweaking options. Temperature control on modern machines is done using a PID (proportional-integral-derivative) controller. PID is a generic term for an electronic controller that maintains a set value using control feedback loops. Many common devices use PIDs – cruise control in a car might use one to maintain a certain speed based on calculating incremental changes of the angle of the road over time. In espresso machines, the PID maintains constant water temperature, controllable to the degree, or 1/10 degree in some cases.
Almost all commercial machines have the option of volumetric water control nowadays, allowing buttons to be programmed to dispense a specific volume of water every time they are pushed, but expect to see these replaced with weight sensitive models before too long. Most of the flagship espresso machines now come equipped with pre-infusion timers, but in an ironic turn of events, it’s the new wave of paddle-controlled machines that are getting professional baristas most excited; these fully electric machines essentially aim to mimic the pressure profiling options that the modest lever machine offers.
BUYING YOUR OWN ESPRESSO MACHINE
The truth is that making true espresso at home is wasteful, messy, time-consuming and costly. It’s far better to go to a café and get someone there to make espresso for you. If you don’t have a café nearby, there are two further options. The first is to accept that espresso is too much trouble, buy a good grinder and try one or more of the traditional brewing options detailed between pages 114–145.
The second option is to join the dark side and buy a Nespresso machine. These machines are basically pressurized coffee makers that produce espresso-style coffee (using the term loosely) using pods containing ground coffee. Buying a Nespresso machine means leaving nearly all of your decision-making abilities at the door. The choice of ‘espresso blends’ are pre-ground, packaged and assigned a suitably nauseating name, designed to evoke an emotional connection. Nestlé grind their coffee in state-of-the art water-cooled roller-mill grinders that operate in a virtually oxygen-free environment, but it can’t beat freshly ground stuff. The good news, however, is that you can now buy empty pods and once you start filling them with your own coffee the dynamic changes entirely, and it might just be you who has the last laugh.
MAKING ESPRESSO
Espresso is a method of percolation through a fine metal filter. What differentiates it from normal gravity percolation is pressure. The espresso machine’s ability to force water through a coffee bed at high pressure means that the coffee itself can be ground very fine indeed, yet still allow proper percolation and balanced extraction. So what? Well, this is of great benefit because very fine coffee grounds means a very quick extraction, since the surface area increases at an exponential rate as the coffee grounds get smaller. It gets better, though. High pressure also forces water into the interior cell walls of the coffee grounds, extracting stuff that gravity alone would never manage, such as emulsifying oils and dissolving sugars. The product is a complex and concentrated representation of coffee, with a body and texture that cannot be found in any other type of brewing.
The trick to espresso, as with every other brew method, is good extraction (see pages 69–71). Sounds simple, but the problem we face with espresso is the intensity of the process. Venturing to get the very best out of a coffee in an exceedingly short space of time presents similar issues to that of washing yourself with a firehose in 30 seconds.
Good extraction in espresso means a balanced array of aromatic and taste compounds. I’m talking structured bitterness, crisp acidity and lasting sweetness – the same stuff that we aim for with any type of coffee
-brewing method. Although all coffees will give up their goods slightly differently, and you can’t beat tasting to check quality, eyeballing the extraction rate can, as a bare minimum, tell you when a shot will definitely taste bad. A double espresso usually weighs around 30–45 g/1-1½ oz and should take 25–35 seconds to manifest in its entirety, based on the needs of the coffee. Where one coffee (and I am about to generalize) – say, a light-roasted Kenyan – may prefer a long, dripping, 30-plus second extraction to overcome acidity, another coffee – like a dark Brazilian – might be better suited to a punchier approach to rein in bitterness.
As a general rule of thumb, it’s the acids that extract first. This is why underextracted (faster-flowing) espresso has a sour taste to it. Next up is sweetness, and this is a harder one to gauge, because roasted coffee contains very little in the way of actual sugar. Sweetness is as much a perception as anything, but it’s typified by good body, associated aromatics and chewy texture. Finally, it’s the bitter flavours that come last – slower dissolving, long compounds that give the cup balance but can easily overpower it in over-extracted shots.
So what of aroma? Aromatic qualities are a large part of what differentiates one espresso for another, but pulling shots with an aim of a specific aromatic profile is nigh on impossible. The aromatics in the cup are not a reflection of the barista’s skill, but that of the roaster and coffee grower. What can be altered is the way in which the aromatics are perceived through the tweaking of the taste profile. A barista can dial up the acidity in a shot, for example, which might highlight orange or grapefruit aromatics. The same shot pulled a little tighter may taste more bittersweet, giving the effect of chocolate orange or grapefruit marmalade.
This chapter covers an in-depth look in to the art of brewing espresso. Before jumping in, though, be sure that you have read the sections on water (see pages 68–69), extraction and brew ratio (pages 69–71) and grinding (see pages 76–83), for a basic understanding of coffee brewing.
WATER TEMPERATURE
As with any brew method, water temperature plays a part. If you’re just starting out, espresso brew temperature shouldn’t be too much of a concern as there are plenty of other factors to contend with already. But brew ratio, coffee roast level, coffee variety and processing all can have an effect on the optimal brew temperature. Not all espresso machines have the function to adjust group-head brewing temperature anyway, but the general rule of thumb is that darker roasts require a lower brew temperature (90–92°C/194–198°F) and lighter roasts a higher brew temperature (93–95°C/199–203°F).
DOSING AND DISTRIBUTING
A dose of ground coffee (ranging anywhere from 14–22 g/½-¾ oz) is placed in the clean filter basket of the group handle. The exact amount you use will depend on the size of the basket itself, as they vary in capacity, but also the desired size and strength of the drink you are making. You’ll know if you’ve overfilled when the portafilter refuses to lock into the group head. Don’t underfill, either, as this leaves too much headspace over the coffee, which can cause the cake to get all mushy, leading to greater inconsistency in flow rate. Be sure to weigh how much coffee you are using, at least to begin with, as this will be used to calculate the brew ratio while the shot is pouring. Note that the grounds may require some degree of redistribution by and at this stage, but it will vary on how the model of grinder drops the grounds in the first place.
TAMPING
Next, the coffee is levelled using a flat tamper. Very fine coffee straight from the grinder is prone to being a bit ‘fluffy’ so we tamp it to remove air pockets and to pack the grounds into an even cake so that it extracts evenly when the water flows through. If the bed of coffee isn’t level, the water will follow the path of least resistance, culminating in both over- and underextraction on opposite sides of the bed during the brewing phase. Compacting with sufficient force to remove air pockets is the second goal. The degree to which you tamp has no effect on the rate of extraction, as an espresso machine will soon be pushing on that same bed of coffee with a pressure of 9 bar/130 psi, which is far more forceful that a human arm.
Tamping is all about tightly packing coffee grounds together for a nice, even extraction.
An espresso dose can be measured easily, by first weighing the empty portafilter and setting the scales to zero.
The tamping stage of espresso is something that has been overemphasized in more recent years, resulting in a kind of gratuitous reverence that it simply doesn’t warrant. I suppose the mystique surrounding tamping spawns from the ineffectual feeling that you get when you brew espresso, where much of the process is concealed from view, meaning that those elements that are in plain sight become overemphasized and contrived.
PRE-INfUSION
It’s typical at this stage to run a little water through the group head before inserting the portafilter. This ensures that fresh brewing water is pumped through from the boiler and makes for better temperature consistency.
The group handle is then inserted into the group head, which locks into place and clamps a rubber gasket over the edge of the filter basket. The seal is relied upon to maintain the necessary pressure for extraction. From here on, in it is important that pre-infusion and extraction follow quickly, since the coffee is now in close proximity to the shower screen and hot components in the group head. Press the button, pull the lever or flip the paddle!
The first contact between water and coffee is the ‘pre-infusion’, where a small amount of water is dispensed on the grounds to wet them thoroughly. Some machines pre-infuse automatically using flow restrictors or, on newer machines, infusion chambers; other machines feature manual controls. Pre-infusion normally takes 3–10 seconds and it’s an important stage, like a warm-up before a quick sprint, which initiates the extraction process, and is thought to even out any inconsistencies there might be in the density of packing of the coffee in the filter basket. The coffee bed expands as it absorbs this first influx of water, which forces any air space that may have been held out of the system. Making adjustments to the length and pressure of the pre-infusion is a difficult skill to master, but offers superior control of extraction from an espresso blend.
The basic rule of thumb is this: the longer the pre-infusion, the faster the subsequent extraction. For example, a 18 g/⅔ oz dose of finely ground coffee with a long pre-infusion may extract the same weight of liquid in the same time as a 18 g/⅔ oz dose of coarser ground coffee that has no pre-infusion. This can be taken to the extreme by grinding very finely indeed and giving a long (10-second-plus) pre-infusion. Exactly why the long pre-infusion makes such a huge difference is not totally understood; some people suggest that the fine grounds become trapped in the puck (the puck-shaped mound of espresso grounds left after a shot is pulled), but my best guess would be that the fines become free-flowing, actively mobile, particles in the wet puck basket, free to move around in a capillary-like fashion during the initial period of low pressure. When the proper extraction begins, the fines are in a state of readiness and make for a comparatively quicker extraction since the path has been cleared. For more on fine grinds, see pages 80–81.
Taking it to the absolute extreme, even the pressure of the pre-infusion is thought to affect how the puck blooms and the extraction that follows. It’s thought that a sudden high-pressure (9-bar/130 psi) pre-infusion has the affect of thumping fines down to the bottom of the puck into a kind of sandbag that restricts flow for the remainder of the extraction. When a low pressure pre-infusion takes place (1–2 bar/14½–29 psi), the bloom is more gentle and the movement of fines less forceful.
EXTRACTION
The next part is the extraction proper, where the system pressurizes and brewed coffee flows from the spout of the group handle. In these first magical moments, you bear witness to the genesis of a unique scarlet-coloured coffee concentrate being brought into the world, never to be faithfully replicated ever again.
The window of tastiness generally lies between a 25- and 35-second extract,
depending on the coffee that you’re brewing. In this time we would expect to produce an espresso shot of at least the same weight as the dose of ground coffee that was used (brew ratio of 1:1) and up to three times the weight of the ground coffee used (3:1). For me, I look for a brew ratio (see pages 69–71) of around 1.5:1, or to put it in usable figures: 27 g/1 oz of espresso from an 18 g/ ⅔ oz dose. Darker roasts tend to use a lower brew ratio than light and tend to carry less body than light ones – a lower brew-ratio helps preserve their modesty.
Always keep in mind that brew ratio is only a function of beverage strength, but that the weight of coffee may slow down or speed up the extraction through resistance. Keeping in line with our understanding of balanced extraction, espresso should, in all scenarios, aim to move 18–22 per cent of the mass of the ground coffee into the cup. If the espresso gushes out quickly, it will taste sour and underextracted. If it drips out too slowly, it may taste bitter and astringent.
A good barista can glean some information from the rate of flow and the shape of flow that emanates from the spout of the machine, telling him or her how fresh the coffee is, as well as how far along and how efficient the extraction is. The pale liquid that appears around halfway through the extraction, known as ‘blonding’, is deemed by many as the stage in which little positive flavour is left to extract. But some good (and important) stuff is extracted towards the end of the run, and we shouldn’t get overly obsessed with the gooey red blobs that feature in the early stages (no matter how beguiling they might be).
When the flow of liquid twists, or seemingly attempts to hang on to the portafilter spout, it’s a sign that the surface tension of the liquid is increasing. This only happens when the extract has a lower level of solubles, typically towards the end of the run. If you see an uneven stream from the start, there’s a good chance that your coffee will be under-extracted. However, visual clues are at best only a mediocre indicator of flavour. The only way to know for sure is to taste your creations.
The Curious Barista's Guide to Coffee Page 13