Want to make an espresso that is as good as Taste itself? Perhaps your physics degree has given you the ideal skill set to make a pretty amazing brew.
WARNING: The following topic may be addictive.
And I do not mean just the caffeine. Once you have fallen down the artisan coffee rabbit hole, there is no going back. As the world’s most popular drink, with the UK alone consuming roughly 95 million cups a day, it is no wonder there are so many styles and varieties out there to discover. As a brief aside, instant coffee will not, I repeat NOT, be discussed in this article. You, dear physicist, are better than that. As much as I, and my all- Italian flatmates, love a frothy cappuccino in the morning, today I bring you the latest scientific lowdown on the king of coffee, the unadulterated perfection that is Espresso. No fancy glasses, frothy milk or watering down to suit the American(o)s. Before I get to the interesting physicsy stuff, it is important to understand how an espresso is made, and how it is different to other coffee. 
From Bean to Cup
Let us start with a bit of myth debunking. It is a common misconception that espresso contains more caffeine than your regular brew. The average cup of coffee contains up to 140 mg, whilst the espresso contains less than half of that. The intense taste difference associated with the espresso actually comes from the way in which it is prepared, coupled with the lower overall volume. I’ll break it down. 
The popular filter coffee method relies upon gravity to extract the delicious flavours from the coffee grinds, as hot water is poured over them in a filter. This water emersion takes roughly 5-10 minutes, and although I have already blacklisted instant coffee, the first espresso was invented with the exact same motivation as its instant descendant – to make coffee much faster! That’s where the comparison ends, however. Hailing from the wonderful Italia, the ‘Caffe Espresso’, literally meaning fast coffee, is made by forcing high pressure water through a bed of ground coffee beans for 25 seconds, producing what is known as a shot. Sitting at roughly 9 bars, this unique high pressure is what creates the signature foam, or ‘crema’, that sits on top of the coffee. The most important part of the process, however, relies upon the coffee itself. An espresso must be made using the finest grind of coffee beans, their pure essence. Now bear with me – the world of quality espresso is complicated and not for the fainthearted. There are a huge number of variables you need to keep in mind whilst preparing your perfect cup. These include grain size and distribution, water temperature and pressure, brew duration and roast type, and let us not forget, personal preference! Coffee connoisseurs have argued over the best possible combination of these variables for decades, but the greatest dilemma facing amateur coffee makers and professional baristas alike is reproducibility. 
Physics to the Rescue!
Picture this. You are at the World Barista Championships (yes, these do exist). You have just created something resembling liquid perfection. You win, becoming no less than coffee royalty. How can you be certain you could create EXACTLY that same espresso again? Did you think about the atmospheric conditions? Did you consider the water chemistry? 
It was these questions that intrigued PhD student, Christopher Hendon, whilst studying materials chemistry at the University of Bath. Joining up with a local independent coffee shop, he investigated the numerous factors influencing the variation in espresso. Now at the University of Oregon with doctoral title in hand, Hendon’s research has certainly assisted him in his quest for a consistent cup. In fact, the results of his many studies may also have had significant economic benefits for the entire coffee shop scene!
Outlined in a report entitled ‘Systematically Improving Espresso’, Hendon’s argument challenges one of the fundamental laws of espresso – the finer coffee grind rule. Shock! The rule itself originates from the assumption that the high pressure water flows homogeneously through the coffee bed during extraction. It follows that the amount of yummy goodness we can steal from the beans, known as the extraction yield, should decrease as the grind size gets larger, simply due to the decrease in surface area. 
Here comes the surprise twist. They had not consulted the physicists. The extent of extraction can be measured based upon the refractive index of the resulting espresso. By using this technique, Hendon’s experimental results showed that the relationship between grind setting and extraction yield was not linear. In fact, there was a peak somewhere in- between course and fine grinds – suggesting an inhomogeneous flow at both extremes. Fundamentally, too fine a grind results in no space for the water to flow, thus actually reducing the exposed surface area.
Not only does this prove that reproducing the perfect espresso using a fine grind is impossible, it implies that we are wasting some of the coffee in the process! Moreover, Hendon estimates that implementing their mathematical model to identify the optimal gauge of grind could reduce the mass of coffee required per cup by up to 25%! Wow!
Give us the Recipe!
Ok, ok. I know that as much as you are interested in going through the many, many lines of differential equations with me, what you really want to know is the end product. What are the perfect conditions for the perfect espresso?
Hendon says that for a highly reproducible espresso, the barista must reduce the mass of coffee for a double shot to 15 g and use a coarser grind. This means that you will also get a much faster shot, cutting down the extraction time to 15 seconds. I implore you to give it a go if you have an espresso machine at home, and please do share the results with me if you catch me in the physics café!
Flawless Formula or Massive Miscalculation?
With sustainability at the forefront of everyone’s minds, the idea that we are cutting down on coffee waste whilst simultaneously making consistently good espresso is awesome! Hendon’s ‘waste reduction protocol’ has even been implemented in a speciality coffee café in Oregon, where they have been able to save roughly $0.13 per drink. The reduced shot time has also allowed café goers to get their caffeine fix far quicker. Overall, the cafe could see an increase in profits amounting to $3,620 per year – huge!
It might be time to wake up and smell the coffee, because the whole procedure may kinda overlook the key variable – taste. Uh-oh. As it turns out, the inhomogeneous flow in the original espresso method may actually be WHY the drink tastes so good. Blockages in the coffee bed create a goldilocks style mixture of over and under extracted coffee which is just right. In the game of imaginative names by scientists, Hendon refers to this perfect mixture as the ‘tasty point’.
All is not lost! With the aid of a beautifully crafted graph, he argues that both this ‘tasty point’ and the economic success of his model may be reached through a method of blending shots. Admittedly, this procedure may only be for the most pedantic baristas among us, as it is nit-pickingly technical. It could, however, easily be executed in large firms, such as the mighty Starbucks (although in my opinion they have a few basic issues to get right first …).
So, do physicists really make the best Baristas? Since its creation in the early 1900s, the modern espresso has been so globally loved, yet also so hugely disputed and diversified. Whilst Hendon’s vision to economically improve the coffee making industry shows amazing innovation, the concept of the best ever espresso cannot simply be a scientific argument. The vast number of variables that go into that tiny cup of coffee provide so much scope for interpretation that quantifying a ‘tasty point’ seems more than impossible to me. Perhaps physicists simply make the best physicists. Sigh, I think I need another coffee.
There’s so much science in coffee – from the design of the machines, to the “9barista” concept, and even the science of freeze drying for instant (yuk) coffee. Please let us know if you’d be interested in a regular series of coffee inspired science articles!
By Rachel Black
Photos by Jodie Wright