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As a SciShow viewer, you can keep  building your STEM skills with a 30 day free trial and 20% off an annual premium  subscription at Brilliant.org/SciShow. If you’re a coffee drinker, you’ve  probably heard a lot of rules about how to brew the perfect cup.

Like, some people say that iced coffee  is an abomination, or that you have to have the greatest Italian-made  machine to make halfway decent java. It gets kind of hard to  separate coffee fact from myth. Luckily, there’s a whole stack  of research that can help.

Brewing a beautiful cup of  liquid gold may be an artform, but it’s also very much a science. And we at SciShow are here to fill  you in on all that rich, balanced, and sometimes nutty research and  show you how to make perfect coffee. [♪ INTRO] It all starts with the coffee bean. Which is the first point to debunk.

Coffee beans aren’t beans at all! Sure, they might look bean-shaped, but  beans are the seeds of legume plants, and coffee isn’t a legume, so those  little nuggets are just normal seeds. But since nearly every English-speaking  person calls them coffee beans, that’s what we’ll stick with in this video.

Ok, so if you’re anything like me,  you spend way too long staring at the shelves in the supermarket only  to end up buying whatever’s on sale. After all, nobody can actually taste  a difference in those blends, right? Well, not so much.

The two most widely used coffee  plants, arabica and robusta, have measurably different amounts  of the four main compounds that give coffee its signature flavors. First you’ve got the alkaloids. Those are compounds that have at  least one nitrogen group in them, and most have a ring structure.

The two main alkaloids in coffee  are caffeine and trigonelline, and both contribute to coffee’s bitter taste. Some studies have found that  unroasted robusta beans have around twice the caffeine that arabica beans  do, but only around two-thirds of the trigonelline, which overall would  make robusta a more bitter bean. The second main group of compounds in  coffee are acids, in particular quinic and caffeic acids and chlorogenic acid,  which is a combination of the two.

And since it has acid in  the name, you can bet that these compounds give coffee  its astringent or sour taste. Robusta has around 1.5 to two times  more chlorogenic acid than arabica on average, so as well as being more  bitter, it’s a little more astringent too. Then there’s sucrose or sugar, which,  unsurprisingly gives coffee its sweet notes, but also breaks down during roasting  to more sweet-tasting compounds.

One reaction in particular, called  the Maillard reaction, happens when amino acids and sugars combine, and  give coffee its browned color and taste. Raw arabica beans have more sucrose than robusta, which gives arabica caramel, fruity, or  fermented aromas when it breaks down. Last up are furans, which give roasted coffee most of its flavor and in particular  those malty or sweet notes.

They’re formed when carbohydrates or  unsaturated fatty acids break down during roasting, and there’s more  of them in roasted arabica versus roasted robusta, giving arabica  beans sweet, almondy flavors.   So picking the perfect bean for the flavor  profile you want in your final cup is pretty vital, even though you might  not be thinking about furans or acid while you’re in the coffee aisle. Ok, but maybe you’re less concerned  about flavor notes and bouquets, and more interested in getting the maximum amount of caffeine as quickly as possible. You’ve got a deadline!

So as you frantically Google how to  jam as much caffeine into your system as possible, you may have  read that lighter roasts pack more of a caffeine punch than darker ones. I’m sorry to tell you that this is  a bit of a myth, and here’s why. See, a lighter roast of  coffee literally just means it was roasted for less time  or at lower temperatures.

And, while some compounds in those beans break down during roasting,  caffeine isn’t one of them. But even though caffeine doesn’t break  down, it can be released during roasting as the pores of the coffee beans close up, gas builds up inside, and the  beans eventually crack open. However, different studies have  assessed the level of caffeine depending on the roast level, and most have pretty  much found no significant difference.

But there can still be a  bit of truth to this myth, depending on how you’re measuring  out your beans for that cup of joe. See, the beans lose mass as they  roast, but they also grow in volume. That means darker roasts get lighter and take up more space than the lightly roasted ones.

So if you’re comparing light  versus dark roast scoop for scoop, you’re probably measuring out fewer beans  and therefore potentially less caffeine. Despite all that, the differences  between a dark and light roast are probably so small, you  wouldn’t notice when drinking it. Alright, you’ve picked your beans and  ground them, and now it’s time to brew.

In scientific speak, many of the properties  of coffee like smell, flavor and texture, come down to chemical and physical kinetics; how chemicals react and how things move. You might not immediately think of  coffee as something that’s moving, but brewing coffee is all about hot water  moving over or around coffee grounds, either because it’s forced with  pressure or thanks to gravity. There are a couple of ways this can happen.

There are infusion methods, like  pour-over, where water flows over the coffee grounds and those grounds soak  in water, usually hot, for a short time before being filtered, all with the help  of gravity to pull the brew through. Infusion methods usually result in  a milder, less harsh cup of coffee and are well-suited to bringing  out lighter, fruitier flavors. Decoction methods, like percolator  coffee, boil the grounds in hot water before condensing the  coffee vapors back into a liquid.

Those high temperatures of a decoction  method mean the coffee compounds are sucked out of the grinds really quickly,  but there’s also not a lot of contact time between the grinds and the water,  so you end up losing some flavor and ending up with a very strong  cup of pretty bitter coffee. Then you’ve got pressure methods, like espresso, where you force hot water over  tightly compacted grounds. Yes, espresso is a method of making  coffee, not a specific kind of bean!

Although you may see bags on the  store shelves labeled “espresso”, it’s not actually espresso unless  it’s brewed into, well, espresso. The combination of slightly different  particle sizes in espresso grinds means those grinds get packed together when you press down on them before  popping them into the machine. And that makes the mix better at  withstanding pressure during brewing.

Then, if there’s good resistance to the  pressure, the energy from the hot water is transferred to the coffee grind block,  extracting more oils with the water, and giving the deep flavor and  creamy texture of espresso. So picking out your method of  coffee concocting matters a lot! If you really want to dial  in particular coffee flavors, you might want to take a hard  look at your water source, too.

Hard water is full of positive ions,  like magnesium and calcium ions, that can grab onto flavorful compounds. And funny enough, those ions might just  make your final brew that much better. See, each of those molecules that gives  coffee its flavor, whether its caffeine or one of the acids, have some  negatively-charged electrons around.

The positively-charged ions in  hard water get attracted to these negative parts of the coffee compounds,  which helps pull them into the water. In a 2014 study, researchers  looked at how well magnesium, sodium and calcium bind to the coffee  compounds like caffeine, malic, citric, quinic and chlorogenic acids as well as a spicy  clove-flavored compound called eugenol. They found that magnesium bound most strongly and closely to all the  compounds, followed by calcium.

Sodium didn’t really cling  onto any of the compounds any more than water molecules did. So according to this study at  least, opt for magnesium rich water if you want to get the maximum  flavor out of your beans. But there’s a catch.

You also need to make sure that water has enough bicarbonate in it to keep  the acidic components in check. So yea, that one coffee shop bro boasting  about how they triple osmosis filter their water might be doing more to  make even less flavorful coffee. So now that you’ve picked out your bag  of beans at the store, you may be staring at that giant coffee grinder thing, trying  to figure out what setting to go for.

Or maybe you took an espresso  class and they told you to take a whole lot of beans  and grind them up super finely. That advice makes sense, since  there’s lots of surface area for the water to pull all those  yummy coffee compounds from. But pulverizing your coffee into  literal dust might not be the way to go.

At least according to mathematics. The Darcy-Forchheimer Law, named  after the engineers that developed it, was originally designed to explain how  water flows through a column of sand. But it works just as well for coffee!

It predicts how water is going to  flow over those coffee grounds, depending on things like the  gradient the water’s moving along, and the area the water’s moving through. And that can tell you something about how  strong or flavorful the brew might be. If the particles are too fine, the water  can’t get through, and that coffee spends ages having the juice sucked out of  it, making it bitter or over-extracted.

Too coarse though and the hot water  just flows right over the grounds, under-extracting the flavors and  leaving you with a sour coffee. In a study from 2020, researchers  created and tested a mathematical model, based partly on the Darcy-Forchheimer Law,  to dial in the perfect espresso setup. When they ground the beans too fine,  they ended up with uneven regions of pressure building up on  that little mound of coffee.

That meant only some of the  coffee was properly extracted, and they ended up with coffees that  were different from one cup to the next. So unlike the typical barista lore,  they found that using less coffee (15 grams instead of the  usual 20) and grinding it more coarsely consistently gave  them much better tasting brews. And just like science in  general, reproducibility is what you want when it comes to your morning cup.

Plus, by using less beans, you  end up with more cups of coffee from your bag and less waste! Win, win! Now, we all know that one curmudgeonly person who insists on ordering their latte extra hot.

Or the one purist who says iced coffee  is an abomination to the coffee artform. Although neither is really 100% right, temperature does play a pretty big role  in how that final cup tastes. The National Coffee Association recommends  a temperature of between 90 and 96 degrees celsius, or 195 and 205  Fahrenheit, just under boiling temperature.

That’s because the temperature of the water dictates how fast all those flavor  compounds seep out of the grind. See, as temperature increases, the water molecules gain energy and interact  more with the coffee grounds. More interaction means more extraction!

Now, some coffee flavor compounds  that are really soluble in water get sucked out of the coffee quickly,  regardless of temperature. These include the quinic, malic and citric acids. But there are also compounds that dissolve differently in water  at different temperatures.

These are mostly the compounds  that give coffee its bitter taste, like the alkaloids we talked about earlier. So with hotter water, you get more of  these bitter compounds leaching out. Extra hot water can also let out  other compounds like pyrazines, which give an earthy or burnt taste.

Which is probably why people  often report that coffee brewed at hotter temperatures  can taste pretty bitter. But scalded coffee is also described  as sour, which doesn’t immediately make sense since the sour compounds  come out regardless of temperature. Coffee scientists, who basically  have my dream job, think that bitter and sour compounds have an additive effect.

So if more bitter compounds like caffeine  get into the water at hotter temps, then this makes the sour  compounds taste more sour. Now, you might be wondering about cold brew. In cold brewing, it’s time doing  all the work, not temperature.

The cool water slooooowly sucks those  flavor compounds out of the grounds. And some compounds, including the  chlorogenic acids and trigonelline, leach out much more slowly. So depending on how long you steep the coffee for, cold brew is generally sweeter and less acidic.

And now that you’ve got your brewer,  your beans, and a whole bunch of science knowledge, you’re ready  to go and make your coffee. All this said, the perfect cup of  coffee is the one you like the best. If you’re looking to improve your brewing game, feel free to bring what you’ve  learned here into the kitchen.

But if your coffee hits the spot, there’s  no science that can tell you you’re wrong! And this one is pretty darn good. I’m gonna grab a refill!

But first, I need to thank Brilliant  for supporting this List Show. Brilliant is an online learning  platform with thousands of interactive lessons in math,  science, and computer science. Like their course in  Measurement, that covers angles, length, area, surface area, and volume.

And that last one might come  in handy when you’re trying to figure out how much coffee you  can fit in your favorite mug. Brilliant courses are even available  offline using their iOS and Android app. So if you’re at one of those  cafes that doesn’t share wifi, you’ll still be able to keep learning.

You can check it out at Brilliant.org/SciShow or in the link in the description down below. That link also gives you a free 30 day trial and 20% off an annual premium Brilliant subscription. Thanks for watching this SciShow video! [♪ OUTRO]