The Science of Coffee

As a PhD student who sees too little of the inside of my eyelids, I must be caffeinated on a regular basis. This morning was no exception and so I found myself in a semi-awake state and craving coffee. Not just any coffee. Duke's coffee.

Duke's Lounge is an amazing cafe in Randwick run by a wonderfully kind and talented barista named Duke. Yes, when you thought he couldn't get any cooler, you found out that his name is actually Duke. This is what my fabulous latte looked like this morning. Beautiful, isn't it?

 Duke's Lounge's owner and barista, Duke (left) and his masterpiece latte (right)

Duke is meticulous with his coffee dosing, the extraction process (not under-extracted, not over-extracted - just perfect), and he makes sure that the milk temperature just right. Something that he said today gave me inspiration for my post. 'Coffee making; a scientific love affair, a logical passion'. So many of us drink this beverage on a daily basis and yet, we never stop to think how the perfect cup is made. I thought that while I educated myself on the history and science of coffee, that I would share my new-found knowledge with you.

When people speak of coffee, they are usually speaking about one of two different types: arabica, which comes from the Coffea arabica plant, or robusta, which is from the Coffea canephora plant. Arabica would most likely have been the only coffee cultivated (due to its wonderful flavour), if it is wasn't for something known as coffee leaf rust (CLR). CLR destroys coffee plantations and is caused by a pesky fungus known as Hemileia vastatrix. However, what's been found, is that the robusta plant has resistance to CLR. This has lead to cross-breeding strategies between arabica and robusta plant species. This means that the plant is resistant to CLR and we can still have the yummy arabica coffee that we love.

Now, I won't continue on with anymore of the science behind the plant (although, if you are interested, check out the book 'Espresso Coffee: The Science of Quality' - it has a plethora of information on coffee harvesting). What I want to move on to is the science behind making your coffee.

Let's talk about water temperature. Having the correct temperature is crucial for a good coffee. If the water is too cold, the coffee will be under-extracted and will be, in a word, disgusting. There's a few reasons for this. Firstly, the heat of the water allows for particular volatile compounds to be released from the coffee, giving it that amazing aroma - for the budding chemists, that includes ketones and aldehydes. Secondly, it allows for soluble compounds to contribute to that beautiful taste, which includes caffeine and sugars. Thirdly (and definitely just as important), insoluble compounds are released from the extraction (such as proteins and polysaccharides), which give us the beautiful coffee texture, and the foam that we know as crema. Although there are many theories on optimal water temperature, it is believed to be around 92 degrees Celsius for arabica coffee.

Ok...so I hope that the purists will stay with me after I mention this one word. Milk. I cannot forget to tell you about the substance that gives us macchiatos, lattes, cappuccinos, flat whites and any other milk-happy coffee that I have failed to mention. Now, I won't bore you with too many of the intricate details, but there is a science to getting the milk perfect. One factor is temperature. Anyone who has had burnt or badly foamed milk in their coffee will agree that it smells and tastes really bad. Baristas tend to heat our milk to around 55 - 60 degrees Celsius. Why?

Well, there's the obvious reasons such as being a good drinking temperature and not giving the milk that sour taste. However, one thing you may not know is that the most stable foam for a coffee is made when the milk is heated at 50 - 60 degrees Celsius. Yep, they've done studies to test this. Basically, foam is made when proteins are present on the surface of the milk. The proteins lower the surface tension between the gas and the water, (which come from the steaming) to allow small gas bubbles to be made. These tiny bubbles make up the foam that we see in our coffee. When the temperature increases through steaming the milk, the proteins unfold and expose what are called 'hydrophobic groups'. These hydrophobic groups are required to keep the foam stable. Even more interestingly, foam made with whole cream milk is less stable than skim milk foam, due to its higher fat content. These fats can interact with the proteins and cause disruption of the bubbles.

So, the take home message is: when making a coffee, remember to have your water temperature just right and to not burn your milk. Oh, and thank science for your cup of caffeinated craziness in the morning!

~ Ness