You probably drink coffee everyday. If you are like me you might be a coffee addict which means two-three cups anytime. Right now I am drinking strong, not very tasty Presidentti Coffee in my office in Helsinki. The only thing that I take for breakfast are morning coffee and a cold walk to work.
Today I started to examine what is exactly in my cup. Since I am in science I started to deconstruct everything in my cup. I have water, milk, and dark coffee. Simple right? Well I looked deeper and found a little more than just that. You can always find more if you are willing.
The surface tension of water at room temperature is 72 dynes/cm. A dyne the measurement of surface tension described as the force required to accelerate a mass of one gram at a rate of one centimetre per second squared. So this unit describes the property of the surface of a liquid that allows it to resist an external force. Water can resist mass being put onto arising from surface tension. This is why it can stay in the cup and I can drink it as compared to (Acetone having a low surface tension and would evaporate and mercury having a high surface tension and probably difficult to drink). The fastest way to do this was with the Kibron (static) tensiometer we have in our lab. All measurements including the brewing of the coffee took me about 15 minutes.
Water changes its surface tension depending on its temperature so colder water means a higher surface tension and hotter a lower surface tension. We use a Moccamaster drip coffee machine which boils the water to 92°-96°C. The water had a surface tension of 58.4 dynes/cm.
The temperature of the water helps in the extraction process in brewed coffee. The lower temperature (below that of 88 C) will make weak coffee too high will make bitter or overextracted coffee. The surface tension, vapor pressure and polarity are perfect for extracting caffeine at that temperature (for reasons I won't explain here). Caffeine can also be extracted by organic compounds dichloromethane or benzene (volatile below 20 dynes/cm) to give you decaffeinated coffee.
Lastly there is the milk. Normally I do not put milk if the coffee is good but this stuff is just merely drinkable. A little milk will make it a little bit more bearable. The surface tension of milk depends on the fats in the milk. Homogenized milk which I used increases the surface tension of the milk. Milk has a lower surface tension than water though (44.9 dynes/cm) . If the milk was left out bacteria cultures might break down the fats into smaller free fatty acids lowering the surface tension even more. These free fatty acids give it a rancid flavor. Luckily, I did not put rancid milk in my coffee.
Since I used drip coffee made from MoccaMaster and not an expensive expresso machine I found this coffee quite flaccid with a surface tension of 46.5 dynes/cm and after adding some milk it lowered the surface tension to 43.3 dynes/cm.
I was wondering why just plain old drip coffee can be so boring while an expresso can delight your senses. So, I read from here that the 'lower surface tension enhances the ability of the liquid to coat the papilla where the taste buds are located, thereby enhancing your ability to sense flavor. The lower surface tension also enables the tiny oil droplets to penetrate the pores of the papilla and slowly release the aromatic substances that have bound to the oils. This accounts for the noticeable after taste of coffee brewed in an espresso machine.' However, it really depends on the wettability of the coffee which is measured using a slightly different device to see how the wettability changes over time on a surface. Espresso coffee beverage may be a effective wetting agent for the oral cavity than regular Presidentti coffee giving it a better taste as suggested by Padday (1978) and further corroborate by Ferrari et al.(2007).
I will forever be chasing that perfect cup of coffee. I might just have to go live in Italy where I only pay 0.80 cents for perfection.
(All images were awesomely taken by my colleague Kim)
