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Coffee Chemistry

October 7, 2022
Written by Julia Raver and Lauren Mellinger

Coffee – an integral part of your morning routine. You wake up and groggily walk to the kitchen to make your morning cup. It has become so integral to our morning routine that a lot of people can’t function without it. Especially in the mornings, having your order messed up at Starbucks or Dunkin Donuts can change the trajectory of your entire day. But how does coffee keep you awake all day? Caffeine is the secret to coffee’s success. 

 

Part 1 – The Chemistry

Caffeine, or C8H10N4O2, is a drug known for its ability to give you energy and boost metabolism. It functions very similarly to xanthine, which is a crystalline drug that is often found in chocolates and teas. This provides an increase in stimulation and wakefulness, the ability to sustain intellectual activity, and decrease in reaction times. In the image below, you can see the similarities between the xanthine and caffeine molecules. The main differences are that caffeine has extra methyl (or CH3 groups) attached to its two rings (National Academy Press, 2001).

Due to the polarity in caffeine molecules, or difference in strength between the nitrogen and oxygen bonds, caffeine exhibits unique bonding forces. It can bond via dipole-dipole forces, where positive and negative atoms of different molecules bond together. Caffeine also bonds via hydrogen bonding with water, which is a strong bonding of hydrogen atoms with carbon atoms of water and caffeine respectively. In layman’s terms, caffeine reacts in the bloodstream by bonding via its nitrogen and oxygen atoms. 

Absorption in the Bloodstream

We can now dive deeper into how caffeine is absorbed in the bloodstream. People absorb caffeine within 45 minutes of ingestion but has an elimination half-life of anywhere from 1.5 to 9.5 hours. This means that if you drink your morning coffee at 8 am, it could still be in your system at 5:30! When absorbed, caffeine undergoes the hepatic first-pass effect, meaning it is not removed by the liver and heads straight for the bloodstream. There, due to the presence of carbons within the structure, caffeine acts as a hydrophilic (or water-liking) molecule and can bond with the water in your tissues. Once bonded with this tissue water, caffeine works with proteins and begin to give you that energy boost you so desperately needed when you drank it at 7am (National Academy Press, 2001). 

Now remembering that caffeine is in fact a drug, it is important to note that it also comes with a certain toxicity level. Luckily for coffee drinkers however, caffeine only becomes fatal when around 10-14g are ingested daily. To get an idea of how much this really is, divide 150 or 200 by your total body weight in kilograms. This gives the toxicity level based on your weight per day (National Academy Press, 2001). For example, if you weigh 110 pounds (or 50kg), 7500 milligrams or 7.5 g of caffeine is your daily toxic dosage of caffeine. An average cup of coffee contains around 100 mg of caffeine so you would need to drink 75 cups of coffee in a day to die of caffeine overdose (Dews, 1982).

Caffeine Appearance

Caffeine is what helps coffee give you energy in the morning but what makes coffee look and taste how it does? Melanoidin, or heterogeneous polymer with a brown pigment, is formed when sugars and amino acids combine at high temperatures (Merriam-Webster). This melanoidin is what gives coffee its nice rich brown color. Depending on the temperature to which the coffee beans are heated, the coffee can taste differently. Once heated above 180 °C, exothermic reactions (that release heat) within coffee beans occur (Rao, Fuller, & Grim, 2020). 

The reactions occurring at these high temperatures are called Maillard reactions, which are defined as reactions between amino acids and sugars. Basically, sugars and amino acids reacted at high temperatures produce brown products, or these melanoidin molecules that create brown pigments (Ellis, 1959). Therefore, different roasting temperatures cause these reactions to go to either more or less completion and are responsible for both the color and taste of your morning beverage. Now that we understand the chemistry behind caffeine, we can look at flavor preferences.

 

Part 2 – The Psychology

Cold beverages have accounted for 75% of Starbucks’ sales in the past quarter (Creswell, 2022). All across the world, coffee drinkers prefer the mellow, balanced flavors of cold coffees. Hot coffee, on the other hand, can only be enjoyed for the fleeting moments between scalding and sour. But why is it that the same beans can produce such different flavors?

The Making

The final cup of coffee is affected by the extraction method — the way water pulls flavor, aroma, and caffeine from the grounds. This is strongly influenced by temperature and time.

For example, in a pour over, water moves over the coffee and produces a brighter tasting coffee. Immersion brews result in a fuller extraction leaving more body in the cup (Kelly, 2018). The cold brew method taking the coffee world by storm involves lower temperatures than your typical drip coffee. To compensate, it needs a much longer extraction time (between 8 and 24 hours) (Córdoba et al., 2021).

Coffee and the Senses

The ability of substances to dissolve out of the grounds and into the water increases with temperature. This causes acids to extract more efficiently in hot brewing. In turn, more sour and bitter elements are released. (Córdoba et al., 2021). Using a lower temperature better retains those delicate flavor components that would degrade or evaporate with exposure to heat (Lane et al., n.d.). Plus, hours of contact with water causes the fats in the beans to emulsify, creating that creamy mouthfeel. Therefore, higher temperatures lead to more nutty, roasty notes — and possibly some unsavory ones as well. Colder temperatures retain flavor compounds and generate a smoother, sweeter beverage (Córdoba et al., 2019).

When you walk into a coffee shop, you are guaranteed to be welcomed by the scent of roasted beans. But you might notice that your cup of cold brew doesn’t smell very strong at all. Since the oils that carry aromas remain in the water, they stay in your coffee. You’ll perceive these as cold brew’s characteristic floral and fruity flavor (Oden, n.d.). Your drink might not have a strong fragrance or bite, but cold brew rewards those who wait (Alkin, 2014).

Is Cold Brew Better?

The novel brewing method also extracts more caffeine with time, which means cold brew is a concentrate. This allows coffee drinkers to adjusted it to their preferred strength and endlessly customize it to their liking. In addition, the lack of acidity may mean cold brew is a better alternative for those with sensitive teeth, heartburn, or GI issues (Rao & Fuller, 2018).

Whether you crave the smokiness of an automatic drip, or the natural sweetness from an hours-long immersion, ultimately, the choice of brew is up to you.

 

Reference List

  • https://www.scienceandfood.org/coffee-brewing-chemistry-hot-brew-and-cold-brew/
  • https://www.sciencedirect.com/science/article/pii/S0924224419305692
  • https://www.nytimes.com/2022/08/02/business/starbucks-quarterly-earnings.html
  • https://www.sciencedirect.com/science/article/pii/S0963996921000387
  • https://www.sciencedirect.com/science/article/pii/S0023643821005168
  • https://doi.org/10.1146/annurev.nu.02.070182.001543
  • https://doi.org/10.1016/s0096-5332(08)60223-4
  • https://blog.lacolombe.com/2018/09/19/brewing-method-affects taste/#:~:text=Immersion%20and%20pourover%20brewing%20each,%E2%80%9D%2Dtasting%2C%20brighter%20coffee
  • https://journals.uvic.ca/index.php/arbutus/article/view/16816
  • https://www.merriam-webster.com/medical/melanoidin
  • https://www.ncbi.nlm.nih.gov/books/NBK223808/
  • https://www.javapresse.com/blogs/cold-brew/hot-brewed-vs-cold-brewed-coffee
  • https://www.nature.com/articles/s41598-018-34392-w
  • https://doi.org/10.3390/foods9070902

 

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Hi I'm Lauren! I'm a second year Chemical Engineering major with a passion for sustainability. I have always loved reading about current events in science and I can't wait to educate more people about new discoveries in STEM!

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.