I take coffee very seriously. It’s probably the most important meal of the day, and apparently the largest overall dietary source of antioxidants in the United States of America. Regardless of whether you believe antioxidants have a health effect (I’m skeptical), that’s interesting!
Unfortunately, industrially roasted and ground coffee is sometimes adulterated with a variety of unwanted ‘other stuff’: corn, soybeans, wheat husks, etc. Across Southeast Asia, there’s a lot of concern over food adulteration and safety in general, as the cost-driven nature of the market pushes a minority of vendors to dishonest business practices. Here in Vietnam, one of the specific rumors is that coffee from street vendors is not actually coffee, but unsafe chemical flavoring agents mixed with corn silk, roasted coconut husks, and soy. Local news reported that 30% of street coffee doesn’t even contain caffeine.
While I’ve heard some pretty fanciful tales told at street side coffee shops, some of them turned out to be based on some grain (bean?) of truth, and local news has certainly featured it often enough. Then again, I’ve been buying coffee at the same friendly street vendors for years, and take some offense at unfounded accusations directed at them.
This sounds like a job for science, but what can we use to quantify the purity of many coffee samples without spending a fortune? As usual, the solution to the problem (pun intended) was already in the room:
Povidone iodine (also called iodopovidone) is an antiseptic listed on the WHO List of Essential Medicines. It is composed of a water-soluble polymer called povidone, hydrogen iodide, and elemental iodine. The elemental iodine forms a complex with povidone that slowly releases elemental iodine that dissolves into the solution that already contains hydrogen iodide.
Chemistry in the Kitchen
Elemental iodine is a fairly reactive substance. In our case, it forms triiodide ions (I3-) in the solution, giving it a dark brown color (yellow in dilute solution) and acting as an oxidizing agent. It also has the very useful property of forming a starch-triiodide complex that is an incredibly dark blue. This lets us use soluble starch as an indicator for the presence of triiodide ions in a chemical assay called an iodometric titration.
Various foods contain a mix of soluble and insoluble starch – in my case I used tapioca starch and it worked reasonably well as an indicator, and it’s commonly available in nearly every local household.
We can now detect triiodide ions in solution very well, but where does that get us? Consider the composition of dry coffee:
Note the relatively high concentrations of both caffeine and chlorogenic acids. While caffeine is probably familiar to most of us, chlorogenic acid wasn’t at all part of my daily vocabulary. It turns out that these compounds are responsible for much of the astringent and bitter taste of coffee, and that they are produced in higher quantities when the plant is under stress. Robusta coffee, as is commonly grown in Vietnam, also has more of these compounds as opposed to the more expensive Arabica variety. Note that chlorogenic acids do not contain or generate chlorine, it’s just an etymological coincidence.
Some people might call these chemicals ‘antioxidants’, but for our purposes they are something related but more specific: reducing agents. Reducing agents are substances that have a tendency to react with oxidizing agents by losing electrons to them. As such, we expect the caffeine and chlorogenic acids to react with the iodine this way. In theory you can make a terrible electrochemical cell with this (let us know how it goes).
In summary, caffeine and chlorogenic acid are major components of coffee. Iodometric titration is a known analytical method to determine the concentrations of both compounds. A combination of tapioca starch and povidone iodine is reasonable candidate for a reagent to perform an approximate titration. Overall it sounds like a reasonable test for the amount of coffee in something, if we create a standard reference solution.
First Reference Tests
As a side note, one study found green coffee beans from Vietnam to have the highest antioxidant load of any coffee they characterized. This bodes well for our test, as it might create a bigger local differential between genuine and fake coffee.
I arbitrarily mixed up a batch and brewed some coffee, then slowly dripped coffee into the mixture while stirring. It appeared to work!
The next step was to prepare standard solutions of coffee and reagent. After some tests, I found that a 1:200 mixture of 10% povidone iodine and tap water mixed with 5 grams of tapioca starch worked reasonably well. For the standard coffee solution, I brewed 20 grams of Trung Nguyên ‘S’ (a common local brand) ground coffee using a traditional ca phe phin and 150 ml boiling water, then diluted the result 30:1 to give about 300 ml of standard coffee solution.
The reason for the 30:1 dilution is twofold. First, Vietnamese street coffee is sold as a thick syrup that is poured over ice when served; it’s way too concentrated relative to our reagent so better to dilute it. Secondly, I suspect the reagent is sensitive to light, so it’s practical to mix lower concentrations of reagent fresh for every testing session.
As it turned out, it took 2.0 ml of the standard coffee solution to completely change the color of the soluble starch in 10 ml of reagent. Armed with that knowledge, it’s time to design an experiment, get on my motorbike, and purchase a frightening quantity of street coffee.
A Coffee Run for Science
Stay tuned – the samples take a while to process, but we’ll have results in a few days time, along with a full description of the research design. I’ll also be visiting what locals call “The Market of Death” to try to source the alleged fake coffee… I’ll try not to get stabbed and dissolved in acid.
In reality, the market is a pretty neat place and I’m often in the area. Chemical safety issues aside, it’s a really exciting place if you love chemistry and aren’t prone to nightmares.
As an aside, there’s no reason this method couldn’t be used to characterize other substances, such as ascorbic acid (Vitamin C). I tried this as well using over-the-counter tablets and they reacted readily, but decided that investigating a longstanding coffee adulteration scandal was more exciting than doing a study on fruit juice concentrations.
It may also be possible to use the solution to detect the illegal addition of sodium metabisulphite to pork, in order to sell it as beef at a higher margin. At a cost of USD 0.002 per test, we might be onto something, and further investigation is required to see if a similar method could be applied practically.