Dear Dr. Brew:
I recently posed the oily coffee question to the Tennessee Division of Water Supply and received the following explanation I thought you might find interesting:
"The Division of Water Supply has received your email regarding an oily substance that appears in your coffee and tea. From your email, it appears that the oily film is caused by the hardness of the water. The attached .pdf file explains the reaction of hard water and coffee. It is similar with tea as well."
The .pdf file (which I have lost) from Chemical Technology and Consulting explains that the calcium in hard water binds with the fatty acids that are released from the coffee during brewing (see transcription below). The lower calcium levels in soft water don’t have this effect which explains why coffee/tea that I brew with bottled water doesn’t produce the oily film. I’m currently in the process of trying to determine which type of water softener is the best to effectively solve this problem in a cost effective, reduced labor manner.:)
Explaining the Oily Film Observed on Coffee. Coffee beans naturally contain coffee oil up to 15% on a dry basis. This coffee oil is comprised of approximately 71% fatty acids or, in other words, oils much like those which would be found in margarines or soaps. These fatty acids contain a hydrophilic area and a hydrophobic area. Because they contain a hydrophilic area, fatty acids are somewhat soluble in water. When water is poured through ground coffee, this naturally occurring coffee oil is dissolved into the water and is carried to the pot which stores the coffee. Here, the oil may remain dissolved and pass unnoticed. This usually the case when the water used is of low hardness. When the water is hard, however, calcium bonds with fatty acids to precipitate them, even though they are usually soluble. Hot water from the coffee maker helps this bond to form more readily, and thus, the precipitated oil is even more pronounced.
A good analogy is soap. Much like the soap molecule, which is sodium or potassium salt of a long chain carboxylic acit (fatty acid), hard water calcium substitutes with the sodium or potassium to form the insoluble scum. In coffee, there is no substitution, but the calcium simply bonds to the fatty acid. The (CH3)(CH2),COOH+Ca2 is the oily film seen on the coffee. It is also the soap scum seen in bathrooms after calcium has bonded with the fatty acid chains in the soap. This is seen in hard water because there is enough calcium present to precipitate the fatty acids. Soft water does not contain enough calcium to precipitate fatty acids. In general, the use of very hard water increases the oily film formation seen on coffee. I hope that you and your readers find this information useful.
Diane, Watertown, TN
Thank you for providing a valuable source of information for coffee lovers everywhere. I had responded to a previous writer and listed a few of the causes directly from the coffee side. I had mentioned that water quality could certainly be one of the issues. Thanks for providing your research to answering coffee issues.
As Red Green would say, "We are all in this together."