MadSci Network: Chemistry

Re: Why does hot water break down coffee faster than cold water?

Date: Thu Feb 11 06:50:09 1999
Posted By: Carl Custer, Staff, Office Public Health & Science, Scientific Research Oversight Staff , USDA FSIS OPHS
Area of science: Chemistry
ID: 917991518.Ch

Hi Kimberly,

I’ll take the risk of sounding like a lawyer -- but -- we need to define “break down”.

1) If “break down” means the dissolving or extraction of the stuff (solutes) from the coffee grounds then lets call it extraction.
2) If “break down” means the disappearance of instant coffee grains when water is added, then lets call it dissolving. The answer to either of the is the same: (I’ll get to it).
3) If “break down” means the off odors that come from a pot of coffee that sets for a while, lets call that rancidity.


For 1 or 2, extraction or dissolving. Hot water will dissolve more stuff (the scientific term is for dissolved stuff is solute) than cold water. Because it dissolves more, it dissolves faster. Thus, a cup of hot water will extract the solutes from a tablespoon of coffee grounds faster than cold water. A cup of cold water can still extract the solutes from that tablespoon of coffee grounds, but it will take longer, maybe overnight.

For #3, rancidity: This bit requires a more complicated answer. Rancidity is the result of a chemical reaction. The speed of a chemical reaction is proportional to a) the temperature, b) the concentration of the reacting chemicals, and c) the reactivity of the chemicals.

What is rancidity, part 2:

I need to establish several facts:

Fact a: Coffee contains quite a bit of oil. You can see those oils come to the surface of coffee beans roasted for “French- style” or “espresso” coffee. The surface of the beans becomes shiny from the oil.

Fact b: Oils (food oil, not motor oil) are a kind of fat. Fats are made up of chains of carbon atoms (usually 8 to 20 carbon atoms in a chain); each carbon atom is linked to 2 Hydrogen atoms and 2 carbon atoms (like kids holding hands).

Food oils have one or more special links in the carbon chain called “pi bonds” (pi is from the Greek and pronounced pie). These pi bonds are two carbon atoms that are missing two of the hydrogen atoms. Fact b-1: Pi bonds, because of these missing hydrogen atoms, are more reactive (see speed of a chemical reaction). For coffee, one of the most available atoms to react with is oxygen (the stuff we breathe) aaa-nd, when oxygen reacts with the pi bond of an oil --- it becomes rancid. So, in a pot of coffee kept very hot on the heater, you have pi bonds circulating to the top where they can react with oxygen. The hotter the coffee, the faster the reaction.

Fact b-2: Another thing is happening, in a hot pot of coffee: the volatile compounds that make coffee taste like coffee are volatilizing (volatilize: another word for evaporate but it’s usually applied to solids that fly off into the air rather than liquids. Remember, most of what we taste is not from food touching our tongue but from smelling the substances that “volatilize” from the food -- that is why food doesn’t have much taste when our nose is stuffed up.)

So, in a hot pot of coffee the rancidity increases and becomes more apparent because the coffee flavors are volatilizing. This “breakdown” doesn’t happen as fast in a thermos flask because: (1) it’s not as hot, (2) there is less oxygen, and (3) the volatile flavors don’t disappear into the air.

If this doesn’t explain what you need to know, is too simple, or too complicated, please do not hesitate to ask for more information.

Carl S. Custer

BTW, my youngest daughter is going to school just north of you in Evanston. She’s learning how cold Chicago winters are.

Here are a few web pages that might be useful: Glossary of Coffee Terminology:

Acerbic: A taste fault in the coffee brew giving an acrid and sour sensation on the tongue. The result of long-chained organic compounds due to excessive heat during the holding process after brewing.

Rancid: A taste fault giving the coffee brew a highly displeasing taste. The rancid flavor of a roasted coffee is caused by the oxidation of the fats.

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