I keep hearing about this “hydrolysis” thing that happens during brewing, what is it? Can you explain the science behind hydrolysis? How is it different than extraction?
Brewed and confused in Baltimore
Dear brewed and confused,
I feel your pain! Brewing is one of the most complex yet under-researched topics in coffee. In our SCAA education, we have always stressed the three “stages” of brewing to be wetting, extraction, and hydrolysis (Lingle 2011). These terms have always seemed mildly scientific but have not yet been well defined and often leave us with more questions than answers.
Let’s consider extraction first. As the coffee grounds are wet, gases and volatile compounds are dissolved and lost, and soluble compounds are simultaneously drawn out from the coffee. This primarily occurs in the form of chemical and physical reactions between the coffee and the water, and we call it extraction. Really, the term can be used in this context interchangeably with brewing, as the extraction process makes up the nuts & bolts of coffee brewing.
When chemists speak of extraction they are referring to the specific separation of a particular substance from a mixture (or whole product). In our case, we are separating many different soluble solids from a ground coffee product. When a product is brought into contact with a solvent, some components of that product behave as soluble and others as insoluble. In brewing, ground coffee is our product and water is the “universal” solvent. In truth, without extraction, we would forever just have coffee grounds in water. It turns out, the term “extraction” that we use today refers to a few different types of reactions that are acting to transfer those coffee solubles from coffee to water over time. All of these reactions are dependent on temperature, time, and agitation, among other factors. In fact, it is more than likely that a coffee goes through more than one of these processes during brewing.
Hydrolysis is one of these types of reactions. It is a general chemical reaction that occurs when water reacts with another compound to alter it or break it down. During extraction, insoluble or large soluble coffee compounds can be ‘loosened’ from the coffee particles via hydrolysis reactions (Cammenga and others 1997). This is one type of reaction that contributes to the extraction from coffee of solubles, typically larger organic acids. There is some debate as to how important this particular reaction is to coffee brewing (Clarke 1987; Thaler 1979). When compounds go through hydrolysis they go through the process of being ‘hydrolyzed’.
The second type of chemical reaction that occurs during coffee extraction are known as dissolution processes. In dissolution, water solubility of compounds allows them to be dissolved into the water and hence extracted into the coffee beverage. Examples include chlorogenic, acetic, malic, and other acids, as well as caffeine and trigonelline (Arya and Rao 2007). There are two types of dissolution processes, named molecular and ionic for their different mechanisms. Compounds such as salts and acids usually dissolve ionically, as opposed to caffeine, which dissolves molecularly. This Dissolution of water soluble compounds happens naturally when water moves over the coffee grounds during brewing.
Finally, the third type of chemical reaction which occurs during brewing is known as diffusion. In diffusion, wherever solutes are not distributed uniformly throughout a solution, there is a concentration gradient. Consequently, solutes diffuse from zones of high concentration to where it is lower. In coffee, this means areas of high coffee concentrations (the grounds) move to areas of low coffee concentration (the water). An important particular case where these concentration gradients result in the diffusion of compounds is called osmosis. Osmosis is the net movement of solvent molecules through a membrane (in this case, a cell wall) into a region of lower solute concentration. This occurs in order to equalize the solute concentrations on both sides of the “membrane” (the cell wall). This movement of molecules via diffusion does not require stirring and would occur even without the active motion of gravity washing the water over the coffee or other agitation.
We’ll explore this topic further in an upcoming article in the digital edition of The Specialty Coffee Chronicle. In the meantime, happy brewing!
Arya M, Rao LJM. 2007. An Impression of Coffee Carbohydrates. Critical Reviews in Food Science and Nutrition 47(1):51-67.
Cammenga HK, Eggers R, Hinz T, Steer A, Waldmann C. 1997. Extraction in Coffee Processing and Brewing. Proc. 17th ASIC. Nairobi.
Clarke RJ. 1987. Extraction. In: Clarke RJ, Macrae R, editors. Coffee: Volume 2: Technology. New York, NY: Elsevier Science Publishing Co., INC. p. 109-45.
Lingle TR. 2011. The Coffee Brewing Handbook, 2nd Edition ed. Long Beach, CA: Specialty Coffee Association of America.
Thaler H. 1979. The chemistry of coffee extraction in relation to polysaccharides. Food Chemistry 4(1):13-22.
Emma Sage is SCAA coffee science manager. Before moving into the coffee industry, she completed degrees in ecology and botany, and dabbled in the wine industry. She enjoys learning all there is to know about the science of coffee (and more importantly, sharing it with you).
(Photo by Glen Landberg)