Why does it require special care to keep pre-brewed coffee fresh?

Coffee is made from green coffee beans that are roasted to produce flavor changes and browning reactions. During the browning reaction, carbohydrates (starch) and proteins react. When they break down, they produce a myriad of aromatic compounds. Flavor-wise, beans are at their peak 24-72 hours after roasting. After that, they lose aroma and flavor rapidly as carbon dioxide is lost.
 
Ground coffee begins to lose flavor almost immediately after being ground due to the faster release of carbon dioxide and essential oils. Grinding coffee also exposes more surface area that can absorb water, a major source of deterioration.
 
How fast coffee becomes stale depends upon storage conditions. The characteristic flavor and odor compounds can volatilize (evaporate) or oxidize in the presence of oxygen, light, heat, and moisture. As they volatilize, they take some of the aromatic coffee compounds with them. Moisture derived from condensation of water vapor from the air due to temperature changes have the same negative effect.
 
Source(s):
Susan Brewer, Ph.D. University of Illinois, Department of Food Science and Human Nutrition
“Modeling the secondary shelf life of ground roasted coffee.” Anese, M, Manzocco, L, Nicoli, MC. Journal of Agricultural and Food Chemistry Volume: 54 Issue: 15. 5571-5576. 2006
 

 
 

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