Do you know why the can of ground coffee smell so marvellous when we open it the very first time, but not so subsequently, and worst by the time we finish the whole can of coffee? The key word is oxidation.
Shortly after roasting, coffee beans are not ground or packed immediately because of the unstable internal structure of the beans. The beans are usually allowed to rest (degass) under proper technological conditions to improve its sensorial properties.
Since roasted coffee is a dehydrated product, the major factors controlling its stability during storage are oxygen concentration, water activity (especially if roasting has been quenched by adding water) and temperature.
During roasting, high temperature causes large formation of carbon dioxide. This forms an effective barrier, which excludes most of the atmospheric oxygen from the cellular structure so delaying the onset of oxidation. But this also corresponds to a progressive loss of its retention capacity in the bean. Among the hundreds of compounds produced by the Maillard reaction, volatiles and melanoidins play a key role in determining the aroma and body of the brew.
Effects of oxygen, moisture and temperature on staling the coffee:
Oxidation a chemical reaction that combines a substance with oxygen. It is responsible for the staling effect of old coffee. This includes the loss of aromatic volatile compounds and formation of off-flavours, and the loss of non-volatile compounds (eg. fatty acids) that are susceptible to oxidation.
Volatile compounds are retained by melanoidins and polar polysaccharides within the cell wall. But once the cell walls are broken down with the grinding of roasted beans, these volatiles can evaporate easily. This is the reason why it is always recommended that we should only grind our coffee beans just before brewing. That explains the opening statement in this topic, ie. coffe grounds bought off the shelf will never give the full aroma and flavor of the product as compared to whole beans ground on demand.
Non-volatile compounds (eg. fatty acids) or lipids are protected by carbon dioxide from oxidation, but this is short-lived. This is evident in the preparation of espresso when old coffee grounds will not produce as much crema as freshly ground coffee. In addition, the loss of lipids will also affect the flow of the espresso brew, resulting in a poor quality cup of espresso.An important contribution to oxidation off-flavours is the formation of volatile aldahydes, eg that gives that pungent smell.
Temperature at which roasted coffee beans is stored and its fluctuation has a direct effect on the rate coffee bcomes stale. It provides thermal energy that is necessary for staling to take place. A temporary increase in temperature can also result in greater solubility of any oxygen present and heightened water activity.
Moisture can contribute significantly to increase rate of non-enzymatic browning and lipid oxidation.
Light plays a catalytic role in many chemical reactions, in the case of espresso (arabica) blends, particularly rich in unsaturated fatty aicds, light triggers auto-oxidation reaction.
Oil migration. After roasting, carbon dioxide tends to push the oil outwards through the cell walls. Once the oil reaches the bean surface, its stickiness tends to aggregate into lumps on the ground, making brewing irregular. It becomes worse with the absorption of moisture.
So does it mean that we should consume the coffee immediately after roasting? Not exactly. Immediately after the roast master stops the roast, the physical and chemical changes continue within the roasted bean due to the high temperature in the bean. In order to minmize such reactions and bring down the temperature as fast as possible to room temperature, fast cool air blows through the roasted beans. Nonetheless, the large amount of carbon dioxide formed during roasting, aided by the brittle cell structure within the beans , continue to be released as a decreasing rate, as what is termed as degassing. At this time, the bean has not yet fully developed its flavor. So the beans should be allowed to rest for a few hours up to a day, depending on the roast style and method, before we can have a fuller cup.
Dissipation of coffee aroma and flavor is inevitable. Even the most advanced packaging method can only slow down the loss of the volatile and non-volatile compounds. Once the package is open, the bean starts its staling process. Though the first impression may be awesome and unforgettable, this does not last and the subsequent cups do not taste as good as the first one.
