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Chemical changes during coffee roasting

coffee roasting

During roasting, various chemical reactions, including Maillard and caramelization occur. Coffee beans attain hundreds of new tastes and aromas as they become brown. Roasting makes the beans so fragile that they can easily be ground. Moreover, physical structure of the bean, becoming porous and allowing water inside, results in water soluble flavors come out and become evident to human senses.

A little more than a third of roasted coffee bean’s constituents by mass, are water-soluble materials. When properly brewed, coffee loses 18 to 22 percent by mass, of which 55 to 60 percent are water soluble constituents, and the rest; oil and cellulose, also known as fine particles.

Acidity gives coffee vibrant, live, sophisticated and bright taste qualities. Although some coffee fans misperceive acidity as negative, coffee will tend to taste dull and lifeless without it. A low-acid coffee will taste smooth and chocolaty when brewed in cold water, but will be considered unexciting and boring if the acidity is too low. Besides many plants, coffee contains the most abundant chlorogenic acids (CGA) 6 to 8 percent by dry weight. These compounds contribute to acidity, bitterness and stimulant effect of coffee. Roasting breaks chlorogenic acids consistently, leaving 50% back in low temperature and 20% in high. Quinic and caffeic acid, two strong phenolic secondary acids decomposed from CGA, contribute to coffee body. Lower concentrations of these two; help liveliness and acidity while higher concentrations make coffee undesirably strong and sour.

The other organic acids, found in lower amounts, improve coffee's flavor but taste undesirable when uncontrolled. Lighter roasts increase the concentration of these acids to top, while concentration decreases consistently as roast goes darker. Organic acids’ decomposition makes darker roasts less acidic, compared to lighter roasts. 3 Citric acid makes coffee sour. Low amount of acetic acid tastes winey, while higher amount tastes like vinegar. Malic acid has a pure and mild sour flavor like apple. Phosphoric acid, an inorganic acid highly found in Kenya coffees, gives coffee its specific and precious acidity. Commonly, it is the altitude, where the coffee is grown, that affect the acidity of coffee, where there’s a correlation between the natural ambient moisture, and type, and name of acids produced by coffee plants. 4 Acidity is measured by pH value, where lower pH shows higher acidity and the higher pH shows lower acidity. Beans are high in acid during the first crack, and the acidity decreases as the roasting goes on. pH value of green coffee is about 5,8 and it decreases to 4,8 during the first crack. pH increases and acidity decreases as roasting goes on. The combination and inter se balance of measurable acids indicate the sensory effects of coffee acidity. Consumers’ sensory perceptions of brewed coffee are closely related to measurable acidity.

Raw coffee's sucrose content has a strong influence on its potential acidity and sweetness after roasting. Sucrose contributes to acidity because its caramelization yields acetic acid. Since ripening of coffee fruit correlates with sugar content, harvesting time is of great importance for growers. Darker roasting breaks down as much as 99% of sucrose, while light roasting degrades to perhaps 87%

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