Roasting Kinetics and Chemical Composition Changes of Robusta Coffee Beans During Hot Air and Superheated Steam Roasting.

As it is known that Robusta coffee beans exhibit more bitterness, less acidity, and lacks sweetness with unbalance flavor as compared to their Arabica counterpart, a means to improve the quality of Robusta coffee beans is desirable. As a step that contributes to the coffee quality, it is interesting to determine if an alternative roasting technique could lead to the desired improved quality. Here, roasting kinetics and changes in pH and composition of Robusta coffee beans undergoing hot air and superheated steam (SHS) roasting at 190 to 250 °C were investigated. SHS roasting led to higher rates of bed temperature increase, moisture loss and decrease in lightness (L* value) of the beans. Evolutions of the moisture content and L* value of the beans could be well described by the adopted kinetic equations. pH, caffeine, sugars, and organic acids contents of the beans were significantly affected by the temperature and degree of roasting. Interestingly, beans roasted in SHS had lower pH, higher sucrose, glucose, and arabinose contents. Their fructose and acetic acid contents were nevertheless lower, indicating that the beans probably exhibited higher sweetness and citrus-like acidity, but lower unfavorable vinegar-like acidity. PRACTICAL APPLICATION: This study illustrates the possibility to improve the quality of Robusta coffee beans via a physical process. Use of superheated steam roasting at appropriate condition can enhance some desirable characters of Robusta beans; such beans indeed exhibit more resemblance to the more sought-after Arabica beans. This should be of much value to a roasting house and blender in such a way that a higher proportion of Robusta beans can be used in a commercial blend of coffee beans.