Coffee flavour and aroma are its essential characteristics that can be sensed only after roasting. Green coffee beans have a sharp and repugnant taste. Roasting is a technological process that involves transforming weak, greenish fragrances of green coffee beans into deeply penetrating aromas which all coffee-drinkers know and love. In the roasting process, aromaticity, acidity and other components of coffee flavour and aroma are created and coordinated in order to find the desired taste of the coffee. This critical level in the coffee production process exposes green coffee beans to a heat from the room temperature up to 220°C or more, expanding the bean and altering its complex chemistry of aroma. A skilled roaster combines interrelated elements such as heat and hot air emissions, air volume, rotation speed, temperature and time, to determine best aromatic qualities the coffee may comprise. For that reason, roaster’s skills are essential in the process of making a high-quality black coffee. Roasting time, at any roasting level, may vary but total roast time is maximum 15 minutes.

In the roasting process, a lot of physical and chemical alterations take place in the coffee structure. The most obvious ones are in pigmentation from green to dark brown, even black. It also comes to moisture reduction and weight loss but roasted coffee tends to regain the lost moisture by absorbing it from the air. In the period between 11. and 12. minute of roasting, hundreds of aromas develop (Maillard reactions). In this time, each moment produces a different aroma nuance. The heat effects increase in volume of the coffee. A very interesting segment of the roasting process is when the coffee releases carbon dioxide that causes coffee cells to explode. The pressure in the coffee cells is up to 26 bars. This process is well-known to roasters as "popcorn bang". Experienced roasters are able to roast coffee by using only their sense of hearing. Roasting releases oil that appears in the central groove of the bean, giving the bean certain radiance. Acidity and sugar are reduced and bitterness is strongly emphasized at the end of the roasting process.

Physical and chemical modifications and side effects of roasting:

• Pigment alterations, from green to dark brown, even black;
• Moisture reduction and weight loss of ca 20% but roasted coffee beans tend to regain lost moisture by absorbing it from the air;
• Development of hundreds of aromas (Maillard reactions) in the period between 11. and 12. minute of roasting. Each moment produces a different aroma nuance;
• Heat produces volume enhancement of 70-100%. Coffee releases carbon dioxide that causes coffee cells to explode, the so-called popcorn bang. Experienced roasters are able to roast coffee by using only their sense of hearing;
• Roasting releases oil and a layer of oil appears in the central groove of the bean, giving the bean certain radiance.
• After roasting, acidity and sugar are reduced and bitterness is strongly emphasized.

Roast styles of coffee

There are different styles of coffee roasts and some of them are known as:

• After-dinner roast – Coffee beans are dark roasted but not long enough to become extremely dark brown in colour. Coffee beans become slightly oily, piquant and give sweetish-sour taste.
• Cinnamon roast – This roasting style produces a light cinnamon brown colour and hazelnut flavour. Coffee brewed from a cinnamon roast often has the sourest taste.
• American roast – Coffee beans become chestnut in colour and brewed coffee tastes like caramel. It is a medium roast style.
• Modern roast – Coffee is dark brown with no traces of oil on the surface. Modern roast produces coffee that tastes like caramel or chocolate and is less sour.
• Viennese roast – Coffee is dark brown in colour with some oily traces on the surface. Brewed coffee has a darker roast flavour.
• French roast – Very dark brown beans with high amounts of oil on the bean surface, bitter taste and a dominant, strong aroma. This is the so-called dark roast.
• Italian roast – This roast produces very dark, almost black coffee beans with very oily surface. It produces a burnt flavour, strong and bitter at the same time.
• Espresso roast – Intended for the coffee brewed in espresso machines. This is also a dark roast.