Roasting
Green coffee beans are not edible. To achieve this effect they must undergo a process of roasting, this procedure represents the fundamental phase that determines the final quality of the product. The process lasts 15 minutes and the beans are roasted inside a specialised machine which creates their characteristic aroma and flavour. During roasting the beans will be treated at over 160° which provokes a series of chemical and physical reactions with proteins and sugars helping to produce the final, unmistakeable coffee aroma. Externally the roasting produces a dramatic increase in the size of the bean and a 20% reduction in its weight. Then there is the separation from the internal membrane and the change to a brown colour which gradually intensifies. Roasting does not alter the level of caffeine present in the bean. The intensity and degree of roasting are two parameters which can vary depending on the preference of the consumer. In Italy the beans are roasted for longer to generate the preferred stronger flavour. Other areas of the world prefer a lighter tasting coffee with a shorter roasting time. Once the correct roast has been achieved it is necessary to stop the heat supply removing the beans to avoid damaging them (which occurs at temperatures of over 240°). The beans are quickly cooled with jets of cold air. At the end of this phase, the chemical, physical and aromatic qualities of the roasted beans will depend entirely on the roasting procedure they have undergone. The roasted product can be divided into three categories. “dark, “medium” and “light”. In reality the discussion is much more complicated as factors like acidity and body are also determined by roasting. It is in this field of roasting techniques that the biggest technological advances have been made. Particularly in the two most delicate areas: the roasting and cooling of the beans. In the latest roasting machinery the beans are not directly in contact with heat but are roasted by a flow of heated air making the outer layer of the bean “fluid” giving an excellent level of roasting constancy and avoiding beans coming into contact with hot metal components. At the end of the roasting the coffee sits for six hours in special containers before being packaged. PHYSICAL MODIFICATIONS The bean will drastically change during the roasting process. In particular we can note: - a reduction in weight of 10 – 15 % due to water loss. - A sixty percent increase in volume due to the presence of gases like CO2 inside the bean. These gases create high pressure (20 – 25 atmospheres) and provoke the swelling of the bean. - A change in the consistency of the bean from elastic to brittle. - A change in colour from green to brown for the caramelisation of the sugars and CO2 and for the creation of pigmented substances generated by a Streeker reaction and dependent on the roasting level. The higher the level the darker the coffee. CHEMICAL MODIFICATIONS The chemical changes that occur in coffee regard the variation in the quantity of some newly formed elements which are fundamental for the characteristics of the future drink. - water reduces from 10% to 1% - sugars reduce from 10% to 2% - azotes pass from 12% to 14% - fats, contrary to the reduction in weight of the bean, increase from 12% to 16%. If extracted in the correct way these are essential for the body, silky consistency and aroma of the drink. - Acidity decreases thanks to the reduction from 7% to 4.5% in chlorogenic acids. For this reason there is an inverse relationship between roasting level and the coffee’s final acidity. - Trigoline reduces from 1% to 0.5% improving flavour and how we digest the drink. - Spermine, putrescine and spermidine present in the green coffee bean and causing a putrid smell, disappear almost completely. - The separation of sugars leads to the presence of piruvaldedes, furfurol and 2-3 butanol, all very important for the aroma. Pirazine and oxazoli also form from the amino acids and piridine from the trigoneline. - Free radical acids like cumarico, degrade in simple perfumed radicals. Terpenods degrade in simple terpens (linolo and others) and in furan. - Many compounds derive from sulphate amino acids like hydrogen sulphate. These compounds have a fundamental importance for the aroma. Furfurtirol, dimetilsulphate, hydrogen sulphate, tiofene lead to the presence of defects like the smell of stagnation or rotten eggs. But ethers, aldeids and ceton give notes of honey, walnut and caramel. Thiazols have the aroma of walnut and roasting. - Fats produce organic compounds which through their reaction with amino, amino acids and protein generate eterocyclic aromatics. - Negative and positive aromas are derived from pirazine (earth, potatoes, grass, walnut, roasting) which are inversely proportional to their concentration. - At high roasting temperatures, piridine produces a bitter coffee with flavours of butter and grass. At low temperatures, they give walnut and roasting aromas. - The pirroli increase with pH, duration, and temperature of roasting. They are beneficial only in small quantities, giving bread and cereal aromas. If in excess, they bring medicinal and petroleum flavours. - The chemical decomposition of sugars creates the furfurilic alcohol typical in black coffee and gives bitter and burnt flavours. - The aldeons with fruit and floral notes increase dramatically. Incorrect roasting produces less pleasant flavours like 2-butanol with a buttery taste. - Excessive roasting increases the phenols of bitter taste. At correct levels phenols give spicy notes such as cloves. - The interaction between amino-acids and sugars considerably increases the richness of aroma in terms of variety of tones. - The new substances created represent 30% of the bean’s weight. In terms of quantity they are principally caramel and CO2 while in terms of quality various volatile molecules play a primary role. These are created in a Maillard reaction at temperatures between 160 and 230 °. This means that the roasting level is fundamental for determining different coffee realities even when beginning from the same blend.
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