Coffees of various origins are usually blended in the trade in different proportions so as to make a cup with varying acidity and taste characteristics. Also, as different batches of coffee are likely to taste different to each other due to the fact that it is a natural product, blending is one way in which constant quality can be achieved. With more than 100 coffee growing regions in the world, each producing beans with distinctive characteristics, proper blending is obviously essential to balance the flavours needed to create a superior espresso. A single coffee bean will generally not possess the complexity necessary for great espresso. Many espresso blends will contain three to seven different types of beans.

Argument still exists among roasters as to which should occur first, the roasting or the blending. Some people believe that roasting each varietal separately, to maximise it's flavour characteristics, and then blending, will produce the best result while others believe that if roasted together, the aromas of the different beans are homogenized during roasting. Blending before roasting certainly has its difficulties in that the homogeneous roasting of beans of different size, weight and country of origin has to be achieved.

When green, coffee keeps for a long time, provided it is protected from damp; keeping it, in fact, improves it. It is entirely devoid of smell. To release the aroma, coffee has to be roasted, an operation which many coffee lovers insist on performing themselves. A good roaster must be part artist, and part scientist, to maintain quality and consistency.

In the development of flavours, roasting is probably the most important of the steps considered so far. Well roasted coffee should be brown, of varying degrees of darkness, but never black. If not sufficiently roasted, it produces a colourless infusion, and is rough and astringent. If over-roasted it produces a black drink, bitter and unpleasant.

In the roasting process coffee beans undergo many pyrolytic reactions which lead to the formation of the substances responsible for their sensory qualities, accompanied by important physical changes. It is during the roasting that the sugars and other carbohydrates within the bean become caramelised, creating a substance which is known as coffee oil. Technically, this fragile chemical is not actually an oil, but it is what gives the coffee its flavour and aroma.

The modern machines used for roasting evolved from crude stone vessels used around 1200AD, through the first cylindrical design about 1650, to computerised roasters now used by major coffee companies. Yet in the 900 years or so that coffee has been roasted, the basic concept remains the same: create a flavourful, evenly roasted bean from the green coffee of the fields.

During the industrial roasting process a small quantity of sugar molasses, or various other products is sometimes added, to 'coat' the berries. This coating, which is permissible by law, gives the berries a better colour and more shiny appearance, prevents the loss of aroma and has the further advantage for the merchant of increasing the weight. Unfortunately this frequently allows him to use inferior quality or damaged grains. Watch out for this when buying coffee!

Speciality coffees, on the other hand, are generally roasted in small batches. The two most common roasting methods are drum and hot-air roasting.

Drum Roasting: Drum-type roasting machines roast the coffee beans as they tumble in a rotating drum that is typically heated by gas or wood. When the desired roast is achieved, the beans are poured into a cooling hopper to keep them from overcooking. There are three main parts in a traditional drum roasting machine: a heat generator, a vessel, where coffee is continuously agitated by rotation of the vessel or by forced heated air, and a cooler where the coffee temperature is reduced.

Hot Air Roasting: The hot-air roaster, also known as a fluid bed roaster, roasts the coffee beans as they tumble on a current of hot air.

Most green coffee is roasted at approximately 400 degrees Fahrenheit. The roasting process causes the coffee beans to swell and increase in size by over 50%, while at the same time greatly reducing their weight.

A lightly roasted bean may range in colour from cinnamon to a light chocolate tan. Lighter roasts are generally not used for espresso since they produce a sharper, more acidic taste than do darker roasts.

Darker roasts, in contrast, have a fuller flavour approaching a bittersweet tang. As the roast darkens, caffeine and acidity decrease proportionately. Dark roasts can range in colour from a medium-chocolate brown with a satin-like luste, to an almost black bean with an oily appearance. The darker the roast, the more you will taste the char, rather than the flavour of the bean. As a result of this, extremely dark roasts will tend to have a smoky flavour and are better suited for brewed coffee rather than espresso.

The amount of oil drawn to the surface of the bean increase proportionately to the length of roasting time.

Beans can be roasted at home by using an ordinary fring pan. Stir often or the beans will burn. A hot-air popcorn popper also does very well. The temperature is just right for roasting coffee, and the motion of the air will keep the beans moving quickly so they don't scorch. At first, the beans will be too heavy for the hot air to move them, so stir them constantly until they start moving.

After roasting, coffee does not keep its aroma for long; it is, therefore, better not to roast or not to buy coffee exceeding one's needs. It is advisable to keep it in jars with well fitting lids or, alternatively it keeps exceptionally well in the freezer if certain guidelines are followed.