Coffee Roasting is a mixture of science and art. It requires scientific understanding of the chemical reactions occurring during the roasting process and how they impact on the flavour in the coffee bean. The artistry is in the experience and sensory perceptions of the roaster, having a feel for the particular coffee being roasted and how to bring out the best flavour.
There are several different types of coffee roasting machines, but the most common is the drum roaster. A rotating metal drum is heated from below by gas burners using low pressure gas. The coffee is heated by direct contact with the hot metal surface of the drum as well as hot air which flows through the system.
Typically the roast will last from 10-15 minutes. Roast time depends upon how light or dark the roast colour but also the profile will vary according to different beans. Bean density (hard or soft), bean size moisture level, and desired shade or colour will all determine for how long and at what rate the beans are roasted. Similar principles to cooking such as grilling a steak or baking a cake apply. Overcooking or undercooking inside is to be avoided. Roasting too fast of too slow will be apparent in the final result and the taste defects noticeable even if the beans have been roasted to the desired colour. The roast colour will depend on what the intended brewing method is for instance, lighter roast – for filter, darker repost – for espresso.
During the process the roaster will measure the current bean temperature with an accurate temperature probe and adjust the temperature according to the desired profile. Generally the rate of temperature increase will be approximately 10 degrees per minute. There are measurable milestones during the roast process. The first phase is drying where the moisture is removed and at this stage then the beans start to change to a yellowish colour. Later as the beans turn brown we hear an audible popping noise called “first crack” and later if the beans are roasted darker a second crack occurs.
Finally at the end of the roast process the coffee needs to be cooled quickly so it is released into a rotating tray cooled with airflow through a fan. It is then placed into bags with a one way valve to protect freshness and allow CO2 gas to escape.
Finding Fresh Coffee
Freshness, freshness, freshness
Someone once asked me what I thought of a famous brand coffee. My response was “I don’t know I have never tried it fresh.” So what is fresh Coffee?
Well think of coffee like any other food or organic substance or life. It’s born, it has its peak and then it declines and it dies. Bananas for example are picked green become perfect when firm sweet and yellow and then they come black and soft before rotting. Somewhere in the middle they are perfect but thats also a matter of personal preference and depends on the purpose…for example, eat raw or use for a shake in which case you may want it more ripe. The same applies to roasted coffee although, instead of days the life span is usually a few weeks maximum.
People often ask how long coffee stays fresh after roasting. Well, usually we degas the beast for several days or perhaps more than a week. This is to let off CO2 which makes the beans unstable for brewing and give a bitey unpleasant taste masking some of the nice flavours. After degassing they evolve but remain fresh for use for another 2 weeks. After 3 or 4 weeks due to oxidation many of the nice flavours and aroma are disappearing and the coffee can be described as stale. Over time you will notice a gradual change in the flavour of your coffee as well the taste evolves and changes during its shelf life. You will soon learn when its peak flavour has arrived.
So when you next choose your coffee beans first thing is to check for the roast date on the bag. If there isn’t one, most likely the coffee is stale. If there is an expiry date that is not helpful because that may be as much as 12 months post roasting. Coffee once grounded oxidises within minutes, so if you really want good fresh coffee buy a grinder and only grind when ready to brew.
Store your beans in a sealed bag preferably with a one way valve so that the CO2 can escape but air cannot enter the bag maintaining freshness as long as possible. Ideal storage conditions are dry cool environment and not in the refrigerator as condensation and moisture are the coffee beans enemy!
Support small local roasters as your coffee is more likely to be fresh and international or long haul transport in hot or extreme conditions is likely to harm the coffee.