HOW TO CONTROL YOUR ROAST

Coffee contains hundreds of aromatic compounds.

Inside of every coffee cherry the seed, fructose, fibers, acids, and proteins contain hundreds of aromatic flavor compounds. These compounds will be developed, transformed and destroyed in the process of roasting.

Naturally occurring organic compounds can be transformed and preserved through the skill and experience of coffee roasters. While you are certainly restricted by the green coffee you are roasting, for better or for worse, you do have some measure of control in determining what flavors emerge. This is why consistent cupping and quality control measures should proceed every roast.

So how do we control our roast to maximize coffee quality?

Roasting is a dynamic process. You should:

Develop a roast plan - what is your batch size or charge weight? Most machines operate best between 50-80% drum capacity.

Start your plan - what is an appropriate charge temperature? Charge temperature is the stable temperature the roaster is preheated to prior to starting a roast. A common charge temp range is 150-200°C (300-400°F). Logically, when you increase your batch size you may increase your charge temperature to accommodate more coffee.

Carry out your plan - how much heat shall you apply and when? Generally we apply little or no heat between charging and the Turning Point (TP). Sometimes, we apply full heat at TP and sometimes we ease into full heat during that 30-120 second timeframe.

Evaluate the roast with ROR- how will you know if the TP, Rate of Rise (ROR) and roast time is accurate? Generally TP occurs within 45-75 seconds. The ROR is a measure of bean temperature acceleration. ROR is typically measured by Degrees/Minute. C°/Min or F°/Min according to your roaster setup.

The ROR is negative, dropping until at TP it becomes zero-0 and turns to positive. Then, the ROR will be very fast as the roast increases temperature quickly (perhaps 20-30C°/Min) from TP to Yellowing (Y). After Y you should see the ROR increase at a medium pace up (perhaps 10-15C°/Min) to First Crack (FC). If you only roast to the end of FC then your ROR should finish very slow (perhaps 3-6C°/Min).

Edit your plan if needed - how do you know if your turning point, rate of rise and roast time is on track and make appropriate adjustments mid-roast? That takes some experience and fast mental math which we will practice together in the Practical Exams.

Evaluate the success of your plan - Did you reach your target end temperature, target development time or development percentage and target roast color? Consider where the roast was strong or may have gone wrong. Record notes to understand how the next roast can be even better, smoother and repeatable.

Confirm with cupping - carry out final evaluation by cupping all coffees to fixed standards. Record strengths and weaknesses in dry fragrance, wet aromas and tastes.

The Sumatra Gayo roast above was charged at 160°C, had a TP near 85°C, had FC near 195°C and was taken into second crack. The roast ended with a drop at 229.3°C for a total time of 12:47.

You will also notice that it speeds through first crack in an effort to preserve a bit more coffee flavor while roasting very dark.

Basic roaster controls & effects

In the next lecture we will begin our Practical Exam Exercise #1. In this exercise you will be responsible to follow 3 prescribed roast plans in order to recognize how variance in gas/heat application impacts your roast time and the rate of rise.

Here is a quick primer on the basic functions you must understand and master.

Fire/Heat - This is the power used to heat the roasting environment. As stated this may be gas or electric powered with atmospheric or electric or infrared burners. No matter your heat source, you will need enough heat to carry your roast to the end without stalling out on power.

Air damper - Air flow is a major source of energy in your roast. We start our roasters with dampers open need 10-50% while later in the roast we generally open dampers 50-100% to finish the roast and blow free any smoke from the drum. These settings vary by roast manufacturer and style.

Drum speed - If you have the ability to adjust the speed of your drum ensure that it is moving sufficient to evenly rotate all coffee and allow the beans to conduct heat evenly to one another. The greatest error is too slow movement. Excessive or variable drum speeds are topics for intermediate and professional roast courses.

Cooling - You should have a cooling tray and cooling fan. If not equipped with a blade or moving arm, you should stir the coffee while it cools. Coffee should cool as quickly as possible to be comfortable to touch - like the temperature of your hand. Cooling coffee stops the roast process by bringing down the internal bean temperatures. Coffee cooling time should never exceed 4 minutes.

Learn to cup coffees like a QC pro.

The best way to understand how to roast better, is to learn to cup your coffees better. The best way to cup your coffees better is to cup them more frequently and approach the cupping table scientifically. Every time you cup your coffees, use the exact same standards. Standards are built upon the Specialty Coffee Association as introduced in the Introduction to Specialty Coffee content and as instructed in the Sensory Training courses.

Consider what quality control habits you can set into place for your QC program and use this attached document link for SCA Cupping Standards and Protocol to help guide you. As always, please ask if anything is unclear.

Understanding Heat.

It's important to understand some basic physics at work in a coffee roaster. How do those beans get hot to turn golden brown with such consistency?

Many rules in thermodynamics apply here.

There are 3 main methods for heat to transfer in a roast. If you can understand where heat comes from and how it is transferred, then your goal becomes clear. You want to channel the correct amount of heat in the right direction to control your roast.

1. As shown above, CONDUCTION is the transfer of heat from contact or touch. This occurs in a roaster when cool green coffee comes into contact with a hot metal drum. We fear the impact of extreme drum heat which causes scorching to the coffee bean face. Later as the roast progresses, conduction transfers heat as hot beans rub against one another in the drum. Bean temperature then is measured by conduction as beans rub against the bean temperature probe.
2. CONVECTION is a primary driver of heat in all roasters. As hot air moves through and around beans it transfers heat effectively throughout the roasting system. Convection also carries with it the moisture and gases present in the roast atmosphere as they are released by the beans. For many sample roasters such as ROEST and Ikawa, or even the Loring production roaster, convection is the sole source of heat production for the roast. This form of heat is most often measured by environmental temperature probes heated by conductive air.
3. The third heat source, sometimes allusive to measure, but always having impact is RADIATION. As roasting progresses through the day the ambient air around the roaster grows ever hotter. As hotter air enters the roaster and as transitional energy between roasts increases, you will notice that roast times shorten and cooling times slow. A skilled roaster avoids burning themselves from the radiant heat emitted from the hot roaster, while accommodating her roast plan according to the radiant heat build up in the system. For example: roast your smallest, low density, low moisture beans first. Roast your large batches and high density/moisture beans later. This form of heat is seldom measured by a temperature probe, but can be inferred from other measures in your roast area.

How can we control so many variables?

You may hear roasters despondently claim, there are too many variables to take into account. However, our goal as roasters is not to learn to control every external variable, but rather to understand exactly which internal variables we pursue.

What is our goal for this coffee and how do we know if we hit the target?

Generally the same great coffee can be achieved on different roasters in different seasons of the year at different roast volumes. Take hope, but keep practicing.