Roast phases explained



Good roaster understands their craft beyond the curve and approaches it based on bean changes. Following the reference is simple, but roasting is more complex than that. There's a lot to be aware of.

The beans undergo a radical transformation. But the change is gradual. You can think of it in 3 steps. Drying phase, Maillard phase and Development Time. Let's see them on the curve:


Tracking software allows you to mark phase changes.


This article refers to concepts explained in the previous post about roast curves. If you haven't read it yet - CLICK HERE



The drying phase

During this phase, there are no chemical reactions happening. The most significant process is the loss of moisture. Keep in mind though - the coffee loses water through the whole duration of the roast.

The phase starts when you charge green coffee into the roaster. Charge temperature is an important indicator here - a control of thermal energy at the start is paramount to consistency.

Green coffee, before roasting.

Your heat application starts immediately as well. You can add gas right off or choose to soak (letting the coffee absorb existing heat).

If you look at the bean temperature curve, the most exciting part of it is the Turning Point (TP). This is the first point the curve indicates the speed of the roast compared to the reference

During drying, there aren't many aromas released. But towards the end of it, you can pick up grassy and green notes.

This phase is very important in profiling. It's establishing roast momentum, building up seed pressure and setting up an inner bean development. Some theories and roast curves are based on that. We’re gonna revisit these in future posts.

The Colour Change - coffee turns yellow.

This stage ends when the coffee goes through the Colour Change (CC). It's subjective when you mark it - but it's vital to be consistent. Personally, I call it when there's no green visible anymore.

The Maillard phase

Named after one of the chemical reactions, it's the phase when most of the transformation happens. The colour changes from yellow to brown. Maillard reaction, Strecker degradation and caramelisation start creating flavour. These can be also called 'browning reactions'

Usually, during this phase, the operator lowers the gas to ensure sufficient development. This phase is for complexity and body. That is due to melanoidins - compounds responsible for viscosity and "brown" flavours - think bread or porter beer.


If you wouldn't spend enough time in this phase the coffee would lack body and complexity.

During the roast aroma will be shifting corresponding to the colour. From green (grass) to yellow (straw, malt) into brown (bread, caramel).


The Development Time

This phase starts with the First Crack (FC) and ends when you drop the batch. The crack is caused by the build-up of pressure, due to water vapour trapped inside the bean.

This causes a rapid moisture release which makes the control of the roast challenging. A lack of skill in managing that is associated with the baking defect. Hence all roasters are extra careful around the First Crack.


Coffee after First Crack.

In Development Time (DT) evaporation and all reactions from Maillard stage are continuing.

The new reaction is pyrolysis. The degradation of organic compounds. A reasonable level of it increases the complexity, but the line is thin. Too much pyrolysis causes burnt, bitter flavours associated with overdevelopment (roastiness).

Most importantly, organic acids are degraded. That is crucial for the control of balance and flavour of the coffee. The longer the DT, the less acidity in the cup. But if you wish to have a complex, multi-dimensional acidity you need to decompose them to a degree.

The art is to balance them all to your preference.

If you go far enough in DT you'll experience the Second Crack. This one is caused by the build-up of carbon dioxide. Coffee gets dark brown, oils bleed to the surface, roasty flavour is dominant. Most of intrinsic flavour characteristics and acidity are lost.

The length of your development time is very likely to be affected by your water quality. Think longer for softer water, but it's complex enough to have a whole course on that.

Aromas during this phase are incredibly varied. It starts off with vinegar (product of caramelisation) - you can use it to mark the FC if you can't hear it. After that, it shifts from floral to fresh fruit to candy, then molasses and dark chocolate. Nearing the second crack you'll experience dry distillates like wood, rubber and carbon.

How do they come together?

It's almost impossible to talk about phases without modulation. Altering their duration shifts flavour and mouthfeel. For example, by extending the Maillard phase you can increase body without roasting darker. Or shift from lime to pink grapefruit by extending the DT. Rob Hoos wrote a great book about it.



An example of poor phase lengths matching.

Once a certain process starts happening, it will continue until the end. Building a lot of pressure in the bean during drying can make controlling the FC difficult. Good inner bean development strategies early on allow for shorter Development Time. It's all connected.

Every single stage is important for solubility and flavour creation. Along matching the curve, getting the same phase lengths is key to consistency. Understanding each step will help you achieve that!


The next article in the series: Curve Explained: Normal

The previous article in the series: Roasting Curve Design

References for the series: Roasting Series References


Photos:
Title - Me in 2018 smelling for First Crack
Curve - light roast El Salvador on P12
Green, Yellow and Brown - Dan Bollinger, CC BY-SA 3.0
Modulation chart - trying to roast Colombia using El Salvador profile

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