Roasting for Bean Density and Moisture

Written by: Stevie McCusker

Not all green beans are created equal. Before we even get into roasting approaches to variety size and shape (cue roasters fleeing at the sight of a pacamara), we need to look at two other factors. Density and moisture. You will have noticed that some beans are as tough as tiny diamonds and others are much softer, sometimes greenish-blue in colour. Fruits are mostly made of water, right? Well, guess what? Your green coffee has moisture locked into its cells too.

Understanding density and moisture and how to adjust your roasting to accommodate both is essential for getting the best from your coffee. Each coffee bean's density can significantly affect the roasting process and the resulting flavour profile. Examining profiles based on moisture content involves understanding how the initial moisture level of green coffee affects the roasting process and the final flavour profile.

First, let's look at the concepts in detail.

Bean density refers to the mass of the coffee bean relative to its volume. High-density beans are typically harder and have more cell structure integrity, often found at higher elevation origins. Think Kenya and Ethiopia.

Environmental factors affecting bean density can be altitude, variety and processing. Beans grown at higher altitudes tend to be denser due to cooler temperatures and slower maturation. Different coffee varieties naturally have varying densities. Processing methods like washing can influence the final density of the beans.

When roasting, there are important factors to consider that will help you manage bean density in the roast profile like heat transfer and development time. High-density beans absorb and transfer heat differently compared to low-density beans. They generally require more energy and longer roasting times to develop their flavours fully. High-density beans can handle more aggressive roasting styles without becoming too overdeveloped or dark, whereas low-density beans need a gentler approach to prevent scorching and ensure development is even.

When roasting high-density beans consider things like higher charge temperatures, longer Maillard phases and carefully controlling the post-crack phase to avoid the release of all that built up thermal energy - the dreaded flick. Start with a higher temperature to ensure adequate energy is supplied. Allow more time in the Maillard reaction to develop complex flavours. Carefully monitor the development phase to prevent over-roasting and ensure your cup profile will be balanced. 

For low-density beans, the opposites seem to work. Lower charge temperature: Begin with a slightly lower temperature to avoid scorching.

Make adjustments based on sensory feedback. Scrutinise the connections between what you changed in the roast and what the sensory data gave back in the cup. Iteration: Continuously refine profiles for each bean type to optimise flavour, balancing between underdevelopment and overdevelopment.

Now let's look at how moisture content impacts your roasting.

Moisture content in green coffee typically ranges from 9-11% in specialty grade coffees. Storing green coffee to maintain this range is crucial for at least the following reasons with your roast: Heat transfer: Moisture facilitates uniform heat transfer during roasting. Chemical Reactions: Water plays a key role in the Maillard reaction and caramelisation (sugar browning) which develop flavour, aroma and possibly body. Consistency: Uniform moisture content ensures consistent roasting across batches. If your coffee is stored somewhere that is damp or too dry, you can be sure your profile will get thrown out pretty soon.

Here are some things to consider, depending on your moisture range.

Roasting High Moisture Content (Above 10%) Extend your drying phase: The initial drying phase, or endothermic phase, of roasting will be prolonged. More time is needed to evaporate excess water before reaching the Maillard reaction. Energy consumption: Higher energy input is required to drive off the extra moisture. Flavour development: Risk of uneven roasting, potentially leading to baked or underdeveloped flavours if not managed properly. Crack: Crack may be delayed or less pronounced. Don't burn your ear trying to hear it.

Low Moisture Content (Below 10%) Shortened drying phase: The beans will dry out quickly, which can speed up the overall roast process. Risk of scorching: Faster drying may lead to scorching or tipping if the heat is not managed carefully. Impact on Overall flavour: Dry coffee can lead to a runaway roast. Too much energy with low moisture has the potential for overly rapid development, leading to a loss of complexity in flavour. Coffee may develop an unbalanced cup, be more acidic or have astringent notes.